GB2530716A - Cascade circuit tester - Google Patents

Abstract

A circuit tester, e.g. a microohm meter, comprises a power supply 1, 2, a voltage generator 4, an energy storage device e.g. a capacitor 9, a monitor unit 21 and terminals 11, 12 for connecting to a circuit under test 13. In use, the capacitor is charged to a known voltage and then discharged into the circuit under test. The response from the circuit under test is measured and used to calculate impedance. Safety of the user may be achieved by means of an alarm, shorting the output terminals of the tester when not in use, measuring the load connected before testing to check for the impedance of a human body, or stepping up the voltage in stages to avoid electric shock. Measurements relating to resonant frequency, faults, break down of insulation may also be made. The circuit under test may be an induction motor or a printed circuit board. The device may also be used to measure conductivity of a material.

Description

Cascade Circuit Tester This invention relates to a device that is used to determine various impedance characteristics of an electrical test circuit, by subjecting it to particular electrical stimuli, in a safe manner, while monitoring and taking a number of measurements as the circuit responds, in it's own unique way, to each applied stimulus, to calculate, using the measured results, the impedance and other numerical characteristics of the circuit, to store and display these characteristics and provide a database and reference for comparison of any circuits tested.

It can be difficult to measure and determine if a fault has occurred when, for example, a large induction motor winding is damaged internally where an electrical conductor is not leaking to earth, but two adjacent turns in a winding are shorted together. This fault causes a motor drive to trip and before going to the trouble and expense of changing the motor, there is a requirement to confirm the motor is indeed faulty, eliminating the motor drive as another possible cause of the trip. Small inductance meters and micro-ohm meters have difficulty measuring accurately large current windings and very low ohm circuits of this nature, to determine if such a fault exists, even when the three motor windings are tested and compared. In practice even the highest current generating instruments, which are currently used to try and determine the resistance of a large motor winding, fail to provide measurements of sufficient discernment, due to problems associated with their method of measurement, restrictions in their usage, the contamination of probe connection points and other problems. This device addresses many of these problems by using a different measurement technique, features of which may be used to overcome, or reduce, the severity of the problems.

In a three phase induction motor the magnetic properties can change when, for example, a rotor bar is cracked or broken, but the windings remain intact, and even the best micro-resistance measurements cannot detect this and therefore they indicate no damage. Accurate inductive and resistive measurements together can identify these faults, and instruments need to use substantial power to do this most effectively, but portable instruments, which use sustained currents when testing, are limited by the weight of their bafteries and the maximum power they are able to provide, Micro-ohmmeters and other instruments, typically measure the resistance of a test circuit by injecting a specific and highly regulated test current into the circuit for a period of time, to allow the test circuit to stabilise, before taking a measurement of the voltage drop across the test circuit, which, depending on the circuit impedance, may be extremely small, when measuring fractions of an ohm, or larger when measuring high ohm circuits, with the same test current. This method of testing has an inherent flaw namely, that the measured voltage and amount of power used, to inject a particular test current into a low ohm circuit, gets less as the test circuit resistance diminishes. In the case of a motor the larger the winding, the lower is its resistance, and more power is required to measure large windings accurately, not less. For this reason micro-ohmmeters have different ranges, where higher currents measure the larger windings, but using increasingly large direct currents also causes measurement problems, as they become more difficult to regulate and stabilise, and any alternating current ripple causes inductive errors. The power wasted, in regulating these large currents, should be used to measure.

When measuring resistance th direct current it must be continuously smoothed, and kept ripple free, to avoid inductance errors. This device does not require current to be controlled, when the natural circuit impedance does this, and it measures with the current constantly changing, and preferably, a low inductance current transfonner, or other device, may be used to monitor these changes and test current samples taken for measurement purposes, as required.

This device does not waste power as it waits for a circuit to stabilise. Measurements start immediately the test circuit is energised, and others may later be taken, until the last bit of energy is absorbed naturally into the circuit. Many different aspects, of the test circuit inductive and resistive response, may be monitored and measured, while the circuit remains active. Individual readings have best accuracy when high power levels, are used to stimulate the test circuit, while the numerous readings provide the facility to compare and evaluate circuits, as never before.

When measuring with this device, a known electrical or magnetic charge is generated and injected into a test circuit which, if it is both resistive and inductive, may produce a voltage response similar to that shown in the graph, figure 2. A number of time measurements may be taken during the overall response period, preferably between key points, such as those indicated in the graph, figure 2, by the numbers 27 to 37, where monitoring circuits can more easily detect these instants in time. Voltage and current levels can also be monitored and measured in the same response period, along with resonant frequency, the wavelength of which may be timed from key points 28 to 32, or 32 to 36, 27 to 3 1 and others, whereby these readings can be displayed and used to calculate the circuit parameters.

TypicaHy, a portable broad application range micro-ohm meter limits itself to provide no more than 500 watts of power, and uses a maximum of 50 volts at 10 amperes of direct current, which means, by implementing Ohms' Law, that full instrument power can only be utilised on a small motor winding of exactly 5 ohms resistance, but such a small winding gets warm, because of the continuous direct current and for accuracy, the readings then have to be adjusted with a temperature compensating factor. The resistance of an electrical conductor reduces by the square of its temperature, and a 10 degree Celsius rise in conductor temperature can double it's resistance, which demonstrates how highly temperature sensitive these measurements are and how it is preferable to cause no circuit heating as measurements occur.

The voltage such an instrument generates across the test circuit, as it is measuring and supplying amperes of test current, into a large motor winding of very low impedance, is very small indeed and the amount of power used is accordingly very small, often less than I watt, which partially explains why these instruments fail to provide sufficiently discerning resistance readings on many of the larger circuits they attempt to measure and why they perform only slightly better than smaller hand held micro-ohmmeters, which typically produce a t ampere test current.

Heavy' specialist portable instruments are available, which can supply several hundred amperes, for use in testing the larger current carrying circuits, but the voltage drops across these circuits still remain low during a measurement, and the maximum power they generate is still low, in comparison to the momentary powers used in testing with embodiments of this invention.

It is evident, when measuring to find both electrical and magnetic faults in any low impedance circuit, that a portable device, which is capable of quickly carrying out a momentary high power, resistive and reactive measurement, and which produces a multiplicity of measurements from just a single test, will be very energy efficient and prove discerningly superior in performance to another instrument, which can produce only a single lower power measurement, after a proportionally longer period of measurement time.

Micro-ohm meters start to measure, but must wait until any test circuit dynamics cease before they can take a reading, whereas this device encourages this dynamic activity, and takes readings while any activity persists. It does not waste this opportunity to evaluate the reactive electrical characteristics of the test circuit, instead it may, for accuracy, enhance the circuit dynamics to induce a more easily measured response.

This device preferably comprises of an insulated portable analogue, digital electrical and electronic test unit, fitted with switches, potentiometers or other controls, with the means to monitor, record, hold, calculate and display measurement results, complete with insulated conducting eads and test probes or clips, designed to make good conductivity connections onto a test circuit. The device can be used to measure, calculate and display a resistive reading, or a reactive reading, or both and many other significant readings, especially on low impedance inductive circuits, with no significant heating of the test circuit.

The measurements may be carried out in a short duration of time, typically taking less than two seconds to fully complete and where much of this time may be taken in generating and storing magnetic or electrical test charges, within this device. Each stored charge voltage is appropriately selected for switching into a test circuit and the natural response of the test circuit to this stimulus is monitored, and various readings may be taken, during the period of time that it takes for the charge to dissipate, until all electrical and magnetic activity ceases. A magnetic charge may be used to initially induce test circuit oscillations and then be removed after it's energy is absorbed into a known circuit capacitor, allowing the circuit to oscillate further, under the influence of the test circuit resistance and inductance, to produce a natural resonant frequency, taking measurements as this occurs.

As an example of how this invention may be used, a simple device to accurately measure the duration of a particular response is shown in Figure, which uses analogue circuitry with high gain comparators to start and stop the timing circuit, but fast digital circuitry could also be used to capture events. It captures the time from when a capacitive charge is switched into a test circuit, until that charge is first depleted to zero volts, The single numerical value this time measuring device produces and displays can be used for comparing circuits, or it could be calibrated to measure resistive circuits in ohms.

In the case of a three phase electric motor, each winding and its' magnetic circuit can be assessed, whereby an undamaged motor will produce similar time readings as each winding is measured, or any difference would indicate that the motor windings are out of balance and the motor is faulty.

Discerning when a motor is faulty or not, is often all that is required of a measurement instrument and these simple time measurements do this most effectively, as they reflect both the electrical and the magnetic properties, of each motor winding, in one quick measurement, The momentary current levels that can be produced by a relatively small portable device, when testing the larger power circuits in this way, can be extremely high, typically 200 amperes or more, depending on the circuit impedance and the voltage and quantity of stored charge used.

These currents are considerably higher than those generated in some other portable micro-ohm meters but, being momentary, they cause negligible heating and are economical in the usage of instrument battery power. The momentary power levels used to stimulate a test circuit with this device, when comparing instruments on a weight to weight basis, are dramatically higher than in any other portable instrument. When a quantity of high voltage charge, is switched into a large motor winding, the test power may reach an instantaneous value of 20 kilowatts, when the test charge voltage has dropped to say, 100 volts, as the current in the circuit has momentarily reached amperes.

When comparing this test power with the maximum power generated by a conventional micro-ohmmeter, where each measures the same fractional ohm circuit, the instantaneous power levels reached, with this device, may be thousands of times greater in magnitude, inducing readings which have more significant figures, due to the much stronger circuit stimulus. The accuracy of measurement is considerably improved, and yet the voltage and current levels, used in the test, all remain safely below the maximum ratings of the test circuit. The speed of measurement ensures that heat build up is negligible, causing little test circuit resistance change.

Many instruments cannot always sufficiently excite the circuit they try to test, as they are limited by their own pre-programmed maximum measurement parameters, which are set for safety in a worse case scenario that rarely occurs. The maximum power generated, when this device measures, is influenced by the test circuit impedance, the charging voltage level, and the capacitance value used in the test. These factors also determine how long an excitation response endures in a test circuit. A minimum amount of charge needs to be used to provide sufficient time to accurately measure the test circuit responses, and there are ways of extending this time period by using the available charge more effectively.

When a test circuit is measured a number of times, using a single charge that gets increasingly powerful after each measurement, the monitoring and measuring of the circuit response progressively produces more accurate readings, as a larger charge take longer to dissipate in the circuit, allowing more time to measure, as well as inducing circuit reactions of greater magnitude, which are more easily measured. There is an optimum quantity of electrical charge, which produces a good measurement and, where this is exceeded, the accuracy of measurement does increase, but with diminishing improvements as more power is used.

If a test charge voltage is of a similar magnitude to the normal supply of the test circuit, it is designed to tolerate this, and current is naturally limited by the circuit to cause no damage. The amount of stored charge may be changed, by using different size capacitors, or by charging them to different voltage levels. The duration and accuracy of a test circuit response is determined by the charging capacitor, but by repeating measurements and averaging the results, then increased accuracies are achieved.

This method of measurement allows other electrica' principles, equations and calculations to be used, in addition to the basic Ohms' Law equation, to produce quantified test results from just one quick measurement.

Electrical charges, that are generated and stored within this device, may be switched into a test circuit in succession, with changes in polarity as required, and they can be arranged so that the magnitude and direction of the test current and voltage may simulate, for a period of time, any alternating current power supply at a required frequency, or the timing and polarity of discharges may be arranged to increase the test circuit response, to more easily measure the circuit parameters.

A series of discharges can be arranged by triggering them in cascade, where a particular event, occurring in response to the initial discharge, can be chosen to trigger the next discharge, which then triggers the next in the same way, and so on, until the required number of discharges are complete. This process is a natural way of timing a cascade of discharge events, where the test circuit itself dictates the response, not a timer, and multiple switching provides more opportunity to monitor circuit reactions and measure them, A particular event to choose after each discharge, for triggering the next, in a cascade of switching, may be the moment all electrical activity ceases, it may be the moment after the first reverse oscillation, as an inductive circuit starts to oscillate, or it may be at the moment of maximum reverse voltage. There are many trigger point options and the best choice of which to use may be selectable, according to the expected response of the test circuit, or as this device decides, after it statistically evaluates the test readings.

This device may measure the parameters of a newly manufactured, or repaired, test circuit using a simulated mains supply, and where the measurements prove the circuit to be within the designed tolerance, as it compares favourably with other good circuits, then that test circuit is highly assured to function correctly and to operate on the mains power supply without insulation failure.

With this device the test current and voltage is constantly changing throughout a measurement, but when the same known quantity of charge is used for a series of tests, on identical circuits, the timed readings all prove consistent and the voltage and current levels are the same at any given instant during each test. This repeatability provides the means to accurately measure the test circuit parameters.

If capacitors are used for storing test charges, it is preferable if they are of a quality none polarised type, to provide good stability and repeatability up to their maximum ratings, while their known values are used in calculations to establish the other test circuit parameters.

A single discharge test can produce a number of different reactive, resistive, voltage, current, time, frequency arid other responses, which together defines the test circuit, so that only another identically reactive circuit will produce a similar set of responses. A selection of the responses may be monitored and measured by auxiliary circuits, which each look for and measure some aspect of the test circuit reaction to a stimulus and there are many from which to choose.

Rather than measure using one large quantity of charge, stored in a single capacitor, it is preferred that such a charge be shared and stored in a number of capacitors, to be sequentially switched into the test circuit in a cascade of measurements, and as this increases the overall measuring time, the accuracy of measurement improves with the greater opportunity to measure, and the charge is more efficiently utilised, From this it may be concluded that, for accuracy, using many stored charges in cascade is preferable to using just a few, but a compromise is required, when individual quantities of stored charge become so small, that they prove insufficient to effectively excite the test circuit and induce an easily measured response, while the consistent and accurate timing of events in increasingly short response periods becomes more difficult and error prone.

The desirable facility of taking a number of measurements quickly, but only using the results of the latter measurements, after the readings have become consistent, may be inbuilt into this device to further improve accuracy by eliminating unwanted test circuit impedances, Even when using Kelvinated test probes additional impedances, caused by contaminated connection points, remain unseen in a test circuit, which introduces measurement errors, but with this device these errors can be eliminated, for high power tests may be deliberately used to break down contamination and establish a good connection, It requires both current and a sufficiently high voltage to effectively punch through and eliminate a contamination layeii When taking a series of measurements, trend analysis may be used on the results to determine if a varying contaminated test probe connection was evident, or had been eliminated, during the tests, whereby only the later consistent results may be selected for display and any bad readings discarded. The use of trend analysis, or other measurement selection process, increases confidence in the accuracy of results and is an option to be used with this device. When repeatedly measuring a contaminated test circuit, the initial measurement response period usually lasts the longest, as it has the highest circuit contamination impedance, but as the test current bums away the contamination, the response times get shorteç until they become consistent, indicating that a good connection has been established.

A further programmable feature of this device may be the facility to measure and cany out a selection of repeated tests, using trend analysis to eliminate bad readings, and to produce one set of averaged and assured readings, from just a single press of the start test button, or to display that the test circuit remains unstable and needs further attention.

Another embodiment of this device can be used for insulation testing, but with an additional feature. It may be connected to the terminals of a single winding and readings taken to measure circuit impedance and to stress the insulation, between the turns of the winding. By repeating measurements and increasing voltage levels it can be determined if the inter-winding insulation breaks down and at what voltage this occurs. This principle of measurement may be used similarly, for high impedance insulation testing, but it is preferable if only a small amount of test chaise is used and current is limited, especially at high voltages, so that in the event of a flash-over, no damage occurs to the test circuit and current does not exceed the safe level.

It is preferable that any test, which could cause shock, commences only after a sound or light warning has been given, or a warning can be continuously given when any high voltage, or powerful charge, has been generated within this device. As soon as a test is completed the output terminals may be shorted, to remove any residual charge. For safet3ç it is preferable if this shorting element is connected at all times, except while taking a measurement or monitoring the test circuit.

In an embodiment of this device, which has the facility to measure as a high voltage insulation tester, it is preferable if a preliminary low voltage monitoring of the connected test load is carried out immediately before allowing a high voltage measurement to go ahead, to establish if the load is suitable to be measured and is outside the resistance range, where it may be a human body connected across the test probes.

Sudden current spikes, reduced rates of voltage charging or sudden voltage drops across the charged test capacitors, all indicate a resistance breakdown has occurred in a high impedance circuit and these may be detected by the monitoring circuits. Repeated impedance readings, taken with an ever increasing test voltage, should remain consistent when the circuit insulation stays intact, but where the higher voltage resistance readings drop in value, the circuit insulation is shown to be faulty.

When testing mid range impedances, at elevated voltages with full current capability, a programmed measurement of stepping up the test voltage, in a series of insulation tests, can act as a final way to forcibly deter a person from receiving a large voltage shock, should they deliberately and perversely overcome all the safety circuits, ignore light and sound warnings and start to measure themselves by holding the test probes, for they will start to feel the lower voltages, as they build up, and be increasingly induced to let go, by these final warnings of higher voltages to come.

In summary, safety is best assured, in embodiments of this device, by maintaining low currents, when using high voltage for high impedance testing, by monitoring test circuits and refusing to carry out high voltage and current measurements, in anything other than low impedance test circuits, and by using safe working voltages, when measuring impedances other than these.

It is preferable if connections to the test circuit are made using strongly sprung and none magnetic probes with multiple contacts, made from highly conductive materials, designed to resist electrical arcing and to share the test current, High conductivity Test Leads of short length are preferably used, as the overall test circuit impedance needs to be as low as possible, where the tested circuit should be a significant part of this overall impedance, for better discernment. Once connected it is preferable if each probe is not moved at all, until all measurements are completed, so that no additional contact impedance problems are introduced.

This device may indicate that no consistent readings occurred while testing; in which case the test circuit connection points should be cleaned and the test probes repositioned before further measurements are attempted.

When taking measurements on similar inductive circuits, there is a requirement to obtain a number of accurate inductive response readings from each circuit, for a discerning comparison to be made and therefore it is preferable if many details of that response is monitored, measured and recorded to do this more effectively Many significant details may be monitored and measured by embodiments of this device, but the choice of what may be displayed is limited by practical considerations, such as cost, types of circuit to be measured, voltage and current levels required, level of accuracy, speed of measurement, size and weight.

Embodiments of this device may range from an instrument that produces only one reading from a single low voltage discharge, to an instrument that has multiple test circuit outputs, where each facilitates a series of multiple assured measurements, using a programmable cascade of high voltage large current discharge events, and displays many results for each discharge, storing the data in a comparison database, with every safety feature instalied.

This device generates and may momentarily store an electrical charge or a number of charges, of a predetermined level and polarity, prior to switching the charge, or charges, into a test circuit.

The period of time, that each charge takes to completely dissipate into a test circuit, is determined by the overali impedance of that circuit. As each charge is suddenly applied to that test circuit with little to limit the electrical current other than the test circuit itself, there is a unique characteristic inductive, electrically resistive and capacitive response and a series of discharges can be arranged to amplify this response or to be switched at key moments to produce other effects.

This device may monitor and measure the impedance response, as the power dissipates in the test circuit, using a number of electrical, magnetic, digital or analogue devices and circuitry.

A multiplicity of time periods, resonant frequency readings, current and voltage levels may be measured and recorded by this device during a single or a cascade of discharge events.

If a capacitor is used to store an electrical charge and the test circuit is inductive, when the two are connected together they form a resonant circuit, or what is also called a Tank Circuit, The theory and operation of such circuits are well documented and a knowledge and understanding of such is assumed here, so that the operating principles of this device can be understood and building embodiments of this device can be achieved, by any competent engineer after reading

this description.

An embodiment of this device can be arranged to work as a micro-ohmmeter and it may be classified as such, though a better classification might be to call it an Impedance-meter, for it measures inductance as well as ohms, but this title is also inadequate, as this device is also used to measure resonant frequency and a variety of timed events, Another title for this device is to call it a Tank Circuit Meter, naming it after the main principle it uses to measure inductive test circuits.

This device may record time periods and take other electrical measurements from when a charged capacitor, or other electrical storage device, is switched to discharge into a test circuit, until the charge in the capacitor is depleted and also while the circuit resonates naturafly and until any oscillations cease, dampened by the circuit resistance, If the storage device is energized, with an exact known quantity of electrical charge, then the time it takes to discharge into a test circuit is consistently repeatable, when testing the same or a similar circuit. The time that it takes for a capacitor to initially discharge fully into an inductive test circuit is dependent on the impedance of that circuit and this time period may be monitored and measured. The extended period of time that it takes for the subsequent resonant oscillations to diminish, under the dampening effect of the resistance in the circuit, may also be measured and this is directly related to the value of the inductance in the circuit. The overall period of time, from the start of a discharge until ail electrical and magnetic activity ceases, may be broken down into well defined time intervals, by monitoring the results of each discharge and recording the individual lengths of time as significant events unfold, while the circuit is electrically activity.

Together these time interval readings define the impedance characteristics of the test circuit and, if required, can be used to mathematically resolve and indicate, in Ohms, the resistance and in Henries, the inductance of the test circuit, while significant time readings, current and voltage levels and resonant frequency measurements may be used simply to compare different test circuits.

The mathematics needed to resolve the time readings into resistance, inductance and frequency response measurements are readily available and calculations may be made using existing electrical impedance equations, including those for tank circuits, and especially those where time is part of the equation.

This device may be calibrated by connecting a certificated inductance and resistance standard across the test circuit terminals and taking measurements, which should indicate the correct value of these standards, otherwise the test capacitance value, which is stored in the memory of this device and used for calculation purposes, may be adjusted, enabling future measurements to read tme.

The many significant time periods, which occur within a single discharge period, may be measured, displayed and used comparatively to determine the differences between similar resistive and reactive circuits. Examples of time periods, which may be regarded as significant, include the time period from first switching the electrical charge, stored in a capacitor, into a test circuit until the capacitor voltage reaches zero, or the time it takes for the curent to reach a maximum value, the time for all, or each, subsequent oscillation to occur and the overall period of time when any oscillations are negative. Another significant measurement may be the recording of the maximum reverse voltage that occurs across the discharge capacitor, when the circuit oscillates due to the test circuit inductance. There are many of these significant events to choose to measure, during any discharge period, but for precise and meaningful measurement results, there may be the qualification that each event must be consistently monitored and measured accurately and that each measured result defines a further aspect of the circuit response.

Standard tests may be set up and maintained as programs in this device, for example, to measure a particular motor type. Periodic measurements may be taken and recorded in this device, to provide a comparison record for each motor. By comparing the latest readings to the historical record, a motor which deviates from the recorded norm or test range standard, may be replaced or regarded as likely to fail. The initial measurements taken from a new motor type, or manufacturers data, may be used as the recorded norm, against which used motors may later be compared.

When measuring, for example, three phase induction motors, it is preferred that each of the three windings be isolated in the motor connection box, so that the other windings do not interfere with the measurements and each winding may be tested in a number of rotor positions and the results averaged. The inductive response varies slightly, caused by magnetic circuit changes, as the rotor is turned to new positions and this averaging method statistically compensates for much of this variance. This device may incorporate the facility to control, supervise and prompt a rotor turning series of measurements to improve the accuracy of measurement in this way, or it may control an external rotor turning device to take a number of measurements, as this turns the rotor continuously or in known increments.

This device may also be constructed with a number of measurement outlets so that, for example, all three phases of a motor may be connected at one time, where each is tested individually, or they may be tested simultaneously, or in other arranged ways, allowing measurements to be taken, as the multiple windings interact and produce different inductive responses.

Another critical example of the resolution and information that can be achieved by this device is the ability to quickly differentiate between two short straight conductive wires of dissimilar length, whereby the wire resistance is measured and the device consistently indicates a different time reading for each wire length. A capacitor will discharge itself through a resistor in five time constants, where a time constant is the capacitance value multiplied by the resistance value. If this device measures the time it takes to discharge it's capacitor into the test circuit, the resistance in the circuit can be calculated using the time reading and the known capacitance value.

This device may determine the lack of inductance, when it detects that no reverse oscillating voltage occurred in the test circuit, during the measurement, or by examining the voltage discharge to see an instant exponential decay after starting the test. Unlike this device, existing micro-ohmmeters do not determine, quantify, or indicate if any inductance or capacitance is present in a test circuit at the same time as they measure resistance, nor can they when they use a basic Ohm's Law testing method to take a measurement.

To frirther enhance the accuracy of this device, the facility to carry out and time a series of discharges into a test circuit can be implemented, where several capacitors can be charged to predetermined levels and discharged into a test circuit in a cascade of switching events, The timing of discharges may be controlled to enhance the inductive response of the test circuit, or they may be controlled to discharge in a particular order and be recharged for immediate use, prolonging a measurement by continually repeating the same sequence, and the direction of discharge into the test circuit can be changed. These functions may be preselected before carrying out a test, or this device can be programmed to experiment by changing the test parameters and storing the various readings from which the most discerning response from the test circuit can be selected, The most discerning response may, for example, be established by displaying the widest numerical range of the measurement results after deciding on the number of readings to display.

This device may be programmed to prompt and control tests on similar circuits, preferably to compare responses and to display measurement results, which are consistent on any one test circuit, but have the widest range of variance from one circuit to another.

When arranging the switching of a number of alternating, or same direction, discharges into a test circuit, each switching event may be timed to best use the residual magnetic and electrical effects of the previous discharge that occurred in that test circuit, during the oscillation period. For example, the instant when the first and most powerful oscillation reversal takes place after a discharge, may be a good time to switch a new charge into the test circuit, for then the charge has to overcome the natural circuit impedance and it may be assisted or opposed by some of the charge eft from the previous discharge.

Initial measurements may be carried out to determine how a test circuit responds and the results used to set up a more discerning test for that circuit. Using this initial information, it may be discerned how best to switch a series of discharges to excite the test circuit in a particular way, where the quantity, direction and timing of each discharge may be used to oppose, assist or nullify any previous charges remaining in the circuit to elicit a different measurable response, as required.

The correct timing for switching each charge in a sequence is essential for best accuracy, as the interacting charges may prolong test circuit reaction times, or reduce them, if a charge is triggered at the wrong moment.

Inductive circuits are usually designed to respond to and work at a particular frequency and therefore, it may be preferable if they are tested with a series of discharges at this frequency, to obtain measurements appropriate to them.

When a number of electrical charges are stored in this device, to be used sequentially for energizing a test circuit, the first stored charge of energy may be switched into the test circuit, until the charge dissipates to a predetermined level, at which time, or subsequently, the switching of a second charge may be induced and a cascade of switching events may continue in this way, until all stored charges have been used. The time it takes to comp'ete this whole cascade of switching events, may be displayed, or each individual charge switching time may be recorded and kept as a typical response for that test circuit. There are many permutations possible, when controlling a cascade of switching events, including voltage level selection, limiting current levels, reversing the direction of discharge, choosing the instants of switching manually, or allowing natural responses to trigger switching and choosing which parameters to display Statistically, the average of four discharge readings may halve the measurement error of a single discharge reading. Using a cascade of measurements, where residual charges remain active, as subsequent charges are switched to mix with these, introduces a wealth of different measurement possibilities.

This device may time the natural response of the test circuit to each electrical stimulus, until each charge is dissipated into the test circuit and display the results but, while this is happening, it may also monitor and record the natural frequency of the oscillations produced, which can be used to calculate the test circuit inductance, This frequency may be displayed in it's own right as a unique response, or the circuit inductance may be calculated from it, using, for example, the tank circuit equation, shown in the drawings as Figure 3.

When using this equation, Figure 3, the capacitance value of the charged capacitor is known, for it is permanently built into this device, Pi is a known constant, the resonant frequency is calculated from it's wavelength, as measured by this device, and by substituting these values into the equation, Figure 3, the only remaining unknown value is the test circuit inductance, which may then be cakulated.

Each of the damped oscillation peaks, produced by an electrical discharge as it dissipates into an inductive test circuit, may be counted as they occur until the oscillations cease, or diminish, under the action of the circuit resistance and this count may also be displayed and recorded as part of the characteristic response of the test circuit.

The following list shows examples of some of the many readings that may typically be displayed using this device.

Display the period of time from when a single stored charge is switched into the test circuit until it reduces to some level.

Display the period of time for carrying out a number of such discharges, in cascade, until all are discharged to a level.

Display the period of time from when an inductive test circuit starts to oscillate, in response to a single discharge, until the oscillations diminish to a level or cease.

Display the period of time when the test circuit voltage is negative or positive during a single or cascade of discharge events.

Display the oscillation frequency.

Display the overall period of time from when the stored charge is switched until it dissipates completely into the circuit and all &ectrical activity ceases.

Display the result of counting the number of oscillation cycles in an inductive test circuit, that occur after a single discharge event, until the oscillations diminish to a level.

Display the time period for a pre-set series of forward and reversing discharges to take place and diminish to a level in cascade and also display parts of each period or the individual results for each discharge.

Display the average timed results of a number of discharges into the same test circuit, but only if each reading is consistent or, if not, indicate the connections to the test circuit may be bad and the test needs to be repeated.

Display the reverse maximum voltage achieved for a single or series of discharges across the test circuit, caused by the inductance in the circuit.

All, or part, of these tests may be carried out simultaneously, or in sequence and the measurement results may be displayed to give each test circuit a unique or typical response that clearly identifies it, but as there is a large number of options when measuring with this device, the selected details of each test may be recorded along with the measurement readings so that the same tests can be repeated in future, to maintain control of the test data and avoid confusion.

The wavelength of the resonant frequency in a test circuit may be found by measuring the time period between two successive positive voltage or current wave peaks and half a wavelength is measured between two successive zero crossing points, as the test circuit oscillates. This device may indicate that the test circuit is not inductive, if no waves are detected and can be calibrated to measure and display the correct circuit resistance.

Figure 1 shows how a single capacitive discharge embodiment of this device may be constructed, using a combination of electrical symbols, circuit connections, electrical building blocks and the reference numbers that refer to these.

It is not shown on Figure 1, but it is assumed that Electrical Building Blocks 4, 14, 16, 19, 21 and 24 are each powered from the Power Supply 1, 2, where 1 is a low voltage DC supply and 2 is the zero volt return path. The Nack dots indicate where circuits are connected together. This embodiment measures the time period from when a single charged Capacitor 9 is switched into the Test Circuit 13, until the charge is absorbed by Test Circuit 13 and the Capacitor 9 first reaches zero volts.

The incoming Power Supply 1, 2 turns on the Voltage Generator 4 when Switch 3 is operated and the ensuing High Voltage Supply 26 is fed through Diode 5 to Capacitor 9, which starts to charge up. The rising Voltage Sample 15, which then appears at the junction of resistors 7 and 8, is fed to Comparator Unit 16, until the Voltage Sample 15 reaches the Reference Voltage 18, which causes the Comparator Output Signa' 17 to switch on. Voltage Generator 4 is immediately turned offi as it also receives Signal 17 and Capacitor 9 is now charged to a specific level, ready for carrying out a measurement, At the same time, Switching Unit 14 receives the Signal 17 and responds by switching Output 25 on, triggering Transistor 10, which causes the Capacitor 10 to commence discharging into the Test Circuit 13 via Connection Terminals 11, 12. Diode S prevents Capacitor 9 from discharging back into the Voltage Generator 4. Diode 6 allows any back electromotive forces to circulate should Test Circuit 13 prove to be inductive.

As Switching Unit 14 switches Output 25, a second Output 22 is also switched on, which causes the Monitor Unit 21 to immediately reset to zero and commence to count a time period.

Comparator Unit 19 receives the falling Voltage Sample 15, which maintains the output Signal 20, as long as Voltage Sample 15 remains above zero volts, Signal 20 enables Monitor Unit 21 to continue counting the time period, while Capacitor 9 continues to discharge into the Test Circuit 13. When Capacitor 9 has completely discharging, the Voltage Sample 15 reaches zero volts, causing Comparator Unit 19 to turn Signal 20 off, which then causes Monitor Unit 21 to cease counting and hold the measured time period, before sending the measured result to Unit 24 to be displayed.

Signal 20 is monitored and used by the Switch Unit 14 to turn off Output 25 and reset itself ready for the next measurement, when it sees that Capacitor 9 discharge has completed.

Switch 3 must be opened and closed again to start a new measurement.

Embodiments of this device may use similar monitoring and control circuitry to measure other significant periods of time during the resistive, capacitive and inductive response period, as the test circuit reacts to a discharge event, thereby economically taking a multiplicity of time and other quantitative measurements, from just one discharge, in the shortest period of time, If the device is used to generate high voltages and substantial charge is present and a risk of shock or burning to operators is possible, then steps to prevent this may be built into the device.

Unlike an Insulation tester, which can give very high voltage shocks as it measures high impedances, an embodiment of this device may only measure low impedance circuits, and therefore the test circuit can be monitored, and measurements inhibited, until a suitable low impedance test load is connected across the output terminals. A low voltage electrical circuit, connected to the test connections on this device, may constantly or intermittently monitor and determine if a suitably low impedance circuit is connected to this device. An adjustable impedance level setting may inhibit measurement, allow measurement, or adjust the level of voltage across the stored charge for safety, or other measurement reasons, depending on what this safety circuit detects, as it monitors the output terminals. The device may indicate when the test circuit is within the acceptable limits. The typical resistance of a dry human body may be several thousand ohms, reducing to several hundred ohms when the skin becomes damp. The test circuit output impedance setting, above which any measurement is inhibited, may typically be set to 150 ohms or less, so that if a person alone, bridges the test terminals, they would not be exposed to an electrical charge. Should they deliberately connect themse'ves onto a high voltage test circuit, and initiate a measurement, their body resistance would be too high to be acceptable to this device and no measurement, or charging, would take place, keeping the person safe, despite their stupidity.

Once a charge has been generated, it is preferable for the measuring process to take place thout delay, to allow no time for the charge to leak away, to limit the charge storage device stress period and to minimize the time when people in the area may become exposed to danger. While the charge is present inside the machine it is preferable if a warning light, a buzzer or other device, warns of the danger of interfering with this device while it is measuring.

A further safety shield may be connected and incorporated with the test probes, which will mask exposure to the conductive parts of test probes and the connection points, and which may inhibit the measurement, if it is not in place, as decided by a monitoring circuit, which may be built into the test leads and probes. A closed loop safety circuit may inhibit a test output from this device, if the loop is opened, as a result of a safety shield being compromised.

A low impedance test circuit, when connected, provides a partially shorted path across the test leads, which preferentially protects against electrical shock, or burns, by absorbing virtually all of the charge.

There is no need to regulate current, when using the measurement principles of this device, and no current ranges are required, as the current is limited by the test circuit impedance itself Current and power levels can be huge, in comparison to those used by other instruments, but are controlled by the test voltage and quantity of charge selected.

The use of a very high voltage and large quantities of charge may not be necessary; when operating this device, but these may be used to electrically punch through and burn away any corrosion barriers and establish a good connection for subsequent measurements. A charge voltage level of 100 volts may typically be generated for use in this device, which will dissipate into the test circuit when a measurement is initiated. The capacity of the charge storage device determines how much current flows during a measurement and the power switching components in this device are preferably rated to withstand any dead short circuit on the output terminals. A series or parallel resistance, or an inductance may be incorporated into the test circuit, to control or limit the current level in the test circuit and this device may be programmed to compensate for it and still measure accurately. This device may momentarily send high levels of current into a test circuit but, as the current is of short duration, there is minimal heating effect and this may be further reduced by using additional series impedances, if required.

It is preferable if this device incorporates a current limiting and monitoring facility only to prevent a dead short, across the output terminals, from causing damage to the output switching components, but the inductive response of the test circuit is damped by this additional circuit resistance and the accuracy of the measurements are reduced accordingly. To avoid this problem, this device may have the facility to carry out an exploratory reduced accuracy test using a low charge to limit current or with a current limiting unit temporarily placed in circuit, such as a half ohm resistor, and then to measure and determine if the output has a dead short across it. When using such a resistor in circuit, Ohm's Law tells us that the output switching devices must be momentarily rated to carry at least 200 amperes, when using a 100 volt charged capacitor, to carry such a current without damage, when carrying out a measurement into a shorted output, even with a protective resistor in circuit. This device, after it has measured and seen such a high current level during a test may inhibit further tests for a period of time to allow it's own power switching devices to cool and to protect them from further heat build up.

It is preferable if the output switching devices, used in this device, are selected to have a very low internal impedance when switched on, to minimize heat build up from repetitive testing, and to allow the test circuit to resonate longer, or they may be chosen so that their internal resistance is suitable for current limiting. The power switching devices can be momentary rated for large currents, which allows the use of small size components of minimum weight If an initial low power test shows there is no dead short in place across the output terminals, then subsequent higher power tests may be carried out with any current limiting devices bypassed or removed from the circuit. In this way, the response from the test circuit may be safely increased, where the inductive oscillations last longeç for they are not inhibited by the added circuit resistance and the accuracy of the measurement is improved.

This device may have the facility to calibrate itself; whereby the outputs are deliberately made short circuit and safe low voltage readings are taken, stored and used as a reference for a dead short condition. When measuring, a dead short placed across the test circuit will be evident by comparing readings with the stored referencç and warnings of this condition may be displayed, or steps taken to inhibit further tests, until the dead short is removed.

During a measurement the time that it takes, for an electrical charge to dissipate into an inductive test circuit is increased when a charge absorbing impedance is removed from the circuit, the circuit oscillates for a longer period of lime, the ensuing results are more accurate and little measurement compensation is required. Unwanted resistances in a test circuit, are present in the outgoing power switching components and connecting leads, but these are known and consistent, and their total value may be subtracted from measurements, allowing the true circuit resistance to be displayed. The smaller these resistances are, the less they affect the circuit response and it is preferred that they do not dominate the test circuit. Unwanted resistance causes a longer measuring time, when the test circuit is purely resistive, as the capacitor discharge current is reduced by it and the charge takes longer to dissipate. In an inductive test circuit, the extra resistance absorbs more charge as the circuit oscillates, reducing this part of the measuring time.

Should a measurement fail to complete and residual charge not dissipate in the test circuit, within an expected time period, for example 1 second, then this device may trigger the placement of a shorting element to absorb the charge and render the circuit safe. This circumstance may occur when the test has been initiated and where the connection probes are immediately removed within the measurement test time period. This event is therefore extremely unlikely and even if an attempt was made to deliberately do this, or even if it had happened naturally, within that fraction of a second after the measurement was initiated, the charge would have been partially depleted and then completely removed within one second by the crow bar circuit.

When the test circuit starts to oscillate, the charge storage device must, preferably, be rated to withstand reverse vottage polarities and therefore a non-polarized capacitor would be suitable for this purpose to prevent damage, and they are usually more stable with time.

There are a number of ways to monitor and determine the total oscillation period, as this occurs.

For example, during a test, while oscillations are occurring, this device may take a voltage sample from the charge capacitor terminals and connect the oscillating voltage via a bridge rectifier to charge a small capacitor, which is drained by a resistor. As long as a voltage continues to charge this small capacitor it is evident that oscillations are still in progress and at the moment when that voltage ceases and the small capacitor voltage drains through the resistor to a reduced level or zero, the end of the oscillation period may be triggered. The time period, from when the small capacitor first receives a voltage signal until the end is triggered, may be displayed.

Fast microprocessor based circuitry may be used in this device for controlling, monitoring and switching events and auxiliary circuits, to capture relevant time intervals, to process measured results, store data and drive displays, or this device may be constructed using combinations of this and other digital circuits, orjust analogue devices and circuitry may be used.

Measuring, capturing and displaying the often extremely fast discharge time intervals, that occur with this device, can be carried out using a fast pulse generator and a high frequency pulse counter, or an interval timer, or a capture and hold voltmeter or other devices and combinations.

The use of fast, complex electrical monitoring circuitry, to capture any timed event during a test.

at the instant it happens, may not be necessary. Simpler monitoring circuits, even if they have inherent time response delays, may still measure time periods accurately, as long as the time delay eror, at the start of a measurement, is the same as the time delay error which triggers the end. There is less requirement for accurate timing, if the monitoring errors cancel themselves out.

If you measure time periods by counting pulses, a fast display unit can typically read and record up to 60,000 pulses a second, but this may not be sufficient to capture enough pulses and discerningly measure the extremely fast discharge times of this device.

Another example arid a preferred analogue method of taking time readings is to charge a sampling capacitor from a constant current source, while a measurement is in progress, and to capture and record the voltages reached as each event occurs. The constant current source may be adjustaNe and controlled to give a range of measurement times to cover any or all of the applications of this device, simply by changing the value of a single resistor in the current source circuit. The sampling capacitor voltage rises linearly th time in such a circuit and it can be monitored and sampled as events take place to provide the time measurements, with the range selected to provide a significant discerning count.

If using a digital display voltmeter; to show and hold measurement results, there may be problems, as the fastest sample rate of such a unit, is typically only 3 samples per second, which is far too slow to accurately capture the readings. An analogue voltmeter also cannot react quickly enough to display and hold a momentary voltage pulse.

To capture, quantify and display the fast time periods on a digital or analogue voltmeter may require the use of separate sample and hold, or other circuitry, and preferably to use a linear rather than a logarithmic time scale, If a time sampling capacitor is charged from a fixed voltage during a measurement the scale is logarithmic, but if it is charged from a constant current source the scale is linear.

The use of a variable constant current source may further enhance this device, for example, different resistors may be switched into the current source generating circuit to provide the different time ranges needed for all applications and to range the measured results so that each produces a sufficiently high reading to be meaningful, but it is below the maximum value for that time range.

Another embodiment of this device may be used for testing and fault finding low voltage circuits, such as those you may discover on printed circuit boards, whereby the measurements are carried out using the same techniques, as previously described, but where the test voltage is limited to a level, for example 5 volts, where no damage is caused to components in the test circuit, The impedance range, over which the instrument can measure, may be increased and the quantity of stored charge may be adjusted and quantified, to avoid excessively long measuring times, such as when the stored charges, used for a measurement, are slow to dissipate into higher impedance test circuits. Using low voltage, the need for protective circuitry to prevent shock to the operator is unnecessary, but some current limiting may be needed to prevent, for example, the fine tracks on circuit boards from being heated. This embodiment of the device may be set up to examine the reactive response of test circuits over the full impedance range, rather than specifically examine only low impedance circuits, When measuring with this device, the readings may be similar to the results produced by a resistance meter, when the test circuit is highly resistive, but where reactive components are also present in the test circuit, this device may prove far more discerning with its ability to produce numerous other readings as it measures.

The use of this device for fault finding or assessing, for example, circuits on a none powered circuit board, complements or exceeds the capabilities of a standard Amps Volts Ohms meter to discover faults and identify response differences when comparing circuits.

A useful instrument used for fault finding components on printed circuit boards is commonly known as a Tracker, which tests with some resistance in circuit and as it constantly injects a small current and alternating voltage signal into an inductive test circuit, the hysteresis voltage loop response is pictorially displayed on a screen. Unlike a Tracker with this invention the test charge voltage is not constantly controlled, as the test circuit reaction to any stimulus is required to be a natural and more discerning response, but this can also be captured and displayed pictoriafly.

The principles of measurement used in this device, can be adapted to evaluate the conductivity of liquids, powders, gases, gels and other materials in a further embodiment, which can preferably measure in the same way as for electrical circuits, but the voltages used, and other settings, may be controfled to have due regard for the volatility, and other properties of the material under test, so that each measurement is safely carried out.

Claims (9)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY

   OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: I. In a device that connects to a low impedance, or any other test circuit, for the purpose of measuring the circuit parameters, by the combination of generating electrical or magnetic charges in a single or plurality of storage units, which are connected into the test circuit to excite it, using electrical power switching devices, or using electromagnetic induction, by monitoring the test circuit and selectively taking electrical, magnetic or other measurements and timing the duration of any electrical or magnetic events, that occur as the test circuit reacts to the applied stimulus, to calculate the electrical parameters of each test circuit, using electrical equations and the measured quantities, to display a selection of the measurements, parameters and calculated results, using a plurality of analogue and digital circuits, appropriately assembled together and housed to perfonn these functions, using further auxiliary circuitry to ensure both operator safety and the integrity of this device, to provide the means to connect to the external test circuits and to have the controls to perfonn any, or all of these functions,
2. 2. The combination defined in claim 1, wherein there is a means to store known electrical charges, preferably with a selectable choice of voltages and capacitances, for the purpose of discharging these into a test circuit to induce oscillations in inductive circuits and test circuit reactions in other circuits, significant events of which may be monitored as each charge dissipates and measurements may be taken, recorded and calculations made, using electrical equations, especially those for capacitive, inductive and resistive oscillator circuits, after which a selection of these results may be displayed, but particularly the time periods of specific reaction events, measured oscillation frequencies and the number of oscillations before activity ceased, reverse current and voltage levels, temperature changes, magnetic parameters, the calculated inductance and resistance of the test circuit, along with other measurable quantities, a selection of which may be used to uniquely characterise each test circuit.
3. 3. The combination defined in claim 1, wherein auxiliary low voltage safety circuitry carries out preliminary tests, by connecting to and monitoring the external test circuit, to establish that it is suitable to be measured and preferably, the auxiliary circuitry inhibits any stored test charge from being switched to the output terminals, or from being generated, until such time as when the external test circuit impedance is detected to be outside the typical impedance range of a human body, should such be connected in the test circuit, before allowing any high voltage measurement to take place, which prevents shock and makes this device intrinsicafly safe.
4. 4. The combination defined in claim, wherein auxiliary low voltage safety circuitry connects to and monitors each external test circuit to inhibit any stored charge from being switched to the test circuit, or from being generated, until such time as when a dead short condition does not exist across the output terminals of this devise, to prevent excessive current from damaging the power switching components, or any other circuits, within or outside of this device.
5. 5. The combination defined in claim 3, wherein if there is a dead short, or a near dead short condition across the test circuit, it is preferred that an auxiliary safety circuit in this device detects this and switches an additional circuit impedance, of known value, into the test circuit, to limit current and prevent damage to components, before allong a measurement to take place, whereby the ensuing result may be compensated by subtracting this known value from each measured reading, before displaying it,
6. 6. The combination defined in claims I and 2, wherein a charge, that has been switched into a test circuit, may be permitted to freely react with that test circuit and fully dissipate its' energy into it, to best ensure measurement accuracy, facilitated, preferably, by the use of optically isolated or other auxiliary circuits, which do not significantly interfere with, or reduce the magnitude of any electrical or magnetic test responses, by extracting power from the test circuit, nor by altering the magnitude or frequency of any oscillations that occur during the response period, except as defined by claim 4, where it is preferable that a known minimal interference component is introduced into the test circuit to prevent component damage.
7. 7. The combination defined in claim 1, wherein a means to generate and store a multiplicity of electrical or magnetic charges is provided, where preferably, each charge is measured and quantified as it is generated to the required level and is used immediately to switch into the test circuit, allowing no time for the charge to leak away, avoiding the need to recharge, preventing the waste of instrument battery power and reducing measurement inaccuracies, as this lack of delay ensures that the actual charge generated and the value of charge used in the measurement calculations are similar minimising errors when calculating the test circuit parameters.
8. 8. The combination defined in claims I and 2, wherein a number of test charges are preferably generated from a rechargeable battery, or other energy storage unit situated within this device, which has sufficient capacity to carry out a plurality of high power tests, for this momentary method of measurement enables a portable embodiment of this device to be light in weight, to be very economical in the usage of power, even as it utilises instantaneous power levels, which are far greater than those used in other micro-ohmmeters, to better excite a test circuit and produce more accurate readings.
9. 9. The combination defined in claims 1 and 5, wherein any electrical power switching component, used in an embodiment of this device, is preferably chosen so that the natural internal impedance of the component, when it is fully turned on, is suitably rated to limit test circuit current and thereby protect itself from damage, should a dead short exist across the output terminals, or an electrical power switching component may be controlled to limit the maximum current, once it has switched on the test circuit charge, or it may be controlled to act as a predetermined quantity of constant impedance in the test circuit, which is preferably arranged by using a power transistor switching device and an auxiliary monitoring circuit, which controls the transistor amplification, while it appropriately maintains the ratio of the transistor voltage drop and the test current as a constant, to simulate a selected circuit impedance for the duration of the test, after which this impedance value may be deducted from the measurements to reveal the true test circuit impedance.tO. The combination defined in claim 1, wherein an auxiliary circuit may monitor the response of a charge as it dissipates into the test circuit but, should this response continue for a prolonged period of time, or if the electrical charge fails to diminish, the auxiliary circuit detecting this may, preferably, cause the measurement to terminate by switching a shorting resistor, or other element, across the test circuit in order to fully absorb the charge, whereby, this device may still determine and show measurement readings, but it may also use these results to determine and indicate the cause of the continuing activity, namely the test circuit was of a very high impedance nature, or it was of a high inductance, low resistance nature producing sustained oscillations.IL The combination defined in claim, wherein a number of auxiliary monitoring circuits are used to take time period measurements, while a test circuit responds to an electrical stimulus, for the purpose of using these time measurements to calculate the electrical and magnetic characteristics of that circuit, but as there are usually component reaction time delays in these monitoring circuits, where each detects and times some event during the test circuit response period, and where the percentage error increases as the time periods get shorter, then it is preferable if components in the monitoring circuits are time delay matched, so that a measured period of time has the same detection delay at both the start and the end of the measurement, which nullifies the errors and produces a time measurement that is corrected for this problem.12. The combination defined in claims 1 and 4, wherein this device preferably gives an audible or visible safety alarm, or both, when a measurement is initialised, prior to generating or switching a test charge capable of causing a shock, and this alarm may be repeated, when auxiliary safety monitoring circuits do not detect a suitable test load, whereby the cause of the repeated alarm may be indicated and the test load examined and corrected before the test circuit is approved and the test is allowed to continue.13. The combination defined in claim 1, wherein an embodiment of this device, from a single start command, may carry out a continuous, or sequence of tests, preferably starting from a safe low voltage level, where the test voltage, or quantity of electrical charge, or both, is continually or successively increased after each measurement up to a safe maximum, and the results may be used to determine, if an insulation breakdown of the test circuit occurred and at what voltage.14. The combination defined in claims 1 and 11, wherein, for improved accuracy, the duration of an event must be reliably timed, but, where digital electronic monitoring devices do not sample quickly enough to correctly measure fast time periods, a preferred analogue method of taking a time measurement may be implemented, preferably utilising time error compensated components in the monitoring circuitry, whereby a sampling capacitor, which may be reset to zero volts before the start of each measurement, is fed from a constant current source as an event occurs, which causes the sampling capacitor voltage to rise linearly to a level, determined by the duration of that event, where the sampled voltage may then be held and displayed on a voltmeter, which may be calibrated by the constant current source and scaled, to correctly display time, inductance or resistance, and where further measurements may be taken, using other time ranges, to find a discerning range, which produces readings with more significant figures, thereby improving discernment between readings.15. The combination defined in claims 1, hand 14, wherein an embodiment of this device may be used for simple numeric comparison tests, preferably using undefined, none calibrated, but consistent numerical scales, where timing ranges may be auto-ranging or they may be selectable where each selection may provide a different time range, a different impedance range or, when measuring three phase motor windings, a selected range may indicate the motor powers that are likely to be more discerningly measured using that setting.16. The combination defined in claim 1, wherein the quantity of charge, the polarity of a charge, the charge voltage level, the number of charges to be used and the timing of successive charges used for a measurement are preferably selectable, programmable, adjustable and controllable to provide the flexibility to cover the whole or some range of measurements, that are required to be measured by any embodiment of this invention, whereby a selection of measurements may be stored and displayed along with a calculation, or estimation, of the accuracy of the results, based on the significant figures produced by the measurements, the variations found in repeated measurements, the design component tolerances and other appropriate factors.t7. The combination defined in claim t, wherein, to achieve best measurement accuracy, it is preferable that the connections made onto a test circuit should introduce no unknown or varying impedances into the circuit, but as contaminated connections can introduce these faults, and cause measurement errors, the voltages and currents generated within this device, may be used to break down and bum away such contaminants, to establish good conductive connections, and this may be brought about by repeatedly measuring, until the readings become consistent, and by using trend analysis, or another selection criteria, to discard unreliable readings, leaving only the consistent readings to be stored and displayed.18. The combination defined in claims I and 16, wherein this device may be programmed to take a pre-selected combination of measurements and utilise the results to discover the settings that are likely to best measure a test circuit, or preferably, this device may be more intelligently programmed to make measurements, and, within safety limits, have the means to adjust and change time ranges, charge voltage, polarity, capacitance, discharge sequence timing and other settings until the program, using the measured results, is guided to discover the ranges and settings, which provide the most consistent, significant and accurate readings.19. The combination defined in claims 1 and 2, wherein, to reduce the risk of shock, the connections onto the test circuit, or any other exposed high voltage areas, may be shielded with a light or mechanical barrier prior to carrying out a high voltage test and preferably, the mechanical shielding is further secured by the arrangement of a fail safe normally closed circuit, which runs through all parts of the shielding and interlocks into this device, so that if any part of the shielding is missing, or out of place, then the normally closed circuit is compromised and for safety, measurements are inhibited until the shielding is secured back in place and the normally closed circuit is restored, and should the shielding be removed during a measurement, the fail safe circuit may also cause a shorting element to be immediately switched across the output terminals to absorb any residual charge and render the test circuit safe.20. The combination defined in claim 1, wherein the repeated use of this device, to provide very high currents, may cause the power switching components to heat up to the point of failure, and preferred methods, to prevent this happening, include monitoring the duration of current levels with an auxiliary circuit, which may temporarily suspend measurements, until a period of time has elapsed to allow cooling, or temperature sensors may be used, to monitor the components at risk, whereby temperatures exceeding a predetermined limit may similarly impose a cooling delay, while these sensors may also be used to display component temperature levels or can be utilised in measurement calculations to provide temperature compensated readings.21. The combination defined in claim 1, wherein standard tests may be programmable and stored for future use, preferably for testing a particular circuit, where the readings are kept in a measurement database as a historical record, and for use in making future comparisons, where the control settings may also be kept and linked with these results, to be later retrieved for frirther measurements.22. The combination defined in claim, wherein an embodiment of this device may be constructed especially for measuring three phase circuits, preferably using three individual testing circuits, one across each phase, with three outgoing cables and connection probes, where in isolation, or together, in a combination of three phase switching, they may be used to measure each of the circuit parameters, or an individual circuit may be switched into each phase sequentially to measure all three phases, without having to change over the connection probes.23. The combination defined in claim I, wherein the requirement to measure the period of time when a test circuit is active, is preferably achieved by taking a test circuit voltage sample, via a bridge rectifier to a small capacitor, which is drained by a resistor, so that when a voltage appears on the capacitor, this shows the circuit is active, triggering the start of the time measurement, and when the capacitor voltage drops back to zero volts, this shows all activity has ceased and triggers the end of the measurement.24. The combination defined in claim I, wherein it is preferable to display fast time periods on a digital or analogue display, where measurements may be captured using separate sample and hold, or other recording circuitry, and if a sampling capacitor is used as a timer; being charged during the measurement period, via a resistor, from a constant voltage source, the scale is logarithmic, but when charged from a constant current source the scale is more useful as a timer, as it is linear.25. The combination defined in claims 1, 5, 6, 7 and 8, wherein a further embodiment of this device may be used for testing and fault finding on any low voltage circuit, such as those found on printed circuit boards where, preferably, the measurements are carried out using a single or cascade of charges, but where the test voltage is limited to a level, where no damage is caused to sensitive components in the test circuit, but the whole impedance range is measurable, and the quantity of stored charge may be adjusted and controlled, to avoid excessively long measuring times, by using quick exploratory measurements to initially evaluate the test circuit, and set up or recommend appropriate settings for a full test.26. The combination defined in claim 1, wherein a number of different time and associated voltage, or current readings, may be taken at intervals during a measurement period, and used to plot and display the results in a time against voltage or current graph, or other pictorial form, preferably showing the response curve as a continuous line from start to finish, by interpolating between the measured points on the curve.27. The combination defined in claim 1, wherein oscillator circuit equations can be used to calculate the test circuit parameters, and as the test circuit capacitance is a known value, for it is inbuilt into this device, Pi is a constant, and the resonant frequency of an inductive test circuit can be measured by this device, then the test circuit inductance may be calcu'ated and displayed.28. The combination defined in claims I and 27, wherein the resistance of an inductive test circuit determines the duration and the number of oscillations produced, when it is subjected to an electrical stimulus, and the time it takes for a charge, stored at a known voltage and capacitance, to first reach zero volts after being switched into the test circuit, may be used to calculate and display the circuit resistance, especially when the circuit inductance has already been established, to assist in the calculation.29. The combination defined in claim I, wherein each winding of a motor may be measured a number of times, as the rotor is turned to a new position after each measurement, and these readings may be averaged, to improve accuracy by compensating for the small variations of inductance, that occur as the rotor is moved, and this device may have the facility to prompt these movements and store the results, for later comparing each phase of the motor, or this device may control an external rotor turning device, which may turn the rotor continuously or in selected increments as measurements are taken, to increase accuracy further, 30. The combination defined in claim, wherein an embodiment of this device may preferably be constmcted and used as a micro ohm meter only, for measuring purely resistive circuits, but a warning may be given if any inductance is detected in the test circuit during a measurement, by the means of monitoring the voltage discharge curve, which should be linear, and any deviation from this may trigger the warning, or by giving a warning when a reverse voltage is detected during a measurement.31. The combination defined in claim 1, wherein the principles of measurement used in this device, can be adapted to evaluate the conductivity of liquids, powders, gases, gels and other materials in a further embodiment, which can preferably measure in the same way as for electrical circuits, but the voltages used, and other settings, may be controlled to have due regard for the volatility, and other properties of the material under test, so that each measurement is safely carried out.32. The combination defined in claims 1 and 13, wherein a series of measurements are taken, first using a safe low voltage charge, but increasing the voltage level after each measurement, may be used as a physical inducement feature, where any person touching the test circuit, who has overcome all other ignored the safety alarms, starts to feel the increasing voltage levels and is induced to let go of the circuit, by the early warnings of higher voltages to come.33. The combination defined in claims 1 and 17, wherein, to establish a good low impedance connection onto a test circuit, it is preferable to use strongly sprung and none magnetic probes, where each has a number of highly conductive connection points, designed to resist arcing and to share the current, and where the use of short straight highly conductive test leads further minimises the overall test circuit impedance.