GB2411734A

GB2411734A - Measuring resistance of a connection

Info

Publication number: GB2411734A
Application number: GB0405097A
Authority: GB
Inventors: Raymond John Vassie
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-03-06
Filing date: 2004-03-06
Publication date: 2005-09-07
Also published as: GB0405097D0

Abstract

The resistance of a connection, such as a crimped connection, is measured by applying a current pulse from a generator 22 to the crimp under test 11. The electric field or voltage produced in the crimp connector by the pulse is measured by a capacitive coupling ring 14 located around the cable at a test point. The capacitive coupling 14 is connected to an electrometer 12 which is also connected to a reference point on the crimp by probe 24. The current pulse may be high value, such as 500A, and short duration, such as 1 ms and may be AC or DC. The current pulse produces minimal heating in the crimp, and the use of non-contact coupler 14 avoids the need to break the cable insulation. An initial pulse may be passed through standard resistance 16 and a subsequent pulse through crimp under test 11. This allows unwanted effects to be cancelled when determining crimp connection resistance. The resistance of such a connection is used to indicate whether the connection is good or faulty. A second capacitive coupling (not shown) may be located further along the cable to measure properties such as wire gauge.

Description

241 1 734

APPARATUS AND METHOD FOR MEASURING RESISTANCE

This invention relates to electrical wiring, and to the measurement of the resistance of electrical wiring and electrical connections. The invention is particularly applicable, but by no means limited, to measuring the resistance of a connection between a copper wire and a crimped connector.

Crimp resistance is the electrical resistance between a crimped barrel (e. g. that of a spade terminal) and the wire to which the barrel is attached. Crimp tooling is designed to ensure that when a crimp barrel is closed under pressure onto copper wire, gas permeability is zero or very near zero, and the interface resistance is at its lowest. Monitoring the pressure on the crimp tool is effective in controlling the point at which the copper wire begins to extrude lengthways since a steep increase in the applied pressure is indicated. By monitoring crimp pressure, moderate variations in copper area can be tolerated because the change in required tooling pressure is detected at the point of extrusion and the tool displacement is adjusted accordingly.

However, problems arise with heavy gauge wire because the force required to close correspondingly heavier crimps is similar to the force required to extrude copper. For this reason, detection of the point of copper extrusion is unreliable and cannot compensate for variations in crimp diameter, wall thickness or changes in the wire copper area. As crimp structures get heavier, closure control becomes unreliable.

Also as wire gauges increase, cost and mass are cut to the minimum while mission criticality often rises (e.g. primary or back-up power feeds). The high current makes bad crimps hot, which accelerates their breakdown, making heavy crimp processes among the most important to control.

Anecdotal evidence from industry workers suggests that heavy wire gauge varies by up to 15% and cast crimp structures are dimensionally variable. The automotive industries in particular use these materials and because they cannot inspect them electrically or non- destructively, more failures occur in vehicle battery links than in many other components.

A German consortium of car manufacturers has reported that 80% of all car faults are electrical in nature and 60% of these are attributed to battery cables and power feeds.

A traditional technique for assessing the integrity of a crimped connection is known as "pull testing", in which the force required to pull the wire from the crimped connection is monitored. This technique assesses the mechanical integrity of the connection, rather than its electrical conductivity. Interface integrity is inferred from knowledge of the interface pressure, which in turn is inferred by how much force is required to pull the wire from the crimp. Thus the pull test is important. However, resistance to pulling may be derived from features other than contact pressure - for instance, the belling out the wire tip; corrugating the barrel; adding grips. These devices, all of which are used in the industry, are effective in increasing pull strength without necessarily affecting contact reliability. For this reason no easy relationship exists between pull force and the inferred interface integrity.

A further problem is that when the crimp pressure is low, the final crimp 'looks' the same as when the crimp pressure is correct and will survive many applications for a year or more. It is not intuitive that crimps must be 100% gas tight and where pressure monitoring on heavy gauge wire is ineffective, quality managers have no objective tools to ensure quality.

In another traditional (4 wire Kelvin) test technique, the electrical resistance of the connection between a crimped terminal and a cable can be directly measured by passing a known current through the wire and measuring the voltage between the crimp and a needle probe inserted through the insulation close to the crimp. The crimp resistance is calculated using Ohm's law. However, this is a destructive (and often sacrificial) technique, as the insulation of the cable is damaged when inserting the probe. A cable that is damaged in this manner is often subsequently unsuitable for many applications (e.g. applications which require the internal wire to be insulated from water), and therefore the cable and terminal are often discarded after the test has been performed. It will be appreciated that, in many cases, this traditional technique does not enable crimps to be tested and then put in (or returned to) service. Accordingly, it follows that, with this technique, the crimps which are in service are not the specific ones which have been tested.

According to a first aspect of the present invention there is provided apparatus for measuring the resistance between a reference point and a test point along an electrical conductor, the apparatus comprising a circuit including a current generator, a voltage measuring device and a coupling electrode responsive to local electric field strength; wherein, when in use testing an electrical conductor: the current generator is operable to cause a current to flow through the reference point and along the electrical conductor through the test point; the coupling electrode is responsive to the local electric field strength at the test point by virtue of capacitive coupling between the electrode and the electrical conductor at the test point; and the voltage measuring device is connected between the coupling electrode and the reference point and is operable to provide a voltage value representative of the resistance between the reference point and the test point when the current generator is operated.

Preferably the apparatus further comprising a reference resistor of known resistance Rsandard connected between the current generator and the reference point, and switching means operable to switch the circuit between a first configuration and a second configuration, the switching means and circuit being arranged such that, in use: in the first configuration, one end of the electrical conductor is connected to the reference point and the other end is disconnected from the circuit, the coupling electrode is positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the other side of the reference resistor from the reference point, the voltage measuring device thereby being operable to record a first voltage value V, in the event that the current generator causes a first flow of current through the reference resistor and the reference point; and in the second configuration, the reference resistor and the electrical conductor are connected in series with the reference point between them, the coupling electrode remains positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the reference point, the voltage measuring device thereby being operable to record a second voltage value V2 in the event that the current generator causes a second flow of current through the reference resistor, the reference point and the electrical conductor, the second current being of equal magnitude to the first current; the resistance between the reference point and the test point being calculable using the values V', V2 and Standard In the preferred embodiment, the resistance between the reference point and the test point is equal to Rsandard (V2 / V,).

Preferably the coupling electrode is configured so as to at least partially surround the electrical conductor at the test point.

S Particularly preferably the coupling electrode comprises a coupler ring configured to surround the electrical conductor at the test point.

Preferably the voltage measuring device comprises an electrometer.

In one embodiment, the current produced by the current generator comprises pulses. The pulses preferably have a pulse length of 1 ms, although this length is by no means essential.

Indeed, other durations of current flow may also be used, depending on thermal and measurement requirements and the impedance of the voltage measuring device.

Also, in an embodiment, the current has a magnitude of 500 amps, although this is by no means essential and other currents may also be used depending on the signal to noise ratio of the voltage measuring device.

The current generator may be configured to send unidirectional or ac current.

The apparatus may be used for measuring crimp resistance, and may be incorporated in a crimp machine to enable quality control checks to be performed during crimping.

In an alternative configuration, the apparatus may further comprising a second coupling electrode positionable, in use, at a second test point further along the electrical conductor from the first test point, the apparatus being operable to make resistance measurements between the reference point and each of the two test points and thereby enable the resistance of the electrical conductor between the test points to be calculated. This apparatus may be used for measuring wire gauge or detecting broken strands within a wire.

In another alternative configuration, the apparatus may comprise a plurality of coupling electrodes arranged in a corresponding plurality of test points around the circumference of an electrical conductor in use and operable to obtain voltage readings between the reference point and each test point This apparatus may be used for detecting an uneven crimp interface.

According to a second aspect of the invention there is provided a method of measuring the resistance between a reference point and a test point along an electrical conductor, the method comprising: positioning a coupling electrode at the test point, the coupling electrode being responsive to the local electric field strength at the test point by virtue of capacitive coupling between the electrode and the electrical conductor at the test point; operating a current generator to send a current through the reference point and along the electrical conductor through the test point; and measuring the voltage between the coupling electrode and the reference point using a voltage measuring device, to provide a voltage value representative of the resistance between the reference point and the test point.

Preferably the method further comprises: including a reference resistor of known resistance Rsandard connected between the current generator and the reference point; including switching means operable to switch the circuit between a first configuration and a second configuration, the switching means and circuit being arranged such that, in use, in the first configuration, one end of the electrical conductor is connected to the reference point and the other end is disconnected from the circuit, the coupling electrode is positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the other side of the reference resistor from the reference point, and in the second configuration, the reference resistor and the electrical conductor are connected in series with the reference point between them, the coupling electrode remains positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the reference point; with the circuit in the first configuration, operating the current generator to cause a first flow of current to flow through the reference resistor and the reference point and recording a first voltage value V, using the voltage measuring device; with the circuit in the second configuration, operating the current generator to cause a second flow of current of equal magnitude to the first current to flow through the reference resistor, the reference point and the electrical conductor, and recording a second voltage value V2 using the voltage measuring device; and calculating the resistance between the reference point and the test point using the values V,, V2 and Rsandard The method may further comprise: positioning a second coupling electrode at a second test point further along the electrical conductor from the first test point; making a first resistance measurement between the reference point and the first test point; making a second resistance measurement between the reference point and the second test point; and calculating the resistance of the electrical conductor between the two test points as being the difference between the first and second resistance measurements.

Alternatively, the method may further comprise arranging a plurality of coupling electrodes in a corresponding plurality of test points around the circumference of an electrical conductor and obtaining voltage readings between the reference point and each test point.

According to a third aspect of the invention there is provided apparatus for measuring the resistance between a reference point and a test point, the apparatus comprising a circuit including a current generator, a voltage measuring device and a coupling electrode responsive to local electric field strength; wherein the current generator is operable to cause a current to flow through the reference point and the test point; the coupling electrode is responsive to the local electric field strength at the test point by virtue of capacitive coupling; and the voltage measuring device is connected between the coupling electrode and the reference point and operable to provide a voltage value representative of the resistance between the reference point and the test point when the current generator is operated.

Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: Figure 1 illustrates a plot of pressure against interface resistance for a theoretical Cu-Cu (copper-copper) joint (Ref. Copper Development Association (CDA) publication "Copper for Busbars", chapter 5, page 82.); Figure 2 illustrates a circuit diagram in accordance with an embodiment of the invention; Figure 3 illustrates a current generator suitable for use in the circuit of Figure 2; Figure 4a illustrates a first example of a coupling electrode; Figures 4b and 4c illustrate equivalent circuits to the configuration in Figure 4a; Figure 4d illustrates a second example of a coupling electrode; and Figures 4e and 4f illustrate equivalent circuits to the configuration in figure 4d.

The present embodiments represent the best ways known to the applicant of putting the invention into practice. However they are not the only ways in which this can be achieved.

Theoretical copper-copperjoint Figure 1 shows a plot of pressure against interface resistance for a theoretical copper- copper joint. Interface resistance is a reliable indicator of the extent of cold welding across an interface (that is, the extent to which the interface looks like a grain boundary within the copper's crystal structure: a discontinuous lattice but with no barrier to electron movement).

Whatever the tooling pressure variation, the status of the copper forming pressure is indicated by the electrical resistance. It is for this reason that crimp specifications universally require resistance measurement at tooling set ups and at audits to establish performance and through life reliability. Resistance measurement has the additional benefit of revealing troublesome oxides or contaminates on mating surfaces - problems that neither pull testing or pressure monitoring can address.

A machine that can routinely measure resistance has essential data for statistical process control (SPC) of the process. If intelligent limits are set, surface finish, cross sectional area and crimp pressure variations can all be screened. With diagnostics, each parameter can be brought under optimum production control.

Measurement of interface resistance The resistance of a crimp on 50 mm2 copper wire is, for example, a few micro ohms, representing a very small percentage of the cable's total resistance. Since variations in the temperature coefficient, the cable length and the cross-sectional area usually swamp changes in crimp resistance, crimp resistance is a difficult quantity to measure.

One embodiment of the invention, as illustrated in Figure 2, makes use of the observation that a high current through a cable 10 generates an appreciable voltage across the crimp resistance. 1,000 amps for instance develops one milk volt across each micro ohm, 20 micro ohms develops 20 milk volts, etc. It has also been found that a pulse of high current, short enough to cause no heating, develops a voltage pulse across the crimp resistance that is large enough for trouble-free measurement of the crimp resistance using a coupling electrode (e.g. the coupler ring 14) and a voltage measuring device (e.g. the electrometer 12). In this embodiment of the test apparatus, the electrometer 12 is grounded to a reference point on the body of the crimped connector 11 using a probe 24, and is also coupled to the wire at the test point (the "exit point" on the cable after the crimp) using the coupler ring 14. A pulse generator 22 is used to supply a constant current pulse of, say, 500 amps, for 1 ms, via a contact 26 at the far end of the cable 10. The electrometer 12 responds proportionately to the magnitude of the resultant voltage pulse across the crimp resistance and, accordingly, proportionately to the magnitude of the crimp resistance between the reference point and the test point.

The metal coupler ring 14 is placed around the cable 10 close to the back of the crimped connector 11, and assumes the voltage of the surrounding field. During a 500 amp pulse, this will be higher than the crimp body by the voltage across the crimp resistance. The ring 14 has a typical capacitance to the copper wire of 5 pF and so the output of the electrometer 12 will be an inverted version of the pulse but at low impedance. This pulse is a useable analogue of the crimp resistance.

The pulse height from the electrometer 12 varies with both crimp resistance and the coupling ring capacitance. This latter quantity varies slightly between cables in any batch and markedly from cable type to type. Hence a method of calibration is required before the crimp resistance can be derived. A standard resistor 16 is therefore provided in the test apparatus, along with relays 18 and 20. By setting the relays 18, 20 to position "A" and having the pulse current flow only through the standard resistor 16 (but with the crimp 11 under test nevertheless remaining connected) a known voltage is obtained through the ring capacitance, for the purposes of calibration. The ring 14 detects the voltage pulse across the standard resistor 16 by virtue of it being capacitatively connected. The electrometer ground must be shifted using the changeover relay 18. Note that no current flows through the crimp 11 during calibration and hence the voltage on the ring 14 is due to the known voltage and nothing else. All other fields are assumed absent, provided there is adequate shielding.

The electrometer 12 should be a low noise electrometer, free of any interference. Screening is important, both for the coupler ring 14 and the electrometer input, to screen against stray external fields (e.g. mains fields and machine interference) and also against the supply cables. In practice, this shielding may be achieved by housing the coupler ring and electrometer inside a suitable enclosure. Appropriate shielding techniques are known to those skilled in the art.

To perform a measurement, two separate pulses of constant current are used. The actual value of the current is not critical, providing both pulses are of equal magnitude. The pulses are typically each of 1 ms in duration, although pulse lengths other than 1 me may also be employed. Furthermore, the first and second pulses need not be of equal duration for example, the duration of the calibration pulse may be 1 ms and that of the test pulse may be 2 me. The pulses may be short (provided they have well defined settling of front edge ringing) or longer (provided there are no excessive thermal effects). The pulses need to be sufficiently fast in light of the impedance limitations of the electrometer, and it has been found that 1 ms pulses provide good results in practice.

With the relays 18, 20 set in position "A", a first pulse is generated by the pulse generator 22 and sent through the standard resistor 16, Rsandard. The first pulse does not pass through the cable 10 under test or the crimp 11. Instead, it is ducted to the standard resistor 16, Rsndard where it establishes a voltage. The crimp 11 under test is connected to this voltage and because there is no current in it, it assumes the voltage across the standard resistor 16.

During this pulse, the electrometer ground is connected to the low end of the standard resistor 16.

The detector ring 14 has an unknown capacitance to the cable 10 and hence the voltage detected by the electrometer 12 during the first pulse will be kVsandard, where k is a constant due to the unknown capacitance. The electrometer output, kVsandard, is stored as V'.

The relays 18, 20 are then switched to position "B" and a second pulse is sent through the crimp 11 under test to measure the voltage across the crimp resistance. During the second pulse, which is known to be equal to the first pulse, the current is ducted through the cable under test and the standard resistor 16. The electrometer ground is connected to the reference point on the body of the crimped connector by the probe 24. The detector ring 14 has not moved, and thus the constant of proportionality k remains unchanged. The voltage detected by the electrometer 12 during the second pulse will be kVCnmp, and this is stored as v2 Any contact resistance or relay resistance in the current path is ignored, as is the standard S resistance 16 during the second pulse. This is permissible because a constant current supply is used. That is to say, the current source is a constant current source. In one embodiment, the constant current source delivers 500 amps, whatever the load. Other values of constant current may alternatively be used. 500 amps or 1000 amps are typical values, given by way of example only.

Using Ohm's law, voltage is proportional to resistance with any given current, hence kVCnmp / kVstandard = V2/ V1 = Rcnmp / Rstandard The k factor, due to unknown capacitance, cancels out, and the ratio of the crimp resistance to the standard resistance (RCnmp Rsandard) is therefore equivalent to the ratio of the electrometer readings (V2:V1). This relies on the two current pulses being of equal current (which is an easy requirement) and that the ring stays in place between pulses (which is also an easy requirement).

The crimp resistance RCnmp is therefore derived by the formula: Rcnmp = Rstandard (V2 / V1) In practice, current pulses of about 500 amps have been used to measure heavy battery cables with excellent results. A sample whose crimp resistance measured 26 micro ohms using conventional 4- wire techniques at 10 amps (needle probe through the insulation, 10mm from crimp), measured 25.9 micro ohms using the electric field detection with electrometer technique. These two measurements, made using the traditional technique and the present embodiment, are in excellent agreement.

It is possible to create a one-pulse test but the current must be accurately known and the capacitance from ring to wire must also be known. In addition, all the stray capacitances around the detector ring must also be known since the slightest change alters the output considerably. The above two-pulse method gives more certainty, with only a small change in the overall cost of the apparatus.

Figure 3 shows, by way of example, a pulse generator 20 suitable for use in the present embodiment. Other pulse generators will be known to those skilled in the art. Here, the pulse generator uses a number of NMOS power transistors in parallel configured in constant current mode. The gate is opto-coupled to a drive voltage (which determines the output current) and the resulting pulse of drain current is passed to the test circuit. The opto- coupler is switched on for 1 ms under computer control to time the pulse. The test current is sourced from a 1 farad capacitor that is constantly trickle charged from any suitable power supply.

Figures 4a and 4d illustrates examples of possible capacitive coupling electrodes. Figure 4a shows an example of a coupler ring 14, which surrounds the cable 10 which is crimped to a connector 11. The crimp resistance is indicated as Ramp. Figures 4b and 4c show equivalent circuits to the configuration of Figure 4a. In Figure 4c, a possible capacitance of the coupling between the cable 10 and the ring 14 is indicated as being 7 pF, by way of

example only.

As Figure 4d shows, the coupling electrode does not need to surround the cable 10, and a "partial" electrode 15 such as the one illustrated may be employed instead. Figures 4e and 4f show equivalent circuits to the configuration of Figure 4d. In Figure 4f, a possible capacitance of the coupling between the cable 10 and the partial electrode 15 is indicated as being 2 pF, again by way of example only.

The coupling electrodes may be encased by an insulating material, and the dielectric properties of the insulating material may be used to enhance the capacitive coupling between the electrode and the cable under test.

The descriptive name that has been given to this technique is "Crimp resistance measurement using electric field detection". The technique has proved accurate, fast, non- destructive and low cost, and early market information suggests crimp resistance measurement may have wide, all-industry interest throughout the world.

The length of cable being tested is not important, provided that it is not too long so as to necessitate a high supply voltage in order to provide the requisite constant pulse current. By way of example, a solid 10 mm diameter copper cable may be 80 m in length and tested using a 20 volt pulse generator. Alternatively, if a 2 mm diameter copper cable is used, then the corresponding length would be approximately 3 m. However, if it is necessary to test a longer cable 2 mm in diameter, then the supply voltage could be increased say to 100 volts (a factor of 5 compared with 20 volts), and this would enable the length of cable being tested to be increased by a corresponding factor of 5.

The equipment can be scaled down to measure smaller crimps, and the detection apparatus S may be integrated into crimp machines to be used as a process control similar to the way closure force is used at present. The techniques may also be used in non-contact measurement of wire-gauge and broken strand detection. Portable measurement sets may be provided to help determine the status of installed wiring The above embodiment uses a rectangular 1 me pulse of high current, which is effectively do (direct current) for the duration of the test. The detector ring voltage follows this pulse by being connected to a high impedance amplifier. However, the amplifier is also susceptible to stray fields and because the amplifier is in wide band mode, the results may be noisy.

However, by using a pulse of ac (alternating current) of say 30 kHz through the cable under test, three benefits may be achieved: a) By filtering the detected signal, interference from external fields (especially mains) may be reduced.

b) The noise bandwidth of the amplifier can fall, and c) The ring size requirement may reduce.

Adopting ac stimulus is expected to increase the signal to noise ratio which will lead to lower currents for test. This in turn enables longer cables to be tested, since the longest is defined by the available voltage to drive the current.

Summary of features

1. Use of the electric field around the cable, coupled to a detector to evaluate the magnitude of crimp resistance.

2. Use of standard resistor and constant current source to calibrate the detector.

3. Calculation of resistance from stored values. Other applications The present testing technique has been described above

in relation to crimp resistance measurements. However, the principles are applicable to other electrical test and measurement techniques, and some examples are given below: 1. Gauge measurement It is known from workers in the field that a major problem for heavy wire crimps is the gauge variation in the wire stock (plus and minus 15% is reported). Currently, there are no practical ways to measure the total gauge of wire bundles in the workshop, whilst in the laboratory the insulation must be pierced, which is a destructive technique.

A single detector ring placed around the cable to be tested just behind the crimp measures the crimp resistance as described. If a second detector ring is placed, say, 100 mm distant from the first and a second resistance measurement is performed, the result will be the same as the first but higher by the additional resistance of 100 mm of cable. By subtracting the first reading from the second and knowing the distance between the two rings, the ohms per metre and hence the wire gauge can be derived.

2. Broken strand detection Measuring wire gauge by comparing the voltage between two detector rings opens the possibility of detecting broken strands within a wire. For example, consider two detector rings 100 mm apart, fixed on a bench or hand held. The cable under test is passed through the rings and either the rings are moved along the wire or the wire is pulled through the rings.

With pulses of constant current flowing through a uniform cable, the voltage between the rings will be constant. One ring will have a constant higher voltage. Offset this constant voltage, then subtract the two ring voltages such that whatever part of a good cable is examined, the output is zero. A high gain amplifier should follow the subtracter.

If there is a disturbance to the cross-sectional area (CSA) of the wire, the resistance and hence the voltage will also change at the disturbance, causing a change to the output of the subtracter. A broken strand within a wire bundle develops a CSA disturbance and may be detectable. This feature would be useful to those examining installed wiring, e.g. in trains, ships, aircraft, infrastructure, etc. 3. Detection of a patchy crimp interface The actual area of contact within a crimp barrel may be unknown. Due to contamination or faulty tooling, only part of the total area may be properly connected. The current distribution in the wire as it exits the crimp is, for a short distance, highest nearest the point of connection. This creates a voltage distribution around the wire giving a crude fingerprint of the quality of the copper/crimp interface. The effect is only present for up to a few millimetres from the crimp exit.

As described above, the present crimp resistance tester uses a detector ring about 10 mm from the crimp exit and is continuous around the cable. If this is split into say four very small quadrants around the circumference of the cable, with each quadrant connected to an independent electrometer, and the whole assembly positioned at the immediate crimp exit, differences in voltage may be detected. Different voltages will infer an uneven crimp interface. This application requires a high signal to noise ratio, since small detector plates are needed. This application would improve crimp inspection and provide diagnostics to manufacturers.

Claims

1. Apparatus for measuring the resistance between a reference point and a test point along an electrical conductor, the apparatus comprising a circuit including a current generator, a voltage measuring device and a coupling electrode responsive to local

electric field strength;

wherein, when in use testing an electrical conductor: the current generator is operable to cause a current to flow through the reference point and along the electrical conductor through the test point; the coupling electrode is responsive to the local electric field strength at the test point by virtue of capacitive coupling between the electrode and the electrical conductor at the test point; and the voltage measuring device is connected between the coupling electrode and the reference point and is operable to provide a voltage value representative of the resistance between the reference point and the test point when the current generator is operated.

2. Apparatus as claimed in Claim 1, further comprising a reference resistor of known resistance R5andard connected between the current generator and the reference point, and switching means operable to switch the circuit between a first configuration and a second configuration, the switching means and circuit being arranged such that, in use: in the first configuration, one end of the electrical conductor is connected to the reference point and the other end is disconnected from the circuit, the coupling electrode is positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the other side of the reference resistor from the reference point, the voltage measuring device thereby being operable to record a first voltage value V, in the event that the current generator causes a first flow of current through the reference resistor and the reference point; and in the second configuration, the reference resistor and the electrical conductor are connected in series with the reference point between them, the coupling electrode remains positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the reference point, the voltage measuring device thereby being operable to record a second voltage value V2 in the event that the current generator causes a second flow of current through the reference resistor, the reference point and the electrical conductor, the second current being of equal magnitude to the first current; the resistance between the reference point and the test point being calculable using the values V,, V2 and Rsandard.

3. Apparatus as claimed in Claim 2, wherein the resistance between the reference point and the test point is equal to Rsandard (V2 / V,).

4. Apparatus as claimed in any preceding claim, wherein the coupling electrode is configured so as to at least partially surround the electrical conductor at the test point.

5. Apparatus as claimed in Claim 4, wherein the coupling electrode comprises a coupler ring configured to surround the electrical conductor at the test point.

6. Apparatus as claimed in any preceding claim, wherein the voltage measuring device comprises an electrometer.

7. Apparatus as claimed in any preceding claim, wherein the current produced by the current generator comprises pulses.

8. Apparatus as claimed in Claim 7, wherein the pulses have a pulse length of 1 ms.

9. Apparatus as claimed in any preceding claim, wherein the current has a magnitude of 500 amps.

10. Apparatus as claimed in any preceding claim, wherein the current generator is configured to send do current.

11. Apparatus as claimed in any of Claims 1 to 9, wherein the current generator is configured to send ac current.

12. Apparatus as claimed in any preceding claim, adapted for measuring crimp resistance.

13. A crimp machine comprising apparatus as claimed in Claim 12.

14. Apparatus as claimed in any of Claims 1 to 11, further comprising a second coupling electrode positionable, in use, at a second test point further along the electrical conductor from the first test point, the apparatus being operable to make resistance measurements between the reference point and each of the two test points and thereby enabling the resistance of the electrical conductor between the test points to be calculated.

15. Apparatus as claimed in Claim 14, adapted for measuring wire gauge.

16. Apparatus as claimed in Claim 14, adapted for detecting broken strands within a wire.

17. Apparatus as claimed in Claim 1, 2 or 3, comprising a plurality of coupling electrodes arranged in a corresponding plurality of test points around the circumference of an electrical conductor in use and operable to obtain voltage readings between the reference point and each test point.

18. Apparatus as claimed in Claim 17, adapted for detecting an uneven crimp interface.

19. A method of measuring the resistance between a reference point and a test point along an electrical conductor, the method comprising: positioning a coupling electrode at the test point, the coupling electrode being responsive to the local electric field strength at the test point by virtue of capacitive coupling between the electrode and the electrical conductor at the test point; operating a current generator to send a current through the reference point and along the electrical conductor through the test point; and measuring the voltage between the coupling electrode and the reference point using a voltage measuring device, to provide a voltage value representative of the resistance between the reference point and the test point.

20. A method as claimed in Claim 19, further comprising: including a reference resistor of known resistance Rsandard connected between the current generator and the reference point; including switching means operable to switch the circuit between a first configuration and a second configuration, the switching means and circuit being arranged such that, in use, in the first configuration, one end of the electrical conductor is connected to the reference point and the other end is disconnected from the circuit, the coupling electrode is positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the other side of the reference resistor from the reference point, and in the second configuration, the reference resistor and the electrical conductor are connected in series with the reference point between them, the coupling electrode remains positioned at the test point along the length of the electrical conductor, and the voltage measuring device is connected between the coupling electrode and the reference point; with the circuit in the first configuration, operating the current generator to cause a first flow of current to flow through the reference resistor and the reference point and recording a first voltage value V, using the voltage measuring device; with the circuit in the second configuration, operating the current generator to cause a second flow of current of equal magnitude to the first current to flow through the reference resistor, the reference point and the electrical conductor, and recording a second voltage value V2 using the voltage measuring device; and S calculating the resistance between the reference point and the test point using the values V,, V2 and Rsandard.

21. A method as claimed in Claim 20, wherein the resistance between the reference point and the test point is calculated as being equal to Rsandard (V2 / V,).

22. A method as claimed in any of Claims 19, 20 or 21, wherein the current produced by the current generator comprises pulses.

23. A method as claimed in Claim 22, wherein the pulses have a pulse length of 1 me.

24. A method as claimed in any of Claims 19 to 23, wherein the current has a magnitude of 500 amps.

25. A method as claimed in any of Claims 19 to 24, further comprising: positioning a second coupling electrode at a second test point further along the electrical conductor from the first test point; making a first resistance measurement between the reference point and the first test point; making a second resistance measurement between the reference point and the second test point; and calculating the resistance of the electrical conductor between the two test points as being the difference between the first and second resistance measurements.

26. A method as claimed in any of Claims 19, 20 or 21, further comprising arranging a plurality of coupling electrodes in a corresponding plurality of test points around the circumference of an electrical conductor and obtaining voltage readings between the reference point and each test point.

27. Apparatus for measuring the resistance between a reference point and a test point, the apparatus comprising a circuit including a current generator, a voltage measuring device and a coupling electrode responsive to local electric field strength; wherein the current generator is operable to cause a current to flow through the reference point and the test point; the coupling electrode is responsive to the local electric field strength at the test point by virtue of capacitive coupling; and the voltage measuring device is connected between the coupling electrode and the reference point and operable to provide a voltage value representative of the resistance between the reference point and the test point when the current generator is operated.

28. Apparatus for measuring resistance substantially as herein described with reference to and as shown in any combination of the accompanying drawings.

29. A method for measuring resistance substantially as herein described with reference to and as shown in any combination of the accompanying drawings.