GB2076237A - Electrical resistance monitoring device - Google Patents

Abstract

An electrical resistance monitoring device is arranged to be capable of detecting changes in the electrical resistance of a load irrespective of changes in supply line voltages to the resistance. The device has load current and load voltage sensors and these sensors each generate an output signal (at 5, 6 and 7, 9) corresponding to the load current and load voltage. Balancing means RV1 balance the load voltage and load current signals so the device is not actuated when the resistance value is normal. An out of balance signal due to a change in the electrical resistance is detected (IC1) and an alarm RLY1 is operated. The device may be arranged such that when the normal supply is disconnected from the load an alternative, low- voltage supply is substituted, so that the resistance can be monitored even when the load is deenergised. The invention finds particular application in detecting the failure of heating tapes arranged in parallel. <IMAGE>

Description

SPECIFICATION Electrical resistance monitoring device This invention relates to an electrical resis tance monitoring device.

In various applications it is necessary to be able to determine the resistance of an electri cal circuit and to monitor such resistance. The determination of the resistance will enable information to be obtained concerning the circuit, for example whether the circuit or a part thereof has failed.

One application for an electrical resistance monitoring device is to detect the failure of electrical heating tapes. Such tapes can be monitored individually but in the case where a plurality of heating tapes is employed it is desirable to have a monitoring device which is able to continuously monitor and detect the failure of any one of the tapes connected in parallel to the other tapes. Such a device needs to be able to detect small changes in load resistance and it has been proposed to detect such changes by a device which de tects changes in the total current to the tapes.

However where this is done errors may arise because current changes due to the usual changes in supply line voltage may be of the same order as those current changes which are to be detected. Consequently the detec tion of current changes alone is insufficient to give the sensitivity required to monitor the resistance.

An object of the invention is to provide an improved electrical resistance monitoring de vice which is capable of monitoring a resis tance and is unaffected by variations in line voltage.

According to the invention an electrical re sistance monitoring device comprises means for generating a signal corresponding to the load current in the electrical resistance, means for generating a signal corresponding to the load voltage across the electrical resistance, balancing means for balancing the load volt age signal and the load current signal so that the device is not actuated under normal load conditions, and means for detecting an out of balance signal derived from the load current and load voltage signals due to a change in the electrical resistance, said detecting means being arranged to actuate alarm means.

Further features of the invention will appear from the following description of an embodi ment of the invention given by way of exam ple only and with reference to the drawings, in which: Figure 1 is a circuit diagram showing the connection of a monitoring device into a cir cuit including a resistance, and Figure 2 is a circuit diagram of the monitor ing device.

Referring to the drawings and firstly to Fig.

1 an electrical supply is applied to a circuit at N, L, the circuit including a load or resistor R whose electrical resistance is to be monitored by a monitoring device M. Power for the device M is applied from the power supply for the resistor R at terminals 2 and 10, and the load voltage to the resistor R is applied to the device M at terminals 7 and 9. A current transformer C detects the load current to the resistor R, the load current passing through the primary winding of the transformer C and the secondary winding having an induced voltage which is supplied to the device M at terminals 5 and 6. Thus the device M is supplied with a voltage across terminals 7, 9 corresponding to the voltage across the load and a voltage across the terminals 5, 6 corresponding to the current flowing in the load.

The load voltage applied at the terminals 7, 9 is rectified by rectifiers D1, D2, D3 and D4 and filtered by resistors R,, R2, R3 and capacitors C, and C2 (Fig. 2). A D.C. voltage appears across capacitor C2 which is proportional to the load voltage.

The output of the current transformer C is applied to the terminals 5, 6 and is rectified by rectifiers D5, D6, D7 and D8. Resistor R7 is the burden of the current transformer and the rectified signal that appears across R7 is filtered by resistors R8 and R9 and capacitors C3 and C4. The voltage which appears across the capacitor C4 is, therefore, proportional to the load current of the resistor R.

The monitoring device has a circuit with a common rail F and the voltage across capacitor C2 is positive with respect to the rail F whereas the voltage across the capacitor C4 is negative with respect to the rail F. These two voltages are joined by a potential divider constituted by resistors R4, RV1 and R5 the values of which are chosen so that the voltage at a slider S associated with RV, is zero if the resistance level of the load R is correct. The potentiometer RV1 is provided so that the device can be adjusted for use over a range of resistance levels in the load R.

It will be seen that a change in the supply voltage at L, N will change the voltage across capacitor C2 but a corresponding change will occur across the capacitor C4 due to the corresponding change in load current. Consequently the circuits will be in balance and the voltage at the slider of RV1 will remain at zero.

If the resistance of the load R changes due, for example, to a fault developing, the load current will change thereby changing the voltage across the capacitor C4. The voltage on the slider S of potentiometer RV1 will become positive, i.e. an out of balance signal will appear at S.

A comparator IC1 is connected to the slider S and compares the voltage on the slider S of potentiometer RV, with a fixed voltage derived from the supply voltage connected at terminals 2, 10. The supply voltage supplies a D.C. voltage suitable for operation of a relay RLY, by transformer T, rectifier D9, D10, D" and D,2, resistor R,3 and capacitor C7 and a stabilised voltage for comparator lC1 is provided by Tr1, R,2 and D,4.

Detection of a positive voltage on the slider S by the comparator IC1 will cause the relay RLY,, connected at 1, 3 and 4, to de-energise and the contacts of the relay will be connected to an alarm (not shown) which is then operated to signal a fault condition in the resistor R.

Resistors R10 and R,1 set the differential of the comparator and a capacitor C6 provides a short time delay so that the circuit is not operated when the load R is switched on and off.

As shown the comparator detects a rise in the resistance of the load R and, if desired a further similar comparator can be added to detect a fall in resistance.

It will be seen that the power for the monitoring device is obtained from the same supply as that for the load. Should the supply fail the relay RLY, will de-energise to give an alarm signal. However it may be required for the load to use a relatively high supply voltage or one with inconvenient access in which case power for the device can be obtained from a different source.

2 Aswitch S, is provided for interrupting the supply to the load R but operation of the switch S, does not remove the supply to the monitoring device.

By the provision of a transformer T2 and a relay RLY2 operated by a resistor R20 (Fig. 1) it will be seen that a voltage can be applied to the load R after the switch S, is operated to interrupt the full supply line voltage to the load R. Interruption of the supply causes the resistor R20 to actuate the relay RLY2 and close the circuit to supply a low test voltage from the supply LN through the step down transformer T2 to the load R. By arranging the ratio of the turns on the transformer to be between 10:1 and 30:1 the test voltage is sufficiently high to enable the resistance R to be monitored by the monitoring device when not under full load and sufficiently low that negligible power is used for the monitoring function. Thus the device is able to monitor the resistance even when the resistance is out of use to detect any fault condition arising during this time.

If the load R is a heating device the switch S, may take the form of a thermostat. With the arrangement shown in Fig. 1 the resistance R will be monitored whether switch S, is open or closed. However it will be seen that, if the transformer T2 is omitted, even if switch S, is opened no voltage is applied to the terminals 7 and 9, no current flows in the current transformer C, the relay RLY, remains energised and no alarm is given.

It will be appreciated that the device is particularly useful in the case where small changes in load resistance need to be detected. One suitable application is to a number of similar circuits connected in parallel where there is a requirement to detect a failure of one circuit. In such an application the change in total current will be relatively small and possibly less than the change due to the usual range in variation of the line voltage. Accordingly detection of a fault in one circuit by measurement of current changes is unlikely to be satisfactory. The present invention overcomes this difficulty by deriving signals proportional to the load current and load voltage, balancing such signals and then detecting an out of balance signal to operate an alarm.

Claims (11)

1. An electrical resistance monitoring device comprising means for generating a signal corresponding to the load current in the electrical resistance, means for generating a signal corresponding to the load voltage across the electrical resistance, balancing means for balancing the load voltage signal and the load current signal so that the device is not actuated under normal load conditions, and means for detecting an out of balance signal derived from the load current and load voltage signals due to a change in the electrical resistance, said detecting means being arranged to actuate alarm means.

2. A device according to claim 1 wherein the means for detecting the out of balance signal includes comparator means which compares the signal with a reference signal.

3. A device according to claim 1 or 2 wherein the load current signal generating means includes means for sensing the load current and generating an output voltage signal proportional to the load current.

4. A device according to claim 1, 2 or 3 wherein the load voltage signal generating means senses the voltage across the load and generates an output voltage signal proportional to load voltage.

5. A device according to claim 3 and 4 wherein the balancing means balances the output voltage signals derived from the loads current and the load voltage so as to derive no output from the balancing means under ,, normal load conditions.

6. A device according to claim 5 wherein the balancing means includes a potential divider which is adjustable to effect an initial balancing of the load voltage signal and the load current signal under normal load conditions.

7. A device according to any one of the preceding claims wherein the detecting means upon detecting an out of balance signal from the balancing means actuates a relay connected to operate the alarm.

8. A device according to any one of the preceding claims wherein the generating means for generating the load current signal and the generating means for generating the load voltage signal include means for deriving d.c. voltages proportional to load current and load voltage respectively.

9. A device according to claim 8 wherein the balancing means includes resistor means for providing a balanced signal position on said resistor means in the inoperative condition of the device, an out of balance signal being generated at said position on a change occuring in the electrical resistance, and said out of balance signal being compared with a reference signal by comparator means.

10. A device according to any one of the preceding claims including switch means for interrupting full supply voltage to the electrical resistance and, upon such interruption, means to supply a nominal test voltage to the electrical resistance is actuated to monitor the electrical resistance when full supply line voltage is not applied to the resistance.

11. A device according to claim 10 wherein the full supply line voltage and the nominal test voltage have a common voltage source, the nominal test voltage being supplied through step down transformer means.

1 2. A device according to claim 10 or 11 wherein the test voltage is applied to the device and to the resistance automatically upon interruption of full supply line voltage.

1 3. A device according to any one of the preceding claims wherein the electrical resistance to be monitored comprises a plurality of resistances connected in parallel.

1 4. An electrical resistance monitoring device substantially as described with reference to the drawings.