CIRCUIT TESTING

Abstract: A method and apparatus for temporarily disabling an RCCB having a magnetically coupled sensing circuit, protecting an AC circuit, and the testing circuit described. The method of disabling the RCCB comprises: connecting a direct current power source to the circuit being protected by the RCCB, and injecting direct current from the direct current power source into the circuit protected by the RCCB, in order to magnetise the magnetically coupled sensing circuit of the RCCB, thereby desensitising the RCCB. The RCCB is rendered temporarily inoperative thus allowing a test to be made on the circuit. In the apparatus a current source (2) is connected to inject DC current into the RCCB in the protected circuit under the control of a ramping circuitry (3, 5). Once the magnetically coupled circuit of the RCCB is saturated a test of the AC circuit is carried out.

Description: The present invention relates to a method and apparatus for carrying out or facilitating testing of electrical circuits, and in particular, though not exclusively to a method and apparatus for testing the impedance in an alternating current (AC) mains supply between the phase and earth lines. The impedance of the phase-earth loop in an AC power supply is very important and all new installations or modifications have to be tested to verify that impedance. Test methods and equipment have been developed to establish the impedance of the phase-earth loop, and a simplified example of one of these is shown in Figure 1. In general terms, the test is made by measuring the voltage between the phase and earth lines with and without a test resistor of known value in circuit. It is then possible to determine the phase-earth impedance. During the test with the resistor in circuit a current, Itest, flows in the phase-earth loop. To obtain a reasonable impedance measurement resolution the value of the test resistor must be low and the value of the test current that flows to earth is correspondingly high. In unprotected circuits these known methods and equipment operate satisfactorily. However, with circuits including circuit breakers for protecting users or connected equipment, there can be considerable problems because the test current can cause a circuit breaker to trip. This can invalidate the test, but worse, it will interrupt the power supply to equipment on the protected circuit. If, for example a computer system is connected to that circuit the cost in terms of data loss and even damage to equipment can be very high. One type of circuit breaker used for protection on AC mains supplies is the residual current circuit breaker (RCCB), which detects any imbalance in the current flow in the phase-neutral loop, usually this will be caused by current passing from the phase line to earth. To accomplish this detection the RCCB has a magnetically coupled sensing circuit which is sensitive to the current balance in the phase/neutral loop. When the test described above is carried out, the test current flows only in the phase-earth loop, so that an RCCB would detect a current imbalance between the phase and neutral lines and be triggered. To avoid this it is necessary to remove any RCCB'S from the circuit to be tested. This is clearly undesirable, time consuming and inconvenient. The present invention seeks to provide a method for temporarily disabling an RCCB protecting an AC circuit, so that tests can be made on that circuit. In another aspect the present invention seeks to provide apparatus for testing an AC circuit, including an RCCB. Accordingly the present invention provides a method of temporarily disabling an RCCB having a magnetically coupled sensing circuit, in an AC circuit, the method comprising: a) connecting a direct current power source to the circuit being protected by the RCCB, and b) injecting saturating direct current from the direct current power source into the circuit protected by the RCCB, in order to magnetically saturate the sensing circuit of the RCCB, thereby desensitising the RCCB, so that the RCCB is rendered temporarily inoperative thus allowing a test to be made on the circuit. By injecting a saturating DC current into the protected circuit, the magnetically coupled sensing circuit of the RCCB which would normally detect any imbalance current, is magnetised sufficiently so that any imbalance current flowing is not effective on the magnetically susceptible sensing circuit and the RCCB does not respond. Thus, a circuit test, for example the phase-earth loop test described above may be carried out without the RCCB being triggered. Preferably the amount of direct current (DC) injected is progressively increased from a low level towards the saturating level. An RCCB might be triggered by a sudden, large DC current, but by increasing the direct current gradually from a low level that can be avoided. Preferably the direct current is reduced progressively to a lower level after having been at the saturating level. In another aspect the invention provides an apparatus for temporarily disabling an RCCB having a magnetically coupled sensing circuit, protecting an AC supply: comprising a DC current source means for connection to the protected supply for injecting a saturating DC current into the supply for increasing the magnetisation of the magnetic circuit towards saturation thereby disabling the sensing circuit of the RCCB, so that a part of the supply protected by the RCCB may be tested without the RCCB being triggered. It is preferable that the apparatus includes a controller means for controlling the output of the DC current source means, and the controller means can increase the current injected into the protected circuit, progressively towards saturation current. By progressively increasing the magnetisation the rate of change of magnetic flux in the magnetically coupled sensing circuit can be controlled to a level which is insufficient to cause the RCCB to respond. Similarly, it is preferred that the controller reduces the output of the DC current source means progressively to reduce the magnetisation of the magnetically coupled sensing circuit of the RCCB. In a further aspect the invention provides apparatus for testing the Phase-Earth impedance of a Phase-Neutral-Earth AC supply with Phase-Neutral lines protected by an RCCB, comprising: a test circuit means for selectively testing between first and second output terminals to determine the impedance of a circuit connected to those terminals; a DC current source means for providing a controllable current output between second and third output terminals; a controller means for controlling the DC current source means to provide a time varying current output, and also for controlling the timing of operation of the test circuit means; whereby, with the first, second and third terminals connected to the Phase, Earth and Neutral lines of the protected supply the controller can progressively increase the current output of the DC current source means to inject into the Neutral-Earth loop of the AC supply direct current to magnetise the magnetically coupled sensing circuit of the RCCB towards saturation, and thereafter, the controller can operate the test circuit means to test the impedance of the Phase-Earth loop of the AC supply without the RCCB being triggered. A preferred embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a schematic diagram illustrating the principle of a known phase-earth loop testing; Figure 2 is a schematic block diagram of a circuit test apparatus of the preferred embodiment of the invention; Figure 3 is a graphical representation of the variation with time of direct current supplied to the circuit to be tested, to magnetise a sensing circuit of an RCCB. Referring to Figure 2, a phase-earth loop test instrument 1 is shown with connections to an AC mains supply with the phase-neutral lines of the AC mains supply being protected by an RCCB. The test apparatus 1 includes a DC current source 2 connected to output terminals N and E, and which is controlled by a current ramp controller 3. The current ramp controller 3 is for controlling the current source, so that the source 2 provides, in general terms a ramped up and then down supply of direct current. The current ramp controller 3 is connected to receive a reference output from ramp generator 5, and to provide a signal level output to a cycle timer 6. Cycle timer 6 is for controlling the timing of the operation of the test apparatus and is connected to provide an initiation control signal to ramp generator 5. The cycle timer 6 is in turn controlled by push-to-test button 4. The test apparatus also includes a test resistor 9, which in series with thyristor 10 and protective fuse 11 is connected between the output terminals P and E. The thyristor 10 has its control gate connected to cycle timer 6 while a measuring circuit 7 is connected to measure the voltage across the P and E terminals to determine the impedance of the circuit connected to the terminals P and E. Cycle timer 6 is connected to measuring circuit 7 to control operation of that circuit. A display 8 is connected to measuring circuit 7 for displaying the determined impedance. A thermal cut-out 12 is provided to respond to overheating of test resistor 9, to disable the test apparatus. The impedance determining circuitry of test resistor 9, measuring circuit 7, and display 8 is in principle as already described in known testers. The control of operation of that test circuitry by cycle timer 6 in conjunction with thyristor 10 represents a significant part of the embodiment of the present invention. More significant still is the function and role of DC power source 2 which can best be described in operational terms, and in the present exemplary embodiment the arrangement shown in Figure 2 will be referred to further. As shown in that Figure, a notional sub-station provides Phase (P), Neutral (N) and Earth (E) lines. Although not shown it will be understood that somewhere in the AC supply system, probably at the sub-station there will be a connection between Neutral and Earth lines. Between the sub-station and the AC mains outlet, an RCCB is connected to detect any current imbalance between the Phase and Neutral lines, as for example would be caused if there was significant leakage from the Phase to Earth. As already discussed above, an attempt to test the Phase-Earth loop with a conventional tester which puts a low value resistance between the Phase and Earth outlet terminals, would result in the RCCB sensing a current imbalance in the Phase-Neutral loop and consequently being triggered. The test apparatus of the present embodiment is connected to the protected AC supply with the output terminals, P, N and E connected to the Phase, Neutral, and Earth lines respectively. A test sequence is initiated by depression of test button 4. The test cycle will be described with additional reference to Figure 3. Initially the current source 2 is off and the test resistor 9 out of circuit by virtue of the thyristor 10 being off. Upon depression of push button 4 operation of cycle timer 6 commences. Cycle timer 6 provides an initial control signal to ramp generator 5, which then provides the reference ramp signal to current ramp control 3. The current ramp control 3 turns on and gradually increases the current output of DC current source 2 so that DC current, I SAT is injected into the Neutral-Earth loop. The RCCB will not respond to the gradually increasing DC current provided the rate of increase in current is insufficient to cause a sufficiently rapid change of flux in the magnetically coupled sensing circuit of the RCCB to trigger the RCCB. The sensing circuit of the RCCB is at its most sensitive at the start of the current injection procedure, and so the current is injected initially at a low rate (see Figure 3) but as the sensitivity of the sensing circuit is reduced the current can be and is, injected more quickly. It is undesirable to keep the RCCB desensitised, and therefore inoperative, for longer than necessary, so this current ramping procedure is preferable to help keep that inactive period to a minimum. When the DC current reaches a determined value the magnetisation of the magnetically coupled sensing circuit of the RCCB will approach saturation, so that imbalance current fluctuations in the Phase-Neutral loop will not be sensed, provided that these fluctuations are of the same polarity as the DC saturating current, or if of the opposite provided they are insufficient to cause the magnetisation to be removed. The current ramp control 3 provides a "Test Proceed" signal to cycle timer 6 indicating that ramping up of the direct current has been completed. On receipt of the "Test Proceed" signal, the cycle timer 6 initiates the testing of the Phase-Earth loop by the measuring circuit 7 which first determines the open-circuit voltage. Then, the thyristor 10 is triggered to bring the test resistor 9 into circuit. The test resistor is brought into circuit on half-cycles of the AC voltage which are in the same phase as the applied DC current so that the effect would be to tend to increase the magnetisation of the magnetic circuit of the RCCB, thereby avoiding the possibility of accidentally removing that saturation. While the test resistor is in circuit the measuring circuit 7 establishes the voltage drop across the resistor, and the Phase-Earth impedance is determined from that measurement and the earlier open circuit measurement. The resulting determined impedance is displayed on display 8. Meanwhile once the test measurements have been completed, the cycle timer 6 provides a further control signal to ramp generator 5 which provides a decreasing ramp reference signal to the current ramp control 3 which consequently decreases the output of DC current source 2 to zero, gradually; again so that the rate of change of flux in the magnetically coupled sensing circuit of the RCCB is insufficient to trigger the RCCB. Thus the current is initially reduced relatively quickly, but the rate of reduction is reduced as time goes on to keep the rate of change of flux below that which would trigger the sensing circuit. Once the saturation current has been removed the RCCB will again be responsive to current imbalance in the Phase-Neutral loop. The DC current source 2 should be driven from a voltage sufficient to overcome any normal AC potential existing between Neutral and Earth, in addition to driving the required current through the Neutral Earth impedance. Conveniently the source may be driven from a switched mode supply running from the Phase-Neutral circuit.

Claims: A method of temporarily disabling an RCCB having a magnetically coupled sensing circuit, in an AC circuit, the method comprising: a) connecting a direct current power source to the circuit being protected by the RCCB, and b) injecting saturating direct current from the direct current power source into the circuit protected by the RCCB, in order to magnetically saturate the sensing circuit of the RCCB, thereby desensitising the RCCB, so that the RCCB is rendered temporarily inoperative thus allowing a test to be made on the circuit. A method as claimed in claim 1 wherein, the amount of direct current (DC) injected is progressively increased from a low level towards the saturating level. A method as claimed in claim 1 or 2, wherein the direct current is reduced progressively to a lower level after having been at the saturating level. An apparatus for temporarily disabling an RCCB having a magnetically coupled sensing circuit, protecting an AC supply: comprising a DC current source means (2, 3, 5 and 6) for connection to the protected supply for injecting DC current into the supply for increasing the magnetisation of the magnetic circuit towards saturation thereby disabling the sensing circuit of the RCCB, so that a part of the supply protected by the RCCB may be tested without the RCCB being triggered. An apparatus as claimed in claim 4, wherein the current source means provides current to cause saturation of the magnetically coupled sensing circuit of the RCCB. An apparatus as claimed in claim 4 or 5, wherein there is a controller means (3, 5, 6) for controlling the output of the DC current source means (2), and the controller means can increase the current injected into the protected circuit, progressively towards saturation current. An apparatus as claimed in claim 6, wherein the controller (3, 5, 6) reduces the output of the DC current source means progressively to reduce the magnetisation of the magnetically coupled sensing circuit of the RCCB. An apparatus for testing the Phase-Earth impedance of a Phase-Neutral-Earth AC supply with Phase-Neutral lines protected by an RCCB, including apparatus according to claim 6 for temporarily disabling the RCCB, and comprising: a test circuit means (6, 7, 8, 9, 10) for selectively testing between first and second output terminals to determine the impedance of a circuit connected to those terminals, the DC current source means being connected between said second and third output terminals, and the controller means being arranged to control the DC current source means to provide a time varying current output, and also for controlling the timing of operation of the test circuit means, whereby, with the first, second and third terminals connected to the Phase, Earth and Neutral lines of the protected supply the controller can progressively increase the current output of the DC current source means (2) to inject into the Neutral-Earth loop of the AC supply direct current to magnetise the magnetically coupled sensing circuit of the RCCB towards saturation, and thereafter, the controller (5, 6) can operate the test circuit means to test the impedance of the Phase-Earth loop of the AC supply without the RCCB being triggered.

Inventors: FLOWERDAY, ANDREW LESLIE, HARRIS, DAVID ROBERT, LITHERLAND, MARTIN BERNARD
Applicants: ROBIN ELECTRONICS LIMITED
Application year: 1988
Main classification: H02H3
IPC codes: H02H3, G01R31