GB2239530A

GB2239530A - Testing AC Installations

Info

Publication number: GB2239530A
Application number: GB8929326A
Authority: GB
Inventors: Edward Smithson
Original assignee: Megger Instruments Ltd
Current assignee: Megger Instruments Ltd
Priority date: 1989-12-29
Filing date: 1989-12-29
Publication date: 1991-07-03
Anticipated expiration: 2009-12-29
Also published as: GB2239530B; GB8929326D0

Abstract

Apparatus for checking an electrical installation (not shown) having an AC supply protected by a residual current circuit breaker (not shown) operates by applying (11, 15, 7) a disabling current to the RCCB, the disabling current (Figure 2) rising at the time of connection and following an alternating form and fading gradually so that on completion of checking the final amplitude is zero. The disabling current saturates the RCCB and has a frequency considerably less than the supply frequency. Checking of the installation is performed by a measurement circuit (12). <IMAGE>

Description

CIRCUIT CHECKING The present invention relates to an apparatus for and a method of checking a circuit of an electrical installation having an AC supply.

Recent introduction of residual current circuit breakers (RCCBs) for protecting electrical installations has raised the problem of measuring the phase earth impedance in the supply of such protected installations. The impedance is normally measured by determining the voltage between the phase and earth lines. When an RCCB is used, the test current can, however, cause the RCCB to trip. For various reasons it is undesirable to break the supply.

In order to overcome this problem the present invention provides an apparatus for and a method of checking a circuit of an electrical installation having an AC supply protected by an RCCB in which a formed disabling current is connected to the circuit to be checked, the form of the current being such that the current rises in amplitude at the time of connection and falls gradually to zero amplitude at a time consistent with the completion of the checking.

The disabling current is such as to saturate the RCCB sensing circuit which normally comprises a toroidal core wound with sensing coils.

Whilst suitably formed DC currents can be adapted to saturate the RCCB sensing circuit, a further problem arises which may be very serious in that the sensing circuit of the RCCB remains partially saturated after the measurements or test are complete. This residual saturation causes the sensitivity of the RCCB to decline, that is the disablement can continue after the test.

Clearly this is a dangerous situation.

In order to overcome this further problem in an embodiment of the present invention the disabling current is an AC current having a frequency considerably less than the AC supply frequency. In this case the form of the disabling current is such that it rises in amplitude at the tine of connection and falls gradually to zero amplitude at the time consistent with the completion of the checking.

The frequency of the disabling current when AC is suitably a low frequency less than a fifth of the supply current, for example, the disabling AC current could be 1OHz down to 1Hz and suitably 5Hz.

The effect of the connection of the low frequency AC current is such as to prevent the RCCB operating whilst "walking" a flux around the BH loop of the RCCB magnetic core to a level which is essentially zero. The RCCB is thereby returned to normal sensitivity at the end of the checking or test.

An embodiment of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a circuit diagram of the apparatus according to the invention, and Figure 2 shows a graph of the AC disabling current against time as used in the apparatus and method of the present invention.

In Figure 1 an apparatus for checking a circuit of an electrical installation having an AC supply protected by an RCCB is shown. The apparatus has test connections 2, 3 and 4 for connection to Earth (E), Line (L) and Neutral (N) lines respectively of the RCCB protected circuit (not shown). Capacitors 5 and 6 are connected across lines 7, 8, 9 which connect to connections 2, 3 and 4 respectively.

Lines 8 and 9 from L and N power a power supply unit 10 which powers an AC generator 11. AC generator 11 is controlled by logic controller 12 by means of RAMP INHIBIT, RAMP STEADY, RAMP DOWN and RAMP UP signals.

Controller 12 is initiated by test push button 13.

AC generator 11 outputs to line 7 through I sense resistor 15 across which is connected voltage level detector 14 which signals conditions A B C and D to controller 12 as will be described below.

Connected to line 7 is SCR 20 with its control gate connected to controller 12 by command line 21. I test resistor 19 is connected in series with SCR 20 and thence to line 18 and line 8 so that SCR 20 and resistor 19 are connected across lines 7 (E) and 8 (L).

A measurement circuit 22 with control line 26 from controller 12 is connected to line 18. An indication of load voltage as measured by circuit 22 and converted from analog to digital signals by convertor 23 is displayed on a digital display 24.

With the apparatus connected to an installation having an AC supply protected by an RCCB, button 13 is pressed.

Controller 12 sends a RAMP UP signal to AC generator 11 to cause the first cycle of an essentially AC voltage to rise from zero (point 30 in Fig. 2). The current rises until the voltage drop across resistor 15 causes detector 14 to signal Condition A (point 31) equivalent to the peak of the first AC cycle. This current flows from L to E.

The controller 12 now changes from signalling RAMP UP to RAMP STEADY, to maintain the current at a level determined by signal condition A. This is adequate to saturate the current transformer of the RCCB in the installation under test.

A loop test is now performed by firstly enabling measurement circuit 22 to determine the "off load" voltage. SCR 20 is now fired by controller 12 for two half cycles. Measurement circuit 22 is now configured to measure "on load" voltage. This is then displayed on display 24 after analog to digital conversion.

Controller 12 now replaces the RAMP STEADY signal (at point 32) with a RAMP DOWN command until the current drops to zero at point 33 and reverses to a reverse peak at point 34 when detector 14 indicates Condition B to controller 12.

Controller 12 now replaces RAMP DOWN signal with a RAMP UP command until the current rises past zero, completing a first cycle to a point 35 where the detector 14 indicates Condition C to controller 12. A RAMP DOWN command is then sent to generator 11 until the disabling current output falls to zero at point 35 when detector 14 signals Condition D to the controller 12. The generator 11 is then sent an INHIBIT signal and the test is finally completed.

The form of the disabling current shown in Figure 2 is such that the current rises in amplitude from the time of connection at point 30 until it falls gradually to zero amplitude at point 33 when the installation circuit checking is completed. The current continues to cycle through points 34 and 35 to point 36 to desaturate the sensing circuit of the RCCB so that at point 36 the sensitivity of the RCCB is returned to normal at the final completion of the test.

If necessary the controller and detector can be arranged to cause a further complete cycle of lower amplitude the level C after point 36.

If necessary the controller 12 and detector 14 can be arranged to continue past point 36 to cause a further complete cycle of current from generator 11 of lower amplitude than level C.

The AC disabling current is suitably between 10 and 1 Hz, that is considerably less than AC supply frequency used in the installation under test which would normally be 50 or 60 Hz.

Claims

1. An apparatus for checking a circuit of an electrical installation having an AC supply protected by an RCCB comprising means for forming a disabling current for connection to the circuit to be checked, the disabling current having a form such that the current rises in amplitude at the time of connection and falls gradually to zero amplitude at a time consistent with the completion of the checking.

2. A method of checking a circuit of an electrical installation having an AC supply protected by an RCCB comprising forming a disabling current, connecting the disabling current to the circuit so as to saturate the RCCB sensor, checking the circuit and disconnecting the disabling current, wherein the disabling current has a form such that the current rises in amplitude at the tine of connection and falls gradually to zero amplitude at a time consistent with the completion of checking.

3. An apparatus or method as claimed in claim 1 or 2 wherein the disabling current is an AC current having a frequency considerably less than the AC supply frequency.

4. An apparatus or method as claimed in claim 3 wherein the disabling current is between 1OHz and 1Hz.

5. An apparatus according to any one of claims 1, 3 or 4 wherein said means for forming the disabling current comprises a generator, a controller arranged to command the generator to output the rising current, a detector arranged to detect the rising current and to indicate a predetermined peak level of the current to the controller at a first condition, said controller being arranged to subsequently command the generator to cause the current to fall to zero amplitude.

6. An apparatus according to claim 5 wherein the controller is arranged to command the generator to output the current at the peak level corresponding to the first condition for an RCCB saturation period.

7. An apparatus according to claim 5 or 6 wherein the controller is arranged to command the generator to cause the current to fall from said peak to a predetermined negative level corresponding to a second condition detected by said detector and to then cause the current to rise to a peak level corresponding to a third condition and finally to cause the current to fall to zero.

8. A method of checking a circuit of an electrical installation having an AC supply protected by an RCCB substantially as described with reference to the accompanying drawings.

8. An apparatus for checking a circuit of an electrical installation having an AC supply protected by an RCCB substantially as described with reference to the accompanying drawings.

9. A method of checking a circuit of an electrical installation having an AC supply protected by an RCCB substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS 1. An apparatus for checking a circuit of an electrical installation having an AC supply protected by an RCCB comprising means for forming a disabling current for connection to the circuit to be checked, the disabling current having a form such that the current rises in amplitude at the time of connection and falls gradually to zero amplitude at a time consistent with the completion of the checking, wherein the disabling current is an AC current having a frequency considerably less than the AC supply frequency.

2. A method of checking a circuit of an electrical installation having an AC supply protected by an RCCB comprising forming a disabling current, connecting the disabling current to the circuit so as to saturate the RCCB sensor, checking the circuit and disconnecting the disabling current, wherein the disabling current has a form such that the current rises in amplitude at the time of connection and falls gradually to zero amplitude at a time consistent with the completion of checking, wherein the disabling current is an AC current having a frequency considerably less than the AC supply frequency.

3. An apparatus or method as claimed in claim 1 or 2 wherein the disabling current is between 1OHz and 1Hz.

4. An apparatus according to claim 1 or 3 wherein said means for forming the disabling current comprises a generator, a controller arranged to command the generator to output the rising current, a detector arranged to detect the rising current and to indicate a predetermined peak level of the current to the controller at a first condition, said controller being arranged to subsequently command the generator to cause the current to fall to zero amplitude.

5. An apparatus according to claim 4 wherein the controller is arranged to command the generator to output the current at the peak level corresponding to the first condition for an RCCB saturation period.

6. An apparatus according to claim 4 or 5 wherein the controller is arranged to command the generator to cause the current to fall from said peak to a predetermined negative level corresponding to a second condition detected by said detector and to then cause the current to rise to a peak level corresponding to a third condition and finally to cause the current to fall to zero.

7. An apparatus for checking a circuit of an electrical installation having an AC supply protected by an RCCB substantially as described with reference to the accompanying drawings.