GB2277598A - Portable appliance testers - Google Patents

Abstract

A PAT (portable appliance tester) is used to apply to a portable appliance (ie an appliance with a mains plug) 11 tests such as CONT (continuity between the appliance frame and the Earth plug terminal), FUSE (continuity between Live and Neutral plug terminals through the appliance), INSUL (resistance from Live and Neutral terminals to the Earth terminal), RUN (operating the appliance), and FLASH (insulation). If the mains plug 14 of the appliance includes an RCD (residual current detector) which requires energization to operate, this isolates the appliance from the PAT and some tests cannot be performed. To overcome this, RCD energization means 12 are provided for generating a high frequency signal, effective to energize an RCD, between the live and neutral terminals of the appliance 11 under test, and superposed on the PAT test signals. (In a development, the HF power is increased for high load appliances, and mains can also be fed through.) The RCD energization means may be included in the PAT, or may be in the form of an adaptor having a mains plug which fits into the mains socket of the PAT and a mains-type socket into which the (RCD-containing) plug of the appliance is plugged. If the RCD energization means is an adaptor, it is preferably battery-powered, avoiding the need for a second mains plug on the adaptor. <IMAGE>

Description

Pro rtable 1ianc Tests The present invention relates to test apparatus for electrical appliances.

Mains-powered electrical appliances are in widespread use in both domestic and business premises. Such appliances range from mundane items like desk lamps and electric kettles to modern electronic items like fax machines and personal computers and their associated components.

The testing of such appliances has in the past been somewhat random and sporadic. Recent regulations, however, effectively require periodic testing of such appliances in business premises.

The test apparatus traditionally used for such testing has usually been a standard multi-purpose meter, and possibly also an insulation tester. With the modern requirement for regular testing, however, test instruments designed specifically for such testing have been developed. Such an instrument is known as a portable appliance tester (PAT). This is intended for testing appliances which are "portable" broadly in the sense that they have conventional mains leads with plugs on the end, so that they can be moved from one place to another merely by unplugging them.

A PAT is normally mains powered. and has a plug by means of which it can be plugged into the mains. It also has a mains-type socket into which the appliance to be tested can be plugged. It also normally has a wander lead for earthing the appliance, and often also has a second wander lead for insulation testing. Such a PAT can normally perform most or all of the usual tests. The test results may be indicated by value readings and/or simple pass/fail indications; in more elaborate PATs, the testing may be partially automated and the test results may be automatically recorded and available for later downloading into data logging equipment.

Typically, a PAT can perform 5 tests, which can conveniently be designated as CONT, FUSE, INSUL, RUN, and FLASH (or similar abbreviations). CONT tests the continuity between the frame of the appliance and the Earth terminal of its plug; FUSE tests the continuity of the circuit through the appliance between the Live (Phase) and Neutral terminals of its plug; INSUL tests the resistance from the Phase and Neutral terminals to the Earth terminal; RUN connects the mains phase and neutral to the phase and neutral plug terminals of the appliance, to allow a check that the appliance performs its intended function; and FLASH tests the insulation (between the phase and neutral terminals and either the earth terminal or the frame or casing of the appliance) by applying a brief stress thereto. The CONT and FUSE tests use a low voltage; the RUN test uses the mains voltage; the INSUL test uses a DC voltage equal to or somewhat higher than the mains voltage (typically 500 V); and the FLASH test uses an AC voltage considerably higher than the mains voltage (eg 1.5 to 3 kV).

Appliances are in fact broadly bivided into two groups, Classes I and 11, the distinction being broadly that Class 1 devices have an earth connection while Class 11 devices do not. Class I devices generally have a substantial metal chassis. while Class II devices do not (and are double insulated, ie have their live parts insulated from any structural metal parts and have those parts in turn insulated from any conductive parts -accessible to the user). It will be realized that not all of the tests are applicable to both classes of appliance; for example, a Class 11 device will fail the CONT test (because any accessible conductive components are isolated from the earth terminal of its plug).

The primary objective of such testing is, of course, to achieve safety in the use of the appliances. There is an alternative approach to achieving safety; the provision of means for detecting a fault while the appliance is in operation and thereupon isolating the device. Traditionally, this has been done by providing fuses at suitable locations in the circuit. More recently, miniature circuit breakers (MCBs) have come into widespread use; also, earth faults are now often detected, and the use of residual current devices (RCDs) has become popular for this purpose.

Essentially. an RCD senses the difference between the currents in the live (phase) and return (neutral) lines to the appliance, and opens a relay in those lines if there is a significant imbalance. The RCD thus indirectly detects any earth leakage current which results from a failure of the insulation of the appliance. One particular application of RCDs is with garden appliances such as hedgecutters and lawnmowers, where there are serious risks of them becoming wet (with the danger of current leakage) and of them cutting their own cables.

RCDs can be applied in various ways. An RCD can be included in the mains supply system, generally in addition to a fusebox or MCB unit. Alternatively, an RCD can be included in the mains connection to the appliance, either as a component in the plug on the end of the appliance's mains cable or as a separate adaptor which is plugged into the mains socket and carries a mains-type socket into which the appliance's mains cable can be plugged.

We have realized that certain types of RCD prevent effective testing of appliances by a PAT. There is one type of RCD which requires a power supply to operate; specifically, that type of RCD includes a relay which has to be powered from the mains supply to close. If the mains cable of an appliance terminates in a plug including that type of RCD, then a PAT cannot effectively test the appliance, because the RCD relay will be open for some of the PAT tests, isolating the appliance from the PAT.

The object of the present invention is to overcome this situation.

Accordingly the present invention provides, for use with a portable appliance tester, RCD' energization means for generating a high frequency signal, effective to energize an RCD, between the - live and neutral terminals of the appliance under test, and superposed on the PAT test signals.

The load on the RCD energization means may be sensed and, in the event of a high load, the high frequency signal output increased, by increasing its power (by using a push-pull transformer circuit) and/or its frequency. Through connections for live and neutral may be provided, with a relay in one of those connections and means for detecting the application of a voltage between live and neutral and closing the relay thereupon. The presence of mains voltage between live and neutral may also be detected and used to disable the generation of the high frequency thereupon.

The RCD energization means may be included in the PAT, or may be provided in the form of an adaptor having a mains plug which fits into the mains socket of the PAT and a mains-type socket into which the (RCD-containing) plug of the appliance is plugged. If the RCD energization means is an adaptor, it is preferably battery-powered (powering it from the mains would require a second mains plug on the adaptor).

An RCD energization means in the form of an adaptor embodying the present invention will now be described, by way of example, with reference to the drawings, in which: Fig. 1 is a simplified block and circuit diagram of the adaptor connected to a PAT and an appliance plug including an RCD; Fig. 2 shows a modification of the Fig. I adaptor; and Figs. 3A and 3B are two sets of waveforms showing the operation of the Fig. 2 circuit.

Referring to Fig. 1, a PAT 10 is connected to an appliance 11 via an adaptor 12. The appliance has a mains cable 13 with a plug 14 containing an RCD at its end. This plug is plugged into a mains socket 15 on the adaptor 12, which also has a mains plug 16 which is plugged into a mains socket 17 on the PAT 10.

For the INSUL test (and the FLASH test, if used), the PAT includes a test signal generator 20 operating at mains frequency for the FLASH test and at DC for the INSUL test, connected in series with a current detector 21. One side of the test signal generator is connected to the earth (E) terminal of the socket 17; the other side is connected, through the current sensor 21, to the live (L) and neutral (N) terminals of the socket 17, which are connected together for this test.

(Of course the PAT also contains further circuitry (not shown) for performing other tests and changing its configuration accordingly.) In the plug 14, the L and N terminals are connected to the live and neutral lines of the cable 13 to the appliance 11 via a pair of relay contacts RL1A and RL1B as shown. These relay contacts are normally open, and are closed by the energization of a relay coil RL1. The RCD includes electronic circuitry for controlling this relay coil. Power for this circuitry is obtained from the L and N terminals via a dropper capacitor C1 feeding a bridge rectifier BR which produces a DC output which is smoothed by a large capacitor C2.

Relay coil RL1 is connected in series with a thyristor SCR2 across capacitor C2 and is shunted by a thyristor SCAR 1. When power is initially switched on to the RCD plug, both these thyristors are initially off. The power drain on the power supply is therefore low, and the voltage on C2 builds up to a high value (peak mains voltage).

To turn the appliance on, thyristor SCR2 must be turned on. This thyristor has its gate connected to its anode via a manually operated reset switch RS SW and a current limiting resistor R2. When this switch is closed, the thyristor is turned on. Cl then partially discharges through RL1, giving a high current pulse which is sufficient to close the contacts RL1A and RLlB. The voltage and current from C1 then drop to a lower steady value which is sufficient to keep the relay contacts closed.

An integrated circuit IC1 is powered from Cl via SCR2, a regulating resistor R1, and a Zener diode ZD1 once SCR2 has been turned on. A current sensing coil LLl is coupled to the live and neutral mains power lines as shown, being wound - to sense the difference between the currents flowing in those two lines, and is connected to IC1. If there is a significant imbalance between the live (outgoing) and neutral (return) currents, the coil LL1 will produce a significant voltage. This will be detected by IC1, which will turn on the thyristor SARI. This thyristor is connected across the relay coil RL1, and will therefore short it out, so that the relays RLlA and RLlB will open, effectively disconnecting and isolating the appliance from the mains. A resistor R3 limits the current drawn from C2 to a safe value when SCRI is turned on. SCAR1 can only be reset by de-energizing 1C1, which requires the power to the plug 14 to be turned off (eg by pulling the plug out of the mains socket, or switching off the mains at the socket).

It is clear that if this plug 14 is plugged directly into the PAT 10, as indicated by the broken lines, the relays will remain open during the INSUL test and the insulation of the appliance 11 itself will not be tested. To overcome this, the plug 14 is plugged into the adaptor 12, which is in turn plugged into the PAT 10. The adaptor 12 has the L and N terminals of its socket end 15 connected together via an HF signal generator 22, which is powered from a battery 23. The N terminal of its plug end 15 is connected directly to the N socket terminal at 15.

In use, the HF signal generator produces a high frequency signal of sufficient power to energize the RCD circuitry of the plug 14 and so close the relay contacts RLlA and RLlB. This allows the test signal from the test signal generator 20 to reach the appliance 11, and the INSUL test is therefore effective.

The signal generator 22 can conveniently consist of an inductor L1 fed from the battery 23 and feeding the L terminal of socket 12 and a field effect transistor TR1 which is controlled by a pulse generator 24; the duty ratio of the pulse signal can conveniently be around 1:1. During the ON periods of TRl, current builds up in the inductor; during its OFF periods, that current in switched into the appliance plug 14.

To charge capacitor C2 adequately before the plug 14 is reset, a fairly substantial voltage is desirable, eg around 150 V. This voltage will be sustained by L1 for only the first part of the OFF period of TRl, with the current through L1 falling rapidly; once the current has fallen to zero, the voltage will fall to the battery voltage (eg 12 V). Once the plug 14 has been reset, C2 will be loaded by the current through RL1, and its voltage will fall to around 60 V.

The voltage produced by L1 will of course match this during the first part of the OFF periods of TR1, and will remain at this level for substantially the whole of these OFF periods. During the ON periods of TRl, the output voltage from Ll will of course be zero, whether or not the plug 14 has been reset.

The RCD energization signal produced by the adaptor 12 is at high frequency for two main reasons. In practice, a convenient value for this frequency is some 10 to 20 kHz.

One is that the power supply of the RCD circuitry of the plug 14 is capacitively coupled, while the appliance itself can normally - be safely assumed to be primarily resistive to the test signal. The RCD power supply therefore presents a low impedance (determined primarily by Ci) to a high frequency signal, while the appliance itself presents its normal impedance to a high frequency signal.

The use of a high frequency signal therefore allows that signal to power the RCD circuitry without having to also supply the normal power consumption of the appliance itself.

The other reason for the RCD energization signal being HF is that that obviates interference with the INSUL test of the PAT. If the appliance is not faulty, any current flowing through it from the RCD energization signal source 22 will be confined to the appliance, and will not enter the PAT. But if the appliance has an insulation fault, then there can be a second path for this energization current, through the appliance into its earth line, and thence through the PAT and back to the RCD energization signal source. If the RCD energize tion signal were at mains frequency, it would be possible for this to interfere destructively with the INSUL test signal from generator 20. By using a high frequency for the RCD energization signal (and making the INSUL test current detector effectively insensitive to that frequency), this danger is obviated.

The adaptor can remain in position for the other tests performed by the PAT 10, though not all of these tests will be effective then. The FLASH and INSUL tests are current limited, so damage to the adaptor is unlikely; if desired, a protective circuit such as a diode (not shown) can be included across TR1.

The CONT test does not require the RCD contacts in the plug 14 to be closed, so the adaptor does not affect this test. The RUN and FUSE tests will simply be ineffective if the adaptor is included in the circuit.

Fig. 2 shows a modification of the Fig. 1 system. In this, the pulse generator 24 is replaced by an oscillator 24, and the inductor L1 by a transformer Tl. The transformer has a secondary winding SEC connected in series with a blocking capacitor C3 across the L and E terminals of the socket 15, and two primary windings PRIMI and PRIM2 each connected in series with a respective transistor TR2 and TR3 across a 6 V power supply. The transistors are driven by the oscillator 24.

In normal operation, oscillator 24 runs at about 10 kHz and drives only transistor TR2, TR3 being held off. This results in the operation shown in Fig.

3A. Transistor TR2 applies a 10 kHz, V square wave to the transformer T1 to operate it in a flyback mode, with "flyback" charging pulses followed by narrow peaks of approximately 350 V. The narrow voltage peaks are broadly similar to those produced in the Fig. 1 circuit.

When the RCD in the plug 14 is operated by the power which it picks up from these narrow voltage peaks, the appliance 11 itself becomes connected across the live and neutral outputs of the adaptor, and appears as a load shunted across the pulses from the transformer. If the appliance is a high power device, ie has a low resistance (eg a 13 A device), with a resistance which may be as low as around 20 Q, this load may well be enough to reduce the amplitude of these pulses to a value below that required to hold the RCD operational.

To prevent this, the transformer T1 has a sense winding SNS which is coupled to a detector 30, comprising a retriggerable one-shot, which is in turn coupled to the oscillator 24. This detector detects the change in conditions resulting from an excessive load from the appliance. When such a load is detected, the frequency of the oscillator 24' is increased to around 60 kHz, and transistor TR3 is driven together with transistor TR2 to drive the two primary windings PRIMP and PRIM2 in push-pull.

This results in the operation shown in Fig. 3B. This frequency increase and push-pull operation together convert the adaptor from something approximating to a current source to something approximating more to a voltage source.

Its output will therefore be increased to a value sufficient to maintain the RCD operational even in the presence of a large (low resistance) appliance load. The turns ratio of the transformer can conveniently be 3:1, so that the flyback charging pulses are of about 18V, sufficient to maintain the RCD operational.

With the Fig. 1 adaptor, the RUN and FUSE tests cannot be made with the adaptor 12 included between the tester 10 and the appliance 12. In the Fig. 2 circuit, the L terminals of the plug 16 and socket 15 are connected together, and the N terminals of this plug and socket are also connected together. This provides a current loop path from the tester through the adaptor to the appliance, so that the RUN and FUSE tests can be performed with the adaptor in position.

The secondary SEC of transformer T1 is connected between the L and N lines. To prevent excessive current flowing through it during RUN and FUSE tests, a blocking capacitor C3 is included in series with it. This capacitor may conveniently be of 1 sLF, presenting a low impedance to the HF signals generated by the transformer but a high impedance (3.3 kQ) at 50 Hz. This high reactance at 50 Hz ensures that it introduces only a small error on the FUSE test.

For the FLASH and INSUL tests, the RCD in the appliance plug 14 has to be energized by the HF signals generated by the adaptor 12. But for these tests, the L and N mains terminals are connected together in the tester 10. To prevent this from shorting out the HF pulses, a normally open relay RL2 is provided in the line between the two N terminals in the adaptor. A voltage detector 31 is connected between the L and N terminals of plug 16, and closes this relay RL2 when it detects a voltage between those terminals. Thus the loop from the tester 10 to the appliance 11 is completed through the adaptor 12 for the RUN and FUSE tests.

A mains voltage detector 32 is also provided, to detect when full mains voltage is -present between the L and N terminals and disable the oscillator 24', to reduce the battery drain. When full mains voltage is present, the RCD in the plug 14 will be operated by it, and the HF pulses are not required, and indeed any HF pulses would be largely shorted out by the mains source, which is of very low impedance.

The voltage detector 31 and the mains voltage detector 32 can conveniently be fed from a common rectifying circuit 33, which can conveniently comprise a bridge rectifier and a smoothing capacitor.

If desired, the battery-powered parts of the circuit can be isolated from those parts which may receive mains signals. This can be achieved by providing opto-electronic coupling at the output of the mains voltage detector 32, and suitable isolation between the output of the voltage detector 31 and the neutral line and between the primary winding PRIM and the other windings (SEC1, SEC2, and SNS) of the transformer Ti.

Claims (10)

Clams

1 For use with a portable appliance tester (PAT), RCD (residual current detector) energization means for generating a high frequency signal, effective to energize an RCD, between the live and neutral terminals of the appliance under test, and superposed on the PAT test signals.

2 RCD energization means according to claim 1, including means for sensing the load thereon and increasing the high frequency signal power output in the event of a high load.

3 RCD. energization means according to claim 2, wherein the high frequency signal output is increased by increasing its frequency.

4 RCD energization means according to either of claims 2 and 3 comprising a push-pull transformer circuit.

5 RCD energization means according to any. previous claim including through connections for live and neutral, a relay in one of those connections, and means for detecting the application of a voltage between live and neutral and closing the relay thereupon.

6 RCD energization means according to claim 5, including means for detecting the presence of mains voltage between live and neutral and disabling the generation of the high frequency thereupon.

7 RCD energization means according to any previous claim in the form of an adaptor having a mains plug which fits into the mains socket of the PAT and a mains-type socket into which the (RCD-containing) plug of an appliance under test can be plugged.

8 RCD energization means according to claim 7 including a battery for powering it.

9 RCD energization means substantially as herein described and illustrated.

10 A PAT including RCD energization means according to any of claims 1 to 6.

ii Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).