GB2277381A

GB2277381A - Portable appliance testers

Info

Publication number: GB2277381A
Application number: GB9407835A
Authority: GB
Inventors: Martin Bernard Litherland; Geoffrey Keith Lawlor
Original assignee: JEKYLL ELECTRONIC TECHNOLOGY L
Current assignee: JEKYLL ELECTRONIC TECHNOLOGY L
Priority date: 1993-04-20
Filing date: 1994-04-20
Publication date: 1994-10-26
Also published as: GB9407835D0; GB9308083D0

Abstract

A PAT (portable appliance tester) 10 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 appliance includes a capacitive mains noise filter on its input, this is liable to be damaged by a FLASH test, which uses a high voltage. To prevent this, the PAT includes means for detecting such a mains filter, comprising means 19 for applying a high frequency (HF) signal between the phase and neutral terminals of the appliance and either its earth terminal or its frame, and means 17 for measuring the resulting HF current. A substantial HF current indicates that there is probably such a mains filter present, so the FLASH test should therefore not be performed. The size of the HF current may be indicated by a meter. Alternatively, the HF current may be compared with a preset value to produce a binary signal; this signal may be indicated to the user and/or used to automatically prevent a FLASH test from being performed. <IMAGE>

Description

Po rtable A Testerbs 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 mundarie 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 and flash 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 divided into two groups, Classes I and 11, the distinction being broadly that Class I devices have an earth connection while Class 11 devices do not. Class I devices generally have a substantial metal chassis, while Class 11 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).

Considering the FLASH test in more detail, this briefly applies a high voltage (typically in the range of 1.5 to 3 kV) between the appliance's earth terminal (or its frame) and its phase and neutral terminals (connected together).

This test stresses the insulation of the appliance to a considerably higher level than it is normally subjected to. The rationale is that if the insulation has a weak spot which is liable to fail within the foreseeable future in normal service (ie under ordinary mains voltage), this stress will detect it by causing it to break down (either temporarily during the test or permanently).

There is a potential problem with the FLASH test. Some kinds of appliance have capacitive mains noise filters built into them, and such filters may be damaged by the FLASH test. A capacitor connected between the phase line and the neutral line will be all right, but capacitors connected between the phase and neutral lines and the earth line may easily be damaged by the FLASH test. Further, the kind of appliance that includes such a filter may well have other components which would also be damaged by a FLASH test. It is therefore important not to use the FLASH test with such appliances.

There are different types of PAT available, some of which have a FLASH capability and others of which do not. In many situations in which the appliances to be tested are likely to include those which would be susceptible to damage by a FLASH test, all appliances can be adequately tested without using the FLASH test at all. By using a PAT without a FLASH capability in such situations, the danger of FLASH test damage is obviated.

Further, even if the PAT being used has a FLASH capability, a reasonably competent user can be trained to recognize the types of appliance which would be susceptible to FLASH test damage, and to avoid using the FLASH test with such appliances. (Alternatively, the user can be trained to use the FLASH test only with certain specific types of appliance.) We have realized, however, that these techniques are procedures are still not entirely satisfactory. There are some kinds of appliance for which a FLASH test is desirable, even if not essential; there are situations in which that type of appliance is present together with other types which are susceptible to FLASH test damage; not all PAT users are fully trained to meet the highest standards of reliability; and even competent users may make occasional mistakes.

Accordingly the present invention provides, in a portable appliance tester (PAT) capable of performing a FLASH test involving the application of a voltage much greater than the normal mains voltage, means for detecting the presence in an appliance under test of a capacitive mains noise filter which is likely to be damaged by the FLASH test, comprising means for applying a high frequency (HF) signal between the phase and neutral terminals of the appliance and either its earth terminal or its frame, and means for measuring the resulting HF current.

lf a substantial HF current flows, this indicates the presence of a substantial capacitance between the phase and neutral terminals of the appliance and either its earth terminal or its frame, and hence that there is probably a capacitive mains noise filter present. The FLASH test should therefore not be performed. The size of the HF current may be indicated by a meter. Alternatively, the HF current may be compared with a preset value to produce a binary signal; this signal may be indicated to the user and/or used to automatically prevent a FLASH test from being performed.

A PAT embodying the present invention can thus be safely used in all situations, without the need for special operator training and without the risk of damaging appliances containing capacitive mains noise filters.

A PAT embodying the invention will now be described, by way of example, with reference to the drawing, which is a partial block diagram of the PAT connected to an appliance having a capacitive mains noise filter.

Referring to the drawing, a PAT 10 is shown connected to an appliance 11, the appliance having a mains cable 12 terminating in a plug 13 which is plugged into a socket 14 on the PAT.

The appliance has a capacitive mains noise filter consisting of a capacitor C1 connected across the live (L) and neutral (N) lines of the power supply, a common-mode choke LLl-LL2 connected in these lines, and a pair of capacitors C2 and C3 connected between these lines and the earth line (E), as shown. These capacitors are all small. The live and neutral lines continue to the rest of the circuitry (not shown) of the appliance 11, which may for example be a personal computer or some similar device which is sensitive to noise on the mains supply and therefore requires that supply to be filtered as shown.

The circuitry shown in the PAT 10 is that required for FLASH testing.

For that, the live and neutral terminals of the socket 14 are connected together, and a high voltage (HV) source 15, controlled by a control unit 18, is connected between the combined live and neutral terminals and the earth terminal. In addition, a flying probe 16 may be connected to any accessible metalwork of the appliance. A current sensor 17 is connected in series with the high voltage source. (It will be understood that the PAT contains further circuitry (not shown) for the other standard tests and other functions, and the connections shown will be broken and replaced by other connections for those other tests and functions.) When the FLASH test is to be performed, the control unit 18 energizes the high voltage source to generate a voltage of around 1.5 to 3 kV, which is applied between the circuitry of the appliance 11 and its earth. If the insulation of the circuitry in the appliance has a weakness, this high voltage will break down the weak spot in the insulation and produce an appreciable current. This current will be detected by the current sensor 17 in the PAT 10, so indicating an insulation failure. This test will detect a potential insulation failure before the insulation would actually break down under normal mains voltage.

The probe 16, which is suitably insulated, may be used to apply the FLASH high voltage (or its return path) to any conveniently accessible conductive part of the appliance 11. This would be done, for example, if the appliance is a Class I1 device, with no effective earth connection at the earth terminal of the plug.

With the appliance shown, a FLASH test is very likely to destroy the capacitors C2 and C3 (and possibly some of the main circuitry (not shown) as well). The PAT 10 therefore contains means for detecting the presence of a mains noise filter in the appliance under test (appliance 11). This means consists essentially of a low voltage, high frequency (LV HF) signal source 19 operating in conjunction with the current sensor 17. The LV HF source may conveniently operate at 20 V RMS at 10 to 20 kHz. The voltage is of course far too low to cause any damage to the appliance 11, or indeed to have any significant effect thereon.

When a FLASH test is initiated in the PAT 10, the control unit 18 energizes the LV HF source 19 for a suitable period before energizing the HV source 15. If the appliance acts as a primarily high resistance load, then the current resulting from this LV HF signal will be small. If, however, the appliance includes a mains noise filter, then the capacitances in that filter will present a low impedance at the frequency of the LV HF signal, and a substantial current will flow. In the appliance shown, for example, there will be two such paths in parallel, one through inductance LLl and capacitor C2 and the other through inductance LL2 and capacitor C3. Although the impedance of the inductances will be higher at the high frequency of the LV HF signal, the current flowing will still normally be large enough for reliable detection by the detector 1 7.

If the current detector 17 detects a substantial current resulting from the LV HF signal, then it sends a signal to the control unit 18, halting the FLASH test and preventing the generation of the potentially damaging HV signal. Damage to the appliance 11 is therefore prevented automatically.

The signal from the current sensor indicating the presence of a capacitive mains noise filter is preferably also fed to an indicator (not shown), to inform the operator that the FLASH test has been aborted (and why).

Obviously, separate current sensors can be used for monitoring the LV HF and the HV signals if desired.

Claims

1 ' In a portable appliance tester (PAT) capable of performing a FLASH test involving the application of a voltage much greater than the normal mains voltage, means for detecting the presence in an appliance under test of a capacitive mains noise filter which is likely to be damaged by the FLASH test, comprising means for applying a high frequency (HF) signal between the phase and neutral terminals of the appliance and either its earth terminal or its frame, and means for measuring the resulting HF current.

2 A PAT according to claim 1 including a meter for indicating the size of the HF current.

3 A PAT according to claim l including means for comparing the HF current with a preset value to produce a binary signal.

4 A PAT according to claim 3 including means for indicating the value of the binary signal to the user.

5 A PAT according to either of claims 3 and 4 including means for preventing a FLASH test from being performed if the HF current exceeds the preset value.

6 A' PAT substantially as herein described and illustrated.

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