GB2278690A - Testing electrical devices - Google Patents

Abstract

The action of an electrically operated relay is tested by comparison with a "standard" relay in a circuit in which the relays are coupled in series so as to be operated by the same test current, and the responses of the two relays are compared. In particular, the time that an electromagnetic relay takes to switch may be tested. The method has advantages that control of the conditions of the test is not so critical, the conditions used may approximate more closely the conditions of use of the relay, and the avoidance of tables. In addition, rather than adjusting the relay under test to a standard setting, the "standard" relay setting is adjusted, and this is particularly beneficial in batch testing. <IMAGE>

Description

Testing Electrical Devices Electrically operable relays have an important function in the protection of electrical circuits. When the applied current is above a threshold value, below which no action takes place, most relays operate with a time delay which is a function of the current amplitude. This is shown in characteristic curve of Figure 1 for the type of relay known as an inverse definite minimum time (idmt) relay, in which response time is plotted against a measure of applied current.

It is not uncommon, particularly in the manufacture of heavy duty relays and/or relays intended to protect electrical circuits, to build adjustments into the relay whereby the characteristic time-current curve can be altered or displaced.

Thus idmt relays commonly possess a number, or continuous range, of different selectable "time multiplier" settings which provide different response times for any set operating current, and accordingly the curve shown in Figure 1 is one of a family of generally similar curves which are relatively displaced along the operating time axis, each relating to a respective time multiplier setting on the relay. While the other axis could be a direct measure of applied current, it is common for this to be expressed as a "plug setting multiplier"; in such a case a relay has a number of adjustable current ("plug") settings and the plug setting multiplier relates the applied current to the current or "plug" setting of the relay.

For many reasons, including reliability and safety, it is necessary to check whether such relays as manufactured have an acceptable time/current response; such testing may also be necessary on relays which have been in service for some time, and may therefore have undergone a change in their operating characteristics.

It is known to test such relays on a routine basis by injecting a standard current through the relay under test, and measuring the time taken for the relay to operate (normally by detecting the associated switching action).

Conventionally, the measured time is checked against the corresponding value taken from published standard curves.

To facilitate an assessment of the results, the normal (or design) "time multiplier" and current settings on the relay under test must be changed to standard settings during the test. This is undesirable because each of a series of relays to be tested must be adjusted for test and then readjusted back after the test. This costs time and money, and requires a degree of skill. There is also the chance that the readjustment may not be properly performed or effective. In addition, damage may be incurred during adjustment, or foreign matter may enter the relay once its protective cover has been removed. Furthermore, the relay is being tested in a condition other than that in which it will be expected to perform when in use, which must reduce the validity of the test to some extent.

If the relays to be tested are not altered to the standard settings, but maintained at their own normal settings, the result from the timing test would need to be interpreted by reference to the published time/current characteristic for these particular settings. Even supposing these are available, this introduces difficulty, particularly when testing on-site with no ready access to such information, quite apart from the danger of misreading the published data or selecting the incorrect set of data. Added to this, the normal settings of the relays to be tested would commonly lead to a very short time measurement during the test (compared with the alternative method where the settings of the relays under test are altered). The measurement of very short times, and the necessary monitoring and control of the test current (which may be large) over such short periods is not practical with existing equipment.

Conversely, if in fact the only source of test current provides a very small current, operation occurs on the leftmost part of the curve shown in Figure 1, and a relatively insignificant change in the relay characteristic curve would give a large change in the time measurement.

This may be undesirable and may lead to difficulties in assessing whether a relay needs adjustment or replacement.

In a first aspect, the invention provides a method of testing the response cf a first electrically operable relay on the application of a test current, in which the operating circuit of =e first relay is connected in series with the operating circuit of a second electrically operable relay having a known response, passing a test current through the operating circuits of both relays, and comparing the responses of the two relays.

In contrast to the prior art, the present invention permits a test relay to be directly compared with a "standard" relay. For batch testing the time multiplier and current setting of the single standard" relay are adjusted so that it behaves as should rie relays to be tested when at their normal settings. It :s an important advantage conferred by the invention that the relays under test do not need to be adjusted for the purposes of the test. Thus apart from the setting of the "standard" relay in an initial step, the method of the invention requires less skilled attention than that of the prior art.

Since it can be operated without requiring alteration of the settings of the relays under test, and with no need to consult reference data, the test can also be faster. It is also not necessary to exercise such close control over the test current, because small changes therein can be expected to have similar effects on the devices being compared.

This is so whether such small changes are current variations occurring during the test (since the relay may respond to the cumulative effect of the applied current over this time) or whether the change is the overall magnitude of the current relative to a standard value of test current which would normally be applied and which would be regarded as relating to a "standard" characteristic or response. In this context, it is possible, as explained below, to provide a test result expressed as a percentage timing error, relative to the response of the "standard" relay, to facilitate a simple accept/reject decision.

The first relay may have any known operating action e.g. it may provide mechanical opening and/or closure of switch contacts by, for example electromagnetic (electromechanical) or thermal action, or it may be an electronic type providing contactless switching. It could also be a hybrid relay, commonly comprising a contactless switching portion in addition to mechanically operated switch contacts. Any of these types may be controlled by a microprocessor. The second relay may be of the same type or of a different type, e.g. one being an electromagnetic (electromechanical) relay and the other being an electronic relay. Where the principles upon which the two relays operate are different, it may be necessary to arrange the test procedure to compensate for this, particularly if the test current is non-ideal, e.g. not truly sinusoidal.

The comparison step may include the determination of the magnitude of the difference between the responses of the relays, and the result may have a sign, or the magnitude may be determined as a ratio relative to the response of the second relay.

The responses to be compared may be the times each said relay takes to reach a predetermined state, for example the time that it takes for a switch forming part of the relay to alter state in response to the test current. Thus the comparing step can include the measurement of the interval between one said relay reaching said predetermined state and the other relay reaching said predetermined state.

In a second aspect, the invention provides apparatus for performing the method according to the first aspect, comprising means for onnecting the operating circuits of the first relay and te second relay in series, means for passing a test current through the series connected operating circuits, d means for detecting and comparing the responses of the wo relays.

While the invention w;l be described in more detail below with respect to the testing of the speed at which an electromagnetically operated relay changes state (switches) in response to a test current, it should be understood that the invention is cape e of wider application.

For example, the response under consideration may be other than a switching action, and/or the response may be gauged other than as a time measurement.

Thus for example, a thermal relay may replace the electromagnetic relay in the arrangement particularly described. If the relay comprises a heater, it would be possible to compare the final temperature attained by a heater in the tested and "standard" devices, or the times at which such heaters reached a predetermined temperature.

Again if the relay comprises a device produces some sort of force or torque, e.g. by electromagnetic action, the amplitude thereof or of an associated limited mechanical movement could constitute the response; if operation of the relay involves a gross mechanical movement, the response could be, for example, the time to reach a predetermined speed or displacement, or a final speed itself. Relays comprising light or other radiation emitting devices could be compared by measurement of final light intensity, or the time taken to reach a predetermined light intensity.

However the invention has been found to be particularly applicable to the testing of the switching action of electromagnetic relays, for example during manufacture or maintenance, and the invention will be more particularly described hereafter in this context with reference to the accompanying drawings in which: Figure 1 shows a typical curve of operating time against current for an inverse definite minimum time electromagnetic relay; and Figure 2 shows a schematic circuit diagram of a circuit useful for implementing the method of the invention as applied to testing relays.

As shown in Figure 2, the "standard" relay appropriately adjusted, a relay to be tested and a test current generator are connected in series. A timer is coupled to switch contacts of both relays so as to detect switch operation of each relay. While the timer may operate to provide indications of the time taken for each relay to operate after the test current is applied, the only measurement which may be necessary is the time difference between operation of the two relays. If the time difference is given a sign, this would indicate which of the two relays operated first. This may be important when considering whether the relay under test is within tolerance, given the asymmetrical nature of the time/current curve.

It would be possible to provide an automatic indication of the time difference as a percentage error, that is relative to the time for the "standard" characteristic under the conditions of the test, and it would be possible for the latter to be entered as a standard value at the start of the test. As has been mentioned above, since the test current is common to both relays, and a comparison between the responses of the relays is made, small changes in the test current are comparatively unimportant, and would be expected to affect operation of both relays to a similar extent. Thus the time difference should not be unduly affected and a valid test can still be performed.

However, a preferred method of comparing the responses of the two relays provides an automatic indication of the time difference as a percentage error relative to the response time for the "standard" relay under the conditions of the test, and includes the step of measuring the latter response time for each test.

It would also be possible to provide a ratio of the two operating times, and again this could be more or less than unity depending upon which relay was first to operate.

It can happen that the test current to be applied to the series connected relays has a non-ideal waveform. For example, if either relay is of the electromechanical type, the relatively high loading it imposes on a transformer within a test current generator may give rise to a nonsinusoidal waveform. If the two relays operate on similar principles, there should be no problem, but if they operate on different principles, current waveform distortion may need to be taken into account. Thus an electromechanical relay may respond to the whole waveform in terms of current amplitude at each instant, whereas some microprocessor controlled relays may be arranged to respond only to the peak current value.

Where the degree of distortion in the current waveform tends to be constant for one or more particular types or groups of relays to be tested, then the measurement circuit could be calibrated to take account of the different responses of the "standard" and "test" relay. A suitable switch could be incorporated into the test equipment to enable the appropriate calibration to be introduced according to the type or group of relay being tested.

If the degree of distortion is not sufficiently constant, an automatic calibration circuit could be incorporated.

This would serve to vary the amount of corrective calibration according to the degree of distortion detected.

The calibration could be based on the ratio of peak to rms current, for example.

Claims (13)

1. A method of testing the response of a first electrically operable relay on the application of a test current, in which the operating circuit of the first relay is connected in series with the operating circuit of a second electrically operable relay having a known response, passing a test current through the operating circuits of both relays, and comparing the responses of the two relays.

2. A method according to claim 1 wherein said comparing includes the determination of the magnitude of the difference between the responses of the relays.

3. A method according to claim 2 wherein said magnitude has a sign.

4. A method according to claim 2 or claim 3 wherein said magnitude is determined as a ratio relative to the response of the second relay.

5. A method according to claim 1 wherein said comparing provides a ratio of the responses of the two relays.

6. A method according to any preceding claim wherein the responses to be compared are the times each said relay takes to reach a predetermined state.

7. A method according to claim 6 as dependent on any one of claims 1 to 4 wherein the comparing includes the measurement of the interval between one said relay reaching said predetermined state and the other relay reaching said predetermined state.

8. A method according to claim 6 or claim 7 wherein said predetermined state is the altered state of a switch forming part of the relay in response to the test current.

9. A method according to any preceding claim in which each of the first and second relays is an electromagnetic type, a thermal type, or an electronic type, or a hybrid relay.

10. A method according to any preceding claim, wherein the first and second relays are of similar type.

11. A method according to any one of claims 1 to 9 wherein the first and second relays are of different types.

12. A method substantially as hereinbefore described with reference to the accompanying drawings.

13. Apparatus for performing the method according to any preceding claim comprising means for connecting the operating circuits of the first relay and the second relay in series, means for passing a test current through the series connected operating circuits, and means for detecting and comparing the responses of the two relays.