I watched a video the other day which mentioned that A-type 30m/A rcd's should be tested at about 54m/A i think it was and that setting your tester to 100m/A and doing a half times test was was acceptable. Is this correct and where does it come from.
Cheers. Gary
It would sometimes be helpful if explanations were published for changes in BS 7671.
I have just had a play with my MFT.
A 30 mA type AC RCD tested on the type AC setting at 1 x IΔn averaged a trip time of 33.2 ms. On the type A setting, it averaged 21.1 ms.
The corresponding results for a type A RCD were 17.9 ms and 8.2 ms.
It is clear then that the tester does something different on the two settings. I don't know what the difference between the two tests is, but testing the type AC RCD at 42 mA on the type AC setting produced very similar results to testing it at 1 x IΔn on the type A setting. So it may be that the tester simply increases the test current by a factor of √2 on the type A setting as the video suggests.
If this hypothesis is correct, it does indeed follow that the simple type AC test is sufficient for both type A and type AC RCDs. Doing the type A test would simply be a waste of time.
Attempting to test at 1/2 x 100 mA failed because the meter states that the RCD tripped and does not give a time.
Incidentally, both RCDs did not trip on the type B setting, presumably because the test current was smooth DC.
A 30 mA type AC RCD tested on the type AC setting at 1 x IΔn averaged a trip time of 33.2 ms. On the type A setting, it averaged 21.1 ms.
All this means, is that the RCD responds to different waveforms with different trip times.
It would sometimes be helpful if explanations were published for changes in BS 7671.
BS 7671 NEVER had a requirement to test with anything other than AC waveform, so there's nothing to communicate. It has been noticed that there is a lot of confusion in the industry regarding the functionality provided by MFTs, and BS 7671 requirements, and therefore this has been clarified in the recent GN3 ...
It is clear then that the tester does something different on the two settings. I don't know what the difference between the two tests is, but testing the type AC RCD at 42 mA on the type AC setting produced very similar results to testing it at 1 x IΔn on the type A setting. So it may be that the tester simply increases the test current by a factor of √2 on the type A setting as the video suggests.
Yes, it does. On the Type A setting, it runs a waveform designed to test RCDs for the additional residual current characteristics (only) for Type A RCDs. That's got nothing to do with the "Type AC" performance times given in BS 7671.
A 30 mA type AC RCD tested on the type AC setting at 1 x IΔn averaged a trip time of 33.2 ms. On the type A setting, it averaged 21.1 ms.All this means, is that the RCD responds to different waveforms with different trip times.
I don't think that it goes so far. All that I can deduce is that either the waveform was different, which caused it to trip more quickly; the current was greater; or of course, a combination of the two.
On the Type A setting, it runs a waveform designed to test RCDs for the additional residual current characteristics (only) for Type A RCDs
Doubtless Graham is better informed than I about the inner workings of an electrician's tester (one could hardly be less well informed).
BS 7671 NEVER had a requirement to test with anything other than AC waveform, so there's nothing to communicate. It has been noticed that there is a lot of confusion in the industry regarding the functionality provided by MFTs, and BS 7671 requirements, and therefore this has been clarified in the recent GN3 ...
I am v. pleased to see the clarification because the proud new owner of Megger's latest would be fully entitled to follow their instructions.
The incipient obsolescence of type AC RCDs is all well and good, but if there is no requirement to test the additional capability of a type A, how do we know that it does what it says on the tin? It can only be an assumption, albeit no doubt a well-founded one.
Indeed - if it helps, then ignoring the safety electronics to cut the test off after 2 seconds, or earlier if the NE voltage moves too far, what is in an RCD tester on the 'AC' setting is little more than a suitable resistor between L and E. The electronic switch that puts it in circuit when you press the button comes on a few volts after the next zero crossing of the selected polarity, and starts a timer counting. The timer is stopped when the Live voltage has fallen below a certain fraction of the expected sinewave, as this indicates the contacts have opened. Some loads that hold the voltage up can fool the tester into thinking contacts are still closed after they have opened.
The A test is the same except now we emulate a resistor of slightly lower value with a diode in series so the RCD sees the rectified but unsmoothed DC waveform rather than a sinewave.
Actually most AC RCDs will respond to the A test waveform to some extent, but not necessarily at the right levels, and performance is not guaranteed.
The steady DC of a B test is far more likely to blind both AC and A types, as the DC detection is a very specific add-on internally.
This is entirely consistent with the results you report.
Mike.
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