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
sounds like cobblers to me. Or more charitably a misunderstanding of the specs.
On a normal AC waveform of a resistive fault or a normal tester a 30mA A type, should still trip reliably at 30mA, just like the older AC type.
Only when tripping on a rectified but un-smoothed waveform is a higher trip level accepted. (as it is the p-p swing that matters)
Mike
That would mean they had really moved the goal-posts then, so that an A type 30mA RCD need not trip at 30mA - EVEN ON A NORMAL AC TEST
(clearly a higher test current for the un-smoothed DC test is a different matter)
This would be silly given the reason for the choice of a 30mA current is the human shock current/mortality curve to Alternating Current and there have been no corresponding changes to improve the robustness of human physiology in the last few years since the last edition of the regs. Actually if anything the 30mA limit and permitted trip time is already a bit high for comfort, protecting neither the elderly nor the very young, both of whom tend to have higher heart rates and greater shock sensitivity, However there is also a trade off against nuisance tripping.
I'm very pleased to say that all the A types I have tested so far do in fact trip at 20-25mA, just like the old ones used to.
Stick with testing all RCDs as if they were AC types at the nominal current (30mA for the instant trip ones) - that should always work as the lowest common denominator.
I think the maker of that video has misunderstood the intent.
Mike.
Just found it again. It was actually 42 m/A.
This is for the Type A Test and is NOT "AC waveform" test, the spec of which is the same for Type AC, Type A, Type B and Type F RCDs. All of these RCDs have the same Type AC requirements.
The RCD should be tested using the Type AC Test.
Take a look at Table 2.17 of the 9th Ed of Guidance Note 3 (Page 96), it's all spelled out in there.
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.
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