But any normal ("AC") RCD will sense all of the waveforms except 6,8 and 9, just at about twice the RMS current that it trips on a pure AC. Sure the DC component does not couple through the transformer core, but the a wavefor of the same peak-to peak amplitude makes it through - just shifted in the Y direction to have an average value of zero, just liek a real AC.

Only an "AC" design with a very undersized core may suffer some loss in sensitivity, but it will not be totally blinded.

That requires the other waveforms.


Those drawings would be much less dangerous if the neutral 'N' was omitted from 7 and 9

It is somewhat interesting to look at those Andy, but you will notice that we are now protecting against faults AFTER the appliance rectifiers, causing leakage to Earth. All of these faults are of a double fault kind, in other words, the fault and the lost Earth connection must both be present to present a trip scenario to prevent danger. If the Earth is satisfactory there is no danger to persons touching anything. Why are we doing this? Appliance faults are nothing to do with BS7671, and the appliance standards should be the ones providing protection. This is feature creep of a fairly major kind, and should NOT be addressed by the fixed wiring standards. It suggests the failure of class 2 products or class one products with an open circuit Earth connection. These are very rare and show a complete change of attitude to product design. Is this an influence from China, where standards seem to be ignored? Adequate product internal fusing would see off 99% of these dangers anyway. Again we are addressing the wrong problem.

found this https://www.blakley.co.uk/sites/default/files/technical_files/TDS13_DC_IMMUNE_RCDs_0.pdf

Showing a graph where 0.1A superimposed DC increases the trip current of a 30mA (presumeably type AC) RCD to about 115mA. 


Regarding the OP's question of whether testing without loads and then re-testing with all loads on would be ok. Not sure whether this would work, since with all loads on, the normal residual currents might be near the trip current of the RCD, but the DC current might increase the trip current threshold. Time and ramp testing on a MFT wouldn't necessarily tell you what was going on.


With regards to whether installing LED lights or induction hobs, on an existing AC RCD, makes the installation less safe, thus not complying with building regulations, who knows? Excellent question though.

Interesting graph Olympus. I wonder what the effect would be on type A RCDs. Maybe I have completely misunderstood but I thought the type A was immune only to DC up to about 6mA.

I will try that Chris. In the first place, VSDs have significant mains filters. The capacitors in these put an asymmetric current on live and neutral as they suddenly charge at anywhere that is not a mains zero crossing, between the live conductors and Earth. The second is that large inrush currents to the electronics also give the RCD transformer problems in balance for a very short period (remember what I said about transformer balance at high frequencies above) and can cause tripping. I have a VSD on my lathe (4kW) and this has so far been fine, but my compressor sometimes does cause a trip on starting. RCDs are not ideal for "bad" loads, and I have been doing some useful experimental work, which will hopefully lead to a technical paper/article sometime. The cure is to fit zero voltage switching to these loads, but it's not cheap. You could also switch your VSD on via a current limit resistor, which you short out after a period, to limit the inrush current. 10 to 20 Ohms will probably work well.

I was perhaps simplifying too much - all non-DC sensing RCDs are a current transformer followed by a peak detector, with varying degrees of low pass filter. So long as the core is large enough not to saturate, in effect this rounds off the more violent changes in current waveform and then measures the peak to peak variation. The rectified waveforms do not have the same RMS to peak relationship as a sinewave, so it is not quite as simple as a 30mA RMS RCD for sinewaves being a 60mA RMS one for half wave rectified waves, but it is close - actually is is the half wave rectified waveform that has the same peak-to peak voltage as the sine wave of 30mA - which is pretty close to 60mA, but not quite.

However, if there is a DC term large compared to the ripple, so that the core saturates, then we are in different territory- more like the last few waveforms in that chart - here the variation needs to be large enough relative to the DC to reduce the total magnetisation for the part of the cycle where they are in opposition, back into the linear (i.e. very much not saturated,but transformer like) part of the magnetic material curve. So if you have a 100mA DC, you need to add enough AC to it that when the AC part of the cycle is in opposition to the DC it can more than cancel the DC - so a result of needing 115mA of AC to overcome 100mA of DC bias is perfectly believable, but the exact amount will depend on the magnetic core properties one that is far from saturating will not be desensitised at all - but it will never be worse than a core that was just saturating on the crests of the original 30mA sinewave and no DC.

As regards the VSD, can you measure the input filter capacitances L-E ; these will contribute to a steady 'leak' that will fool the RCD.  Larger VSDs , especially those designed for TT countries (Japanese designers tend to remember this better than EU ones)have  options to configure the filters for lower leakage - essentially the filter capacitors are wired L-N and then N-E but not L-E directly.

It is also possible that the inrush of charging the DC bus is actually only an L-N current but so much larger than the on load current that the RCD mis-reads it (if the L and N windings are not quite symmetrical on the core, and they never are, then that imbalance can cause a trip on high currents - say the paths are balanced only to 99.9% -  a 100A inrush looks like a 100mA imbalance.) 

Take care Lyle, the current you are considering to block the transformer is smooth DC. As Mike says only 6, 8 and 9 are possible problems, what a loop tester does is not any of these diagrams. The important thing to notice about these diagrams is that they depend on two problems to be dangerous, first a broken PE connection (or perhaps a TT installation) and a fault to connect the electronics to the Earthed exposed conductive part. It is not dangerous if the PE is in place even if the RCD doesn't trip, and in all these cases the CPD or an internal fuse should provide ADS. RCDs are not the only protection mechanism in most installations. TT installs are a rather special case, and only require a single fault to be dangerous, but that is always the case under many conditions anyway. In most installations RCDs provide "additional protection" only and are not the main form of fault protection and ADS, this is the CPD.

The Blakley paper is interesting but again nothing like the situation we find in domestic premises. The DC traction supply of 750V is quite smooth, is of a high voltage, and very large power availability. Circulating currents in Earth conductors and even through the Earth itself are inevitable, but the range will not be very great away from the railway. It looks to me as though someone is looking for a reason to define a potential problem in domestic premises, which has none of the conditions cited in the Blakley paper. Domestics do not have traction supplies, complex Earth paths etc. Earth bonding is by definition to a single point per premises, which do not have a great extent. Circulating DC current will probably be very small, particularly as many domestic items are class 2. I also see no reason to blame electronic devices, many ones complying with the EU directives on power do not have the internal circuitry shown in BS7671, and higher power items like induction hobs certainly don't. It MAY be possible to work out a fault which "could" produce some DC in the case of a fault, but this could not be much as the Fuse/CPD would trip due to excess power consumption.


I would like someone to explain to me why this is suddenly a problem, we have had lots of electronics in homes for at least 30 years, and RCD protection of the simplest kind (1 or 2 per property, type AC) have proved to be useful, and the number of accidents is tiny. Are we again trying to reduce 1 per million to zero per million by throwing money at the problem? Would this make any difference anyway?