Type A R.C.D. 6mA tolerant.

For example will a type A (or “better”)  trip at low enough DC currents such that an upstream type AC won't be blinded by DC currents low enough to be passed?

A-types aren't guaranteed to trip at all on pure d.c. - their only guarantee is that they'll still operate within spec if the pure d.c. component of the residual current happens to be below 6mA.

B-types will trip on pure d.c. but need a considerable pure d.c. residual current to do so - over 60mA for a 30mA (nominal a.c.) rated device I believe - so won't be blinded by pure d.c. and will trip for very significant d.c. faults, but pretty useless for protecting upstream A (or AC) types from being blinded if there's any possibility of a d.c. fault (or leakage) producing a d.c. component of the residual current between 6mA and 60mA.

There are devices that are intended to look for d.c. residual currents and trip if they exceed 6mA - RDC-DDs - Residual d.c. detecting devices - which are good for protecting upstream A-type RCDs (provided there are no other sources of d.c. residual currents) - as mentioned in 722.531.3.101 in AMD1.  Sometimes they're sold combined with an A-type RCD as an “EV” type.

   - Andy.

I think that we are getting into very deep unknown territory here, and it is far from helpful to anyone. Let's review what we expect from an RCD protected circuit, and why we may have an RCD. I will not include TT installations here as they have another completely different set of requirements.

We fit 30mA RCD protection to final circuits as a personnel protection measure as additional protection against direct contact. We assume that the circuit has a satisfactory CPC, and therefore a short circuit from live to Earth should open the CPD. This should have nothing to do with the RCD, but we expect an L-E fault current of many (several) Amperes, and therefore the RCD being less sensitive is fairly irrelevant. Now we get to the slightly more tricky point, say the appliance has electronics incorporated that uses a rectified supply, does this present a danger. It seems that manufacturers are suggesting (via their instructions) that it does, and want “fancy” types of RCD protection. Now the question, what is the actual risk from a DC fault, remembering the additional protection clause? The appliance has a normal CPC and Earthing, so how does this fault occur in a dangerous way? The RCD protection of final circuits in BS7671 never says that these are a primary protection method against Earth faults (except for TT) so why do we need additional protection that works for DC (or some version of not pure AC)? The only reason can be if an RCD shares several circuits or several loads, one of which has a DC Earth fault and another with direct human contact that needs additional protection, that is a double fault scenario. You will see that the DC fault current must also be fairly small so that the CPD doesn't open, and yet large enough to prevent full RCD sensitivity or speed of operation.

We have here a contrived “what if” situation, of the type that I particularly dislike. The risk of direct contact is very small, the risk of a suitable DC fault also ought to be very small, so the overall likelihood is minute. I can think of many double fault scenarios that are much more likely, say a broken appliance CPC and an earth fault in the same device, or a broken CPC and an exposed appliance lead conductor. These may well be rendered fairly safe with an RCD, as we all understand. However, this does not change the requirement that somehow the person needs to be exposed to a very unlikely scenario, that used to be considered and proved to be very unusual. I have a feeling that we are seeing the result of the increasing “back covering” by everyone, if one specifies the “best” possible RCD protection criticism is impossible, and all risks must be avoided however tiny.

If one examines the circuit diagrams of a number of consumer devices, the number of places where fault currents could be small (> 6mA) and not large, and between an incoming conductor and the electronics without causing any other failure is very small. Asymmetric current from the mains supply is also quite difficult to arrange, particularly if the circuit is not to be broken in some way by the heat generated. DC current in both mains conductors cancel in exactly the same way as AC ones in the RCD core, and highish frequency ones will still operate the RCD. Those promoting these ideas need to explain exactly how such faults as they describe can reasonably happen,because in most cases I think that they cannot, for sound engineering reasons.

We fit 30mA RCD protection to final circuits as a personnel protection measure as additional protection against direct contact.

Agreed (for TN systems) - so we're not talking about earth faults of negligible impedance at or before the appliance, more screwing a utensil rail into a concealed cable kind of thing - where we really do want the RCD to trip out normally (well within 300ms at 30mA or 40ms at 150mA kind of thing) and even a small reduction in the RCDs performance could be very significant.

So bearing in mind that an RCD might be covering half the house (or even the entire house) and someone screws/drills into a cable, is there anything about any of the electronic appliances downstream of the same RCD that could reduce it's effectiveness? Either an uncleared fault (d.c. → N perhaps) in  one of the appliances, or just cumulative leakage currents? Many electronic devices are renowned for high leakage currents (everything from induction hobs to PCs) with the filtering trying to stop nasty spikes escaping the equipment as much as protecting the equipment from incoming noise - can we be sure that there are no non-sinusoidal components to the leakage current? As the residual current seen by the RCD will be the sum of the shock current and the leakage currents from connected equipment, there seems to be a possibility there to my mind.

        - Andy.

davezawadi (David Stone): 
Now we get to the slightly more tricky point, say the appliance has electronics incorporated that uses a rectified supply, does this present a danger. It seems that manufacturers are suggesting (via their instructions) that it does, and want “fancy” types of RCD protection. Now the question, what is the actual risk from a DC fault, remembering the additional protection clause? The appliance has a normal CPC and Earthing, so how does this fault occur in a dangerous way? The RCD protection of final circuits in BS7671 never says that these are a primary protection method against Earth faults (except for TT) so why do we need additional protection that works for DC (or some version of not pure AC)?

This is more or less what I was asking. I don't see how direct contact could arise in a boiler - it's not like the frayed flex on Mrs P's steam iron. So it could only be indirect contact. Even if somebody put a 13 A fuse in the line, a 10 ohms fault will blow it reasonably promptly. In that case, I envisage some electronics which amount to say a 9 ohms resistance and allow 1 ohm for the fixed wiring, but wouldn't that give a touch voltage of only 23 V?

Physics always did make my head ache, but I am struggling to see the danger.