Regulation stating a type AC RCD can not be upstream from a type A RCD

Further to Dave's I'm not convinced that there is a credible  L-E (or N-E  but not both at once ) fault path that puts un-smoothed DC into the loop (which all that an A type is sure to detect) . Certainly it is nothing  like AC leakage that occurs by design in common mode filters.

The car chargers do use the CPC in the link from car to charger to carry a control signal that is uni-polar - not really a continuous DC like a battery but certainly a DC of sorts.. But even though this flows in the CPC, how exactly that would saturate an L-N measuring RCD is not at all obvious. Certainly in normal operations it won't and a credible fault where it does is likely to involve enough fault current to trip everything  else as well.

In many ways things like an LE fault on the lamp side of the traditional filament lamp dimmer switch are more of a hazard, as the series triac can fail in  way that makes it fire assymetrically and the light fitting may be class 1 , There  are a few appliances like cheap hair-dryers that use a series diode for the hi/lo heat setting, but these are all double insulated and have been since about 1970, and before that rectifiers would have been more expensive than creating  a heater coil with a centre tap contact. (time  was when 5s was the cost of a single diode). But we are still looking at double fault to danger at least to get that 'blinding' fault path.

Also  even in odd cases when it occurs 'blinding' is not all or nothing affair, more of a de-sensitising, usually more or less pro-rata, so to blind a 30mA RCD to a 100mA fault, something like 100mA of DC is needed, as with much less the core is pulled out of saturation on the  half cycles that oppose the DC bias, and an AC is detected in the trip side. Not great, but  you do need a lot of unsmoothed DC to make the situation dangerous.

I think the risk is misunderstood and often overstated.

Mike.

The thing is though, the Wiring Regulations mean we need to be replacing or installing additional consumer units to carry out minor electrical work such as adding an additional socket outlet or LED lights, if you are going to comply.

Just thinking aloud - say we had something with an internal d.c. supply, not galvanically isolated from the mains ('cos it's cheaper) that could under certain fault conditions send a limited current (a few 10s of mA perhaps) to PE - I've got in mind something like an LED bulkhead with an LED strip stuck to an Earthed steel dish.

Do I have in mind (much simplified) something like:

That of itself wouldn't be hazardous, but it seems like it could disable an AC type RCD, so loosing additional protection not just from that circuit but any other circuits that rely on the same RCD (as in split load CUs), and the first fault could persist indefinitely. It'd still need a 2nd fault for danger, but where we have additional protection we'd expect something rather better than just the old-fashioned two-faults to danger, which we're not really getting any more.

   - Andy.

There was a thread on here recently about boiler manufacturers stating in MIs that type A RCD should be used. I queried this with a manufacturer, asking why they used products with DC leakage, the DC component magnitude and why they were introducing something onto the market which was potentially dangerous, given that most installations in the UK had AC types fitted. Here is their slightly unsatisfactory reply

"The design of the pump used in our (and all other manufacturers of ErP appliances) boilers requires a type A or type B RCD as defined by the pump manufacturer.

The design of these pumps is to ensure compliance with ErP regulations. The leakage current is <3.5mA. Type AC RCD’s are now very limited in their use and are suitable for resistive, capacitive and inductive loads without any electronic components.

Most modern electrical equipment used within properties now comes under the requirements for a Type A RCD including power supplies for Class 2 equipment, washing machines, LED lighting, induction hobs etc."

 Primary simple, reliable, protection is afforded by good solid earthing and bonding in association with overcurrent protective devices. (Excepting TT installations).

Z.

mapj1 said:

The car chargers do use the CPC in the link from car to charger to carry a control signal that is uni-polar - not really a continuous DC like a battery but certainly a DC of sorts.. But even though this flows in the CPC, how exactly that would saturate an L-N measuring RCD is not at all obvious.

It's not obvious, but with an N-PE fault between EVSE and vehicle, there's a path to share the DC steady-state or pulsed current back to PE through the supply N-PE link (in PME installations, at the service head, so we are talking similar impedances and sharing is a definite mechanism for this to occur).

As David says, not all Type AC RCDs can be saturated, but some can, and this can be demonstrated easily.

AJJewsbury said:

Just thinking aloud - say we had something with an internal d.c. supply, not galvanically isolated from the mains ('cos it's cheaper)

If you think about it, even in galvanically-isolated power supplies, all you've drawn is what's up-front in a switched-mode power supply - rectifier straight onto mains, and a relatively large reservoir capacitor. MIcrowave oven HT supply may have half-wave or full-wave rectification of mains up-front, etc.

Zoomup said:

Primary simple, reliable, protection is afforded by good solid earthing and bonding in association with overcurrent protective devices. (Excepting TT installations).

But that doesn't offer additional protection only ADS for a fault of negligible impedance.

As you say OM, the manufacturers don't understand either, it is as though there is some push to type A, B or F RCDs. Now I have deliberately opened this can of worms, because I actually care about what is in the regulations. I see a problem (and recently there was another one about IT supplies) because it appears that the regulation chain has no persons who can actually analyse the proposals coming from Europe. I wonder why this is? Thanks Mike for making your points, I completely agree, and the reason must be either snag shifting or money. I don't recognise the term ErP. Is this a European thing? So now a pump manufacturer has DC? leakage of 3.5mA or less but wants a type A? Please explain why that might be.

I am now going to reply to Andy J. The design of modern SMPS does not have a rectifier into a smoothing capacitor because the "rules" say they must achieve virtually perfect power factor. The rectifiers feed the first switch mode stage which chops up the rectifier output throughout the cycle so takes a sinusoidal current in phase with the supply. Thus good power factor. What happens after that is conventional. Small consumption appliances do not need this stage because there is a limit to the requirement in terms of power but an 800W microwave with a SMPS would have to comply. Essentially we have a load which must take equal current from L and N whatever happens to it's output, the mains cannot have a DC component! A fault to Earth would simply cause more mains consumption, not DC! The Earth wire might carry DC, but that does not flow as DC through the RCD, simply in the Earth-supply TX N loop, which is not seen by the RCD! Interesting, try sketching a circuit diagram, there is a double wound transformer in the way in the appliance. Until someone makes a complex SMPS without a transformer there is basically nothing to worry about, as long as it is not possible to contact the DC output and the class 2 transformer is not faulty, even if it does operate at 100 kHz or so. You do need to realise that an Iphone charger or similar has this class2 transformer, but it is very small due to the high frequency of operation.