Tricky - there certainly were rumours that for a period that what were effectively A-type RCDs were badged as AC types - or they might simply have been some improvement on AC-types but not entirely meeting all the A-type specification. On paper it's simple though - if it doesn't a A-type on the box, we can't claim it is an A-type.

A-types aren't simply about DC blinding - if you want something entirely immune to DC then you'd need a B-type not an A-type. A types are sensitive to half-wave rectified residual currents, which AC types might miss - so would be better for earth faults after power control electronics - rather than relying on large fault currents and the transistors/triacs exploding kind of ADS - which might not always work especially on TT systems.

Problem is that Schneider only have  a 63A type A rcd for over £100 before vat.

Suitable RCD of a brand of your choice in a separate enclosure, for the new/modified circuit?

   - Andy.

It is not simple - there are a number of ways to make an RCD, and there are some that involve permanent magnets that require a bipolar triggering signal to ensure a trigger. Others just see the 'pulsed DC' as an AC with a baseline offset - which in a manner of speaking it is, and trip when the AC equivalent is about the same, so 70mA p-p is about 25mA RMS..

An RCD that tripos on the ripple DC test like a type A (though I'd check both polarities) is likely to offer similar protection to a modern A type - indeed many A types are just older designs re-badged,  However it would be a brave decision to rely on that without knowing what is in the RCD, or indeed if earlier or later models of the same part will react the same ' we reserve the right to improve..'

How likely is this sort of fault is hard to say. Actually a hard fault to earth on the wrong side of a rectifier is not the issue - if a fuse does  not fail, the diode soon will, and then either the fault is interrupted or converted to the full AC sort.

The problem is the high impedance fault, perhaps the path between the rectified DC and earth with a human being in the way, where the current is limited to a fraction of an amp, as then it may be that nothing operates at at all.

Given that at the moment sockets for small appliances fed by no RCD at all attract a C3, one may consider that half an RCD is better than none (or at least an RCD that operates on half the faults and not the others), and should attract at worst the same level of seriousness.

However, as RCDs slowly become more ubiquitous, I can see that liberal attitude changing.

Mike.

Actually a hard fault to earth on the wrong side of a rectifier is not the issue - if a fuse does  not fail, the diode soon will, and then either the fault is interrupted or converted to the full AC sort.

But perhaps not a TT system where the Earth fault current could well be a lower than the normal load current.

The problem is the high impedance fault, perhaps the path between the rectified DC and earth with a human being in the way, where the current is limited to a fraction of an amp, as then it may be that nothing operates at at all.

Indeed - one scenario might be an earth fault with a broken c.p.c. - the kind of situation we'd normally expect additional protection by 30mA RCD to help with.

    - Andy.

Actually a hard fault to earth on the wrong side of a rectifier is not the issue - if a fuse does  not fail, the diode soon will, and then either the fault is interrupted or converted to the full AC sort.

But perhaps not a TT system where the Earth fault current could well be a lower than the normal load current.

The problem is the high impedance fault, perhaps the path between the rectified DC and earth with a human being in the way, where the current is limited to a fraction of an amp, as then it may be that nothing operates at at all.

Indeed - one scenario might be an earth fault with a broken c.p.c. - the kind of situation we'd normally expect additional protection by 30mA RCD to help with.

    - Andy.

AJJewsbury said:

Indeed - one scenario might be an earth fault with a broken c.p.c. - the kind of situation we'd normally expect additional protection by 30mA RCD to help with.

Broken casing and accidental contact with live parts downstream of a rectifier is also possible ... not a 'fault' in BS 7671 terms, but 'additional protection' Reg 415.1, the requirement for additional protection being invoked by a number of other Regulations for different circumstances/applications (e.g. 411.3.3, 411.3.4, 522.6.202, 522.6.203, 701.411.3.3, etc.)