100mA RCDs TT system

a glorified grommet

Pretty much, yes. That's what prevents contact between the tails and steel enclosure at the point of entry. Leaving the outer sheath on the tails inside the enclosure is deemed to provide the equivalent of double/reinforced insulation inside (as per 412.2.4.1) - then some manufacturers even add a clamp to ensure that the bare ends of the conductors can't pop out of the incomer's terminals. So hopefully meeting the requirements of 531.3.5.3.2.201.


You're quite right that manufacturer's don't directly claim Class II compliance - usually using words like "similar to Class II". I gather part of the issue is that true Class II equipment never connects exposed metalwork to a c.p.c. (and any PE conductors have to be insulated as if they were live parts) - that way Class II equipment is immune to importing shock hazards from elsewhere in the system and so can be specified where that would be a problem - e.g. outside of an equipotential zone or some special locations (BS 7671 doesn't really define the requirement for Class II equipment, but 412.2.2.4 hints at that). Since the CU case still has to be Earthed to provide ADS for faults on the outgoing wiring (and possibly internal wiring), we can't claim Class II for the enclosure itself - but that's not what is required - we just need to provide protection according to section 412 for the incoming tails.


(As I mentioned before, personally I don't like that approach. The consequences of a L-PE fault before the first RCD is particularly nasty - imposing a hazardous voltage on the entire installation for a long duration (almost certainly until someone suffers a shock), so I think exceptional care is needed for this particular case. Also if we've now decided that CUs have to remain safe even if they catch fire internally, for the safety (from shock) of the entire installation to rely on plastics that'll melt at the first sign of a fire doesn't seem entirely consistent to me. But what I think is logical and what BS 7671 (or the law) requires/permits aren't the same thing.)


   - Andy.

a glorified grommet

Pretty much, yes. That's what prevents contact between the tails and steel enclosure at the point of entry. Leaving the outer sheath on the tails inside the enclosure is deemed to provide the equivalent of double/reinforced insulation inside (as per 412.2.4.1) - then some manufacturers even add a clamp to ensure that the bare ends of the conductors can't pop out of the incomer's terminals. So hopefully meeting the requirements of 531.3.5.3.2.201.


You're quite right that manufacturer's don't directly claim Class II compliance - usually using words like "similar to Class II". I gather part of the issue is that true Class II equipment never connects exposed metalwork to a c.p.c. (and any PE conductors have to be insulated as if they were live parts) - that way Class II equipment is immune to importing shock hazards from elsewhere in the system and so can be specified where that would be a problem - e.g. outside of an equipotential zone or some special locations (BS 7671 doesn't really define the requirement for Class II equipment, but 412.2.2.4 hints at that). Since the CU case still has to be Earthed to provide ADS for faults on the outgoing wiring (and possibly internal wiring), we can't claim Class II for the enclosure itself - but that's not what is required - we just need to provide protection according to section 412 for the incoming tails.


(As I mentioned before, personally I don't like that approach. The consequences of a L-PE fault before the first RCD is particularly nasty - imposing a hazardous voltage on the entire installation for a long duration (almost certainly until someone suffers a shock), so I think exceptional care is needed for this particular case. Also if we've now decided that CUs have to remain safe even if they catch fire internally, for the safety (from shock) of the entire installation to rely on plastics that'll melt at the first sign of a fire doesn't seem entirely consistent to me. But what I think is logical and what BS 7671 (or the law) requires/permits aren't the same thing.)


   - Andy.