RCD Protection for CCTV

Generally if you have LV (e.g. 230V) live conductors only covered with single insulation that, if the insulation broke down for whatever reason, could energise an exposed conductive part, then the part needs earthing and you need to arrange things to provide automatic disconnection of supply (ADS) in the case of a fault to such an earthed part, or indeed to the c.p.c. of the circuit. On TN system, where fault currents are reliably high, a fuse or MCB will typically disconnect adequately quickly. On a TT system that less likely to be the case. If the earth loop impedance is 50Ω say, 230V will only generate about 4.6A - so a 6A MCB is unlikely to trip at all. Normally for TT some kind of RCD (not necessarily a 30mA one) will be needed - not only for the equipment itself, but usually for the circuit(s) supplying it too. That need not necessarily be per-circuit - it was common enough in the old days to have a single RCD "up-front" covering an entire installation, or as an incomer for a distribution board (often a 100mA or more, time delayed). If it is a TT installation, there should be some means of providing ADS for all the parts upstream too.

30mA RCDs for "additional protection" are a different matter (although the same device may be used for both additional protection and ADS in some cases) - depending on the circumstances (e.g. location and if the circuit supplies certain other types of equipment) such protection might be required, but probably not if it's just for CCTV.

   - Andy.

Generally if you have LV (e.g. 230V) live conductors only covered with single insulation that, if the insulation broke down for whatever reason, could energise an exposed conductive part, then the part needs earthing and you need to arrange things to provide automatic disconnection of supply (ADS) in the case of a fault to such an earthed part, or indeed to the c.p.c. of the circuit. On TN system, where fault currents are reliably high, a fuse or MCB will typically disconnect adequately quickly. On a TT system that less likely to be the case. If the earth loop impedance is 50Ω say, 230V will only generate about 4.6A - so a 6A MCB is unlikely to trip at all. Normally for TT some kind of RCD (not necessarily a 30mA one) will be needed - not only for the equipment itself, but usually for the circuit(s) supplying it too. That need not necessarily be per-circuit - it was common enough in the old days to have a single RCD "up-front" covering an entire installation, or as an incomer for a distribution board (often a 100mA or more, time delayed). If it is a TT installation, there should be some means of providing ADS for all the parts upstream too.

30mA RCDs for "additional protection" are a different matter (although the same device may be used for both additional protection and ADS in some cases) - depending on the circumstances (e.g. location and if the circuit supplies certain other types of equipment) such protection might be required, but probably not if it's just for CCTV.

   - Andy.

AJJewsbury said:

but probably not if it's just for CCTV.

Except if it's street furniture or similar that comes under Section 714 ? I wouldn't want to be the one that decided to leave out additional protection for a CCTV column accessible to the public (even if on private land) after this tragic case: www.bbc.co.uk/.../uk-england-london-56758749

Except if it's street furniture or similar that comes under Section 714 ?

I did think of that, but my reading of 714 seems to suggest additional protection is only required for lighting (714.411.3.4). I take the point that the risks are likely similar (although the most similar, a street lighting column, is specifically excluded from the requirement), and of course the designer is always free to go beyond minimum BS 7671 requirements if good judgement requires.

   - Andy.