For my sins I've been involved in caravan electrics again, and with the recent introduction of the 18th, it's got me thinking about SPDs and how they might be fitted in a touring caravan - or indeed any other similar situation such as a mobile or transportable units.
Obviously the simplest approach is just to find a way to avoid fitting SPDs at all - I guess it wouldn't be stretching a point too far to regard a touring caravan as 'a single dwelling unit' and in a lot of cases they wouldn't contain a lot of valuable electrical equipment so the 'opt out' of 443.4 could work a lot of the time. However there may be times when it doesn't, or indeed the owner simply prefers to have the reassurance of some SPDs.
So if we did have a situation where SPDs were required on a caravan, how would we go about fitting them? Caravan supplies have to be treated as rather an unknown - as of course the could be plugged in almost anywhere (even abroad) - so we can't make any assumptions about the type of earthing system (could be TT, TN or even IT), polarity (as many foreign sockets aren't reliably polarised), or indeed the even the presence of an earthed neutral at all (some areas still provide 230V between two phases). Hopefully the pitch supply provides 30mA RCD protection, but again we can't safely assume that.
For TT system I think the preferred way of connecting SPDs is upstream of the first RCD and in connection type 2 (CT2) arrangement (SPDs L-N and N-PE). With caravans though we can't reliably identify which of the supply conductors is N (indeed neither might be), so a CT2 arrangement can't be done. Which I think means we need to go for a CT1 arrangement (SPDs L-PE and N-PE) and be connected downstream of an RCD. However we then need an RCD with an immunity to surge currents of at least 3kA 8/20 - which the usual (AC and A type) 30mA RCDs don't provide - and we're obliged to provide non-delayed 30mA RCD protection at the caravan inlet (and at the pitch supply too) - which feels like a bit of a catch 22 situation. The only glimmer of hope I can see is that some manufacturers seems to suggest that some (of course more expensive) types of RCD may provide suitable immunity - e.g. B, F, AKV and EV types - so if 30mA versions such types were fitted in the caravan (and pitch supply) we might have a route to compliance (if an eyewateringly expensive one), but is this really what JPEL/64 have in mind?
In BS 7671 terms the caravan seems to be treated as its own little installation and the supply inlet (or possibly the far end of the supply lead) the origin of that installation. The regulations for SPDs seem to have been written with permanent buildings in mind however, and some of the requirements for SPDs at the origin seem a little OTT for a little caravan that will in practice always be a sub-circuit from another installation.
I get the impression that incorporating SPDs into installations that aren't within permanent buildings hasn't really received much consideration yet - should we expect new sections (e.g. 721.443 721.534 for caravans) to address this in due course?
- Andy.
