DIY Doctor

RCD - same as earth fault detector?

Postby chris_on_tour2002 » Wed Apr 16, 2008 10:10 pm


i've just bought underfloor heating for our toilet and, upon reading the installation instructions, have decided to get someone in to do it. i have a couple of questions that i'd like resolved before i get someone in to see if its feasible otherwise the kit is going on ebay!

firstly, the installation calls for an RCD. if possible i don't want to put in a separate RCD in a box in the toilet, and we have an earth fault detector near the consumer unit. is that an RCD? whats the difference? it is very sensitive and isn't just connected to the sockets - a lightbulb blowing can make it trip.

also it calls for a fused spur coming from an RCD socket. does the socket have to be RCD or is it ok if the circuit is protected somewhere else (at the consumer unit for instance).

i want to put the fused spur in the toilet, about 500mm away from the toilet and just above the floor. the backbox will be the steel type, recessed into the wall. can i do this? it is well away from any plumbing and the toilet is the only plumbing fixture in the room - no basin or owt else.

is there a regulation as to minimum height above the floor it can be? i was thinking about 6 inches.

i'd like to know this as i want to do the positioning and chasing out so i can have things exactly where i want them before i get a spark in. the chasing in will continue above the box to another box about shoulder height for the thermostat.

one final question, when running a spur, must it come from a socket, or is it possible to cut into the loop under the floor, put a 30A junction box in and tee a spur in that way? just redecorated all the rooms, plastered and put nice chrome sockets in - really don't want somebody to hack into the walls to put in a spur!
Rank: Project Manager
Posts: 1024
Joined: Thu Aug 30, 2007 9:14 pm


Simply Build It

Postby ericmark » Thu Apr 17, 2008 9:18 am

A residual current monitor (RCM) is not a protective device but it may be used to monitor residual currents in an electrical installation. An RCM produces an audible or audible and visual signal when a preselected value of residual current is reached. This would be an earth fault detector but since you say yours will trip I think it is wrongly named.
Residual current device (RCD). A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions. Also called residual current circuit breaker (RCCB).
These as the name suggests use current but before these were used there was a voltage trip and these were often in there own box very close to the meter with a large yellow test button they had a problem that if extra earth wires were fitted it could stop these working and could cause the whole protective system to stop working. Since you say near the consumer unit rather than built in I wonder if it is one of these old units.
The name “Earth Leakage Circuit Breaker (ELCB)â€

Postby sparx » Thu Apr 17, 2008 12:45 pm

Hi, it seems like your 'earth-fault detector' is an RCD under another name ie America = Ground-fault-current-interuptor GFCI,
Spain/Germany = Differential Interuptor etc.
If in doubt why not use an RCD Spur which is a combined Fused connection unit & RCD which fits in a single 35mm deep metal socket box, I use lots of them and now price down to around £22.00.
BTW check length of thermostat probe capiliary or sensor flex it may limit the height you can fit body of 'stat, presuming it's not an air temp. sensing type which are not good with U/F heating systems.
regards SPARX
Rank: Project Manager
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

Postby chris_on_tour2002 » Fri Apr 18, 2008 10:41 am

thanks very much indeed for these most comprehensive of answers!

ericmark - based on what you said i double checked the component that i thought was an earth fault detector and it is indeed an earth leakage circuit breaker, 63amp load and a 30mA trip.

i also have the wylex-type consumer unit with type B circuit breakers. so it appears to me that my system is suited to underfloor heating without the need for a separate RCCB on the appliance itself, only a fused spur.

thanks for the help!
Rank: Project Manager
Posts: 1024
Joined: Thu Aug 30, 2007 9:14 pm

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides
  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!

  • Related Topics