DIY Doctor

underfloor heating - almost ready to go!

Postby chris_on_tour2002 » Sat Apr 19, 2008 9:23 am

ok, so i now have my underfloor heating kit and i am ready to install. here is my plan:

tee into the ring via a 30amp junction box and run 2.5mm cable to a fused spur. then to thermostat etc. the upstairs ring is protected by a type B mcb on the consumer unit and i have an earth leakage circuit breaker as well, rated at 63amp load, 30mA trip.

does this sound ok? might be being paranoid, but the installation guide stipulates a 30mA RCD. what i have i think is fine, i could insert a fused spur RCD i know, but they are frankly a little bulky and ugly so would rather not fit unless necessary! of course they are to keep us safe so if needs must i will fit one but would rather not if not required.

also i've been told that having two RCDs on the same circuit can be a nightmare as they constantly make eachother trip. is this true?


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Postby sparx » Sat Apr 19, 2008 7:15 pm

Hi chris, whilst it is not true that having 2 or more RCD's in line will make them trip more what is true is that the most likely one to trip first will be the main one not the second one.
Reason being the local one is only 'seeing' leakage from whatever is connected to it ie your heating mat, but the main one also sees same thing plus any other leakage say from a fridge, computor, kettle etc. so assuming both RCD's tripped at identical leakage current the main one will reach trip point first.
Having said all of that, most main RCD's I have tested usually trip around 23-27mA & most RCD spurs trip @ 17-19mA even though all are rated for 30mA nominal. The makers of rcd spurs & 'plug-through' & 'plug-in' power breaker types play on safe side as they know they are only protecting a single piece of equipment which should not in normal use get anywhere near 30mA leakage, but ona house main RCD several circuits each leaking a few mA's can soon 'tot-up' to near the limit, only neading a small additional amount to go over the rating.
If you have a working RCD main switch then there really is no need for a second one, likewise with a shower, both manufacturers say must have RCD protection, which you have,
sorry for long reply but want to put you at ease abt your set up.
regards SPARX
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Postby chris_on_tour2002 » Sun Apr 20, 2008 8:57 am

please don't apologise for giving a comprehensive reply! totally answered my question, ready to start tomorrow, many thanks for your help.
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