Fault Protection (complicated)

[trey]

The situation is an electric shower (9.5kw@240V, 10mm2 cable, 9m long, conduit and clipped direct, no thermal insulation) and electric boiler (10kw@240v, 16mm2 cable, 10m long, clipped direct between floors).

To prevent overloading the 100A main fuse, the shower and boiler are connected using an "off the shelf" relay unit designed for preventing shower+boiler working together. The relay is on the load side of the shower isolator and when the isolator is on, the relay opens the volt-free thermostat circuit causing the boiler to stop heating. This has all been wired in accordance with regs by competent person. The boiler is on a 50A MCB, the shower is 40A mcb and both protected by 63A RCD. The rest of the house is on the other RCD.

Now whilst this works fine, I am a little concerned as to the effect of a fault. Should the relay fail to open the circuit, the boiler and shower could operate together - 82A (45A + 37A) through the RCD and combined with more than 18A (eg cooker) from the circuits on the other RCD, bye-bye main fuse and/or damaged RCD.

One existing fault prevention is that the relay and shower light are both powered from the same side of a 3A FSU and so if the fuse blows the light won't come on. No light = check fuse. (for more details of light see below).

However, there is still the possibility of failure in the boiler or the actual contactor used in the relay device. I would proposed removing the 40A MCB for the shower and using the 50A MCB to power both the shower and the boiler. That way, if any thing fails and they both come on together (82A) the MCB will trip before anything else.

Has anyone come across anything similar?

The long term solution will be removing the electric shower and replacing with an unvented hot water system. But this won't happen for ~12 months.





As an aside, the shower light / relay are wired thus:

10mm2 (7m) -> 50A isolator -> 10mm2 (1m) ->60A junction box->(1)/(2)

(1)->10mm2 (2m)->shower
(2)->10mm2 (30cm)->FSU (3A)->Relay device and 13W low energy IP65 shower light.

 

[ericmark]

Many of the older star-delta starters and reversing contractors had mechanical interlock between both contactors but today most rely on the electrical interlock only.
And the chances of both the relay failing and something else being used at the same time are very low and if it were my house I would be happy to leave as it is.
Many houses if you added all the loads would be double the fuse value well over what diversity allows but one gets very few incoming fuses blow.
Personal I would leave as it is.
You could used a star/delta starter contactor with mechanical interlock and it would ensure both can never run together but I think that's going OTT.
Eric

 

[holmslaw]

..

 

[trey]

Thanks for your replies. The 3A supply is fed from the shower supply, it's the isolater being engaged that triggers the feed.

I think I'm going to go with my proposal as a solution to the correct way to fail if both units operate at once (a very likely scenario as we often shower at times when the heating will be on).

One problem I can see, and perhaps this isn't a problem, is that a 50A type B MCB will not trip in the 20 mins it takes my other half to shower if it has ~80A going through it. However, during this time the RCD will have has a sustained current of 80+ A going through it and with a rating of 63A this could limit the lifespan.

Anyone have any info on RCD lifespan vs current (there must be plenty of people with 63A RCDs protecting a whole house with peak I > 63A).