Shower trips MCB 2 mins after shut-off

[Helli]

I have an 10kw electric shower which is run through a double pole pull switch which is connected to an individual 40A MCB + RCD box connected direct to the meter. All cables are 10mm.

About 2 minutes after the shower is switched off the MCB trips. A new shower has been fitted and still the MCB (not RCD) trips. After fitting the new shower I've discovered the MCB trips even if the double pole pull switch is turned off immediately after the shower.

Any suggestions greatfully received.

Thanks

 

[mattthesparks]

What size is the circuit breaker.

 

[Spark123]

The fact that the DP switch is open and it still trips means the fault will be between the CU and the DP switch.
It could be a number of things from a faulty MCB to a damaged cable or something overheating. The MCB is undersized anyway.
Really speaking you need an electrician to sort it.

 

[bhm1712]

might be worth noting that the cable size is at capacity as well, unless it is clipped direct to a wall and not buried in any walls.

10mm cable buried = 43 amps.


of course, voltage differences make a slight difference to the max current of the appliance. Depends whether it is rated at 10kw at 230 volts or 240.

 

[GaryMo]

might be worth noting that the cable size is at capacity as well, unless it is clipped direct to a wall and not buried in any walls.

10mm cable buried = 43 amps.

A cable buried in plaster is also reference method C, same as clipped direct so same current ratings apply.

See number 57 and 58, table 4A2.

10mm t&e, reference method C is 64 amps.

 

[ColJack]

my guess is a loose connection in either the CU or the switch ( much less likely than the board unless it's a very short run ) causing heating of the cable / breaker..

 

[Helli]

Thanks to all for your suggestions. The shower is rated about 10KW at 240v and the 10mm cable runs under floorboards so plenty of ventilation. I'll fit a new DP switch, 45A MCB and check connections.

Thanks again!

 

[ColJack]

sure it's 10KW and not 10.5KW? showers tend to be rated 7.5, 8.5, 9.5 and 10.5KW..

anyway.. 10KW at 240 = 41.6A.. at 230 = 43.5A.. so either way over the 40A the breaker is rated at.. so it's getting warm and the thermal trip is going out..

you say "2 mins after it's switched off", but does it trip if you leave it on for a little longer than usual?

 

[Helli]

You're right, I think it is 10.5KW at 240V. As for when it trips, it's fine whilst the shower is running but a couple of minutes after it's shut down (and even if the DP switch is turned off) the Circuit Breaker trips. So it seems it's heating up after everythings turned off which I don't understand? I'll leave the shower running longer as you suggest to see if it trips whilst running if it's just a set time from start to trip.

 

[Spark123]

anyway.. 10KW at 240 = 41.6A.. at 230 = 43.5A..

The ratings on showers are usually the 240v rating I imagine as it sounds better in the sales literature.
A heating element is pretty much a linear resistive load, it doesn't take more current as the voltage drops. A 10.5kw shower will draw about 44A @ 240v but only about 42A @ 230v (9.6kw.)

 

[ColJack]

I know all that, stop teaching your granny to suck eggs..
I don't know however if they have started marking up showers for 230V yet.. after all that's what we're supposed to use..

 

[Steve]

Inside my 2003 vintage Triton, the rating is given at 8.7-9.5KW, for a 9.5KW shower.

 

[Helli]

I tried leaving the shower running last night for about 15 minutes (much longer than usual). Again, no tripping the MCB until about 2 minutes after shower and DP switch turned off. I'm going to fit a 45A MCB anyway but am still puzzled why the 40A one fitted doesn't trip whilst the shower is on. I'd have thought that if there was an overload or overheat this would happen before the shower's turned off not after. Guess it doesn't matter if the new MCB cures it, I just liike to understand what's happening.

 

[ColJack]

truly.puzzling.

 

[LoveRocket]

Your circuit breaker is fecked. Get a new one.