Faulty consumer unit

This shower has its 10.5kW rating at 240V for that reason, rather than the nominal 230V. At 230V it takes 9.6kW:

With this kind of shower, the inlet water is more or less a constant temperature, aside from a short leg of room temperature water at startup. There's some seasonal variation but once the water is flowing the temperature the element sees is roughly the same throughout a shower. The output temperature can vary based on changes in water flow (either deliberate via the dial, or other taps/toilets/appliances in the house).

Agreed, that's my best guess of what's happening.

Theo

OP mentioned 30A for the shower.

Oh yes, I missed that in the OP. That will be the problem then. 30A is way too low, only enough for a 7.2kW shower.

Theo

No, it will run for a long time on a 10kw load, ~43 amps. Contrary to popular belief, a 32 amp breaker does not trip at 33 amps, at 10kW/43 amps, it will run for roughly 10000 seconds+ (nearly 3 hours) , it will be down to the thermal element inside the circuit breaker as to when it trips, Manufacturers data will show that. So, for a typical shower user, the overload of a 10kW shower on a 32 amp circuit will never be noticed. The OP said it took out the whole CU, which means it tripped the upfront RCD. That is not an overload / circuit breaker issue, it is an earth fault issue, which could be live to earth or neutral to earth, either of which will trip the RCD. Anyway, a 30/32A CB isnt going to trip with a 10kW shower.

I took '30 amp circuit' to mean 32A cable - I'd imagine the cable gets quite toasty in places after a 10 minute shower.

Whether that'd trip anything, not sure - you seem to suggest not. Which doesn't help with the cause of the fault, but does suggest the cable at least needs sorting out. If indeed it is 6mm - perhaps the OP could confirm . . .

I think to give a more reasoned answer we could do with a bit more detail about what devices are actually fitted to the CU, since the term "trip" is a bit unclear.

A standard CU would have a "main switch" as its master control - there is no trip capability with these - it is just a manual switch. So it sounds like you may have a RCD[1] in the main switch position.

There was a time fitting "whole house" RCDs like this was common - since it gave significant extra protection from electric shock. However it also proved to give a much larger risk of nuisance trips, and it is particularly inconvenient since you lose all "discrimination" - any earth fault on any circuit will trip all circuits, not just the one with the fault.

Now if just the shower trip cuts out, it is likely that it is protected by a normal MCB[2], or a RCBO[3] (unlikely given the age).

Assuming it is a normal MCB it is either seeing too much load, or getting too hot. (modern MCBs have two separate trip mechanisms - one is thermal using a bi-metal strip that will respond to a combination of current and time - too much for too long will cause it to trip. The other trip is magnetic and uses a solenoid rigs to snap open the switch "instantly" when seeing a current surge above a certain threshold)

So either the shower is drawing too much current for the MCB, or there is a loose connection near it that is getting hot and heating the whole MCB - in effect sensitising it so that it trips at a lower current than intended.

A 30A MCB would only support a 7kW shower. If the unit has been replaced in the past, could it have been replaced with a more powerful model?

(In reality a 30A MCB would support a small overload for a fairly long period of time)

It is worth checking for signs of over heating, and also checking the terminations on the shower circuit are still good and tight.

A photo of the CU so that we can see the devices fitted might help.

It may not need a whole new CU (although there may be other reasons why you might want one), but it sounds like some diagnosis and testing is in order to find out what is actually happening.

See also:

Not really. The cable is rated to run at 70 deg C at full load. This will take quite a long time to reach 70 deg, probably one hour+, as the cables are generally rated higher than BS7671 allows. In general, there is a 20% over-spec allowance built in, add in that a

No, if in the plaster of the walls, or surface mounted, 6mm T+E will carry 47 amps forever. If it is in any insulation, then the current carrying capacity is reduced, as its thermal emitting properties are reduced, so it can get hotter more quickly. As for it tripping, if it was the RCD that tripped, it would not be an overload problem, it will be an earth fault.

I have a 30ma 'trip' on the Consumer Unit main switch.

OK, thanks. Does make the 10mm cable an electrician asked me to install seem like overkill for a 10kW shower then. Wonder why the shower manufacturer recommended 10mm. Anyway. There, as they say, it is :-)

As I say - 10mm is only what the electrician recommended to me, although this was partly for future proofing, as well as required for a 9.5kW shower.

As it happens, for other reasons, I'm not using him. Or installing an electric shower - another story . . .

A so called "whole house" RCD - long since deprecated, but you are certainly not alone in still having an installation like that.

-- Cheers,

John.

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I have one - quite normal in the late 1970s.

Even more thanks for all these helpful responses.

I'm going to have it all looked at by a professional!

In the '70s more likely to be a huge VOELCB, rather than an RCD

No, it's an RCD - it says so on it. Made by MK.

So showing the stupidity of the wiring regulations which permit old installations to remain in place even though they have long been obsolete...

Dave

How many RCDs should one have, and what should they control?

(I have one for the cooker circuit and the gas boiler, and one for everything else.)

Why's that then? They havent been made obsolete, just that better ways are now available. If it trips, its a nuisance as all power is lost, but it isnt unsafe.

I would say latest best practice is to have no RCDs, but instead to have one RCBO, so a combined RCD and MCB, per circuit. That way any tripping only affects a small part of the system .

Dave

My house was built 1993.