Electric Shower tripping MCB

Blimey! Thanks for the explanation Stefek.

Could you please explain why a motor which runs at a lower that nominated voltage, draws more current to keep the power output the same?

Mike.

Reply to
Mike G
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Thanks Stefek, for your comprehensive reply.

Just to fill in a couple of blanks, the cable (10mm2), the RCD and the

40amp MCB were all installed at the same time as the shower. There was no previous electric shower. There is definately nothing else on the same circuit. The total length of the cable run is about 6 metres.

I will get my brother to test his RCD. Is the built in test button a sufficiently good test to rule out a problem with the RCD?

I believe the shower is still within guarantee, so if the RCD works and all the connections look sound, we'll get someone out to have a look at it.

Cheers Simon

Reply to
smb

I recently went around this exact loop when installing a shower.

It was a similarly rated Triton model to that whose spec was mentioned by Ano poster(s) - i.e. 8.7KW at 230V, 9.5 KW at 240 V.

Anyway, when rating the cables etc. I thought I'd better measure the local voltage. Turned out to be 248 Volts under moderate load, which implied a draw of a shade under 41 Amps, IIRC, for the shower. (Making the assumption of a linear resistance change with increasing voltage - not 100% correct, but a reasonable approximation here).

Should also say the load of the shower would pull the actual supply voltage down a bit, so reducing the current actually drawn... never checked how much.

Anyway, as the Triton installation guide said "40/45 Amp MCB" for this nominal 9.5 KW unit, I fitted the latter.

Given my problem finding a **competent** electrician, I'd think it would be worth you checking the local voltage **but only if you feel you can do this safely**, just to rule this in/out as a possibility.

BUT, as has been correctly stated, MCB's don't just trip out as soon as the rated current is exceeded - typically a so-called type B will allow double the current for 60 seconds or more. How fast does it trip ??

"coherers"

Reply to
Coherers

I've got a multimeter that can handle upto 700 volts ac, so I'll take it round there and get a read for the voltage.

My brother gets up first in the morning and usually has a shower OK without it tripping. When his wife get in the shower a bit later on, it often trips after 5 min or so. Not always, but increasingly often.

The RCD has been tested, using the built in test button, and appears to work correctly.

Reply to
smb

very good explanation here

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Reply to
Chris Oates

load; it states 'it does this' but glosses over 'why'.

Reply to
Bob Eager

Best to test the voltage when the shower is on if you can ( But your test points not nearby!)

Reading graphs for various MCBs, I figure it would need to be drawing *at least* 50A for 5 minutes to trip a "B" or "C", which seems unlikely. However, really depends on the exact characteristics of the type in use. And it could have wandered off spec, hence it starting to trip where it never did before.

If it is drawing over 40 A, and the shower manufacturer allows for it, replacing the MCB (£6 or £7 IIRC) could be a fast and cheap diagnostic/fix. However, as I recall not all consumer units brands take the 45A flavour and the 50 A is a definite no-no....

However, following up the other posters' comments on safety, I would seriously recommend no-one uses the shower for its intended purpose until you have got to the bottom of this one.....

Reply to
Coherers

diagnostic/fix.

'wandering' is what mine did - I brought home the clamp meter and found it tripping after 4-5 minutes at 36A

Reply to
Chris Oates

[SNIP a load of tosh]

Wrong the power will go up with the suare of the voltage. e.g. voltage goes up 10% poer taken will go approx 20%. Yes, the heater resistance will change with temperature but not to a great extent.

Reply to
BillR

That means tha the MCB is tripping thermally, due to a sustained slight overload, rather than magnetically, due to a heavy overload.

Serious question: How long does your brother take in the shower? If he is one of those people who takes quite a while, this could explain the problem.

And how soon after him does his wife shower?

Reply to
Chippy

I don't know their showering habits well enought to give an answer. I will have another conversation with them to find out. Thing is, they have had the shower for several months, and their morning routine has not changed recently, so why should it start tripping and why should it trip more often as time goes on.

Simon

Reply to
smb

To be fair, I think the article assumes a backrground knowledge of electrical engineering. Where term like "power factor" and "magnetic saturation" would be concepts the reader is expected to know about beforehand.

Reply to
Ed Sirett

I can only suggest that the thermal part of the MCB is becoming 'tired' due to repeated tripping and/or lengthy periods of being heated to an 'almost trip' state.

If the usage cycle of the shower is such that it normally has almost but not quit reached a trip state, it wouldn't take much cahnge t cause the problems that you have described.

A few minutes extra use each day, your sister-in-law's shower following your brother's more closely, a slight change in line voltage

- any ro all of these could affect the situation.

One simple test I'd suggest - turn on the shower, leave it running, and see how long it takes to trip.

Reply to
Chippy

Cheers. Radio silence from me is because I'm in the US for a week; just chewing through uk.d-i-y backlog while considering wording of major document I need to start on tonight ;-)

OK, so the more worrying possibilities become less likely (phew!) It's possible then that the positioning of the MCB between other loaded MCBs is the problem - they're warming one another up and making the shower one trip. But that's not nearly as likely as a fault in the shower, methinks...

Sadly not: if the test button doesn't make the RCD pop, the RCD *is* faulty for sure. But if it does pop, the RCD could still be out of spec: too sensitive, not sensitive enough, take too long to pop (not as if you can reliably press the test button for exactly 20 milliseconds, is it!). But in your brother's case it's an individual MCB which is popping, not the RCD, right? To "properly" test an RCD needs a "proper" RCD tester, which full-time electricians can spread the 700-quid cost of over years of work, but which thee and me aren't going to shell out on! It'd make a good school *design* project, though, using suitable resistors to make a 10mA,

15mA, and 30mA load, and a gravity-dependant timing element such as a steel ball running down an inclined channel with a conductive section: lots of good simple applied maths working out how to do various short timing intervals. Course, Health-n-Safety being wot it is, you couldn't ever *build* one with the kids :-(

Hope you/he get it all sorted... Stefek

Reply to
stefek.zaba

Better yet, consider the situation if you were to ever cut the supply! to maintain the somewhat dotty theory that the current will vary to keep power constant as the voltage changes (I = P/V where P is constant) the current I must become infinite!!!

Conclusions, either: a) When you turn off a shower (or a light bulb) the power dissipated remains constant and the current is infinite. OR b) The constant power idea is tosh and it is the resistance that stays constant.

Please avert your eyes if you are offended by someone shamelessly waving their qualifications about - I don't like to do it but it seems reasonable at this point... I have a masters degree in electrical engineering, came top of my class, was awarded a prize by the Institution of Electrical Engineering (the IEE) and my grasp of electrical theory leads me to opt for answer "b". Anyone think they have any better ideas?

Reply to
Matt Beard

Yes. The resistance remains very nearly constant.

Reply to
Ed Sirett

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