Shower tripping mcb

Risteard

cpbackhouse said:

I am not an electrician but .... at some very rough maths if its 9.5kW assuming 230V supply it could easily be pulling more than 41A which I would hope would trip the MCB rated at 40A. 
Obviously not a professional opinion but sounds like the MCB is doing what its designed to and might need upgrading. 
Surprised the electricians who have been out haven't mentioned this though so I might be missing something obvious.

It wouldn't. A circuit breaker must not operate at below 1.13 times its rating, but must operate within the conventional time (1 hour) at 1.45 times its rating.

That said, the rating of the circuit breaker should not exceed the load. 50A devices are much more common than 45A devices. Very few makes will have a 45A device available.

coffeehound

@will155 can you say the sequence of events in which the changes were made?  Was there previously a lower power shower installed?

I'm not an electrician, but: if the first electrician made the decisions on cable gauge for the 9.5kW shower and chose to not uprate the mcb accordingly, then IMO you could rightly ask him to sort it out.  

The fact is that the 40 amp mcb is too low for the shower.  I would guess the decision whether the cable installation is able to support a higher value mcb is for the electrician who installed it to decide, if it can't be properly inspected.

If the cable run is not able to support a higher-load mcb, then you either need to fit a lower power shower (say 9kW), or install 16 sq mm cable run + higher rated mcb.

Eldi_Dos

What did the instructions with the new shower say was required.

coffeehound

Hmm.  From Risteard's info, the breaker should not be tripping below 45A.  Since two showers have caused the same result, it sounds like the existing MCB is faulty.  Worth getting that swapped as a low cost test.

grumbler

coffeehound said:

Hmm.  From Risteard's info, the breaker should not be tripping below 45A.  Since two showers have caused the same result, it therefore sounds like the existing MCB is faulty.  Worth getting that swapped as a low cost test.

Depends on your voltage that, theoretically, can be as high as 253V.

If the nominal power is for 230V, then at 253V it will be 11.495kW and the current will be 45.43A

ETA:

All are less than £10

coffeehound

Hi Grumbler.  The 9.5 kW is for 240 volts supply according to Mira; they quote a lower figure for 230 volts (presumably other makes are the same).  That gives an impedance of 6.06 ohms.  At 253 volts that impedance would conduct 41.7 amps, so still well under 45A.  

knightstyle

MCBs work on heat so if the consumer unit is above a radiator then the ambient heat in winter will cause the MCB to trip at a lower current than the rating.

fenwick458

OK I wouldn't advise buying a £10 test instrument from china it won't be calibrated and will probably not be accurate. 
I think we need to know why you have had 2 showers fitted, and why there has been 2 electricians out and yet none of them have been able to fix this seemingly simple problem. what did each of them do?

bris

The set up is right, 40amp MCB, 10mm cable and 9.5KW shower will be installed in millions of homes without problem, assuming a typical cable run.

The fault is elsewhere, have they ruled out a faulty MCB? Have they ruled out the cable being damaged during installation?
Have they used a muti function tester on it? The tests should show a problem.

grumbler

bris said:

Have they ruled out the cable being damaged during installation?

It has to be some very special damage - that triggers the MCB only when the shower is switched on.