electric shower trips MCB

[daveob]

i installed a new 9.5Kw gainsborough shower in the bathroom, i used approx 12m of 10mm twin and earth cable. i wired in the 40A MCB into the consumer unit and clipped the cable around the top of the wall, which was then covered by coving. The cable then ran up to the loft and across to above the bathroom. In the loft it is under insulation. the cable then drops to outside the bathroom where i installed a 45A isolator switch. the cable then runs throught he cupboard to the shower. when running the shower on cold it runs smooth but when i start to turn up the temperature the MCB trips.

i checked all connections in switch and consumer unit

the MCB was given to me by an electrician in college but it is not new, the only item in the installation that is not new.

could the MCB be faulty or do i need a 45A one?

Do the regulations stipulate you need an RCD aswel?

thanks

 

[industryspark]

9500W / 240 = 39.5A

so your right on the border with the mcb you have, having said that though it shouldnt be tripping as you have said, i.e when you try turning heat up at all. the 40A mcb should be fine.

mcb could be faulty- it does happen. anychance you can get hold of another to try? not that expensive.

no regulation requiring rcd..... HOWEVER most sparks wont fit a shower without rcd protection and in fact most manufacturers state an rcd should be fitted.

strongly recommend you fit an rcd. also have you had the circuit tested? does it comply with bs7671 in terms of zs, continuity of cpc's ect....

have you assessed the current carrying capacity of the cable, followed permitted routes?

diy electrics involving showers are in my opinion a big no no.

 

[ricicle]

Good call on the Rcd fitting I-spark

 

[Spark123]

Can you get hold of a clamp on ammeter?

 

[ricicle]

If your supply voltage is slightly high then the shower could pull just over 40A but this would take a while to trip (havn't got table to hand)

 

[JohnD]

It may be worth (if you can ) doing an insulation test on the cable in case it has a nail or something through it.

Does the shower unit have a high/medium/low control on it? If so, then nothing but the "high" setting should be anywhere near tripping the MCB (though I agree that it shouldn't trip anyway).

There is also a possibility that your chum found the MCB in his box of "old bits that will come in handy one day" and it is a faulty one - though this is not very common.

What make is your consumer unit?

 

[jondiy]

Don't rule out the possibility of a faulty shower.

Could be a short in the shower heating element. Gainsborough Showers - people either love em - or hate em!

 

[slippyr4]

If the shower is rated at 9.5KW at 230V, which is entirely possible, and if the shower can be thought of as a purely resistive load, which seems appropriate, and discounting any change of resistance of the element with temperature...

Then ohms law says the shower has R = (230^2) / 9500 = 5.57 ohm.

Running a 5.57 ohm load on a 240V supply, which is probably what the OP's voltage is closer to, then the current is 43.1 A

That's probably enough to trip a 40A MCB reasonably quickly. It needs a 45A mcb.

*******************

edited to add that i think it's unlikely that the R tolerance on an element is better than 5%, so the current could even be as high as 45A at a real 240V.

 

[daveob]

i've ordered another 40A MCB legrand, trying to order a 45A one but i am having problems finding a wholesaler that stocks legrand. any suggestions?

as for the RCD, i will install one because of your advice. if i could get an RCBO it would be much more easier to install but i do not want to invest in one of the these until i am certain what current it is tripping at!


the shower is 9.5kw @ 240 v and 8.7kw @ 230. if i had looked on this site before buying the shower i would have stayed clear of the make i bought.

thanks for your advice so far, can't wait for this to be resolved

 

[daveob]

how will i know if it is a faulty shower?

 

[ebee]

Is the shower rating at 230 or 240?
Life used to be easy because we were all on 24o in the UK and its (ex) colonies.
Then harmanisation brought all Europe on 230 =/- lots of tolerance so we are now notionally all the same but none of us has actualy changed the real voltage we use.
Shower manufactures always quoted power rating at 240 but were made to change for 230 , most now quote power ratings at both voltages.
So , is your shower 9.5KW at 240v or 9.5 KW at 230 therefore making it 10.34Kw at 240 which might conceivably trip the MCB, especially if it`s a tadd sensitive or if your actual voltage is well above 240 anyway and even more so if you shower for 60 minutes rather than 6 minutes

 

[RF Lighting]

the shower is 9.5kw @ 240 v and 8.7kw @ 230.

 

[ebee]

I would not expect it to trip then unless you`ve an usually high actual voltage.
I `ve seen loads od 8.5KW showers on B32 running no problem although strictly incorrect as the MCB is operating slightly overcurrent (240V)