electric shower tripping

[neilyboy]

Been asked to find out why a new shower is tripping a new cu rcd,
new shower 9.8kw mira
supplied with 10mm T&E
40amp mcb protected by 63a RCD
Its a new cable thats been run in and I have no equipment to test the continuity etc I went right back to basics ,missed out the 45amp DP switch and it still tripped,then wired it straight into the DP isolator switch of the cu and the shower started straight away I should also say that this cu is fed from the main house but the shower did not trip the main cu.
The mira instructions say to fit a 30a rcd

Anyone any ideas ?

Ta

 

[telectrix]

1.Neutral conductor in the wrong N bar.

2. L-E or N-E fault.

and going back to basics means continuity and IR testing. and it's a 30mA RCD not a 30A.

 

[neilyboy]

Thanks for the reply as I said I have no proper testing equipment I have a fluke ,continuity was fine with this,and yes you are right it is 30ma ....really should proof read posts I'll check the neutral is in the correct bar next time I'm out..

 

[Deleted member 9648]

You need to IR test ALL circuits, not just the shower. If there in a N-E fault anywhere on the installation this can cause an RCD to operate when a load is switched on such as the shower, this gives a false impression that the shower is at fault.

 

[neilyboy]

ok got that, would this occur even if the cu is split load ?
and if there is an N-E fault elsewhere why is this not tripping the rcd

things were much easier in the 70's sparky wise....a meggar for testing and a wire coat hanger and a piece of string with a plug chain for fishing cables...

 

[Specialist]

Is it the MCB, RCD or both knocking out ?. Just googled Mira 9.8 kW install manuals & they specify a 45a MCB.

 

[davesparks]

A 41A load on a 40A ocpd is not a good start!
Without test equipment you are going to waste a lot of time and effort.

If I've read the op correctly, which I hope I haven't, you have bypassed the protective devices by connecting the circuit directly to the main switch of the CU?

 

[Specialist]

Think you've read it right Dave, if not we both need to learn to read. That's how I read it as well.

I don't really understand going to look for a fault without test equipment either, maybe I'm getting too old for this.

 

[neilyboy]

Really should take more notice but I was more concerned theat the rcd was tripping and didnt (or cant )remember if both had tripped I have a feeling the the mcb is 40a as it was one of those pre-populated CU's that he bought
and Ive just done a quick calc for a 9.8kw appliance and its 40.833333333a...

 

[Specialist]

So as Dave pointed out, it's a 41a load on a 40a MCB & not good.