New shower tripping RCD

[dog1255]

Help required please
I have installed a 9.5kw shower , 10mm new cable, new 50 amp switch, 40 AMP MCB on CU.
Power goes to the shower as shower light is on ..but as soon as I turn on the shower in an attempt to introduce water the RCD trips ( not MCB ) .
Also noticed that the neon on the isolation switch does not light up.
Water is connected with no leaks.
Wiring in sitting in wall channels but not yet covered.
Shower is no more than 5 mtrs from CU.
I fear that the CU has too much power going through it.
I have the following MCBs on the same buss bar ...32 16 16 16 16 16 32 ....and now 40 amp for shower . I am aware that this exceeds the 80 amp 30 mA 230v circuit, so I switched all other mcbs off to just test the shower, with a view to adding another CU ..but still trips the RCD .
Is this just purely to much AMP making it trip ....irrelevant if I switch off all other MCBs ?

[Someone-Else]

Is this just purely to much AMP making it trip

Nope.

An RCD is not an MCB and the two work very differently.

In short:
An MCB will operate (Trip) if too much current is drawn.
An RCD will trip if current goes (or leaks) to earth

I would check all your connections, particularly in the isolation switch, check to see if the Live or Neutral cable has been squashed into the earth cable.

[Neelix]

New circuit , no testing, trips RCD

Now remind me why I waste so much money on books, test equipment , insurances, update training …..

[ericmark]

As @neelix says, you need to test, it may be there is nothing wrong with shower it could be a neutral - earth problem on some thing else. But only way to find out is test.

[dog1255]

Thanks SE ....fitting 10mm into the switch was very tight. I have tried to re-work it .....not switched it back on yet , we'll try and let you know. I take it that fitting 10 and 16 mm into an isolation switch is a problem and there are no other means to make this a little easier?

[Someone-Else]

Make life easier and have 10mm all the way

and 16 mm

[OnlyMe]

If it was a NE short on a shared RCD then the RCD would trip due to the load on the other circuits.

[ericmark]

If it was a NE short on a shared RCD then the RCD would trip due to the load on the other circuits.

This is why I said need to test.

On another forum there has been a thread about grouping many circuits on the same RCD, and questions to if this is against regulations, the fact it is debated shows how contentious it is, I think we all agree RCBO's is the way to go, but as to if a RCD and many MCB's is non compliant is not so easy.

[Neelix]

If it was a NE short on a shared RCD then the RCD would trip due to the load on the other circuits.

This is why I said need to test.

On another forum there has been a thread about grouping many circuits on the same RCD, and questions to if this is against regulations, the fact it is debated shows how contentious it is, I think we all agree RCBO's is the way to go, but as to if a RCD and many MCB's is non compliant is not so easy.

Dual RCD boards are only fitted by amateurs imho

[ahfix]

Dual RCD boards are only fitted by amateurs imho

Care to explain more? Isn't this how the majority are installed ?

[Neelix]

Dual RCD boards are only fitted by amateurs imho

Care to explain more? Isn't this how the majority are installed ?

Dual RCD's don't minimise nuisance tripping

BS 7671 18th edition 531.3.2

[ericmark]

Dual RCD boards are only fitted by amateurs imho

Care to explain more? Isn't this how the majority are installed ?

It does depend on home size, with a caravan for example we have one RCD feeding all, and a two up two down house could likely be OK with a two RCD board, many old homes had Wylex fuse boards with three or four fuses, cooker, ring final, immersion heater and lights, to use just two RCD's would be enough, however four RCBO's will likely cost less, so what would be the point?

As the size of the home increases so the natural leakage also increases, we are allowed 3.5 mA leakage maximum for an item plugged in, and 10 mA for item hard wired, and the no fault leakage for a 30 mA RCD must not exceed 9 mA, it is near impossible to assess what the leakage will be, but clearly danger and financial considerations must be taken into account, so an emergency light top of stairs will likely take care of safety, and a RCBO supply to fridge/freezer will take care of the financial.

But in the main either circuits likely to trip need removing from general RCD, or essential circuits need removing. So you really need at least three RCD's. But you also need to fit a type tested distribution unit called a consumer unit, and there are very few with three RCD's and a RCD costs around £25 so three looking at £75 a MCB around £4 and a RCBO around £15 so £11 more for a RCBO so two RCBO's cost came as two MCB's and a RCD.

So although my 14 RCBO consumer unit did cost more than a high integrity consumer unit with 6 MCB to each RCD and 2 RCBO's the difference is very little. My CU is 18 module long, the twin RCD one would be 22 module long, and mine has one neutral bar, the twin RCD type needs three neutral bars. So we are looking at less than £50, and a freezer full of food is around £300 so it does not make economic sense to fit a twin RCD consumer unit.

Also the calling of an electrician to come back because the board is tripping is not going to help him find more work, he wants a satisfied customer. And there is some debate to if a duel RCD consumer unit complies.

Can't say duel RCD consumer units don't comply, it would depend on size of property etc. But

Every installation shall be divided into circuits, as necessary, to:
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation

is nearly impossible to comply with when we simply don't know what protective conductor currents will be as we have no idea what equipment will be used. So not worth an electrician taking a chance.

[LadySpark]

Dog, keeping it simple - MCBs (fuses) protect appliances from too much power. RCDs protect people. RCDs normally trip with a wiring fault, probably your CPC (earth wire). I would focus on checking your wiring as you are not overloading the RCD if nothing else is turned on. As you didn't test, I am guessing that you are in another trade, maybe a plumber or doing your own home, and used existing wiring, which is why you have 16mm wire joining to 10mm wire. My suggestion would be to revisit your connections on the 16mm wire. Honestly, I struggle with 16mm wire. It is thick and heavy and a total pain to get in right. Check that you have the green and yellow wire fully connected. Check that you have trimmed the exposed wire on the blue and brown wires so that the plastic sheath goes right up to the connection with no copper showing as powerful appliances arc if you leave exposed copper sticking out. Hope you get it working safely.