We have recently had a 17th edition British General 10 way split board consumer unit installed, also a new Creda 9.5kw shower unit. Our problem is that each time we try the shower unit it trips out the 80amp RCD. Having had the shower unit tested, and installing a new RCD, the problem still persists. HELP.
As to RCD it will have three ratings.
The power it can take continuous which is 80A
The power it can take under fault conditions until fuse/MCB opens about 4.7KA is normal could be higher.
The amount of un-balance it will allow normally rated 30ma and this means maximum of 30ma and it can be anywhere between 15ma and 30ma.
It is the latter that is the important one. The unbalance is caused by either line or neutral leaking to earth. With any line leaking to earth it is normally easy to find what is faulty but when neutral leaks to earth it is not so easy.
Because the neutral is often not switched and it is at nearly the same voltage as the earth the item which is faulty may not be the item which when turned on causes the RCD to trip.
When a shower is turned on all the cables between the point where the earth and neutral split will have a volt drop as there is resistance in the wire. Without this volt drop you could connect the earth and neutral together and the RCD would not trip but once we have this volt drop any parallel path will start to have current flow so the RCD will trip.
The earth to neutral fault could be anything connected that has an earth and neutral connection so a toaster left plugged in could cause the RCD to trip when the shower is used even though the toaster works OK.
So first step is to un-plug (Not just switch off) everything plus switch off any isolators / fused connection units like those feeding cooker and immersion heater then try your shower.
If it still trips it could be a wiring fault but you can't go much further it needs meters to find the fault. However if it doesn't trip then you know some thing that was left plugged in was at fault.
Two ways to proceed if you think it was something plugged in. One is of course plug in each item then try shower and the other is to have each item checked with a meter. Since it may be more than one item the latter is the only sure way to test.
My house has had RCD protection for 20 years and once or twice I have needed to hunt for a faulty item but if we get a new item in the house and then the RCD starts to trip we will soon realise what it is. But when a house has not had RCD protection then there could be quite a few items that are faulty.
The mineral insulated heating element is a common cause. They are found in your cooker, washing machine, tumble drier, dishwasher, immersion heater, kettle etc. The other is damp dust or spiders which could be inside a socket outlet. One would hope on fitting a new consumer unit all fixed wiring would be tested however a spider or wasp could crawl into a socket at any time. Normally find that sort of problem when there is a supply to a shed or garage.
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