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not a problem the RCD breaker had tripped but won’t reset until the new circuit breaker is switched off. I’ve separated the main cable going into the switch and the same happens, the only way I can get it to work is to disconnect the earth wire from the consumer unit then everything works fine including the water heater.
| Rashers Member Since: 21 Jun 2015 Location: Norfolk Posts: 3514   
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Hi Defender SVX.
Yes, with the main switch on the distribution board on, and making the assumption that you have a TN-C-S (PME) earthing system (like 90% of the UK), there will be continuity between the Neutral and earth. They are one of the same thing the other side of you electricity meter. (I will talk latter on faults between neutral and earth, but as this connection is before the RCD, this is why it doesn't create a fault) Firstly, if you are supplying the heater with a 50 Amp Circuit Breaker, the switch should be rated to 50 Amps also. 9.5kW is roughly 41 or so Amps so this isn't an immediate issue, but I will just highlight that to you. The RCD works on an imbalance between what goes up the line (live) cable and what goes back down the neutral. If any electricity goes down to earth, there becomes an imbalance in the currents and the RCD trips. I am assuming the RCD is on the Main Switch of the consumer unit? The Circuit Breaker will only be single pole so it doesn't switch the neural. if you have a neutral-earth fault on the circuit, this will also cause a small imbalance and cause the RCD to trip. First thing first, turn he MCB supplying the water heater off and remove the neutral to the water heater from the neutral terminal and see if the RCD stays on. If it does, you have a neutral earth fault somewhere - perhaps it may even be the heater (though they should have been well tested before dispatch). If the RCD still doesn't reset, you have a fault somewhere else. Turn all MCBs off and then turn on the RCD. It should reset. Turn each MCB on one at a time and when the RCD trips, you know which circuit has the problem. I very much doubt the RCD is weak. They do have a maximum number of trips when the springs and mechanism get weak (or if they are outside they usually corrode in the on position). Unless you are very unlucky, I would disregard this option in the first instance. Possible, but unlikely. Bizarrely, touching a neutral and earth together on a circuit which is switched off will often trip the RCD. A royal pain in the bum if you are in a loft and your lead light goes off 
Hope that helps. Take it steady and take care. |
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18th Nov 2020 7:35pm |
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| custom90 Member Since: 21 Jan 2010 Location: South West, England. Posts: 20453
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I’m only a layman, but I’d double check the wiring.
Also check the voltage, is the shed on any long cable run? If so, you may find if there is any significant voltage drop that the current draw is increased. And thus, that could be causing a nuisance trip by current draw exceeding the supply allowable draw. There are three pin plugs that you can get to plug into a socket to safely tell you that there isn’t reverse polarity present and I think voltage as well which would be safer. I doubt the RCD or MCB’s are at fault, if the wiring is all correct and polarity currently fitted then I’d be thinking about voltage drop and increased load exceeding allowable supply load. As I said I’m not an expert, but these things are typically the issue. Since you originally fitted the initial current draw from stone cold could well be slightly increased causing the trip. Shorts tend to be an instant trip, and won’t reset. Overload can be sporadic either after a short period of time or instant. Neutral could be grounded too that maybe an issue. If nothing else there is some ideas to think about. Otherwise seek advice from a qualified electrician.  No Guts, No Glory.
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18th Nov 2020 8:07pm |
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