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Standby Generator RCD Tripping


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Just installed a 15KVA single phase standby genset into a domestic property last week with manual transfer switch. Fixed wiring at property is PME and is up to date, having recently been re-wired. All circuiys protected by 30mA RCD or time delayed RCD at the distribution board.

The generator also has a 30mA RCD protecting the 63A/230V outlet - presumably to protect locally plugged in appliances rather than as standby power for a fixed installation. Generator is approx 50M from house, connected via 35mm csa SWA 3 core cable.

This where I run into problems with BS7671 recomendations. If I install as earth rod at the generator end and connect the earth core of the SWA cable to the house earth wiring (at the transfer switch), the generator RCD will always trip instantly when switing across to generaor power. Disconnect either the earth rod or the earth connection at the transfer switch and the same thing happens. Disconnect both and it doesn't trip and this is the way I have left it at present after taking advice from various experts and the genset manufacturer and trying different things over a 10 day period.

I've done a few similar installs before - but with no generator RCD - and all has been well with the earthing arrangement as recommended.

What worries me about this installation is that it might be in breach of Regulations - if I read them literally. The genset has a neutral-earth link at the alternator. In theory, therefore it should be operating at zero potential. In effect, I am operating with a floating earth. In the event of a utility power outage, is it safe to assume that the house PME arrangement will provide adequate path for fault current? Would it have been better to bypass the RCD at the generator end? Just wondering if anyone has similar experiences because 2 genset manufacturers I've spoken to say they hit this problem increasingly - sometimes at the genset and sometimes at the distribution board.

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the best thing to do is change the TNCS to TT at the house, and have the genny feeding house as TNS. you will need a rod at house to main supply, and because of distance, an earth rod at genny (NOTE: this does not make the genny TT!). the earth from genny can be connected direct to CU & other earth rod. change over switch will change L&N only between mains and genny. you should not have any problems with RCD's. if possible, genny RCD should be changed to 100mA TD if its 30mA. just so a fault in house doesnt take out feed

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Thanks - that's an interesting solution - is it one that you have tried previously? Are there any other solutions that allow me to kep TNCS at the house? Going to TT seems like a backward step - would I need to inform the Utility? Would obviously need ensure the impedance is within limits for TT and apply additional rods at the house until it is.

A bit more background: the genset I installed was a replacement for one that had died after 30 years service. Initially, I didn't touch the earthing configuration or the transfer switch/genset supply cable. It is only L+N switched. The earth core of the genset supply cable was originally bonded to the steel enclosure of the transfer switch which was also bonded to the CU via main earth terminal. The only variable I introduced to the original setup was an RCD at the generator. If I bypass that and re-connect the genset earth rod, would I breaking Regs? With no locally connected load, surely the RCD isn't protecting anything (apart from SWA cable & Xfer switch?).

I presume you are telling me that with the earthing arrangement previously installed, I had parallel earth paths (which had existed for previous 30 years) and what you are suggesting removes that possibility.

So, in summary, when I read the Regs, there are 2 things I cannot determine for definite:

a) Whether I must have the generator RCD there when there is no locally connected load - many smaller gensets aren't fitted with one.

B) Whether I am definitely breaking Regs with the 'floating earth' arrangement that I currently have at the genset end, given the existence of the PME arrangement back at the house.


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you do not have to inform anyone of going from TNCS to TT. if you look around the forums, there are many threads on TNCS-TT, mostly because TNCS can be dangerous (one fault and your earth could be higher than 230V, and your RCD is guaranteed not to trip)

you should install a rod at the house, and get rid of the TNCS. its a safety issue to DNO if you dont

(for example, if your earth between genny and house fails, your earthing is now using DNO neutral fomr service head, down to a rod somewhere. not if DNO are working on this 'dead' cable, it isnt really dead anymore)

you should have an RCD at genny, but not necessaraly a 30mA one, just because its a genny doesnt mean its less dangerous than the mains!

do not make a floating earth. this should only be used under controlled conditions, your house it not controlled enough. if you want more info on floating earth, check the IT earthing section of 7671.

and finally, the RCD tripping under normal use, and not tripping when earth is removed, suggests that a fault is using the earth, and RCD detects it and trips. by removing this earth, you may be leaving something live

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