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Petrol Generators and Earthing

A customer has asked me to install a change-over switch for a 8kW petrol generator to his house consumer unit to be used in the event of power loss from the grid.

The user manual for the generator states that it is a "floating earth configuration which means that the Neutral of the alternator is not connected to the Earth of the machine". It then mentions that you can only use 1 type of class 1 equipment or more than 1 type of class 2 equipment etc.

The generator has 2x 115V 16A CEE yellow output and 1x 230 230V 32A blue output. The generator has "overload protection which will cut off power if it exceeds the maximum energy capacity" but NO RCD.

When the generator runs I get 90V between Earth and the Neutral pins and 90V between Earth and the Line pins, this would suggest that the protective earth (centre)-tapped to the winding.

The earth pins of the CEE form sockets generator are connected to the frame and a ground stud/pin. The user manual states that BS 7971:2008 requires the frame to "be properly connected to an approved earth ground".

1. I can't see what function any earthing would provide as the N and E are not connected, if they were connected it would result in catastrophic failure of the machine. Is this correct?

2. Secondly I am correct in thinking that this generator is not suitable for connecting to a house, and he would be better off getting a new machine with correct type earthing arrangement, or are there alternative ways of connecting this generator safely?

  • Do you have a web link to the genset datasheet?

    Data sheet 

    User Manual 

  • It might be worth having a careful look at the way the changeover between 115 & 230 is done on the Gen. I've seen these windings tapped in such a way that its not possible to modify the set such the NE bond is at one end of the winding in 230V mode but yet correctly at the 55-0-55 tap when in 110V mode - I've had to completely disable the 110V option to make this work.

    As others have said, without connecting the 'earth' to one end of the winding, there will be no path for an earth fault current to take & hence an RCD or MCB will not operate in the event of such a fault. Indeed with such a small genset, its doubtful it will be able to deliver enough current to operate anything bigger than about a 10A MCB in the instantaneous region - more likely that either the prime mover will stall or the voltage will collapse.

  • Hmm not the most useful documentation is it. I'd try grounding the neutral via a modest resistance - in the old days I;d have said a light bulb, now in the world of the LED  it probably has to be a small appliance like a fan or a heater, to see if the neutral shifts to 0V and the live goes up as you'd expect.

    M

  • I know that it doesn't help, but customer seems to have bought something which is totally unsuitable for it's intended purpose.

    "Fuel should be emptied before storage for 30 days or longer." As I said!

    96 dB - you wouldn't want that under your kitchen window for very long.

    Perhaps in this situation the best response is to walk away?

  • It is currently a floating supply that will not operate RCDs or MCBs with Earth faults. It is an IT supply without Earth fault monitoring.

    Well, that's not quite true. A floating system will deal with "faults to the cpc of the system" but unless there's fortuitous earthing, not faults to Earth, and additional protection as we know it for earthed systems won't be guaranteed to operate.

  • All of this is covered in BS 7430, quite adequately. Other guidance also covers it.

    1. I can't see what function any earthing would provide as the N and E are not connected, if they were connected it would result in catastrophic failure of the machine. Is this correct?

    If N and E are connected, then imagine an accident in which the line conductor is accidentally "spiked" to the ground. What happens then, is "exposed-conductive-parts" are immediately at full line voltage, which is obviously very dangerous. It's imperative if you intend to operate this as a floating system, for the generator to have an RCD immediately at its output terminals, just in case this happens - at least the poor person touching the "exposed-conductive-parts" stands a chance. This is illustrated in A722 of BS 7671.

    2. Secondly I am correct in thinking that this generator is not suitable for connecting to a house, and he would be better off getting a new machine with correct type earthing arrangement, or are there alternative ways of connecting this generator safely?

    This kind of generator is definitely suitable for connecting to a house, either stand-alone, or as a switched-alternative of supply.

    BUT

    (a) If stand-alone (no grid supply), a consumer's earth electrode is required to make it a proper TN-S system - either connected to MET, or directly to the generator frame and neutral; or

    (b) If a switched-alternative to the grid supply, the installation must comply with Regulation 21 of ESQCR. All live conductors must be disconnected from the Grid when the generator is in operation, and in accordance with Regulation 551.4.3.2.1, a consumer's earth electrode is required. The switching arrangement should also consider that both BS 7671 and ESQCR prohibit two N-E connections at the same time, so this generator must only be connected by a change-over switch.

  • All of this is covered in BS 7430, quite adequately. Other guidance also covers it.

    Unfortunately I don't have access to a copy and at £289 don't think I will be buying one any time soon.

    1. I can't see what function any earthing would provide as the N and E are not connected, if they were connected it would result in catastrophic failure of the machine. Is this correct?

    If N and E are connected, ...

    N and E are not connected on this genset. My preference would be to have a NE link and install an earth electrode making it a TNS system. Then connect the generator by CEE plug and socket with change-over switch.

    ... then imagine an accident in which the line conductor is accidentally "spiked" to the ground. What happens then, is "exposed-conductive-parts" are immediately at full line voltage, which is obviously very dangerous. It's imperative if you intend to operate this as a floating system, for the generator to have an RCD immediately at its output terminals, just in case this happens - at least the poor person touching the "exposed-conductive-parts" stands a chance. This is illustrated in A722 of BS 7671.
    2. Secondly I am correct in thinking that this generator is not suitable for connecting to a house, and he would be better off getting a new machine with correct type earthing arrangement, or are there alternative ways of connecting this generator safely?

    This kind of generator is definitely suitable for connecting to a house, either stand-alone, or as a switched-alternative of supply.

    BUT

    (a) If stand-alone (no grid supply), a consumer's earth electrode is required to make it a proper TN-S system - either connected to MET, or directly to the generator frame and neutral; or

    (b) If a switched-alternative to the grid supply, the installation must comply with Regulation 21 of ESQCR. All live conductors must be disconnected from the Grid when the generator is in operation, and in accordance with Regulation 551.4.3.2.1, a consumer's earth electrode is required. The switching arrangement should also consider that both BS 7671 and ESQCR prohibit two N-E connections at the same time, so this generator must only be connected by a change-over switch.

    But you would still need a NE link on the generator for option (b)?

  • That's what I meant.  Maybe I didn't word it clearly enough.

  • But you would still need a NE link on the generator for option (b)?

    Yes !! ( If not how do your RCDs ADS etc work without a sense of neutral being near ground and live not being. ? )
    and some sort of RCD at the genset after the NE split please, perhaps a 100mA delay type if you do not want one-out all-out tripping. When I have done this in the field I have used a clear bodied plug into the genset with the NE link and a label visible inside

     the plug. (this example is a WALTHER-210306CL other makes exist.)

    The very next thing after the plug is an RCD.

    Mike

  • Are we sure it`s not a fake?

    Chinese copy or whatever!

    Would a decent manufacturer make such a simpler error as :-

    "SOUNDE LEVEL 2 x 16A 230V, 1 x 32A 230V

    OUTLET SOCKETS 96dB(A)"

    How loud is 16A x 2 + 32A and how many watts is a 96dB(A) for socket outlets