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Petrol Generators and Earthing

sparkiemike

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?

  • 0 in reply to sparkiemike
    sparkiemike said:
    But you would still need a NE link on the generator for option (b)?

    Part of the change-over mechanism that also (practically simultaneously) disconnects the line conductors.

    It's also possible to have a permanent N-PE link (which you could add yourself) provided that the changeover switch is "break before make" on all poles in both directions.

  • 0 in reply to sparkiemike

    That will be perfectly OK if done well, but realise the CPC through the RCD box to the genset is also in the loop for faults to terra-firma, as well as the earth rod and the MET and be very careful to make this secure, If there was an electrode at the genset (either only or as well as one at the MET) then  you will trip the RCD on holding a live wire even if the plug CPC came detached,  Size and connect the CPC  to the plug with that safety criticality in mind.

    Mike,.

  • 0

    Thanks for all the replies. The distributors/importers of the equipment have confirmed that installing a N-PE will do no damage the generator. I have one more question regarding the connection of the earth electrode. Does the earth electrode need to connect to the generator by a separate  conductor or can it be done through the "generator distribution cable", as shown in the schematic below.

  • 0

    That should be enough to convince the purchaser that it is not suitable for its intended use.

    Under that section perhaps, but what about the alternative approach outlined in the following section?

       - Andy.

  • 0 in reply to mapj1
    mapj1 said:
    I think in this case the house is seen as a single class 1 item  ;-)
    ebee said:
    Hmmm. I must admit I`d never considered that interpretation Mike. 

    Well ... not that far from the mark ... except in this case BS 7430 tells you that you should connect an earth electrode to the MET so the "single class I item" is in fact referenced to terra-firma ... and the MET should also be connected to the neutral of the generator.

    The same sorts of considerations apply to a mobile & transportable unit, but in this case it may be acceptable (if the generator is not touching or otherwise connected to Earth, and no leads will be run outside the unit to power equipment outside) to not use an earth electrode (although this is usually preferred anyway).

    If using the system as a switched alternative to the grid, ESQCR requires isolation of live conductors from the grid, and that the installation as a whole must comply with BS 7671. Additionally, BS 7671:

    (a) prevents duplication of N-E connections in the consumer installation (this may well trip RCDs anyway); and

    (b) has conditions about switching of protective conductor arrangements and timings.

    Hence, a switching mechanism as I described earlier is required.

  • 0 in reply to mapj1

    Hmmm. I must admit I`d never considered that interpretation Mike. 

  • 0 in reply to WiredScience

    I think in this case the house is seen as a single class 1 item  ;-)

  • 0

    Hi everyone, it's my first reply since the forum was updated,but have been lurking in the shadows :-)

    The instructions clearly state 

    4.6. Floating earth portable generator use.

    4.6.1. This range of generators adopt a floating earth configuration which means that the Neutral of the alternator is not connected to the Earth of the machine.

    4.6.2. It is therefore IMPORTANT that you ONLY use the generator to supply equipment in the following combinations.

    4.6.2.1. One or more of CLASS II equipment

    4.6.2.2. Only ONE item of Class I equipment

    4.6.2.3. One or more of CLASS II equipment and only ONE item of Class I equipment.  

    That should be enough to convince the purchaser that it is not suitable for its intended use.

    For my own use I would be engineer a N-E link solution, but that would void the warranty, and as others have stated the fuel storage & noise levels make it a poor choice.

  • 0

    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 

  • 0 in reply to sparkiemike

    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

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