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Generator earthing

Baldyhugh
Hi


I have a tractor driven Backup generator to install on a farm shortly and have a query regarding the earth spike. Does it need to go directly back to the MET or can I connect it directly into the generator earth terminal, the latter being Quite a lot simpler   In my case? 


thanks

  • Former Community Member
    0 Former Community Member
    Won't the farm be TT anyway?


    Will there be an "earth wire" accompanying the cable route to the C/O switch from the generator? 


    Will there be a RCD at the generator end?


    Regards


    BOD


  • 0
    The farm is 200a single phase TN-S and as such does not have an earth spike. There will be an earth cable coming from the generator as part of a 3 core cable to the changeover switch Which will incorporate an rcd. In theory connecting the spike to the generator earth terminal will suffice as this earth then goes back to the MET via the 3 core cable but was unsure if the spike cable should go directly to the MET
  • 0
    If more than 3kW then BS 7430 says you need an independent earth electrode for the generator with an earth resistance of 20 ohms or less.
  • 0
    No Hugh that isn't right. If the supply is TNS you don't need a spike, as you have an Earth connection which should give the same protection as the mains supply. I assume the farm has a transformer close by for its supply, which will have an Earth mat or whatever. Everything should have RCD protection already, (section 705) 705.411.1. All you need is a change over switch in the tails (only phases and neutral) between your generator and the mains supply, and a 300mA RCD in the generator supply to provide Earth fault protection to the connecting cables etc. and the existing Earth will work normally as long as you connect it! I expect a tractor unit will be at least 40kVA, so all your normal OPDs up to 40A or so should operate normally. If the supply is bigger (for a grain dryer or similar) then it is likely the Generator may not power the entire load, some warning is needed because you will stall the tractor or cause mechanical damage somewhere. Ask any further questions if you need.


    regards

    David
  • 0
    In response to John, an Earth spike is not necessary if there is another adequate Earth connection. A defect in BS7430 which makes assumptions which are not necessaraly correct, it is assuming a very small generator which is free standing. I'd like to see the Earth "spike" at a 2.4 GW power station! Hugh, You latest post is correct. What size is the tractor generator, might be up to 100 kVA if a big tractor? It is still not 200A though. See above. Bigger tractor generators are usually 3 phase, and a local TX supply is too, are you sure it is real TN-S?
  • Former Community Member
    0 Former Community Member
    Most of the PTO driven standby generators I worked on were Froment (spelling) with the orange lamp to set speed. These were 15 kVA and on dairy farms. Others just had a PTO driven vacuum pump for standby. Not really applicable these days with everything dairy wise geared round electronics. I'd be surprised to now see anything much bigger on a PTO as typically a dedicated generator is used to supply full load demand.


    Regards


    BOD
  • 0
    Relying on the DNO earth (TNS or other) may not be wise if the power is off completely, as it may be off because the supply cable is damaged or because the DNO are working on the earth. Supplementary generation like solar panels that cannot run as an "island" does not have this issue.


    In reality you will have plenty of local  electrodes in the form of plumbing, and barn foundations and  and so on in the ground, maybe the odd container on skids but you may well have nothing that is labelled as an official one.

    I assume the wire from genset to buildings will include a CPC ?

    Regardless if the genset sports an electrode and an RCD that should be fine, whatever the DNO supply does.

    Mike.
  • 0
    David


    i went last year on the tourist mini bus trip inside the magic mountain pump storage power station in Dinorwic. I notice at the various stopping places thick and wide copper bus bars going in to the rock. I assume this was the earthing  for the generator.
  • 0
    davezawadi (David Stone):

    In response to John, an Earth spike is not necessary if there is another adequate Earth connection. A defect in BS7430 which makes assumptions which are not necessaraly correct, it is assuming a very small generator which is free standing. I'd like to see the Earth "spike" at a 2.4 GW power station! Hugh, You latest post is correct. What size is the tractor generator, might be up to 100 kVA if a big tractor? It is still not 200A though. See above. Bigger tractor generators are usually 3 phase, and a local TX supply is too, are you sure it is real TN-S?


    David,


    I agree with Mike, this is not correct if it's a TN-S earthing terminal provided by the DNO. See Regulation 551.4.3.2.1:




    551.4.3.2 Additional requirements for installations where the generating set provides a switched alternative to system for distribution of electricity to the public (standby systems)
    551.4.3.2.1 Protection by automatic disconnection of supply shall not rely upon the connection to the earthed point of the system for distribution of electricity to the public when the generator is operating as a switched alternative to a TN system. A suitable means of earthing shall be provided.





    The reason for this, is that the supply cable may break.


    But good question whether it's "real TN-S".


    The added difficulty with a DNO-provided earthing terminal is the need to provide a switched N-E bond as well as ensure adequate isolation from the live conductors of the grid as part of the supply changeover arrangements - which, if the generator has its own electrode completely, a simply "All lines and neutral" changeover switch is probably best to achieve all of this.


    Regulation 21 of ESQCR would apply.


  • 0
    David


    I was always taught that if you have an alternative source of supply eg. Backup generator then you must always have a reference earth connection that is within your control. The theory behind this being that if you are running your backup generator due to a supply failure, the dno may well have removed their earth connection as part of their supply failure works and if I am relying on it, it may not be available. This has been well debated on this forum

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