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Generator EFLI values

gibb01

Hi All,

Apologies if this has been answered previously & forgive me for my lack of knowledge on generators.. However we are currently involved in a project on a new school building that has a 275Kva backup generator. Due to delays with the DNO, The generator is likely to supply the building on a permanent basis until at least the end of the year. 

It's currently situated around 20 m from the building and we have parallel 95 m4 core SWAs from the generator to the ATS switch. 

We have a specialist earthing company coming in to give us an earth array for the generator & They seem to think they can give us around 1 ohm resistance. 

What i can't quite grasp, Is whether I'm right thinking that that one ohm value on the earth array has nothing to do with the loop Impedance we will be getting to the building as this Should solely be based on our L-E resistance back to the star point of the generator? 

If I am completely missing something & our Ze value at the building when fed via the generator will be 1ohm minimum, then we need to ensure correct final circuit protection (IE 30mA RCBOs) on the majority of final circuits that will exceed the maximum ZS values to meet disconnection times.

Any advice would be greatly appreciated.

Thanks,

Tim

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  • 0

    As already pointed out the generator needs to be suitably rated for continuous use rather than for standby use.

    The generator does need an earth electrode to comply with BS 7430. However the electrode is not in the earth fault path if you distribute  separate earth and neutral from the star point of the generator. So you do not need a very low value of earth resistance perhaps 5 or 10 ohms will do the job.

    I did a quick check in Amtech and using generic values a 275kVA generator should give you around 2kA of fault current  and a Ze of around 0.11 ohms. As pointed out by mapJ1 you would only have to ensure that the fault current from the generator would operate the over current devices in time for higher rated circuit protection. If there is a problem with this ensure the generator has adjustable earth fault protection you can set up for 20A or so. This could be a problem if there are already neutral earth faults on the installation that have been there from day one as they have fabricated test certificates.

    JP  

  • 0 in reply to John Peckham

    Well for a small rural pole-pig transformer in the UK the neutral/ earth electrode resistance can be as high as 20 ohms, and pass the DNO requirements, but of course the copper path for the fault loop should be quite a bit less than quarter of an ohm, at least at the transformer end of the overhead lines.
    Mike.

    Earth fault relays can usually be set high enough to not false alarm on NE shorts further down the wire than a few ohms ;-) It is better to know in advance if you have them though.

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  • 0 in reply to John Peckham

    Well for a small rural pole-pig transformer in the UK the neutral/ earth electrode resistance can be as high as 20 ohms, and pass the DNO requirements, but of course the copper path for the fault loop should be quite a bit less than quarter of an ohm, at least at the transformer end of the overhead lines.
    Mike.

    Earth fault relays can usually be set high enough to not false alarm on NE shorts further down the wire than a few ohms ;-) It is better to know in advance if you have them though.

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