Generator EFLI values

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.



  • Broadgage mentions a recent change to the arrangements on red diesel, which seriously affects generators for commercial use. The notice is here:

    but is fairly unhelpful in this case. There are some options and exceptions and you can apply to have the condition removed, and this would seem reasonable where there is no alternative mains supply. The change in the regulations is in my view highly unreasonable, as fuel duties for road vehicles was the previous definition. Bringing in almost all other plant and machinery with IC engines (including boats) seems to be another tax grab, or perhaps something to do with the "Green" agenda, but there is of course no other option to most portable generators, site plant and vehicles and boats of any size. I would expect the changed to be challenged at some point by a group of fossil fuel users, I assume the government thinks electric 1000 HP Bulldozers are available, along with 50 acres of solar panels to charge it, but then it could do very little work as they normally operate 24/7 and this change would cost the operator about £3,000 per day!

    Back to the OP. The EFLI of your design is nothing to do with the Earth electrode resistance, and excessive effort to get this very low is unnecessary. I suggest Foundation Earthing should be examined as the most economical for a new building, which is obviously of significant size. Earth Fault Loop Impedance in this context is simply the resistance between the live supply cable and the Earth wiring in the building, which will vary widely with circuit size and length. It is simply the number we use to ensure that an Earth fault at a point of utilization in the installation will trip the circuit protective device with a suitable time-span, typically 0.4 seconds for final circuits and 5 seconds for distribution circuits. It therefore has nothing to do with Electrode resistance. The Earth resistance to true Earth may well be a number of Ohms, and within reason is of no consequence, 5-10 Ohms is fine and easily obtained with foundation Earthing because these are usually wet and of very significant surface area. The Earth connection is a solid cable to all the reinforcement, this all being connected usually by welding.

  • So far as I can see, it could be covered under:-

    2.4 Heating and electricity

    Rebated diesel may be used for non-commercial premises for:

    • heating
    • generating electricity
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