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.

Thanks,

Tim

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  • Hi All,

    Many thanks for all the helpful & informative responses. We are also looking at a temporary supply from an adjacent building so the generator can be turned off at night - however this is likely to be 60a single phase & I'm yet to see how we can get this to function with the generator ATS swtich (Which I imagine has phase failure relays - So not sure if it would recognise a single phase supply connected to it) 

    So as I understand it, Our Ze value into the building is likely to be fairly low & equivalent to that supplied from TNCS system via a standard DNO supply.  So we should therefore hopefully meet all our Zs limits and disconnnection times on final circuits protected via MCB's. Where we have RCD (Or RCBO in this case) protected circuits, this is essentially the reason the Generator needs to be earthed as a referance point for any leakage current that flows.. 

    I assume that as we have Bonded structural steel within the building back the the MET this would also act as the same thing essentially? We also have a Lightning protection system in place

    With regards to the use of the generator, I'll ask the question back to the client Who provided it in the first place. The likely hood being is it's not going to be loaded anywhre near it's maximum & there was talk of it requiring regular monthly servicing - especially if it's running at a fairly low % & there was also talk of load banking at regulat intervals if that was the case. 

    Thanks again for all your help so far. 

    Tim

Reply
  • Hi All,

    Many thanks for all the helpful & informative responses. We are also looking at a temporary supply from an adjacent building so the generator can be turned off at night - however this is likely to be 60a single phase & I'm yet to see how we can get this to function with the generator ATS swtich (Which I imagine has phase failure relays - So not sure if it would recognise a single phase supply connected to it) 

    So as I understand it, Our Ze value into the building is likely to be fairly low & equivalent to that supplied from TNCS system via a standard DNO supply.  So we should therefore hopefully meet all our Zs limits and disconnnection times on final circuits protected via MCB's. Where we have RCD (Or RCBO in this case) protected circuits, this is essentially the reason the Generator needs to be earthed as a referance point for any leakage current that flows.. 

    I assume that as we have Bonded structural steel within the building back the the MET this would also act as the same thing essentially? We also have a Lightning protection system in place

    With regards to the use of the generator, I'll ask the question back to the client Who provided it in the first place. The likely hood being is it's not going to be loaded anywhre near it's maximum & there was talk of it requiring regular monthly servicing - especially if it's running at a fairly low % & there was also talk of load banking at regulat intervals if that was the case. 

    Thanks again for all your help so far. 

    Tim

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