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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

    I would prefer manual changeover between the generator and the restricted capacity mains supply

    Each morning start generator, observe correct operation. Then move the manual changeover switch from "external mains supply" to "generator.

    Each evening switch from generator power to external mains supply, then stop the generator.

    Connect to the changeover switch a small distribution board with say 6 circuits each of 16 amps.

    One circuit for fire alarm, one circuit for intruder alarm, and four lighting circuits. each to supply perhaps a few dozen self contained maintained emergency lights.

    Alternatively you might be able to run limited essential lighting 24/7 from the external building. 500 cheap bulkhead fittings each containing a 6 watt LED bulb is only about 3 kw or maybe 25 amps in total allowing for poor power factor. It could reasonably be argued that no other emergency lighting is required with two completely independent supplies. This has the merit of extreme simplicity. 

    A small manual changeover switch or automatic changeover relay would still be needed for the alarms. 

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  • 0

    I would prefer manual changeover between the generator and the restricted capacity mains supply

    Each morning start generator, observe correct operation. Then move the manual changeover switch from "external mains supply" to "generator.

    Each evening switch from generator power to external mains supply, then stop the generator.

    Connect to the changeover switch a small distribution board with say 6 circuits each of 16 amps.

    One circuit for fire alarm, one circuit for intruder alarm, and four lighting circuits. each to supply perhaps a few dozen self contained maintained emergency lights.

    Alternatively you might be able to run limited essential lighting 24/7 from the external building. 500 cheap bulkhead fittings each containing a 6 watt LED bulb is only about 3 kw or maybe 25 amps in total allowing for poor power factor. It could reasonably be argued that no other emergency lighting is required with two completely independent supplies. This has the merit of extreme simplicity. 

    A small manual changeover switch or automatic changeover relay would still be needed for the alarms. 

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