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.



  • Many large generators have two different ratings, prime power and standby power. They are not usually different machines but simply different ratings applied to the same machine.

    The higher standby power rating is generally for a few hours at a time and also for a limited number of hours a year. This rating presumes that utility power or similar  is the main source, with the generator being reserved for breakdowns etc.

    The lower prime power rating is generally applicable when the generator is used as the main power source, for example in places without utility service. The prime power rating is for continuous use, but NOT for continuous use at the full load. It is presumed that the load will vary between 100% load and 50% load. This reflects likely use to power a home, hospital, hotel or the like which is most unlikely to need full load continually.

    If a generator IS required to run continually at full power, then a third and lower rating is applicable, known as "base load" this presumes almost continual use, often for 8,000 hours a year. These sets are sometimes of a slightly different design, with for example duplicated oil and air filters, in order that these may be replaced without stopping the engine. 

    In the situation described, the prime power rating is likely to be correct. This does not preclude latter use of the same machine for standby purposes.

    As has already been said, consider the logistics of fuel supply. Don't worry about fuel costs, it may well be cheaper than mains power at present.

    Presume a load of say 200 kw. At present non capped prices this will cost at least 60 pence a unit or £120 an hour.

    A modern diesel generator should produce about 4 kwh per litre of fuel burnt.  200 kw will therefore be about 50 litres an hour of fuel. That will cost about £100 an hour. 

    Another factor to consider is emergency lighting. Presuming that the engine is to be shut down at night, then standard self contained emergency lights wont be much use as the batteries will be fully discharged each night. Options include.

    A central battery system, with a manual "all lights off" control to be operated when the engine is stopped each evening.

    Standard self contained fittings with a 24/7 power supply. Either a limited mains supply from neighbouring premises, or a second much smaller generator to be run overnight.

    Or if two supplies are available, you might be able to argue that no other emergency lighting is required.

    Modified self contained fittings with a "remote off" facility.

  • And in the same vein.... Intruder & Fire alarm systems....

    Although both will have battery backup, and will run through the night without issue, they might not like being discharged EVERY night for a few months.

    What about weekends? The fire alarm will have enough battery capacity for 24 hr ride through but what about the 60 hrs from, say, 6pm Friday to 6am Monday?

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