Generator EFLI values - Wiring and the Regulations BS 7671 - IET EngX - IET EngX
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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.



  • Depends what you and they  mean by 'earth' resistance !

    ~You are correct that in a TN-S system, and this will be - copper all the way back via the CPCs  to the star point,  the loop impedance will not rely on the connection to terra-firma proper.  The only time that current path is needed is for faults to things that are not bonded to the CPC - tripping RCDs when some one picks up the severed lawnmower flex etc.

    1 ohm sounds very good for an electrode resistance  If they do mean the resistance between the gensets electrodes to the plate at the end of the universe, then I  presume it is a buried mesh/ mat or an array of many electrodes - and that the soil is either clay or salt marsh.

    Do be aware that the Zs or output impedance is a funny thing with a genset, as low current test results from a test meter scaled up and real short circuits do not match so well as they do on the wider 'mains' network - and what happens rather depends on the loading at the instant. The genset will have some sort of regulator that alters the currents in the exiting windings to maintain a constant voltage as the load varies, and more or less fuel is also thrown in  but the  response time of all this is very slow compared to a suddenly appearing  fault. The practical effect is that the PSSC may be  lower than a test meter indicates. 

    Even so a 275kva genset (what is that - 350A load current per phase or so ? ) should be perfectly up to blowing fuses and tripping trips at the 60-100A level without too much of a splutter, perhaps giving a kA for the cycle or so it takes to do so. If you have anything larger than this that needs to 'instant trip' for safety of life reasons, you may need an earth fault relay or equivalent. Normal MCBs for lights and heaters should be OK.


  • A couple of things to think through.....

    Generators come in two flavours - 'prime' & 'standby' and as such have different ratings. A 'prime' set is capable of delivering its rated output full time. A 'standby' set is not - it will be rated for a number of hours. I suspect your set may be a standby rated one and will not be capable of delivering the power you expect full-time.

    Is this is intended to run 24/7 until the mains is connected ? What about servicing? It will need to be to be serviced approx once a month.

    Fuel will be a headache. If this generator is running at anything close to full load its going to be drinking close to 1 litre/min. ie. around 400 litres for a 8 hours day. Considering that red diesel cant be used to run gensets anymore & with the current cost of white diesel - somebody needs to be thinking in terms of £500+ per day for fuel......

  • 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?

  • 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
  • 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. 


  • 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. 

  • 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.


  • 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.

    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.