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

Parents
  • 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.

    Mike

Reply
  • 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.

    Mike

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