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66kVA Generator Earth

Former Community Member
Former Community Member
I am involved in a project requiring a 66kVA generator as a standby supply to to a control panel. I've read quite a few posts about earthing, but came across what seemed to be a contradiction regarding the earth electrofde resistance in Topic Title: life safety generator earthing.


I realise that a separate earth electrode is required, but what should be the resisstance of the earth rod. In the post it reads "The earth rod resistance needs to be low enough to operate your ADS promptly for an LE fault, even if the DNO earth is absent", but in another it says "Provided the generator supplies a TN-S system (as would be usual), the rod won't be involved in passing earth fault currents, it merely references the N to earth, so it's resistance to earth isn't at all critial. From memory BS 7430 recommends < 20 Ohms".


Can anyone clarity?




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  • Former Community Member
    0 Former Community Member
    There is no anomaly here, you just need to think through what we are trying to do.

    This is presumably LV, so for a public supply, in the UK the star point must be solidly earthed. For a standby supply, you may lose the DNO earth reference if the neutral is opened, and if you cannot provide an equivalent point, your protection scheme may no longer work as expected. Even if you don't lose the neutral connection altogether, an increase in the fault path or the earth loop resistance will reduce the fault current, increasing the trip times.   On the other hand, if you are within a building, any fault current is likely to have a metallic return to the star point, so the fault current does not need to flow through the actual earth, ie the soil outside, and a figure of <20 ohms is adequate to maintain the neutral potential at a low level relative to any other random bits of structure.

    Hope this helps
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  • Former Community Member
    0 Former Community Member
    There is no anomaly here, you just need to think through what we are trying to do.

    This is presumably LV, so for a public supply, in the UK the star point must be solidly earthed. For a standby supply, you may lose the DNO earth reference if the neutral is opened, and if you cannot provide an equivalent point, your protection scheme may no longer work as expected. Even if you don't lose the neutral connection altogether, an increase in the fault path or the earth loop resistance will reduce the fault current, increasing the trip times.   On the other hand, if you are within a building, any fault current is likely to have a metallic return to the star point, so the fault current does not need to flow through the actual earth, ie the soil outside, and a figure of <20 ohms is adequate to maintain the neutral potential at a low level relative to any other random bits of structure.

    Hope this helps
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