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Voltage (Uo) for calculating permissible (Zs) earth loop impedance and disconnect times

Former Community Member over 5 years ago

Guys,

Ignoring BS7671:2018 appendix 3 for the purposes of simplifying this discussion.

Private installation, TNCS earthing system where the main transformers taps are set to give phase/phase voltage (U) 416Vac RMS on the secondary side.

As ye are aware 416V/(√3) = 240Vac RMS line to earth voltage Uo.

I am also being told from an inspector that the permissible disconnection time in table BS7671:2018 41.1 is 0.4 seconds.

However when I look at table 41.1 it is stating that if Uo is 240Vac the permissible disconnection time is only 0.2 seconds.

I am being told by an inspector that we have to use 230Vac when performing permissible earth loop impedance calculations as per BS7671:2018 clause 411.4.4 Zs × Ia ≤ U0 × Cmin.

However if we perform the calculation using 230Vac will will get a reduced permissible Zs. This would seem to suggest to me that we could be failing Zs values that allow enough current to flow in the event of a fault to trip the fault protective device.

Is the inspector wrong ?

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0 gkenyon over 5 years ago

AJJewsbury:

And before that (for as long as I can remember), it was 240V +/- 6% - i.e. 225.6V to 254.4V - i.e. an even higher upper limit - and we still used 0.4s disconnection times in those days.

True the specification was originally intended for public supplies - but the underlying physics (and physiology) of shock protection would be the same whoever owned the supply so I see no logical reason not to adopt the same approach. The definition of voltage, nominal in part 2 of BS 7671 (especially the NOTE) would also seem to suggest that the same fundamental approach can be taken regardless of supply ownership.

- Andy.

It doesn't matter for disconnection times - as per my previous post, you're on the right side of safety.

However, where the actual upper voltage is 264 V, not 254.4 V (the latter would make three shades of no difference over 253 V upper voltage of course), things start to err on the wrong side of caution when calculating fault currents (by as much as 4 %), earth electrode resistances for diverted neutral currents (by as much as 2 %), etc.
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0 gkenyon over 5 years ago

AJJewsbury:

And before that (for as long as I can remember), it was 240V +/- 6% - i.e. 225.6V to 254.4V - i.e. an even higher upper limit - and we still used 0.4s disconnection times in those days.

True the specification was originally intended for public supplies - but the underlying physics (and physiology) of shock protection would be the same whoever owned the supply so I see no logical reason not to adopt the same approach. The definition of voltage, nominal in part 2 of BS 7671 (especially the NOTE) would also seem to suggest that the same fundamental approach can be taken regardless of supply ownership.

- Andy.

It doesn't matter for disconnection times - as per my previous post, you're on the right side of safety.

However, where the actual upper voltage is 264 V, not 254.4 V (the latter would make three shades of no difference over 253 V upper voltage of course), things start to err on the wrong side of caution when calculating fault currents (by as much as 4 %), earth electrode resistances for diverted neutral currents (by as much as 2 %), etc.
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Voltage (Uo) for calculating permissible (Zs) earth loop impedance and disconnect times

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