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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 Former Community Member over 5 years ago

AJJewsbury:

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.

The table is based on nominal rather than actual voltage - 230V nominal is usually taken to be anything within the -6% to +10% range - i.e. between 216.2V and 253V. It's very common for DNOs to have far above 230V at their transformers to compensate for voltage drop in the distribution system - typically a 433/250V. So as long as your supply doesn't go above 253V (during times of low load say) I'd say it's acceptable to use the 230V nominal column (e.g. 0.4s for TN small final circuits).

- Andy.

@AJJewsbury

Thank you for your response. As far as I am aware the voltage tolerances that you are referencing above stem from 1988 when the European electrical standards body CENELEC agreed on harmonization of low voltage

electricity supplies within Europe as further detailed in BS7671:2018 appendix 2. My understanding is that this applies to public electricity supply systems. Does it also apply to private installations such as we are discussing here?
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0 Former Community Member over 5 years ago

AJJewsbury:

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.

The table is based on nominal rather than actual voltage - 230V nominal is usually taken to be anything within the -6% to +10% range - i.e. between 216.2V and 253V. It's very common for DNOs to have far above 230V at their transformers to compensate for voltage drop in the distribution system - typically a 433/250V. So as long as your supply doesn't go above 253V (during times of low load say) I'd say it's acceptable to use the 230V nominal column (e.g. 0.4s for TN small final circuits).

- Andy.

@AJJewsbury

Thank you for your response. As far as I am aware the voltage tolerances that you are referencing above stem from 1988 when the European electrical standards body CENELEC agreed on harmonization of low voltage

electricity supplies within Europe as further detailed in BS7671:2018 appendix 2. My understanding is that this applies to public electricity supply systems. Does it also apply to private installations such as we are discussing here?
Cancel
Vote Up 0 Vote Down

Cancel

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

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