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

Former Community Member
Former Community Member
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
    If it was a curve of disconnection times versus voltage in the regs and not a table no one would care, as 230V and 240v would have times very similar.

    But it is not really a sudden step in risk between 230v and 231, more of a gentle rise in risk from quite safe at about 50V to really very bad for you by perhaps 1500V , from which 2 handy values have been converted into safe times for the table.


    The origin of the 0.2 secs or 0.4 secs  is all about shock currents, and the time it takes to destabilize a human heart. For shocks that last longer than about half a heart beat, a lower current is dangerous, compared to shorter duration events, where we can stand quite a lot more peak current. (we still swear a lot though...)

    The assumption on a TN x system is that on fault the appliance will be mid rail between L and E, and the voltages will drop equally along the wiring there and back.

    So a barefoot victim gets exposed to about 120V, and maybe we can have an argument about reduced CPC in twin and earth.

    For a TT system we assume most of the volt drop is in the earth path, not the live, so the voltage from faulty kit to earth is more like 200V plus, so the maximum safe exposure time is reduced.


    For both cases the safe time has to be reduced for 400V, and again by 690, and again by 1200.

    Hope this helps.

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  • 0
    If it was a curve of disconnection times versus voltage in the regs and not a table no one would care, as 230V and 240v would have times very similar.

    But it is not really a sudden step in risk between 230v and 231, more of a gentle rise in risk from quite safe at about 50V to really very bad for you by perhaps 1500V , from which 2 handy values have been converted into safe times for the table.


    The origin of the 0.2 secs or 0.4 secs  is all about shock currents, and the time it takes to destabilize a human heart. For shocks that last longer than about half a heart beat, a lower current is dangerous, compared to shorter duration events, where we can stand quite a lot more peak current. (we still swear a lot though...)

    The assumption on a TN x system is that on fault the appliance will be mid rail between L and E, and the voltages will drop equally along the wiring there and back.

    So a barefoot victim gets exposed to about 120V, and maybe we can have an argument about reduced CPC in twin and earth.

    For a TT system we assume most of the volt drop is in the earth path, not the live, so the voltage from faulty kit to earth is more like 200V plus, so the maximum safe exposure time is reduced.


    For both cases the safe time has to be reduced for 400V, and again by 690, and again by 1200.

    Hope this helps.

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