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permitted tolerances for 230V supply and automatic disconnection time

Hi All,


BS7671 says the permitted tolerance for low voltage  (230V)  supply is +10%/-6%. 


And table 41.1 for max disconnection time states that for a TN system,  if the system supply is less than or equal to 230V ac,  the disconnection time is 0.4s and if the voltage is greater than 230V ac, the disconnection time is 0.2s


Do the above two concepts contradict? how could one account for an increase in voltage for a short time at the source as two different disconnection times has to be considered?  Should it be based on the worst case of 0.2s?


Many Thanks for your response.
  • It is accepted practice that the disconnection times are set for the nominal supply voltage - so a  nominal 230V supply, even if it happens to be more like 253 volts on the day of the test, only needs ADS to give a disconnection in

    0.4 seconds on a TN system

    (because the likely shock voltage is a potential midway between L and PE as the L path and CPC path are assumed equal resistance), and

    0.2 seconds on TT,

    (where it is assumed that the exposed metal will be pulled up to nearly the full supply voltage, due to a much higher resistance earth path than a live path) 


    These times are all set by the fraction of a heartbeat that can be interrupted by various levels of current without starting a cardiac fibrillation. It hides some very large approximations about body contact resistance (i.e. how much current actually flows into the body for a given voltage, which is among other things contact area and sweat dependent) and how sensitive different sized people with varying state of health actually are. As an aside, there is some evidence to suggest winter, with less sweaty skin, and more clothing, is a safer time of year for shocks in the UK.


    In reality of course the ADS time should be a smooth function of shock voltage, not stepped like this , so for a 10% increase in voltage you may expect a 10% reduction in maximum exposure time.  However, that would be too simple.... so if you design a nominal 250V system, then you will need to use the same figures as for 400V.

  • There are two answers to your question.

    First a disconnection time of 0.2 seconds will satisfy both above 230V and below 230V since the table is the max disconnection times.

    However standards are always written considering the system nominal voltage and when the voltage is quoted it should be allowing for the permitted variations (+10%/-10% for ac, +10%/-6% for dc). It can't be expected that the system will have a voltage monitoring device to adjust the disconnection time when the voltage goes into positive tolerance.

    (Edit: I was interrupted while writing the above - If I had seen Mike had responded I wouldn't have bothered.....his was much more detailed)