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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.
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  • 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)
Reply
  • 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)
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