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18th Edition Help BS7671

RB1967

Good day,

After years of designing subsea oil and gas equipment, I've decided to return to my original electrician trade (self-employed).  I'm carrying out the online training and have found a question with which I need some guidance.  I cannot find in the 18th edition Regs causes of severe temporary overvoltages—any advice gratefully accepted.  

Thanks

Richard

Parents
  • 0

    I'm not sure you are looking in the right place - the regs tells you to plan your design more or less, its not really a fault find aid.
    How  big an over-voltage are you looking at and how long is its duration?

    However predictable (sort of regualr) short duration over-voltages are usually related to the switching of inductive loads like motors, where all the stored energy suddenly gets 'dumped' . Less predictable but often more devastating, are currents induced by nearby (or direct !) lightning strikes. Longer duration over voltages usually relate to network faults or generator faults, where a substation transformer tap changer jams, or the neutral -earth bond fails (and up to 400V appears where 230 was expected), or on gensets problems with the regulator/ speed stabiliser, usually coupled with frequency going walk about as well....

    Mike

Reply
  • 0

    I'm not sure you are looking in the right place - the regs tells you to plan your design more or less, its not really a fault find aid.
    How  big an over-voltage are you looking at and how long is its duration?

    However predictable (sort of regualr) short duration over-voltages are usually related to the switching of inductive loads like motors, where all the stored energy suddenly gets 'dumped' . Less predictable but often more devastating, are currents induced by nearby (or direct !) lightning strikes. Longer duration over voltages usually relate to network faults or generator faults, where a substation transformer tap changer jams, or the neutral -earth bond fails (and up to 400V appears where 230 was expected), or on gensets problems with the regulator/ speed stabiliser, usually coupled with frequency going walk about as well....

    Mike

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