Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about prospective short circuit (PSC) values of protective devices, is answered by SELECT:
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| The IEE's 'Electrical Installation Design Guide'. |
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Question: With a three-phase TT supply, how do we define and calculate the prospective short circuit ( PSC) values of protective devices at the design stage. Also, how do we calculate the size of the main earth?
Answer: The calculation of the PSC for three-phase fault is quite complicated, but - as a rule of thumb - it is generally 2 x single phase PSC. You could either enquire via the distributor, calculate or measure.
For the size of the main earth, you can either select via Table 54.7 in BS7671: 2008, or use the adiabatic equation.
For reference, you may want to refer to the latest Electrician's Guide issued by the IEE (www.theiet.org) entitled: 'Electrical Installation Design Guide'. This is readily available and costs about £20.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link: www.voltimum.co.uk/consult.php?universe=consult.index.questions
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