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:
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.
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