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bethelec
can someone please explain why you multiply the answer by 2 on a 3 phase supply and do you record the highest or lowest reading and why. thanks
Discuss pfc in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net
can someone please explain why you multiply the answer by 2 on a 3 phase supply and do you record the highest or lowest reading and why. thanks
With 3 phase, the PFC will always be the PSCC.
Following on from Dillb, when you take a measurement of PFC on a loop impedance tester, the instrument is simply utilising the loop test and then applying ohms law to display the result in Amperes instead.
So what is the difference with a test between two phases? Well the resistance of the loop under test is typically the same, however the voltage that would occur in a fault between two lines will be 400v instead of 230v. So instead of applying the 230v in the ohms law equation we should use 400v.
But as our tester was already using the 230v source we have to convert it from 230v to 400v. As Dillb stated, this is more accuratley done by multiplying the value by the square root of 3.
However, as the purpose of the fault current measurement is for verification of selected protective devices breaking capacities, simply doubling the measurement as a rule of thumb provides a slight over estimation - which results in 'over compliance' consideration.
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