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A BS 88-3 fuse protects a 20A radial socket outlet circuit.the circuit is wired using 2.5mm single core cables installed in a 16m length of PVC conduit. A separate protective conductor consisting of a 1.0mm PVC cable is used. Assuming that no correction factors are aplicable and that the value of Ze is given as 0.5 ohms, determine whether the circuit complies with IET Regulation for shock protection? The nominal voltage (U0)may be taken as 230 volts

2 Answers from MyBuilder Electricians

Best Answer

Firstly you need to look up the maximum value of Ze for a 20amp BS88 fuse. From memory is should be above 0.50 ohms, but I don't have the time to look it up in BS7671:2008 at the moment. You need to consider the adiabetic equation as the CPC is only 1.0mm so after checking the cable size is ok and on the basis that the Ze listed is higher than 0.5 Ohms, you can then check the current time curves to see whether the circuit will disconnect in less than 0.4 seconds.


Answered 9th Jan 2015

Cable ID / REF number = , Cable type = Standard cables, Phase size = 2.5 mm², Earth size = 1 mm², Cable length = 16 mtrs , MAX Disconnection time = 0.4 Sec, 80% of Zs = 1.416, Calculated Zs = 0.990 - Protective Device WILL operate in the required disconnection time


Answered 9th Jan 2015

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