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Fault Current Sufficient to Operate a Protective Device.

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I was undertaking some work today in a chalet park. I was testing R.C.D.s. These are supplied in each holiday chalet at the chalet meter position before any consumer unit. The chalet owners are responsible for any internal electrics after the park's R.C.D.


I wondered why R.C.D.s were first installed by the park owners. After all an R.C.D. or R.C.B.O.s could be incorporated in the consumer units. The supplies to the chalets is TT. O.K. so we do need an R.C.D.


But why TT the permanent wooden chalets? This is not a caravan or camp site at all. And the chalets are not transportable. They are not residential park homes according to the B.S. 7671 definition.


Then it dawned on me. At the last chalet, the remotest from the main intake room the P.S.C.C. is only 400 Amps. It is protected by a B50 type M.C.B. So, under fault conditions the fault may take a long time to clear. Evidently a TT system is safer.


So, what effect would a heavily loaded distribution cable have on the operation of the B50 M.C.B. if the park was fully occupied and the supply Voltage dropped? Could it ever become dangerous and prevent a B50 M.C.B. from not opening, or opening too slowly,  at the remotest chalet say due to a short from L to N?


Z.






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    I see little if any danger.

    If the cable is into the chalet is rated for at least 50 amps, and is protected by a 50 amp MCB then the cable should be safe against thermal damage.

    The cable does not "know" the difference between a short circuit current of say 300 amps on a high impedance supply, and a gross overload to 300 amps on a low impedance supply. In either case, the MCB should protect the cable against thermal damage.


    Earth faults are more dangerous since a person could be exposed to about half line volts for a dangerous time whilst waiting for the MCB to trip. In this case that is handled by an RCD.


    I would however consider this installation to be almost certainly unsatisfactory as regards voltage drop.

    A short circuit current of 400 amps implies a 10% voltage drop at a load current of 40 amps, or 12.5% at the full load of 50 amps.

    Volts from DNO could well be 220 volts actual.

    Voltage at intake to chalet could be about 195 volts.

    Voltage at point of use could be below 190 volts, almost certainly too low.
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  • 0
    I see little if any danger.

    If the cable is into the chalet is rated for at least 50 amps, and is protected by a 50 amp MCB then the cable should be safe against thermal damage.

    The cable does not "know" the difference between a short circuit current of say 300 amps on a high impedance supply, and a gross overload to 300 amps on a low impedance supply. In either case, the MCB should protect the cable against thermal damage.


    Earth faults are more dangerous since a person could be exposed to about half line volts for a dangerous time whilst waiting for the MCB to trip. In this case that is handled by an RCD.


    I would however consider this installation to be almost certainly unsatisfactory as regards voltage drop.

    A short circuit current of 400 amps implies a 10% voltage drop at a load current of 40 amps, or 12.5% at the full load of 50 amps.

    Volts from DNO could well be 220 volts actual.

    Voltage at intake to chalet could be about 195 volts.

    Voltage at point of use could be below 190 volts, almost certainly too low.
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