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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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    mapj1:

    Quite right, PSSC of 400A, assume supply voltage at origin of 240- effective Zs willl be no better than 240V/400A or about 0.6 ohms.

    Now there is no time limit for disconnecting L-N faults, so those are OK, but to fire a B50 smartly on an earth fault could take as much as 250A, and if it is a C50 then 500A.

    An RCD or RCBO removes these concerns, totally.

    I'm with Broadgage, folk will notice the lights dim when the neighbours put the kettle on. If it is a real problem is less clear, appliances vary in how well they behave on low voltage.

    It may be the transformer is private to the site and the taps are set a bit high to allow for this.

    M.


    Hello Mike,

                            there us no site transformer. The supply comes in from overheads. There are five overhead lines on poles. I will take a closer look when I get time. The main earthing terminal is connected by a black 25mm single cable coming up from the ground next to the main underground incoming main supply cable.  I do not know its origin.


    My next investigation will be of the 25mm2 twin underground S.W.A. cable supply to the most remote chalet. It comes from a distant distribution board and is protected by a C63 M.C.B. This is the cable with the P.S.C.C. of 400 Amps at its end. What will an earth fault produce if somebody drives a metal pole through it?


    Note. E.F.L.I. at main intake room = sub 1.0 Ohm. P.S.C.C. = 1.68kA.


    Z.




     


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  • 0
    mapj1:

    Quite right, PSSC of 400A, assume supply voltage at origin of 240- effective Zs willl be no better than 240V/400A or about 0.6 ohms.

    Now there is no time limit for disconnecting L-N faults, so those are OK, but to fire a B50 smartly on an earth fault could take as much as 250A, and if it is a C50 then 500A.

    An RCD or RCBO removes these concerns, totally.

    I'm with Broadgage, folk will notice the lights dim when the neighbours put the kettle on. If it is a real problem is less clear, appliances vary in how well they behave on low voltage.

    It may be the transformer is private to the site and the taps are set a bit high to allow for this.

    M.


    Hello Mike,

                            there us no site transformer. The supply comes in from overheads. There are five overhead lines on poles. I will take a closer look when I get time. The main earthing terminal is connected by a black 25mm single cable coming up from the ground next to the main underground incoming main supply cable.  I do not know its origin.


    My next investigation will be of the 25mm2 twin underground S.W.A. cable supply to the most remote chalet. It comes from a distant distribution board and is protected by a C63 M.C.B. This is the cable with the P.S.C.C. of 400 Amps at its end. What will an earth fault produce if somebody drives a metal pole through it?


    Note. E.F.L.I. at main intake room = sub 1.0 Ohm. P.S.C.C. = 1.68kA.


    Z.




     


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