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origin of the installation

Dave Miller

please forgive this daft question, and I accept that I may be reacting to a non-existent problem

if I'm reading the wiring regs correctly, then we can protect the origin of an installation against overcurrent (whether overload or fault) by using the distributor's protective device with their consent. However, I can't see an equivalent provision to protect the origin of an installation against electric shock by similarly using the distributor's protective device for automatic disconnection of supply

I'm thinking of industrial supplies where the meter is in a substation at the edge of the site and the main DB is in a building on the other side of the car park, so the only protection for a couple of hundred yards of incoming cable is whatever the distributor has fitted at the substation

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  • 0

    This rather depends on the supply impedance - what you are really saying is the company fuse can clear a modest L-N fault (over-current may or may not be dead short) in 5 seconds or so but not an fault to earth in 0.4 or 0.2 seconds. On a TT supply this situation is definitely true and an RCD or earth fault relay is needed at the origin. With a TN-s or TNC-s supply it may well be true that a short L- CPC would clear the fuse. However, it needs some measurement/ analysis and may or may not need something on the customer side to protect that sub-main across the car-park.
    Mike

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  • 0

    This rather depends on the supply impedance - what you are really saying is the company fuse can clear a modest L-N fault (over-current may or may not be dead short) in 5 seconds or so but not an fault to earth in 0.4 or 0.2 seconds. On a TT supply this situation is definitely true and an RCD or earth fault relay is needed at the origin. With a TN-s or TNC-s supply it may well be true that a short L- CPC would clear the fuse. However, it needs some measurement/ analysis and may or may not need something on the customer side to protect that sub-main across the car-park.
    Mike

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  • 0 in reply to mapj1

    thanks, Mike

    It's a TNS system from a distributor's breaker, well above 63A, and I think that the cable across the car park would be classed as a distribution circuit. Would that require a disconnection time of 5s for a L-CPC short?

    I'm probably splitting hairs but, assuming that we can get the clearance time down to whatever the right level is, do the wiring regs allow us to use the distributor's protective device to provide protection against electric shock by automatic disconnection of supply? I'm concerned about the lack of a specific provision in the regs, when such provisions are made for protection against overcurrent

    Dave 

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