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Surge protection on an mcb on a TT

aligarjon

Hi guys, i fitted a replacement CU today on a TT system. I have used a main switch with double pole rcbo's. The pre installed surge device is protected by a 32amp mcb.  It basically has a loop of 6mm inside the unit aprox 5 inches long that loops from the breaker, behind the meter tails into the top of the surge device.   I was a bit concerned because of the circuit breaker, i was tempted to order another rcbo, but after reading through the manufacturers bumf it says its fine on a breaker or fuse on a TT system.  I presume this is because it is part of the assembly ? Or is it because the link is so short and it's pretty much impossible to cause a problem.  A bit like meter tails in the metal box if installed correctly. Any thoughts please.

Cheers  Gary

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    In principle I'd be happy about an SPD before an RCD on a TT system - that way you don't have a delicate RCD exposed to the full surge currents. Everything before the RCD should be protected by double/reinforced insulation however - so I'd expect the wiring between the MCB and SPD to be at least insulated & sheathed. Single insulation where it can touch the inside of steel box where ADS won't function isn't my idea of good.

    Strictly speaking the internals of the CU would conform to its product standard rather than directly to BS 7671's concepts - but I'd suspect they're not so different in practice on this point.

       - Andy.

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  • 0

    In principle I'd be happy about an SPD before an RCD on a TT system - that way you don't have a delicate RCD exposed to the full surge currents. Everything before the RCD should be protected by double/reinforced insulation however - so I'd expect the wiring between the MCB and SPD to be at least insulated & sheathed. Single insulation where it can touch the inside of steel box where ADS won't function isn't my idea of good.

    Strictly speaking the internals of the CU would conform to its product standard rather than directly to BS 7671's concepts - but I'd suspect they're not so different in practice on this point.

       - Andy.

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  • 0 in reply to AJJewsbury

    "In principle I'd be happy about an SPD before an RCD on a TT system - that way you don't have a delicate RCD exposed to the full surge currents."     A.J.                                                                                                               High Voltage surges can originate from within installations and don't always come from outside.

    https://www.youtube.com/watch?v=eq8mKTaBDPY

    Z.

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