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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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  • 0

    Does it comply with the CT 2 arrangement of Appendix 16? I mean, does it have a 3a L to E? Or is it a single M.O.V?

    It's got two indicator windows, so presumably contains two SPD devices within the one package (which is quite normal).

    Zooming in, the text on the front seems to be headed L-N and N-PE - which would suggest CT2 connections internally (if it were CT1 they'd be L-N and L-PE).

    If that brown wire is single insulated, it does look to be rather close to the bare metal tails support bracket though.

       - Andy.

  • 0 in reply to AJJewsbury

    Interesting to learn what the difference is between CT1 and CT2 SPD's and why it's required, I knew I had to use CT2 SPD's on TT systems but not why.

    Then looking at data for the fusebox unit, expert electrical say TN only, which worried me for a minute as I have installed quite a few on TT systems. The looking at Mastertrade and eventually Fusebox's literature it shows TN and TT system compatible  https://user-5zq6k5a.cld.bz/FuseBox-2021-Brochure/4-5/. From memory I think FB upgraded their SPD's about 12 months ago, maybe Expertelectrical haven't updated their website

    As others have said the wires still represent a risk, but what do you do if the SPD operating is going to trigger an RCD. Is it acceptable for a voltage surge to cause the entire installation to be shut down? Personally having installed 20 or 30 so far I have found that as the cable is very flexible it tends to just move away from anything that may damage it, unlike 25mm tails which are always a fight. The cable is unlikely to snap due to poor stripping as it's already prepared at the correct length and strands welded together at the end. This leaves coming loose, which has a small chance of happening.

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  • 0 in reply to AJJewsbury

    Interesting to learn what the difference is between CT1 and CT2 SPD's and why it's required, I knew I had to use CT2 SPD's on TT systems but not why.

    Then looking at data for the fusebox unit, expert electrical say TN only, which worried me for a minute as I have installed quite a few on TT systems. The looking at Mastertrade and eventually Fusebox's literature it shows TN and TT system compatible  https://user-5zq6k5a.cld.bz/FuseBox-2021-Brochure/4-5/. From memory I think FB upgraded their SPD's about 12 months ago, maybe Expertelectrical haven't updated their website

    As others have said the wires still represent a risk, but what do you do if the SPD operating is going to trigger an RCD. Is it acceptable for a voltage surge to cause the entire installation to be shut down? Personally having installed 20 or 30 so far I have found that as the cable is very flexible it tends to just move away from anything that may damage it, unlike 25mm tails which are always a fight. The cable is unlikely to snap due to poor stripping as it's already prepared at the correct length and strands welded together at the end. This leaves coming loose, which has a small chance of happening.

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