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Name the breaker. (Dorman Smith BS3871 Type D).

Sparkingchip

I an trying to determine the maximum Zs for the spare 16 amp MCB with a 5 second disconnect time. 

The maximum Zs of the SWA distribution circuit will be less than 1.4 ohms using a third core as the CPC not including the armour, so it will be lower.

Edit- taking Ze as 0.8 ohms TNS.

professional-electrician.com/.../

  • 0

    Presumably all the information I need is on the side of the MCB.

  • 0

    Need to see more but could be Doorman Smith.

    JP

  • 0 in reply to Sparkingchip

    If not you could go at it the other way - it will be a type 1 or 2 in domestic, so a lot better than a C type, and comparable to a B type.
    Taking the 1,4 ohms figure, I get PSSC to earth of 230/1.4 or 160 Amps - ten times the breaker rating of 16A. You are in luck , and in the 'instant trip' part of the curve for a type 3 as well. ;-)  Or you could  worry about Cmin, and hot cables, and only be really sure about types  1,2,

    The older (10,15 30 series not 10 16 32) of type 1s. As hosted on TLC website.

    And same idea for type 3 where  160 amps is right on the line between instant and thermal. I'd expect that maybe in a more industrial setting.

    Mike.

  • 0 in reply to John Peckham

    Definitely Dorman Smith Loadlimiter Board & MCBs. I'm not sure which age/model though.

    regards, burn

  • 0

    it will be a type 1 or 2 in domestic

    Probably should be type 1 or 2, but you do occasionally come across odd things, like type 4s.

      - Andy.

  • 0

    If so then a quick look with a dental mirror at the side of the breaker my be needed - but I bet it is not the end one you want is it...

    M..

  • 0 in reply to burn

    do you mean loadmaster ? Load limiter were Din rail and bus bar I thought ? EDIT and had the rating on the front...

  • 0

    https://www.ebay.co.uk/itm/202819824817?hash=item2f3900ecb1:g:cj4AAOSwZQ9dyRWv

    or,

    www.ebay.co.uk/.../255009847313

    Z.

  • 0

    It's in a flat above commercial premises and I guess the same electricians wired the whole building,  note the lights are on a 16 amp MCB which is not to be expected in a flat.

    I need to add a 30 mA RCD, but I want to do it at the other end of the circuit in a garage consumer unit where I can ensure it is accessible to a wheelchair user, not high up in a wall in a cupboard. The highest rated MCB in the CU it will supply will also be 16 amps, so I'm not completely relying on this old device. 

  • 0

      @Burn

    JP and Burn are correct it is a Dorman Smith consumer unit. I tried reading the side of the MCB with my new Boddington 1000v insulated inspection mirror and taking photos with my phone,  but stopped short of removing a MCB, it's not a consumer unit to get over involved with and as Ze is 0.20 ohms and Zs is 0.59 ohms there isn't a problem. 

    Could it be a Dorman Smith BS3871 D16 MCB, was there such a thing made, as it looks like a Type D to me?

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