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Discuss MEM 3-phase board in the Commercial Electrical Talk area at ElectrciansForums.co.uk.

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  1. hightower
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    hightower Guest

    Morning all,

    As the regulars know, I work in a school part-time (while sparking part-time), and I'm going to do a bit work for them - they need power dropping in for 11 PCs. No problem, 2 RFCs should do to share the leakage currents of the machines, bit of dado trunking round the room, RFC covers the additional earth return path needed, job's a good 'un.

    What's more, there's a DB in the room too so the run is an absolute doddle. But where I need help is with finding a suitable RCBO for the board. It's a MEM 3-phase board, (see attachment), and the following ways are spare:

    Y1 (but as you can see, the dual module of B1 sits in this space)
    Y5 (but again, the dual module of R5 sits in this space)
    R6
    Y6 (protective device installed, but not in use)
    B6

    There-in lies my problem. If I'm to use these BS3871 devices I need four spare ways, and I only have three. So I've got two possible solutions:

    1) Somebody suggests a single module RCBO that I can use in this board (opening the board up, there's not a lot of head space between the breakers and the enclosure, so really not sure this is a solution.

    2) B5 is only feeding a solitary CCTV camera, it's wired in 2.5mm and on a 6A. I could drop this cable in with say, B4 - Alarm Spur in Corridor, or any number of 6A breakers feeding lights.

    So I guess option 1 would be my preference, but does anybody see a problem with option 2 should option 1 not be feasible?

    Also, a sub-question, just out of interest. Every lighting circuit from this board has 2.5mm down as the cable size - I'm guessing they've done this to account for voltage drop on longer runs. Question is, why have they wired the CCTV in 2.5mm too - it's literally 3m from the board.

    View attachment 35354
     
    Last edited by a moderator: Jan 27, 2017
  2. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    Location:
    cheshire/staffordshire
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    Telectrix
    option 2. shift the CCTV over to another way.
     
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  3. hightower
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    hightower Guest

    Thanks Tel. I'm thinking a Type B breaker too would be fine instead of the Type C that is sitting spare. I know the PCs will have a higher startup current but there's only 6 per circuit and I can't see them knocking a Type B off.
     
  4. hightower
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    hightower Guest

    After searching, it seems you can't get Type B of these....
     
  5. gizmo spartan
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    gizmo spartan EF Member

    Have you read up on bs3871 breakers?

    Not sure I would want to use them.
     
  6. hightower
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    hightower Guest

    I'm not sure what you're referring to with them - you got any material I can read through?
     
  7. gizmo spartan
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    gizmo spartan EF Member

  8. hightower
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    hightower Guest

    Well I understand they are obsolete but I thought you were referring to something dangerous about them.... just because they are discontinued doesn't mean a safe, usable circuit cannot be designed with them.

    I'm not going to upgrade a whole DB just to stick a few more sockets in a room.
     
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  9. gizmo spartan
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    gizmo spartan EF Member

  10. Rosco
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    Rosco Electrician's Arms

    Location:
    Maidstone
    I've been told before not to put rcbos on sockets if they are designated for computer circuits only due to the possible high earth leakage
     
  11. hightower
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    hightower Guest

    School stipulates it.
     
  12. Rosco
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    Rosco Electrician's Arms

    Location:
    Maidstone
    Fair enough, I'm only going by previous when doing office fit outs
     
  13. hightower
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    hightower Guest

    The values for maximum Zs aren't listed in the current regulations, but that doesn't mean BS 7671 frowns upon their use. BS 7671 basically states that an overcurrent protective device or an RCD may be used for fault protection under ADS (which this BS 3871 would tick the box for) and that the characteristics of the protective devices and the circuit impedances shall fulfil the following requirement: Zs x Ia <= Uo

    Zs = 0.7ohms for the sake of argument. My design tells me it would be far lower though.
    Ia = 224A if I've read the table correctly

    0.7 x 224 = 157 which is lower than our Uo of 230v. Looks compliant to me, unless I'm missing a regulation that specifically bans their use?

    EDIT: Sorry, working off a slightly older regs. I know BYB introduces Cmin too, but even with this factor applied it will be compliant.
     
  14. Vortigern
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    Vortigern Electrician's Arms

    Location:
    England
    Business Name:
    F.H. Electrical
    Cant you take another sub board from the main board?
     
  15. hightower
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    hightower Guest

    I could probably take a sub board from this board, but we'd still have the same debate about reusing BS 3871s to protect the sub board circuit. I can't take another sub board from the main main board if that's what you mean.

    It's not an issue to remove that 6A circuit and then I have 4 spare ways - which is what I need. In fact, the CCTV camera is supplies is dated and it'll be on a todo list somewhere anyway. When it does get replaced it will be with a POE IP camera, so moving it off temporarily isn't a major problem.

    Was just hoping members could recommend a replacement breaker that fits, but I've done a fair bit of research today and it seems we're now up to MEM3 boards which offer no backwards compatibility.

    Has anyone got any of these devices that I could buy off you?
     
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