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Flakey 16mm sub main

Grumpy
It's a 70's property, TT, extended in 2011. They obviously took one look at the ancient Wylex fuse board and decided to split the tails in a Henley and run a 16mm t&e sub main into a new split board supplying the extension.

The water and gas bonds 10mm, yet to be verified, go into the extension board and I've been asked to replace the original board. British gas have installed two new rods (600mm apart?!).

So, the way in which the sub main is installed is non compliant, exposed cores etc, and in my opinion the tails should be split into a switch fuse and the 16mm run from there. I'm thinking that RCD protection can be omitted as the 16mm runs in stud walls and under the floor.

This is an ugly job and I don't like it but I don't want my prejudices to incur additional cost for the client. So am I right in thinking that the 6mm cpc in the t&e is sufficient for the earthing and bonding for the replacement board (as it's TT) and can I just replace the old Wylex board and leave the flakey sub main as is?

Furthermore, if the 16mm t&e did run <50mm and required RCD then how would that work? Is the answer don't run t&e in those circumstances?

  • 0
    No. As it is TT, there must be an RCD before the T&E submain. In order to provide discrimination, your best option is to fit a time delay “S” type RCD before the tails. If you like, there is nothing to stop you using that as a main switch, although you may choose to have the board you are replacing connected before the RCD. 


    Regards,


    Alan.
  • 0
    My inclination would be to put a 100 mA S type RCD in a REC2 enclosure in the consumer's tails, which covers everything.


    Had it been a TN earthing system, the sub main should have been protected by a switch fuse.


    For a TT system, a 6 mm earthing conductor, or CPC downstream of it, will be more than enough. Equally, unless there is an exceptionally good contact with terra firma through the gas or water pipes, bonding need be no bigger.
  • 0
    For many years I would have told you to install an upfront 100 mA S-Type Time Delayed RCD.


    However we are now having to change our specs from Type AC RCDs to Type A RCDs and I doubt you will get a Type A 100 mA S-Type Time Delayed RCD.


    So I will suggest that an upfront Type A 300 mA RCD is a better choice, as they are more readily available.


    Andy Betteridge
  • Former Community Member
    0 Former Community Member
    If you are fitting a new board couldn’t you just take the 16mm into the new board and run off a 45/50amp circuit ?


  • 0

    Sparkingchip:

    For many years I would have told you to install an upfront 100 mA S-Type Time Delayed RCD.


    However we are now having to change our specs from Type AC RCDs to Type A RCDs and I doubt you will get a Type A 100 mA S-Type Time Delayed RCD.


    So I will suggest that an upfront Type A 300 mA RCD is a better choice, as they are more readily available.




    Is BS 7671 telling us that? ?

  • 0
    It is telling us RCD type A is better than type AC for circuits with non transformer isolated electronics -  nowadays that even includes mains fired LED lights, so indirectly, yes.


    And, do I understand this 16mm submain is only protected by the DNO fuse ? Apart from no RCD, that is safe, but the DNO will not like it being more than 3m long and relying only on their fuse.  In a perfect world, both earth leakage and overload ADS would be at or near the origin.


    As an aside, to join meter tail to T&E neatly in the past, I have used an adaptable box  to form an enclosure for  line taps  Nowadays materials being cheaper,  cost less of an issue I might use a DIN box and rail terminals.


    But you are replacing the old hot wire board, give the sub-main a 50A RCBO in that and satisfy the single point of isolation requirement at the same time.
  • 0

    My inclination would be to put a 100 mA S type RCD in a REC2 enclosure in the consumer's tails, which covers everything.


    Had it been a TN earthing system, the sub main should have been protected by a switch fuse.





    To be clear - the 16mm² submain will need RCD protection to provide ADS (as it's TT) - and - overcurrent protection (e.g. by means of a switchfuse).


      - Andy.
  • 0

    However we are now having to change our specs from Type AC RCDs to Type A RCDs and I doubt you will get a Type A 100 mA S-Type Time Delayed RCD.




    You may be interested to know that such as thing as an 'AS' appears in the Dopeke listings.

    so 100mA time delay, type A with 100A contacts is DFS2 100-2/0.10-AS



    and there are many others in the table.

    here - 'time delay type A'      however, I have not seen such a thing in the wild yet so it may still only be 'vapourware' but at least the wheels are turning,

    lets hope the other makers follow suit.


  • 0

    mapj1:

    But you are replacing the old hot wire board, give the sub-main a 50A RCBO in that and satisfy the single point of isolation requirement at the same time.




    A double pole 100 mA S-Type RCD main switch in the first consumer unit with the distribution circuit to the second consumer unit on a MCB, then  the second consumer unit can be split with two double pole 30 mA RCDs or have DP RCBOs, as can the first consumer unit for the final circuits.


    I would consider that to be sensible, but the RCD main switch may have to come out of the consumer unit and go into the tails upfront of the CU, because devices may have to be mixed and matched from different manufacturers if you use Type A RCDs.


    I have some Schneider 300 mA RCDs in stock with one on the van plus one of another make, though I cannot remember which make off hand, but it has a matching 63 amp MCB with it just in case it’s needed. Those be installed alongside other manufacturers devices in the same enclosure on the same busbar as they won’t line up correctly, so would need to go in their own enclosure on their own busbar.


    It is much easier when all the devices are from the same manufacturer and can go into the same enclosure, but that is not how it seems to work at the moment.


    Andy Betteridge 

  • 0
    Thank you all, I'll be sorting it next week.

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