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Main fuse or use meter switch, or other thoughts

Former Community Member
Former Community Member
Hello, I am wiring a sub main to my detached garage. I am doing this under a Building Notice with a Building Control appointed inspection through an approved contractor company. The approved company provide the inspection and test, but are not there to provide advice.  Hence, would appreciate your thoughts. I have been looking at how I will connect my sub main to the supply. I currently have a DNO box containing the incoming feed, main fuse and a meter. This is within an open to air porch. The tails from the meter pass directly to a second box below with a 100amp Proteus DP breaker. The two boxes are butted up together. The output from the breaker goes into 25mm2 SWA that then progresses to the main house consumer unit elsewhere. My original plan was to take the output from the breaker, split it with Henley blocks, then feed a second breaker feeding the new sub main. However, there simply is not the room within the box to do this. The best solution would be for a larger box beneath the DNO box. To do that requires power off. I have two choices for that, pull the main fuse or use a single pole isolator that is built into the meter (Ampy 5258K). 

I could request the DNO pull the main fuse ( would they do that for someone doing it DIY under a Building Notice?). However, I like to take my time on this sort of thing and do not want a short time slot to get it all done. I do not want to pull the main fuse myself and appreciate that I am not allowed to. That leaves me with the built in isolator? would you do this using that? whats the risk?
  • 0
    Well, as with all isolation before working, check that the power is on before you start and check again that it has actually gone off when you operate the isolator.

    But first make sure you have all the bits to do the job -relying on being able to do  a quick nip to the wholesalers with it all open and bare ended  is not sensible.

    As a practical point, do not do it at any time when if it were to  go wrong help may not be available - in that sense  being the only chap in the house at 4pm on a Sunday is far less useful than doing it at 10AM on a weekday morning - that way if it does not turn off, or turn back on again, you can then 'phone your metering company and hopefully get a visit before the freezer melts.

    This is however the sort of thing the meter isolator is intended for, and liek the company fuse it is single pole. Just as good practice, switch off all heavy loads before isolating, so there is not a large current to interrupt.

    Planned carefully, the risk is almost nil.

  • Former Community Member
    0 Former Community Member
    Thank you. I appreciate your time to respond. I had read some views online that expressed caution over using the meter isolator, however, they failed to articulate why. I will certainly ensure there is no load on the isolator during switching and conduct checks to ensure it has actually isolated. Take your point about timing, have fallen for that before with plumbing. 


    I have a supplementary question, which would be considered best practice, split the tails from the meter, each to a separate breaker or split the output from the 100A breaker?  I was planning on the latter, but with a new box, the former appears to be an easier route (can leave the 25MM2 SWA intact).
  • Former Community Member
    0 Former Community Member
    Apologies, ignore the last point on leaving SWA intact. I will still have to disconnect, but may not have to adjust length.
  • 0
    That meter has a mechanical isolator- some are more like a remote control contactor operated by software from the meter company. There I'd be slghtly  wary of the software clicking it back ion early or late, and would probably put a shorting link on so that if it did come on, we'd know in short order !  (Rather like the earthing of lines being worked overhead - you do not expect the power to be put back on, but if it were  to happen, it would become obvious.)
  • 0
    I am a bit confused by "100amp Proteus DP breaker". If it is an MCB, in what is it housed? Or is it an isolator in the consumer tails?


    I would probably protect the new distribution circuit with a switch fuse.


    A couple of photos would be good. They are easy to post in here.
  • Former Community Member
    0 Former Community Member
    Apologies, my use of descriptor was a bit poor. It is a 100A switch/fuse unit in its own metal case. This is mounted to a board in a further case below the DNO provided box.29e27b55fcc17c992f6319e4b689f667-original-incoming-mains-2.jpg
  • 0
    It would be unusual, but not unknown, to have two switch fuses. If you do this make it very clear which one is isolating what. The regs do prefer a single main switch for the whole installation if possible.  The other is to just switch the SWA from here, and say the indoor isolator is the one on the consumer unit.

    In any case the key is clear labels  - even if it just says 'outbuilding supply' in sharpie pen.
  • 0
    Thank you - that's clearer. I had been wondering how the SWA was connected.


    Much the easiest approach would be to split the consumer tails in the meter cabinet and feed the new tails into another box with a switch fuse (assuming that there is room for it).


    What I have at home is an industrial switch fuse in a second cabinet. SWA goes under the floor to the main CU. The installer had installed a second (32 A) fuse in the box of the switch fuse, which then fed the garage and outbuildings via 4 mm² cable. (I cannot photograph it right now 'cos it isn't convenient to turn everything off.)


    AFAIK, nobody makes a 2-way switch fuse.


    (Incidentally, odd that the installer has chosen to strip back the outer sheath in order to identify the cables!)
  • 0
    You can use the meter switch to turn off the supply but turn the isolator off first. Install a Henley block and install another switch fuse for the new cable. The meter tails need redoing so the inner insulation isn’t showing. Wylex make an DP isolator with twin outlet terminals which would be best to use if you can get hold of one.

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