The French way of doing this side-steps the isolation problem quite neatly I think. What the metering arrangement does there is to supply a pair of volt-free contacts (rather than a switched supply). The electrician then uses a low rated MCB in the DB to power whatever contactors they want, switched by the meter's volt-free contacts. Turning off the main switch then automatically isolates everything as everyone would expect. They then repeat the same scheme when there are two or more tiers of DBs - each DB provides a pair of volt-free contacts to each DB it supplies - so again each local simple incomer isolates everything within that enclosure.


So you could emulate that approach by using the off-peak switched supply to operate just a contactor (in its own little enclosure say) - and use the contacts of that to control the DB mounted contactors which are powered from an MCB in the same DB.


Otherwise, you could do something equivalent to the traditional "dual-tariff CU" - i.e. two separate main switches in the same enclosure and lots of labelling!


   - Andy.

Thanks for the reply. The idea of creating volt free contacts from the switched supply will probably be the neatest option. As said, I had thought of running the switched control feed through a auxiliary contact bolted on the side of the DB main switch, but if the property three phase main switch is operated the switched control feed would still be live as taken before the main switch and strictly speaking the switched feed would be using the neutral from another circuit.

I had thought of running the switched control feed through a auxiliary contact bolted on the side of the DB main switch

I have seen something similar to that done (an aux on an MCB to isolate a "pilot" wire that was run alongside a heating circuit) - but I would have reservations about doing that under UK regs. Generally I'd expect an isolator to reliably isolate all poles - e.g. not be possible to open it fully if one of the contacts were welded - as well as having sufficient clearances (e.g. 3 or 4mm) - I'm not sure if a clip-on auxiliary would achieve that.

   - Andy.

If you don’t need to switch the neutral on the off peak main switch, could you use a four pole main switch with the time switch feed running through the fourth pole rather than the neutral to supply a three pole contactor all upfront of the off-peak distribution board?


I presume metering services only have single phase contactors, but there must be a lot of three phase off-peak installations.


Andy Betteridge

Thanks for the replies. Point taken about the aux contact providing adequate isolation. Equally if I have a contactor box providing the potential free contacts I'll effectively have three supplies in the same enclosure.

I was trying to recreate the original set up in as much originaly each three phase supply was protected by a 30A switched fuse, so I was using a four pole MCB as the DB main switch and switching the neutral, but I could run the control feed through the fourth contact and run the neutral directly to the neutral bar.

The company providing the replacement meter were only able to provide a single pole contactor.