Sensing supply presence when using a generator - ideas needed Featured

Yes, I mentioned the possibility of using a fuse or MCB in that link in an earlier post (a difficulty with branched discussions), but there's still a small risk for the wire from the supply to the protection device (unless it's 25mm2, which would be silly!).

These discussions have been very useful for making me think through what's actually needed. If the supply goes off, I need to: move and connect the gen, start it, make sure any large loads are off, and flip the changeover switch. Despite the relatively complex electrical system here* there's no need to flip all the RCD/RCBOs as I had first thought, only to check the loads and remind SWMBO not to switch anything on without asking first - the gen will stall if anyone forgets, so that's not a problem. I like your idea of having a low power kettle available - it's now on the shopping list! When the supply comes back it's not a good idea to have the reversion happen automagically because it will cause a glitch when I might be doing something critical; I want to control when the glitch occurs so an auto-changeover switch is not really appropriate in a domestic situation (regardless of the safety, or otherwise, of the device).

So, the plan is now: a GEYA 2P manual changeover switch

another kettle alternative is a camping gas cooker

It would not need to be 25mm^2 since you are not expecting it to carry full load long term. All it needs to do is survive long enough to open the circuit breaker in the event of a fault. So you could calculate the minimum required size using the adiabatic equation.

e.g. if your prospective fault current is 1000A, and the protective device can operate in 0.1 sec, and the insulation is PVC then the minimum conductor size would be:

s = sqrt( i^2.t ) / k = sqrt (1000^2 . 0.1)/115 = 2.75mm^2

(you used to commonly see a thin set of tails split off from the main one to feed a time switch in old E7 style supplies with a automatically switched load - they might have 25mm^2 tails, but the split off supply to the switch would be significantly smaller)

It does depend a bit on how fast it can switch. If it can do it in less than 50ms, most loads will not notice.

Remember you will also need a neutral earth bond on the genset to keep some loads happy. Since you are already TT, that makes life easier.

I was thinking of the (miniscule) risk of damage to the section of wire before the MCB, rather than from a fault after the MCB, where the protection device would be the DNO fuse (but I had also forgotten about the adiabatic equation!).

Perhaps, but it's an unknown and I'd prefer to be in the loop.

The house connection is TN-C-S (it's the external sub mains that are TT) but I don't know whether it should be called PME or PMB. We have an overhead 2-wire feed to a pole (no visible traffo), where I presume the N is grounded, and then a short underground connection to the house. I've added an additional earth rod "to be sure to be sure" ;-) Thanks for the reminder about checking N-E strapping on the gen.>

The wire to the MCB could be meter tail sized - looped in and out of the

Indeed... Why I went for a bog standard manual transfer switch. I figured I was going to manually start the generator, possibly fettle the load, and then switch it in. So since I need to manually stop it, there it not much hardship in needing to switch it back manually.

(I could also imagine a case where you leave the gen running for hours without even realising that it switch back to mains some time ago!)

Yup you are free to add more earth connections to the PME earth (i.e. another of the "multiple"), so with adequate RCD protection you can rely on your earth in the event of a loss of supply (where you should not rely on the DNO earth during a power cut).

Without it, you might find a boiler won't light (many use flame rectification to prove ignition).

I run my gen in an enclosure, and so gave it big air vent on one end, and a large fan on the other. The fan runs from one of the outputs on the generator, so I included the bond in the fan wiring.