Power cut questions

[charles]

Link box, I suspect rather than fuse box.

At least these people could keep their freezers cold.

[charles]

a 40 ish YO development of 25 bungalows near us was all on the one pahse.

[Tim Watts]

There are link fuses in the road, usually on street corners where a cable branches - typically 300-500A per phase.

But in this case, they've probably just chopped a common cable that does both you houses.

Separate 16/20A radial would be easiest - don't need a ring for such a low power application - unless you were planning on a mosnster generator.

What you must take into account though, is that technically if the supply fails - you can no longer rely on the supplier's earth conductor (if you have a TN-S or TN-C-S installation)

CAVEAT - this is beyond my level of qualification, so the following is the conclusion I came to when considering the same as you:

You will need to connect the generator's radial circuit's earth/CPC to the MET (the main earth terminal (near your meter or in the CU).

But in addition, *I believe* you should probably consider a secondary earth rod wired to the MET. I *believe* this is going to be mandated in the 18th Edition regs anyway - but I'm not sure if that's be finalised.

The last 2 para's need some verification by others - but that's my understanding.

Your other option is a UPS.

[Tim Watts]

Apart from it's a more complex installation.

There is something reassuringly simple about a secondary radial with a few sockets in strategic locations...

[Tim Watts]

Might be a shared supply running along the boundary - loop into one cutout and out to the other next to it, or behind it on the party wall.

That's still done in blocks of flats where there are a load of meters next to each outer in a rise.

[John Rumm]

Where I was previously, They did two halves of two semi's on the same phase. So you shared a drop wire and a phase with the non attached adjacent property, but the cojoined half semi was on another phase.

[ARW]

On the gfs street (PME supply) it 4 houses (two semis) to a phase. The supply pops up at the back of one house and then runs external to all 4 houses with a catenary between the non joined houses.

[Dave Liquorice]

Except it ain't that simple. You have to make sure that there is absolutely no way, ever, at all, even for a ms, energise the "dead" incomer from your generator/UPS. That's fairly easy with a proper, double pole, three position, break before make, on-off-on chnage over switch.

The tricksy part is earth. You can't assume that the suppliers MET is still connected to earth. Household wiring assumes single phase "live" and return path "neutral" along with the protective earth, many appliances MUST be earthed. You could install your own spike but you have to ensure that has a low enough impedance and you bond the generator chassis and one of its phases(*) to that spike. effectively creating a "live", "neutral", "earth" supply. However you shouldn't connect this locally derived earth to the MET. They could be at different potentials, right up to being connected to a supply phase. So you ought to break the connection between the MET and the house hold earth, except the regs don't allow breaks in the bonding wire let alone a switch...

When I drag the genset out here, I run temporary extension cables from it to the appliances that need to be kept running, essentially just the fridges/freezers and CH system. The two phases from the genset float, if some one touches one that gets dragged down to "earth" and is fairly safe. The danger comes should the other phase also have a connection to earth, depending on the impedances you could get a full strength belt.

A suitably sized overload protective device solves that one. Unless you have some slow learners...

(*) "Single phase generators" are really two phases @ 180 degrees with niether phase connected to the chassis.

[John Rumm]

That's less of an issue with modern all RCD installations.

You may need to do that anyway for things like your CH boiler, where many will use rectified flame sensing.

It would be acceptable on a PME supply (and in fact may become a requirement for new installs at some point). Obviously TT setups would be fine. TN-S would be more problematic.

Debatable whether that can be called two phase - its more like a single phase transformer isolated output.

[tabbypurr]

is 2 phase still used in the grid? I know it was widespread pre-war.

NT

[Dave Liquorice]

How many places are "all RCD"? And we don't know what the OP has.

I'm not sure I'd like to come into contact with the chassis of a "must be earthed" appliance with a L to chassis short and an CPC impedance above about 7k... True enough with an RCD protection you don't need a low impedance earth path to make sure the overload protection is tripped in a sensible amount of time in the event of a fault.

Poncy gas things, oilers have jacobs ladders. B-)

Under supply fault conditions?

[David]

Just to note that the house is all RCBO.

Cheers

Dave R

[John Rumm]

If you are installing your own spike, then I would aim for a significantly lower impedance than 7k...

I have always managed to get under 15 ohms in these parts (clay soil), and normally closer to 6 or 7 ohms.

Must admit to never having looked at how the control systems on an oil burner work...

Yup, for all RCD installs on either TT or TN-C-S with earth spike. (earth spike working and tested)

[newshound]

Agreed. I should have been more explicit. But I don't think I would wire an "independent" radial for the generator myself, I would use a *proper* changeover switch, with overload protection, and a good tested earth spike!