Using a circuit 2 ways

sparc · May 9, 2014

A customer wants to have generator in or near garage that can be used to power a single 'special' socket in the main house in the event of a power cut. This socket would then be used for the fridge/freezer, BB router, and some lights.

Sounds fairly straightforward, with SWA being run from garage outside wall of kitchen, dedicated socket in there, and terminated in garage via wall box and flex to a commando plug. The latter would be plugged into generator when in use.

However, there is already SWA from CU in house to a 2-way disti board in garage.

As customer not keen on a dedicated supply cable just for emergencies I'm wondering how I could use the garage supply cable. The socket in house for power cuts could be installed in cupboard next to CU, as this is in kitchen, so easy to run some leads into there on temporary basis. At each end of the circuit would need to be a 2-way switch, to connect circuit for either use.

However, in the garage the commando plug is an issue. If - after a power cut - the home owner switches back from generator there could be instance when the supply cable is being powered from the house CU and switch not reset in garage meaning the pins of the commando plug are live.

Are the any ways to safely use a circuit in both directions for this type of application - and switches or isolators that allow this set up?

Richard Burns · May 9, 2014

It would be much easier to have a dedicated supply.
Also the generator will not easily be able to power computer equipment unless the generator has a pure sine wave rather than modified sine wave output with automatic voltage regulation.
If you installed a mulitpole changeover switch in the supply to the garage such that it disconnected the incoming supply and connected the special socket and also via an additional cable (that you do not want) that disconnected the garage circuits and connected the input from the generator then you could do it, but really it will be cheaper and easier to have a dedicated and separate supply because otherwise you would also have to switch the earthing so that faults on the special socket/generator were not transmitted to a dead network where someone is working on it.

sparc · May 9, 2014

Good points. I hadn't considered the sine wave output issue, tbh.

Not being able to run the BB router wouldn't be a massive issue, but it would be better to have a solution that allows it. It's not so much an issue for laptops and tablets, as they could run off batteries for duration of the power cut, connected to BB router via the wi-fi. So if router is okay with genie output, all else should be okay.

edexlab · May 9, 2014

Might seem obvious but if they do want the generator in the garage, be sure its exhaust is vented properly, I've seen several recently which had no provision for this and I mentioned it to the customers , one actually said yeah the fumes in the house made him feel ill!,
I remember a news story a few years ago where a whole family died in the US because of an unvented genny running in the garage and they were sleeping on the first floor

sparc · May 9, 2014

Yes, that's a very good point, and one I raised with customer when discussing possible siting for the generator. They're well away from other houses, so sound isn't going to be a problem for them, or their neighbours, if its outside the garage.

Richard's view that is dedicated circuit will be best is right, and was the first solution I posed. I then put my foot in it by musing out loud about the existing garage circuit supply cable. However, if I can't come up with a solution to use it safely then it's clearly not viable.

It seems odd, given how common the demand for set-ups like this - in rural areas like mine, which can suffer 4 -5 power cuts a year in the winter - that there isn't a neater way of doing this. (Other than a dedicated circuit.)

Richard Burns · May 9, 2014

For a sensible solution the whole power supply would have an automatic changeover switch that on loss of power would switch over to a generator capable and suitable to take the house load and never backfeed the grid.

Knobhead · May 10, 2014

sparc said:
Are the any ways to safely use a circuit in both directions for this type of application - and switches or isolators that allow this set up?

In a word, no.

sparc · May 10, 2014

Tony said:
In a word, no.

That's seems to be the general consensus - at least with regards this particular circumstance.

As I understand the problem, with 4 pole changeover switches at either end - and assuming appropriate overcurrent and additional protection is in place, as needed - the solution would work, but does not FAIL SAFE.

In other words, it is open to incorrect operation by user, with wrong changeover switch positions either having generator pushing power to grid (on which a DNO spark maybe be working to fix outage), or the grid feeds power to generator - or a commando plug, making the pins live.

But - indulge me a bit longer here, please - is there no technology available that would make this work safely?

I appreciate it is probably not cost effective for domestic situations like this, but I'd be interest to know if a such does exist. It's not hard to imagine 'intelligent' changeover switches able to communicate with each other over an extra core.