We don't know the history of that very old switch. We don't know the earthing or cicuit protection arrangements. One party could be working on something connected to the supply when the other turns it on. Possibly other 'unknown unknowns'.
Changeover Switch
- Thread starter russellsmiler
- Start date
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That's true, and has previously been pointed out. I thought you were talking about the actual idea being dangerous.
And earthing and circuit protection concerns are generic - they apply no more or no less to having a transfer switch than any other aspects of the installation, and therefore they are irrelevant. You can't cite unknowns like that as a reason for declaring that a switch is likely to be unsafe. If you do then you would also have to tell anybody who asks about running a supply to an outbuilding that a cable taken from his CU is likely to be unsafe and therefore he should not run a supply to an outbuilding.
The switch will be upstream of the CU in the outbuilding, which it seems reasonable to assume will have a 2P switch as the incomer. Operation of that whilst someone is working on something is no more, or less, of a possibility with a transfer switch in place, and a transfer switch does not make it more or less likely to happen. So it's irrelevant to the discussion of a transfer switch.
In other words you can't come up with anything which is specific to the provision of a transfer switch to back up your claim that it is likely to be unsafe or open to misuse.
That might be correct, but I don't have enough information to state that it is safe, so I'm applying the precautionary principle. We don't have much information about the physical arrangements, layout, etc, so it might be capable of being safe... or it might not. The outbuilding will have an installation that can be supplied from one of two sources, depending on the position of the switch, and it is quite possible that the circuit protection will be different for each of those sources. It might be adequately protected in both cases... but it might not.
There is certainly a foreseeable risk that someone might take the centre"off" position of the transfer switch as sufficient isolation to work on the system (or something connected to it) when someone else decides to energise it.
But don't you see that the same precautionary principle will (or should) lead you to declare every idea proposed here as unsafe? New lights, cooker circuits, additional sockets - you name it there will be things which need to be considered, but that does not make the idea of having new lights, cooker circuits or additional sockets unsafe.
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I see where you're coming from BAS, but this particular case is unusual. In the usual cases of power to outbuildings, there is a 'standard solution' that is accepted as being safe. In this case there isn't.
So with no switch, you would assume that the circuit protection for the cable was safe?
You would assume that the earthing arrangements were safe?
You would assume that the provisions for safe isolation and prevention of inadvertent energising of the outbuilding circuits were satisfactory?
You would assume that the earthing arrangements were safe?
You would assume that the provisions for safe isolation and prevention of inadvertent energising of the outbuilding circuits were satisfactory?
BAS, why would I make those assumptions? Would you?
If the OP had simply asked how to supply power to his outbuilding, you would I'm sure have replied with your usual "teach a man to fish" list of informative links. What would you say if he'd asked how to supply power to his outbuilding from his neighbour's house? Or if he'd asked how to supply power from his house to his neighbour's shed?
I still can't get my head around the earthing issues, which I suspect might be a problem depending on the physical layout of the wiring. Where would you connect the CPC of that part of the installation in the outbuilding?
Edited to correct silly
typing error!
If the OP had simply asked how to supply power to his outbuilding, you would I'm sure have replied with your usual "teach a man to fish" list of informative links. What would you say if he'd asked how to supply power to his outbuilding from his neighbour's house? Or if he'd asked how to supply power from his house to his neighbour's shed?
I still can't get my head around the earthing issues, which I suspect might be a problem depending on the physical layout of the wiring. Where would you connect the CPC of that part of the installation in the outbuilding?
Edited to correct silly
typing error!Yes, there are definitely some potential issues with the earthing. A lot depends upon the recurrent question of whether one exports earths from the two houses or TTs the outbuilding supply locally. If the former, one imagines that CPCs from the two supplies (installations, properties) would be joined within the outbuilding (regs don't allow them to be switched), so I suppose that they would represent extraneous-conductive parts where they enter each of the houses, and therefore theoretically would require a 'bonding' path from the points of house entry to the respective METs of at least 'main bonding conductor' CSA (i.e. at least 10mm²). Furthermore, with 'exported earths', there may well be a need for main bonding of other extraneous-conductive-parts (e.g.local pipes, building structure) within the outbuildings - this time requiring a path of 'main bonding CSA' from outbuilding back to both houses' METs. The 16mm² SWA should be adequate (whether using third core or armour), but if that were fed by, for example, 16mm² T&E, the CPC in that (6mm²) would not be adequate as a bonding path back to the METs. These issues are not unique to the 'changeover switch' method - if the '2 sockets' method were used, although the outbuilding would only be plugged into one at a time, the wiring to the other socket would probably constitute an extraneous-c-p as far as the outbuilding was concerned, hence may well need bonding to the other property's MET!
Not straightforward. Locally TTing the outbuilding would presumably make life a fair bit simpler.
Kind Regards, John
Local TT is probably the ONLY way to do the project. As John correctly points out the "earths" can NOT be switched.
And if they were connected to each other this could lead to problems caused by high currents in the "earth" wires from houses to shed due to differences in the voltages on the two neutrals in the house.
Terminating both supplies directly onto the same switch will create a problem if the switch has to be worked on. Each incomer to the shed needs to have an isolator in the shed where it can be seen by anyone who is working on the switch. If the isolator is in the house it could be turned on by someone unware that someone is working on the C/O switch in the shed.
That is why the two contactor method is better than a change over in the switch, The contactors cannot be relied upon to isolate ( unless they are forced separation contacts ) but terminating on separate contactors is safer ( less hazardous ) than terminating on the single switch.
Cutting costs by buying second hand items from E-bay may save money in the short term but the risks that creates in the longer term of damage or injury must outweigh short term cost cutting.
Whilst I certainly think that local TT is the simplest option, I don't think there is any significant risk of what you suggest above. Even if it's TN-C-S, it's hard to see how the neutral voltages could differ significantly - the OP told you back on page 1 that we're talking about a pair of cottages which share a 2-wire overhead feed, with one cottage fed via the other.
Kind Regards, John
No I wouldn't.
Which is why I still don't understand why your "precautionary principle" leads you to say that having a switch is likely to be unsafe and so should not be done, but when asked to apply that same principle to declare any supply to any outbuilding as likely to unsafe and so not to be done you say "In the usual cases of power to outbuildings, there is a 'standard solution' that is accepted as being safe."
I don't think that electricity knows who owns the building it's in.
And many of them are generic to any outbuilding supply.
Still haven't seen anything which makes a transfer switch likely to be unsafe.
That's a very good point.
Only really the ubiquitous question about 'export earth or TT?', and consequences thereof. Everything else I wrote results from the (very unusual) situation of connections to two different house's earthing systems entering a common third building.
One real risk with a centre-off switch has been mentioned (people inappropriately using that position for 'isolation') - although that risk does not exist with a non-centre-off transfer switch, such as the one you illustrated. I think that a generic risk of any way of achieving what the OP wants is that, without mechanical locking, there is going to be a risk of 'the other person' rendering live something which the first person thought was dead.
Kind Regards, John
I don't think I've suggested that having a switch is likely to be unsafe, but having two alternative supplies to an outbuilding might be unsafe without additional precautions, such as the isolator(s) in the shed that bernardgreen mentioned.
As I've said, I can't decide how important the earthing issues might be.
Incidentally, Klockner-Moeller changed their name to Moeller in 1999, so that switch shown by the OP is likely to be at least that old.
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