Changing the direction of an outbuilding cable

[adam_stubbs]

I have a customer with a farm who has a barn containing a large 3 phase generator which powers various equiptment at that location. A single phase from this also supplies a ring main and lighting radial for the building. That part of the setup has a generator changeover switch so that the sockets and lights can also be powered by the main site supply without starting the generator. This mains supply comes via a 50m cable from the farmhouse.

Due to the rural location the customer often has power outages and would like to be able to power the house from the generator. The most vanilla way to do this of course would be to run another armored cable from the farmhouse to the barn to supply directly from the single-phase output side of the barn change-over to a changeover switch at the properties meter tails. However the cost of running the cable from the house to the barn would be significant, and the customer does not understand he cannot reuse the existing cable.

The reason is of couse that with switch-gear at both ends of the barn-house cable there would be nothing to prevent switch configurations that left the mains directly connected to the generator.

Has anyone encountered this before and know of a standard approach. To my mind there are a few options, both centered around the design that there must exist some physical thing that can only be at one end of the cable or the other.

- dual pole 100amp key switch isolators at locations in each building, with a single key that cannot be removed in the on position
RISK ASSESSMENT: customer could duplicate key
RISK mitigation: note attached to the key prohibiting this, with reasoning.
- add plug and socket configurations at each end of the cable, with the same gender on each end of the cable (probably female). provide a gender changer which is used to
make one end, and only one end, of the cable become the power-in side.
RISK: The end of the tail that connects to the cable will be live and male when setting up.
customer could make up their own tail.
Risk migigation: User has to be considered skilled/educated to the risk, supplies can be rcd protected.
Warning against additional tail would need to be displayed.

- add two supplier cut out housings (one to live and one to phase) to meter tails before consumer unit. Add matching cut outs that bridge the barn generator-changeover switch.
supply two non-fused cutout links (the red sort) which must be moved to whichever location is to be the supply.
RISK: customer could purchase additional red links. A single link could be moved. links could be removed/placed under load causing arcing.
RISK migigation: warning notice. Possibly connect the two links to each other via a tether, so both have to move location.

 

[snowhead]

Is the generator rating enough to power both the house and whatever it's there to supply at the moment?
Is the cable, house to barn, suitable for the likely load?

- add plug and socket configurations at each end of the cable, with the same gender on each end of the cable (probably female). provide a gender changer which is used to
make one end, and only one end, of the cable become the power-in side.

RISK: The end of the tail that connects to the cable will be live and male when setting up.
customer could make up their own tail.
Risk migigation: User has to be considered skilled/educated to the risk, supplies can be rcd protected.
Warning against additional tail would need to be displayed.

Absolutely unthinkaable and unacceptable.

Given the above statement you are out of your depth of knowledge and should seek more experienced assistance with the issue.

There are solutions to the issue that allow safe reconfigurations of cables and supplies.

 

[davesparks]

As far as I can see there is no safe way to do it in an installation which is not under the supervision of a skilled person.

Whatever interlocking you put in place will get defeated by someone who doesn't understand the dangers, especially on a farm.

Remember that it is not just the users of the installation who could be at risk here but the poor sods trying to repair a network fault could be the ones who end up dead as a result of someone defeating interlocks on this setup.

 

[timhoward]

I can't immediately see any solution that doesn't involve at least running control cable between them and adding multiple points of future failure. I can easily imagine the cost of designing and implementing this exceeding the cost of running the required cable.

The only compromise solution I can see is that he accepts that the barn power always comes from generator when required and a changeover switch is fitted at house. Even this requires some careful circuit design - the cable may be sized to supply a few lights and sockets currently but not be suitable for a shower, an oven, a hob etc. if the supply direction is reversed.

He can't have his cake and eat it.

 

[davesparks]

Will the generator be able to cope with the house load from a phase balance point of view?

If it is a 3 phase generator and has a whole house load on one phase and no load on the other two phases it may not last very long.

 

[adam_stubbs]

There are solutions to the issue that allow safe reconfigurations of cables and supplies.

I am intrigued, as potentially may be other people who read the thread in the future.
Depth of knowledge wise, as I recall some exposure to live contacts is permissable as long as the person is deemed skilled. None of us would be allowed to work on CUs otherwise after all. I do doubt however that any farm person or anyone else who encountered the setup might be considered sufficiently skilled though, so I wouldn't have considered it a very good option.
I think also there is the very real risk of someone later touching it and causing issue without being aware of the risks of their actions. Its actually fairly unusual these farm sorts even ask for help, usually they do stuff themselves, including some interesting things I've seen over the years such as bridged fuses. One chap proudly told me that a bit from a screwdriver fits very nicely into a 13amp plug when you 'need' to use bigger things on it like a welder ;-)

 

[adam_stubbs]

Will the generator be able to cope with the house load from a phase balance point of view?

If it is a 3 phase generator and has a whole house load on one phase and no load on the other two phases it may not last very long.

very true. The place in question doesn't have any electric showers or cookers (it is old school with a range and stored hot water). However he would need to be aware of this risk. Also care would need to be taken in the design to avoid power being distributed to other parts of the site.

 

[pc1966]

You can get automated systems for reverse-power protection, etc, but I strongly suspect it would end up more expensive than simply running in another cable.

As above, making the barn generator-only (maybe for lights having some solar panel + charger + battery + inverter so it can be used a lot of the time without starting it up) and then the home being either mains or barn-generator but NEVER both by means of a transfer switch would be the cheapest option.

Also remember it needs its own means of earthing. If the buildings are TT then already present, otherwise it needs an earth rod as under loss of mains conditions you cannot assume the DNO earth connection would still be present.

 

[adam_stubbs]

You can get automated systems for reverse-power protection, etc, but I strongly suspect it would end up more expensive than simply running in another cable.

As above, making the barn generator-only (maybe for lights having some solar panel + charger + battery + inverter so it can be used a lot of the time without starting it up) and then the home being either mains or barn-generator but NEVER both by means of a transfer switch would be the cheapest option.

Also remember it needs its own means of earthing. If the buildings are TT then already present, otherwise it needs an earth rod as under loss of mains conditions you cannot assume the DNO earth connection would still be present.

That's interesting that reverse power protection equipment is available, it is not the sort of thing we need on a day to day basis. probably as you say it will be too expensive, and running another cable is likely to be more cost efficient, especially if he is willing to dig the trench himself with the tractor. I will look into costings nether the less, as at least it does show willing.

Buildings are both TT, and earthing rods are present.

I agree protecting operators in the network is essential, thus this discussion as to a way we can prevent my leaving them to find their own solution.

losing the ability to power the barns ring main and lights (plus hydrollic vehicle lift and large air compressor) from the house may not be palatable to them, but the fact this is one of the few good options might further reinforce the suggestion of a new cable.
Such a cable can of course be protected by a suitable breaker on its supply side to protect it from overload.
Lets not get into voltage drops on this thread though ;-)

 

[Julie.]

The only practical way other than running an additional power cable would be M&E interlocks.

Essentially this is the mechanical shared key you suggest with additional signal exchange, you would not be able to energise the generator to the supply if the mains supply is energised. This still requires cables, but is just signal rather than power.

Mechanical alone is generally not acceptable these days, especially by untrained persons.

These keys are not copied easily however as long as you get a proper solution from castell or whoever.

A reverse power relay wouldn't be suitable, if there is any possibility of parallel operation with the supply then you have all the requirements of g59 to content with (rocof or dv/dt etc).

 

[adam_stubbs]

The only practical way other than running an additional power cable would be M&E interlocks.

Essentially this is the mechanical shared key you suggest with additional signal exchange, you would not be able to energise the generator to the supply if the mains supply is energised. This still requires cables, but is just signal rather than power.

Mechanical alone is generally not acceptable these days, especially by untrained persons.

These keys are not copied easily however as long as you get a proper solution from castell or whoever.

A reverse power relay wouldn't be suitable, if there is any possibility of parallel operation with the supply then you have all the requirements of g59 to content with (rocof or dv/dt etc).

Thanks Julie,

Yes I looked at the revers power protection, and they didn't look suitable they seem mainly for the purpose of protecting generator sets themselves.

That's interesting that mechanical alone would potentially not be sufficient. Its quite a niche situation in question here, but are there some similar rules/regs that you are referring to?

I think if there was a need for a signal cable then probably he might as well have a new power supply cable, which would end up cheaper than the key-switch gear + the electronic actuator

Thanks again, at least there isn't some simple solution I hadn't thought of, this is one of those cases where there isn't a cunning solution to the problem!

 

[Julie.]

I haven't anything specific, but in recent years everything I came across would require M&E rather than just electrical or mechanical.

Castell have quite a selection of stuff, I would imagine they are likely to have something that can retrofit most industrial isolators.