Calling the electricians

[Tim Lamb]

I need to connect a tp&n supply to my new workshop. I have an existing

16mm2 supply to a neighbouring barn. For space reasons this is protected by a 30 amp switch fuse fitted with HRC fuses (anything else would need a bigger hall cupboard).

At the load end I have a 30 amp isolator. I suspect the outlet terminals will not accept a second cable.

Is there a cheap way round this that meets the regs.? I envision a metal clad box with three terminal blocks but am hugely out of date with what is permitted.

regards

[Owain]

You might be able to use henley service cutouts as fairly neat way of going from 2 x armoured to tails.

x 54141-03

1 x 54143-08 1 x 54318-02

Owain

[Dave Plowman (News)]

Fit a CU? You might be able to find a two or low number way basic one being thrown out.

[Tim Lamb]

Umm.. I hadn't wanted to fit fuses at this point because of discrimination issues and I still need to maintain the wire armouring as the earth connector. The new cable will be at least 10mm2 and I hoped to couple into the cable without disturbing the existing distribution arrangements (I have a loop of spare cable ready for such alterations).

regards

[Dave Liquorice]

I'm not a spark but can't you just put the incomer in the barn into four henley blocks and then feed the outgoing cable and the existing barn kit to the other terminals of the henley blocks? The termination of the armour for earth connection might be problematical but doesn't "exporting" the earth open another can of worms anyway? Better to give the barn and workshop their own earth spikes and fit

100mA time delayed RCDs to each of the CU's.

[Tim Lamb]

This is more or less my plan. I'll terminate the SWA in a galvanised adaptable box and link the earth via lugs on the glands.

I am rather hoping this will not be vetoed by the cognoscenti:-)

Exported earths are a different kettle of smelly fish. Inserting a local earth rod would not be difficult but the cable is hugely overrated for the duty and the wire armouring may be adequate.

Why the long time delay?

regards

>

[Dave Liquorice]

I'm just aware that exporting an earth to another building is best avoided, even if the earth loop impedance is satisfactory from the fault clearance time POV. The actual earth potential in one building may be different to that in the other, it becomes smelly as you say...

'Cause 100mA doesn't provide shock protection only earth fault protection. You need fast acting 30mA RCDs for shock protection. So to provide discrimination between the cascaded RCDs the 100mA upstream one needs to take longer to react.

[Tim Lamb]

err.. Currently there is no upstream shock protection. The fusing is only intended to protect the cable. Local 30mA protection will be used for ring main and lighting supplies.

Three phase m/c tools will be on individual mcb's backed by HRC fuses.

regards

>

[Dave Liquorice]

Upstream in relation to the 30mA RCD. B-)

Earth in barn and workshop provided by local spike.

Incomer --- Henley --- Barn CU - 100mA RCD - 30mA RCBO - Ring | - MCB - Machines | - MCB - lights | Block --- SWA --- Workshop CU - 100mA RCD - 30mA RCD - Ring - MCB - Machines - MCB - lights

I don't like lights being on RCDs. Though I don't think you can easily avoid it with the 17th Edition regs. B-(

[Tim Lamb]

Should that be RCBO for the workshop ring? One attribute of urban fringe farming is that it is never dark! Most workshop use is daytime anyway so lights on RCD not that big an issue. No one is employed to work there so emergency lighting not mandatory.

Thanks and regards

>

[Dave Liquorice]

Yes that would be preferable. Typo B-)

And you never get to see a decent star field.

I'd still not share it with a ring, the chances are that it'll be a handtool or extension cable that causes a trip due to some misfortune and the last thing you want is the lights to go out as well. Even if you aren't plunged into darkness semi gloom isn't goood either.