Electrical question....

[stephenten]

I currently have a 32A B curve RCBO in my house consumer unit.

6mm2 cable runs from this RCBO to the garage which is a run of less than 3m. Currently nothig is connected as a load to this 6mm2 cable.

I plan to connect the already buried 10mm2 SWA cable to this 6mm2 cable via a junction box. The SWA is 10mm2 due to the voltage drop across the garden.

I would like to then put in a shed CU at the other end of the SWA which is at the bottom of my garden.

This Shed CU would then have 3 x 6A RCBOs for a shed lighting circuit, a greenhouse lighting circuit, an external floodlighting circuit and a 20A RCBO for a double external socket.

We may want to add further circuits of up to 32A for a hot tub in the future.

The house has PME earthing (In old money) and there is a MET in the meter cupboard

One small fly is that the shed is a metal yardmaster so I plan to supplementary bond the shed metal work to the SWA's earth conductor (I propose to export the earth)

Now How do I sort out some form of discrimination between the house CU's 32A RCBO and all the RCBOs in the shed CU as I've never seen a time delayed 40A type C curve RCBO to replace the 32A RCBO with?

[John Rumm]

At risk of saying you probably don't want to start from here, but...

Short answer is you can't. Even if you had a time delayed RCBO (not even sure such a thing exists), then that would not offer protection for the T&E at the head of the submain. This would need it unless protected in some other way (say by burial >= 50mm from the surface or by earthed metallic shield).

You also have a second problem with the T&E in that its 4mm^2 earth is inadequate to function as a main EQ bonding conductor for the PME supply on its own.

You mention a greenhouse as well - and these are notoriously difficult to extend a PME EQ potential zone into since there is such easy access to alternative independent earth references.

I think I would be inclined to replace the short run of T&E with earth-shield or similar, or protect it in earthed metal conduit. Replace the head end RCBO with a normal MCB. Run the PME earth as far as the CU in the shed, but then isolate that there - so all it is providing earth fault protection for is the sub main itself and not the shed. Make the shed a TT installation with its own earth spike. Now you have no need to export the PME EQ zone to the metal shed or difficult greenhouse.

Discrimination will also be better since earth leakage at the shed end won't be able to trip a RCD element at the head end.

Adam might be along shortly with a better idea?

[ARW]

A couple of things. The OP has not actually said that the 6mm requires RCD protection. If it does then the last time I checked the 6mm earth shield cable had still not pass the required tests.

You can always do a deviation from the regs and still have buried T&E less than 50mm behind the plaster.

I agree about making the shed a TT supply.

[stephenten]

The 6mm2 cable runs from the back of the consumer unit through a double skin wall straight into a ceiling-floor void for 2.5m and then straight through a double skin wall. The cable is then direct clipped on the wall at a height of 2.5m towards the garage door, past the garage door and then along the opposite side wall.

There will be a cooker switch style isolator which is how the 6mm2 cable will connect to the 10mm2 SWA.

I cannot see a situation where a rcbo is required in the house consumer unit....

The SWA is then buried under lawn before coming out into metal shed to feed the second consumer unit.

[stephenten]

P.S....

The metal shed and metal framed greenhouse are both on a patio which in turn is sat on sharp sand followed by 100 mm mot1 hardcore.

The nearest bit of earth to hammer an earth rod into is the lawn and this rod would be at risk of lawnmower or strimmer damage.

Only alternative is to drill right through patio, sharp sand and hardcore and use an extra long earth rod...

I was going to use 20mm plastic conduit on all the cable runs between sub consumer unit and all the electrical accessories to improve both mechanical protection and implement "double insulation" between the conductors and the metal structures.

This is addition to using IP66 sockets and switches along with IP66 fluorescent lighting battens in both structures.

S.

[Marland]

Surround it with something like a bit of pipe/tubing.

Decent drills and long enough bits can be hired.

Not a lot of point in asking for advice if you are concerned about the difficulties of implementing it and are looking for excuses why you can?t.

GH

[John Rumm]

In which case you can probably argue that a deviation is acceptable, and feed the T&E from a non RCD protected B32 MCB.

ok

Technically speaking, all cables should now have RCD protection unless they are mechanically (or otherwise) protected from damage. Surface mounting is in some cases an acceptable way of protecting it (i.e. you are less likely to drill into it etc).

[John Rumm]

Then dig a small pit and put in a protective enclosure for the rod and connection to it.

Yup, that would probably be easy enough - in fact you can probably just hammer drive it through the sand and MoT.

I would be inclined to start with a normal 4' rod, and then test that once installed before deciding if you need to couple another rod to it.

Plastic gives some mechanical protection and can make a surface wired job look neater. Trunking is a bit easier to install.

Pick whatever is appropriate for the location. Inside a shed you would be ok with a lower IP rating normally so long as it doesn't leak.

[tony sayer]

Plenty of decent earthing stuff here:)

[ARW]

Is it 3 core SWA and how long is the run?

[John Rumm]

Quick brain fart correction, the CPC on 6mm^2 T&E is only 2.5mm^2 (its

10mm^2 that has the 4mm^2 CPC)

[Roger Hayter]

I don't think you want to double insulate the circuits you are using outside.. At least, it might be ok if the only things connected to it were double-insulated light fittings with cord operated ceiling switches, but if you are having any other wired appliances, let alone sockets, then you want the metal building bonded to the earth you are using for the local sockets. As discussed, their are theoretical dangers if this is the exported house PME earth, which is why you are being recommended to have a local TT earth stake for the outside circuits.

[stephenten]

Yes it is three core SWA in the new three phase colours so I will have to use coloured tape on both ends....

The cable is 20 m long from end to end but about 5m is above ground and 1.2m below down to the trench so the physical lateral distance between garage and shed/greenhouse is only about 13.8 metres.

[ARW]

Go TT.

[ARW]

OK so you could actually export the earth for main equipotential bonding purposes by adding a 10mm earth between the CU and the SWA. However IMHO both the shed and the green house should be supplied from a TT supply.

[stephenten]

Ok so this now leads to a question of:

What are the pros and cons of exporting the main house earth Vs going TT with an earth rod?

S.

[John Rumm]

(see the subsequent section on exporting an equipotential zone as well)

[stephenten]

Incidentally I can buy RCBOs that have a short flying white wire which is a functional earth.

As I understand it, if there was ever a loss of earth, the RCBO then trips.

So can such an RCBO be used in a TT install as the electrode resistance will be much higher than a electricity board supplied earth via what I know as PME in old money

[ARW]

The fly lead is there so that the RCD part of the RCBO will still function if the neutral is lost.

They are fine to use on TT supplies.

And that reminds me of an NICEIC inspections I had. The inspector chose a house where we had added circuits to the CU using RCBOs and there were already existing RCBOs. We did not over sleeve the white fly lead with green/yellow sleeving but the RCBOs that were already installed were oversleeved.

The inspector asked "Why did you not over sleeve the fly lead?" and I replied "Because it not an earth it's a functional earth and it's colour should be white". That was the correct answer.

[stephenten]

When you say neutral lost, do you mean neutral before the RCBO or after the RCBO?

In any case, if there was loss of neutral, the load would not work as there would be no return path for the current from the live, i.e. an open circuit surely?