On 29/04/2019 23:17, wrote:
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?

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?

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/