Outbuilding Earth TT Question

[acvc]

This is my first TT/Outside job combined so any comments on my plan would be great ta.

Job: Power to outbuilding from house which is TT (Ze 36ohm).

Design current: 35.5A
Volt Drop: 3%
CCCapacity of SWA 50A (Ref meth D), 64A for 10mm t+e (ref meth C).

Plan:
1. Split tails and main earth.
2. L+N into 63A 100mA rcd, and earth into metallic block.
3. Take 10mm t+e - not using the cpc - and a 10mm single core earth from the metallic block, to outside the house where
4. It goes into a IP56 adaptable box and joins 10mm three core SWA.
5. SWA runs 40m - 6 of which underground - to outbuilding.
6 Inside outbuiding into JB where SWA joins 10mm t+e - not using the cpc - plus 10mm single core earth, all to garage c/u
7. C/U with 30mA mainswitch/30mA RCBOs for each way.
8. Test and anticipate a possible second earth rod for outbuilding.

 

[Spudnik]

In a word, no.

If you split the tails in the house, you should use a switchfuse to protect the SWA leaving the house.

Ideally, a spare way in the CU is better with say a 40A MCB, with no need for RCD preotection.

Then the outhouse CU must have an RCD main switch, or RCBO for the sockets, unless any cables are buried less than 50mm etc etc.

 

[acvc]

Ah Thanks Jason.

While you were replying I was busy editing the OP, sorry!

I realised that there was no protection for the cable running out ....a point which is debated a lot in the forum and I overlooked.

What you think of the revised plan then?

 

[sparks1234]

What type of consumer unit are you using?

 

[acvc]

Most likely a Wylex - plastic and I need five ways - sockets, a heater, lights+smokes, alarm and outside light. The customer doesn't mind paying more for rcbos.

 

[Guest123]

As this is a TT system....would the sub-main not need RCD protection somewhere in the order of 100mA TD as the loop impedance would be too high for the fuse/MCB alone to achieve disconnection in the required time???

 

[acvc]

Hi Lenny.

Yes I'd put it through a 100mA rcd in the house, straight after it's split off from the original feed.

I presume this is what you mean ....

Thanks