Protection and Discrimination for Twin and Earth to Sub Main

[badgerfarmer]

Hi,

Could someone help me out with some guidance on the following please.

N.B, this will be done by a spark, and thus be signed off under part P, but I want to see if it is feasible / what my options may be.

Currently I have a TNC-S supply to the CU (all RCBO's) that has 2 spare ways left. Ideally I would like to add 2 sub mains, one for my aquarium, and one for the detached garage.

As I understand it it's necessary to provide protection for the cable even if it is in safe zones, unless it is more than 50mm deep (not possible in this situation) it's also not practical to run SWA all the way to the CU, so I am looking for ideas. Obviously there is metal conduit, but are there other options? Is there such a thing as a time delayed RCBO? as I guess that would be the ideal solution.


Thanks,
Badgerfarmer.

 

[1john]

You can surface mount the cable, use MIMS/MICC or ali tube ect thus avoiding the use of an RCD. When you get sparks around to quote they can have a proper look at your property and see if they can find some routes that may the use of T&E an option.

 

[ricicle]

If a time-delayed RCBO were available it would not provide the required additional protection.

 

[badgerfarmer]

If a time-delayed RCBO were available it would not provide the required additional protection.

Because the disconnect if the cable was damaged between the CU & sub main CU would not be quick enough? I was under the impression discrimination & protection was often provided by a time delayed RCD in situations like this? But I don't have enough ways spare for that.

Surface mount is not an option, I will look in to MIMS/MICC, Currently the ceiling in the hall is down and I was hoping to get the cables up so I can get the ceiling back up and decorate the hall etc over the bank holiday. I haven't spoken to the spark yet, but doubt he will be able to advise till after the bank holidays. Was planning on putting 16mm2 T&E up for the garage, and 6mm2 for the fish tank. Currently the garage is on 6mm2, but the wiring in the garage, and the garage CU is/are ancient, the roof leaks the lights have exploded in plumes of smoke when turned on... So am going to get a new bigger sub main put in for when the roof is fixed, and it gets rewired. for the length of the hall I can meet the 50mm, but after that it gets complicated. Damn houses!