Is this the correct way to do a three phase supply to a

Aug 19, 2023

Question:

A customer is having a new 3 phase supply to his house, the earthing is a TT - the 3 phase head is going into a meter cabinet on the outside wall of his house. Inside I am going to install 3 100amp switch fuses; 1 to serve the house in SWA, another phase to serve log cabin in SWA and the last phase to serve a garage in SWA.

Each building will have its own rod, am i correct?

Each sub main will it need an rcd up front in the cabinet?

The Zs for each submain will need to be with in the max Zs for the protective device in this case 100amp switch fuse?

Please help im being pushed from pillar to post.

Answer:

Hi there, lots of questions.

So on a TT system you only need a single electrode, though there is no problem with having multipe.

It is quite difficult to avoid an RCD on a TT system, not impossible but in this case I would suggest it may be impractical. So it is likely that any circuit (final or distribution) would have to be on an RCD. Whether it is best to do this with a MCCB/RCD module set up or individual RCD/MCBs at the origin will be up to your design, clients requirements and costs.

The Zs could be measured against the maximum permitted for an RCD as per table 41.5 as it is unlikely that you will achieve the required values for the fuse.

Of course, it is not possible for anyone to design an installation for you if they have no indemnity insurance, which ECA do not have, so the above is just guidance. But hopefully this will get you steer you in the right direction.

Gary Parker - ECA