A customer having a new 3-phase supply to his house where the earthing is TT provides questions on RCD protection for this Voltimum reader.
This Q & A is one of thousands posted in our Technical Expertise area, and answered by our Voltimum Experts.
Question:
Hi, I need you to clarify something for me. 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'm going to install three 100 A switch fuses - one to serve the house in SWA, another phase to serve a log cabin in SWA, and the last phase to serve a garage in SWA. Each building will have its own rod.
Am I correct in doing this?
For each sub main, will it need an RCD up front in the cabinet?
Will the Zs for each sub-main need to be within the max Zs for the protective device, in this case a 100 A switch fuse?
Allen
Answer:
Gary Parker - Senior Technical Support Engineer, ECA
On a TT system you only need a single electrode, though there is no problem with having multiple.
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, client's 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 steer you in the right direction.