James asks this question, which is about a TT earthing system, the circuit protection used, and which column to fill in with the completion certificate. It has been answered by the NICEIC (Theme – Regulations and Legislation):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: I’ve come across a problem when trying to fill out a completion certificate - if the earthing system is a TT type, and fault protection is under RCD protection and not using an MCB, is the maximum system loop impedence (Zs) to BS 7671:2008 column the maximum for an RCD or an MCB? In other words, is it 1667 for a 30mA RCD, or the maximum for the MCB?
It's that long ago since I've come across a TT earthing system that I can't remember which figure goes in the Max Column.
Answer: If the design of the installation is relying on an RCD for fault protection then this value can be used if being relied upon.
On this occasion, Table 41.5 of the Wiring Regulations should be consulted to ensure that compliance is met.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link. Experts from leading organisations provide online answers to your technical questions on a broad range of subjects. Our searchable database of existing Q & As now contains over 3,500 entries; you can browse through them here.