David asks a question about an electrical installation having a TT earthing system, and would this need RCD protection. His question has been answered by the NICEIC (Theme – Regulations & Legislation):
Question: I'm looking some information on RCD protection. I have a 16mm three-phase and neutral S.W.A. cable, about 70m long, supplying a three-phase distribution board. I had planned to protect the supply cable with a three-phase 80A MCB. Note that the means of earthing will be from an earth spike joined to the armour of the S.W.A. cable.
As it is a TT earthing system, will I need RCD protection on the supply cable? If so, what rating of RCD would be suitable, or is the MCB OK on its own?
Also the distribution board will be in a glasshouse, supplying circuits to six three-phase heaters, and three-phase motors for ventilation operation. As these are fixed equipment and are inductive in load, would a 300mA RCD fitted to protect all of the circuits be suitable?
Note: I was wanting to use 300mA RCD to try and avoid unwanted tripping due to inductive loads switching on an off.
Answer: Working on the basis that the electrode is at the supply end of the proposed sub-main, then an RCD will be required at the supply end and also at the distribution board end.
At the supply end - Both conditions of Regulation 411.5.3 must be considered before a suitable RCD can be selected. This involves installation and measurement of the electrode resistance, and then a calculation using the formula listed in the regulation. This RCD must also be designed to trip in a maximum time of 1s. Typically this could be anything from 100mA to 500mA depending upon the electrode resistance. This device should also be time delayed so as to permit the downsteam RCDs to trip first in the event of a fault on the final circuits.
The distribution board end - again, RCD protection should be designed according to Regulation 411.5.3, but this time the maximum disconnection time should be 0.2s.
Provided that the final circuits are not embedded in the walls (where 30mA protection is required), RCDs at the load end would typically be from 30mA to 100mA again, depending upon electrode resistances.
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