David asks a question about a three-phase distribution board in a glasshouse, and whether it would be possible to fit a 300mA RCD? His question has been answered by SELECT (Theme – Distribution Equipment):
Question: I plan to fit a three-phase distribution board into a glasshouse. The board will supply six separate heaters, each of which has a three-phase motor for fan operation. There will also be three off three-phase motors for ventilation. All socket outlets will have 30mA RCD protection, by means of an RCBO fitted into the distribution board.
The installation is on a TT earthing system. I plan to fit a 300mA S-Type RCD on the supply to the three-phase distribution board, then - if possible - fit a second 300mA RCD to protect the three-phase distribution board. Note that the earth spike is below 167 ohms, so a 300mA RCDs should operate OK.
Basically, is this level of protection acceptable for a glasshouse environment - given the need for unwanted RCD tripping? Does BS7671 require 30mA RCD protection for all circuits wired in an environment such as a glasshouse where conditions are damp?
Also, because the loads are mainly inductive and may cause unwanted tripping of a 30mA RCD, would it be possible to fit a 300mA RCD?
Answer: From your explanation it would appear you will be compliant.
The 300mA RCD should be of the S type or have a time delay to avoid nuisance tripping. In addition, all live conductors should be isolated - i.e. the three line conductors and the neutral conductor.
Note that all equipment installed in the glasshouse should be IP44 rated.
BS 7671:2008 Section 705 deals with agricultural and horticultural premises. Further information is also available in Guidance Note 7.
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