Im doing some temp site supplies to cabins and wanted to clarify the following.
The supply is from an outside water proof box housing the electrical meter and main consumer unit with a Ze of 39 ohms. From this consumer unit 30ma RCBOs feed two swa for cabins and site offices. The consumer unit has a linked main switch. Is this ok or should it be an RCD as requested by the local electricity supplier.

thanks

Eamonn

Personally if it was temp site supplies I'd go with a time delay RCD at the front end just to keep everyone happy.

I agree that a main switch followed by RCBO's would work (providing it's a plastic distribution board) but I'm just not comfortable with it


Stu

Far be it from me but I don't agree with Stu. I thought the 100 mA RCD-S stemmed from the days when lighting circuits etc didn't require 30 mA RCD's but couldn't otherwise meet the required disconnection times on a TT system and the S was to provide discrimination for the 30mA RCd's. So, if as the OP suggests, all circuits have 30mA RCBO's then I can't see a need for a 100 mA RCD main switch. If it's not an insulated DB then the tails would need the 100mA RCD.

Whats on the end of those swa's - I strongly suspect DB's - probably with a 30mA RCD main switch or RCBO's.

Use a 100mA S type to protect the submains as stu suggested - ideally a simple 100mA TD main switch at the first DB - MCB's to feed the swa sub mains and 30mA RCBO's for any other cicuits locally (Site lighting ?, welfare facilities ?, gatehouse ?

What are the safety implications of the site offices etc all going off in one fell swoop on a bleak December afternoon with guys all getting out straight into the construction zone - you are the designer - you have responsibilities under CDM if nothing else - think it through and do it properly

regards

OMS

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Failure is always an option

Sorry if i made this unclear what the electricity supplier is saying that if RCBO s are used for circuits on tt or tn systems a rcd must also be used to provide disconnection of both line conductors to prevent current flowing back through neutral in the event of a fault as RCBOs are only single pole

I would have assumed you are using DP RCBOs anyway because a SP RCBO would not achieve discrimination for a time delayed RCD in the event of a N to E fault anyway

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

no single pole RCBOs on installation and currently no RCD