Sub main and TT

The disconnection time is one second rather than the five seconds required for a TN earthed installation, which you will achieve with a slugged 100 mA RCD or a 300 mA RCD.


I have been using 300 mA Type A RCDs, but those who know about these things have said that using a S-Type 100 mA Type AC RCD upfront of a 30 mA Type A RCD is not an issue.


It’s no wonder that people get confused about “types” of RCDs ?

Is the supply at the origin TT or are you making an island of TT spurred from an TN-s or c-s network ?


If the latter the armour may be protected by the suppliers earth, and then if you like it that way the RCD can be at the load end, which may be preferred...


If it is all TT, then yes an upfront RCD is needed to protect the cable regardless.


100mA delay is a relatively expensive but a good idea, certainly if you need it to discriminate against 30mA instant trip devices further down the line.  An upfront 30mA instant is possible, and cheaper of course but a pain in some situations, as of course it all goes off at once in a fault. (thought some  emergancy light is a possible mitigation)


Mike

Thank you both

Lewdon do a switch fuse where you can switch the main switch to an s-type if you want a fuse instead of a circuit breaker up front.


Gary

I'd too keep the fuse rather than an MCB for the submain - otherwise you'd likely loose discrimination with downstream MCBs/RCBOs on L-N faults.

  - Andy.

Good point Gary and Andy, thanks.

The 100 amp 100mA Lewden 2 pole S type is now type A. Also shop about, particularly if you have a trade account, as you can get these sub £40.   Some are charging £80+

Yes it's a type A.  I paid £36.88 for the switch fuse and £29.00 for the time delay rcd from Kew Electrical.  Both plus VAT.


Gary