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Sub main and TT
Grumpy
90 Posts
Question
I am dismayed that I have to ask but it is so long since I worked in a TT area that I am doubting myself.
I'm running an SWA submain to an outbuilding so instead of the usual switch fuse should I be using a 100mA S RCCB and a 63A mcb?
The armoured passing directly through the outside doesn't necessitate a 30mA RCD does it (as I have been so advised)
Apologies and thanks.
 
10 Replies
Sparkingchip
4016 Posts
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 🙄
mapj1
3421 Posts
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
Grumpy
90 Posts
Thank you both
 
aligarjon
102 Posts
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
AJJewsbury
2499 Posts
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.
Grumpy
90 Posts
Good point Gary and Andy, thanks. 
Grumpy
90 Posts
aligarjon:
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

Is that the basic SP&N switch fuse that TLC have on their website Gary?

Alcomax
298 Posts
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+
aligarjon
102 Posts
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
aligarjon
102 Posts

Yes that looks like it.  Price looks a bit steep though.  Worth shopping around.

Gary

Grumpy:
aligarjon:
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

Is that the basic SP&N switch fuse that TLC have on their website Gary?

 

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