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I went to a property yesterday very large upstairs flat. Meter & service head in the hall way down stairs, tails into an 30ma rcd then travel over 4m to cu upstairs in the cupboard. Cu is dual rcd 30ma. Problem is probably obvious, when a light pops the rcd trips downstairs and is a hazard as no light when coming downstairs. Flat has recently been rewired, TT earthing system. First of all there is no discrimination it will be pot luck which rcd trips, there should also be a switched fuse next to the meter as tails over 3m.
Tails are not surface mounted & travel through the fabric of the building. If it was TN i would say, use 25mm Swa as the tails, armour earthed & also a seperate core for an earth. (and no rcd at the intake).
Q.1. Would this be correct & acceptable?
Q.2. If the tails were surface mounted in a TN installation i would just use standard tails 25mm (no rcd), would this be correct & acceptable?

However the installation is TT, therefore could the 30ma rcd be changed for 100ma s type? And could the tails be left? (The tails may not be deeper than 50mm in the building fabric.) Would the tails have been better installed in s.w.a with 100ma s type front end.

Is there a better way of doing this?

Sorry quite a few questions there for a friday morning. There are 3 of us having a debate over this. Maybe the chaps who installed it, should have had this debate :25:

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Put a 30mA time delayed RCD in, or put a 100mA RCD downstairs - its only protecting the cables against mechanical damage and the main board upstairs has 30mA RCDs installed (need to check the regs for 100mA in hall)

Put Emergency Lighting in the hallway at the top and bottom of the stairs - these will stay on - these also should be on a separate source to what the flats are on - plus the Fire officer will want to see this anyway.

Each flat and hallway electric needs to be separated so nothing should be feeding off the same RCD in the hall - Ideally a 3-phase - 1 phase per flat

Amp David

If his tails are less than 50mm in the wall and don't have any protection as defined in the regs, they should have additional protection by a 30mA RCD.


  • Thread Starter Thread Starter
  • #4
Hi there i have never seen a 30ma time delayed rcd, 30ma is for additional protection therefore what use would time delay be. Also this is just one upstairs flat (extremely large one. (it is not a shared building and has no landlord supply.)


Well , looking at this , i'd say your problem is the cable and theres no easy fix.
Bs 7671 requires a 30mA rcd protection for cables buried in the wall so swopping to a 100mA for the sub-mains isnt going to comply.
SWA would be ok , but i'd prefer surface mount pvc trunking for the tails on a TT system with no rcd at the mains , just a switchfuse instead.
Depends on the customers budget and whether they would be ok with visible trunking in the hallway

Deleted member 9648

Only way I can see out of this is ditch the existing tails and run surface plastic trunking with new tails and either no RCD at the intake,just a main switch and OCPD......(complies)...or preferably a T/D 100ma and OCPD which will give back up RCD protection but good discrimination. Alternatively use surface SWA instead of tails in which case the 100ma T/D will be needed for earth fault protection to the SWA,unless of course you have TN values of Ra as seems to be the norm for everyone in the UK apart from me.
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