To use an RCBO or not to use an RCBO .

[alexander jackson]

Right Men, hear me out on this please.
I have a series of Rolec power pedestals with 2x 16 amp socket outlets on each one, each socket os protected by a C16 amp 30 mili amp double pole RCBO.
The supply cable is SWA and id inside an underground duct direct from the DB to the pillar, each pillar (there are two) has its own supply.
Now back at the DB each pillar is fed from a 32amp rcbo 30 mili amps instead of what I think should be just an MCB.
We all know that 100 miliamp time delay would be better at the Db to stop nuciance tripping which it does, and when I was doing RCD tests last week it was nigh impossible to keep my temper down with the rcbo at the board constantly tripping.
My question is does this installation need an RCBO at source when the cable is in a duct and completely protected.
Cannot quite see the reason.
Many thanks
Alexander.

 

[telectrix]

SWA cable does not require RCD protection unless a TT system where a RCD provides fault protection due to high Ze/Zs.

 

[Richard Burns]

The SWA if correctly installed is a cable with an earthed metallic covering so would not require 30mA RCD protection even if installed in a wall. As the cable is also in a duct there is no requirement for 30mA RCD protection.
As Telectrix states only if the installation at the DB is a TT system, without an ultra low Ra, would an RCD for fault protection be required, in that case the rating of the RCD would need to be low enough to operate with the existing Ra value and because of the 30mA RCBOs at the final circuits the RCD would need to be selective to allow discrimination.
The existing arrangement is not too bad from a compliance point of view because each RCD is protecting the same circuit so there is no nuisance tripping whichever one goes, but as you say the testing is a real nuisance.

 

[Andy78]

Check out the beama guide to RCDs and also bs7671. They are both very informative regarding basic issues surrounding RCDs like discrimination and requirements for use.

 

[alexander jackson]

Thank you for your replies. Just as I suspected and I will be speaking to my boss to replace these RCBOs with MCBs to make testing less trouble as RCBOs at source seem pointless in this installation.