RCBOs or MCBs?

[Donkmeister]

Hi,

I'm planning a full rewire (discussed with Buildings Control, will be properly tested and notified, Part P).

However, despite scouring my copy of the 17th edition, the diynot wiki, the forum and the TLC-direct website, I'm still unsure of what to do with RCBOs! I'm looking at getting a dual-RCD "17th edition" consumer unit.

1) Am I right in thinking an an RCBO should be fed straight off the main switch, or should it be protected by a main RCD?
2) is an RCBO always preferable to an MCB, or should they only be used for specific conditions (such as where you want an RCD-protected circuit that isn't going to trip the main RCD if it faults).

The circuits I'm planning on putting on RCBOs are:
1) Fridge-freezer
2) outdoor lighting circuit
3) feed to the shed consumer unit (will be wired via adaptable box and SWA)
4) central heating

 

[JohnD]

RCBOs do not need to be fed from an RCD (and should not be - in the event of a fault they will probably both trip at the same time*). Worse still, with single-pole RCBOs, an earth/neutral fault will not be broken so will cascade down the the RCD even when the RCBO has tripped.

An RCBO is preferable to an MCB where the MCB is one of several fed from a single RCD. However it is a more expensive way to do it. If you are doing the work yourself you can consider using some of the money you save to get good quality components, and RCBOs on every circuit. in this case you will not need an RCD as main switch.

*unless you have a time-delayed RCD. You have no need for one if all your circuits are already protected by RCDs.

 

[Sailbadthesinner]

1 Yes RCBOs should all be fed from the main switch not the RCD.

2 Preferable but expensive best on the circuits you don't want nuisance tripped by other faults ie freezers or those you foresee being troublesome ie outside circuits (not that any new wiring should ever be)

The other option is to group the critical circuit MCBs (freezers, video recorders, PCs etc) on one RCD and the higher risk circuits (outside lights, sheds, heating etc on the other RCD.

Best of Luck
Sailbadthesinner