RCBO's Vs RCD & MCB

[rolyberkin]

Can someone assist please?

In my strive to understand all things electrical am struggling to find a reason over and above cost why it would be advantageous to use a RCD's and MCB's over a board fitted with all RCBO's?

Am I missing something or is it purely economics, the advantage of a pure RCBO board in my view being the continued operation of all other circuits if one circuit develops a fault.

 

[davesparks]

The advantage of an RCD over the common rcbos is that the rcbo doesn't break the neutral as it is a single pole device, an RCD breaks all poles (so double pole in the case of a single phase unit)
However RCBOs which break the neutral (SPSN - single pole with switched neutral) do exist but are less common.

But does this matter? In my opinion yes but there is no requirement in the regulations for the neutral to be broken.
An RCD or RCBO will detect a fault between neutral and earth and subsequently trip.
In the case of an RCD it disconnects the fault from the installation as it breaks the neutral.
A single pole RCBO however will disconnect the live only but leaves the fault connected. So we have a device which detects a fault on one conductor then disconnects a different conductor to the one with the fault on it. This is obviously completely illogical but also will allow a diverted neutral current from the entire installation to flow via the fault to earth. In large installations with high neutral currents this could lead to overheating of the neutral of the faulty circuit.

People argue that a fuse or MCB also does not disconnect the neutral, but these devices also do not detect faults in the neutral nor are they ever going to give a false impression that a fault has been disconnected when it has not.

 

[rolyberkin]

Many thanks for the reply, makes sense now.

 

[Paul.M]

Great post by Dave btw.

Here is a good example:- I'm doing a house rewire atm, the new owners work from home a lot and are changing one of the bedrooms into an office. They have had nuisance tripping affecting there work in the old house (lamps blowing/accumulative earth leakage) and do not want this to happen in a house they will live and work in for the next 20 years.

It is only a 3 bedroom semi but this is how the circuits have been designed, may help a few of you?


40A 10mm Shower
32A 6mm cooker
32A 2.5mm RFC kitchen
32A 2.5mm RFC downstairs
32A 6mm feed to loft for future conversion
20A 2.5mm radial to office/bedroom
20A 2.5mm radial to 2 bedrooms
20A 4mm 3Kw wall heater (only because I have some 4mm spare and it is better for this load)
6A 1.5mm downstairs lights
6A 1.5mm upstairs lights
6A 1.5mm smoke detectors

None RCBO

6A 1.5mm alarm (in conduit on the same wall as the CU,,, no point in having an RCBO)
32A 6mm SWA feed for garage (only 10m away) 2 lights, 3 sockets + 4 outside lights. T&E from CU run under the floor void (2ft away from any finished surface btw) to an external adapter box. 6mm SWA buried 12" under flag stones.

Garage CU WILL BE PLASTIC due to it being a better unit for its environment, IP65.

Single RCD 2 way board.
20A 2.5mm radial sockets
6A 1.5mm lights.

The use of RCBO's Vs RCD's boils down to the design of the install, the demands of the Regs and the requests of customers. The rest we work out lol. But it can be expensive!!!