Are MCBs and RCDOs interchangeable?

[fex10]

Could an older consumer unit that has an inputter switch and MCBs have the MCBs replaced with RCBOs? Or do the RCBOs need a special consumer unit?

Also am I right that not all brands of MCB can be mixed with other brands due to the position of the live bar that distributes the live?

 

[PrenticeBoyofDerry]

You should not mix MCBs or RCBOs that are not type tested by the manufacturer for the board, regardless of busbar suitability!
Not all boards will take RCBOs, what type of board and what code of MCBs are present?

A few pictures would be very helpful!

 

[bernardgreen]

It is best to create the consumer unit with compatible items from the same manufacturer. That help with retaining type approval of the consumer unit.

RCBOs are almost twice the height of MCBs and have four connections at the top thus considerable more space is need for wiring an RCBO than is needed for wiring an MCB.

The four connections are Live to circuit, Neutral to circuit, Neutral feed from bus bar and functional Earth.

Yes you are right, different makes of MCB do have different positions for the live bus bar. Some are compatible but not all.

 

[JohnW2]

The four connections are Live to circuit, Neutral to circuit, Neutral feed from bus bar and functional Earth.

Methinks a small edit is called for!

Kind Regards, John

 

[davelx]

It also varies between different RCBOs. On my Eaton units, there is one connection at the bottom - the live busbar input, and three at the top: the incoming Neutral on a flying lead which connects to the CU Neutral terminal block, L and N to the load on terminals. There is no functional earth. From looking at other makes I think this is probably the most common configuration.

This shows the size difference between MCB and RCBO quite well:

IMG_0617

• davelx
• 27 Oct 2015

 

[bernardgreen]

Methinks a small edit is called for

No, four connections at the top and of course the Live connection to the Live bus bar at the bottom.

OK I will concede to "Neutral feed from bus bar" should be "Neutral feed from the neutral bus bar"

 

[bernardgreen]

RCBOs without a "functional earth" would have to be non electronic (*) as they could not trip if the Neutral supply was disconnected.

(*) non-electronic in that the trip mechanism is operated directly by the energy from the sensor's output winding without any form of electronic amplification or conditioning.

or maybe they are eletronic and rely on a battery or super capacitor ( power store ) to power the electronics when the Neutral is disconnected.

 

[JohnW2]

Methinks a small edit is called for

No, four connections at the top and of course the Live connection to the Live bus bar at the bottom. OK I will concede to "Neutral feed from bus bar" should be "Neutral feed from the neutral bus bar"

Fair enough - I misunderstood. I'm not used to hearing the neutral bar of a CU described as a "bus bar".

Kind Regards, John

 

[JohnW2]

RCBOs without a "functional earth" would have to be non electronic (*) as they could not trip if the Neutral supply was disconnected.

That's technically correct, but that doesn't stop their being a fair bit of debate, with some manufacturers saying (at least in the past) that connection of the functional earth was 'optional'.

I think the point is that the co-incidence of neutral being lost at exactly the same time that an L-E or L-CPC fault on the circuit arises is 'vanishingly improbable' .

Kind Regards, John

 

[ban-all-sheds]

Could an older consumer unit that has an inputter switch and MCBs have the MCBs replaced with RCBOs? Or do the RCBOs need a special consumer unit?

Also am I right that not all brands of MCB can be mixed with other brands due to the position of the live bar that distributes the live?

If you are thinking of replacing your MCBs with RCBOs you'd be better off replacing the whole CU rather than keeping what is essentially just an old plastic box. It will probalby be cheaper overall.

 

[ban-all-sheds]

From looking at other makes I think this is probably the most common configuration.

You get some designed for a 2P busbar.

 

[ban-all-sheds]

RCBOs without a "functional earth" would have to be non electronic (*) as they could not trip if the Neutral supply was disconnected.

That's technically correct, but that doesn't stop their being a fair bit of debate, with some manufacturers saying (at least in the past) that connection of the functional earth was 'optional'.

I think the point is that the co-incidence of neutral being lost at exactly the same time that an L-E or L-CPC fault on the circuit arises is 'vanishingly improbable' .

Kind Regards, John

 

[JohnW2]

<a quote from 'Commentary on IEE Wiring Regulations'>

"IEE Wiring Regulations"?

I'm not sure what point you are trying to make. Without mentioning the possibility that a 'functional earth' connection can avoid this problem, they do comment that a lost neutral can prevent operation of the 'RCD mode' of an RCBO - but we knew that!

What I don't understand is why this (as I said, I would think 'vanishingly improbable') 'simultaneous lost neutral and earth fault' scenario only ever seems to be mentioned in relation to RCBOs. However, given that all, or virtually all, 'domestic' RCDs these days involve 'electronic tripping' (in 'RCD mode'), I would have thought that the issue is at least as relevant to RCDs - yet I never see any mention of that, and I've never personally seen an RCD with a functional earth connection. Am I missing something?

Kind Regards, John

 

[bernardgreen]

The last RCD I opened up had no electronics, the output winding on the sensing toroid connected directly to the solenoid in the trip mechanism.

Almost certainly required carefull adjustment of the mechanics to get the trip to operate on the specified difference current. So expensive to produce. Electronically driven solenoids would not need mechanical adjustments so production costs would be much lower. Whether this is the reason that RCDs have electronics is hard to determine

 

[JohnW2]

The last RCD I opened up had no electronics, the output winding on the sensing toroid connected directly to the solenoid in the trip mechanism.

Was it an 'old' one? It's many years since I opened one and didn't find electronics.

Almost certainly required carefull adjustment of the mechanics to get the trip to operate on the specified difference current.

Indeed - and I would imagine probably appreciably less reliable than ones with electronic sensing in terms of long-term maintenance of the trip threshold.

Electronically driven solenoids would not need mechanical adjustments so production costs would be much lower.

Possibly - although there are obviously 'swings and roundabouts' as regards their relative cost.

Whether this is the reason that RCDs have electronics is hard to determine

Again, possibly, but that was not my question. As I said, what I don't understand is that we hear 'fuss' about the effects of a 'lost neutral' on (electronic) RCBO function, but not in relation to (undoubtedly much more common) 'electronic' RCDs - and why, if manufacturers have felt the need for 'functional earths' for many RCBOs, why not also for RCDs?

Kind Regards, John