UK MCB or RCBO to feed a second (garage) consumer unit

[Mark7124]

I'm a home owner with basic electrical knowledge; I needed my electrics 'brought up to code' in order to have an EV charger installed about 18 months ago. The main fuse was upgraded, a smart meter installed and a new consumer unit installed (also the garage consumer unit was also replaced) . So I have a consumer unit populated with RCBOs except for 1x MCB which supplies the consumer unit in the garage, is the correct or should it be 'protected' by an RCBO?
The reason I'm asking is because another electrician has recently done some work and said that it should be an RCBO instead and not an MCB. I'm on a TT earthing system if this has any baring on the situation.
Any advice would be appreciated.

 

[westward10]

If you have a TT earthing system it is not likely fault protection will be provided by an mcb alone so an RCD/RCBO is generally fitted by default. Can you show a picture?

 

[DefyG]

If the MCB is protected by a time delayed RCD and armoured cable it could still meet disconnection time of 2s!
Coordination between mcb's is another aspect to consider though.

 

[Mark7124]

If you have a TT earthing system it is not likely fault protection will be provided by an mcb alone so an RCD/RCBO is generally fitted by default. Can you show a picture?

Photos attached:
IMG_1023 is the garage consumer unit (I know it needs a couple of blanks),
IMG_1024 hopefully shows the TT earth coming through the wall into the consumer unit,
IMG_1025 is the main consumer unit, and circuit 3 is the feed to the garage consumer unit and he the one the breaker in question.
Cheers

 

[westward10]

That circuit to the garage almost definitely needs RCD protection. It is not impossible the mcb alone provides fault protection but TT systems are notoriously unstable and can't be relied upon so an RCD is normally the default method.

 

[Mark7124]

@westward10 thank you for your help, I will get the MCB changed for an RCBO.

 

[DefyG]

@westward10 thank you for your help, I will get the MCB changed for an RCBO.

Should really be a time delayed RCD which you won’t get in RCBO guise!

 

[Mark7124]

So I need a time delayed RCB in the main consumer unit as well as in the one in the garage consumer unit?

 

[westward10]

You would have to replace the main switch with a compatible 100A, 100ma time delayed switch. You can fit an RCBO at that circuit however you would lose selectivity with the RCD in the garage but that wouldn't be the end of the world.

 

[Risteard]

Personally I would opt for an S-type (time delayed) RCCB upfront of the distribution boards to give some protection against the tails coming into contact with the metallic DB. I would fit the RCCB in an insulated enclosure. (It's not a "similar switchgear assembly" to a DB in my opinion where there is only an RCCB present.) However the existing RCBOs should really be neutral switching to prevent a fault on one of these circuits from taking out the upfront S-type RCCB as well.

P.S. There are a couple of blanks missing from the Proteus DB, which should be addressed with urgency as this is hazardous.

 

[brianmoooore]

Although I agree with Risteard that a S type RCD in an insulated enclosure would be preferable, doubling up as the external isolator that's become popular these days, looking at the pics, it might not be practical for reasons of space.

 

[EricMark]

To my mind, if the garage is fed with SWA, I would not fit an RCD/RCBO in the house.

 

[westward10]

To my mind, if the garage is fed with SWA, I would not fit an RCD/RCBO in the house.

Provided ADS is maintained without it.