Which RCD for Garage Consumer Unit, mains coming from house

Squiral1 · 17 Sep 2020

Hi All,

I have a armoured cable going from the main house to detached garage.
In picture 1 & 2 you will see the armoured cable in the house consumer unit attached to a RCD B32 (Garage sockets)

Now the question: Should the armoured cable going into the garage consumer unit be connected to a same B32 RCD? Is that ok? or should it be a different RCD?

See picture 3 for the garage consumer unit.

I will have a sockets ring circuit and lights circuit in the garage#

Thanks Very Much for the advise.

winston1 · 17 Sep 2020

The garage feed is on a 32A MCB which fed from a RCD above the green label. You don't need a second RCD in the garage.

In fact you don't even need a CU in the garage, the sockets ring can be connected directly with the lights via a switched FCU with a 5A fuse used as the light switch.

EFLImpudence · 17 Sep 2020

It is an MCB - Miniature Circuit Breaker - not an RCD - Residual Current Device.

The RCD is the one above the green label.

Squiral1 said:
Should the armoured cable going into the garage consumer unit be connected to a same B32 RCD? Is that ok? or should it be a different RCD?

Not really. There is no need for another MCB of the same rating.

Also, it is not ideal to have the garage supply on the house RCD - it will trip with a fault in the garage.

I will have a sockets ring circuit and lights circuit in the garage#

Why so many cables, then?

AndyPRK · 17 Sep 2020

It's typical to have a 40A RCD in the garage CU. (or 63 or 80 etc. is fine too)

Taylortwocities · 17 Sep 2020

AndyPRK said:
It's typical to have a 40A RCD in the garage CU. (or 63 or 80 etc. is fine too)

Yes, but only if the feed is from a non-RCD protected way. Otherwise it is pointless having a second RCD in the outbuilding. As the OP will discover the first time he presses the test button on either RCD (or if there is an earth fault).

@Squiral1 Where is the earth for the SWA armour coming from? Can't see any evidence of it at the outbuilding end......

AndyPRK · 17 Sep 2020

yes and no. No harm in having it. The test buttons don't always trip each other.

One of the cores maybe used at the earth. Black

Taylortwocities · 17 Sep 2020

AndyPRK said:
The test buttons don't always trip each other.

One of the cores maybe used at the earth. Black

Doesn’t always trip both, but it does quite a lot!

I wasn’t looking at the internal cores. I also assumed that, as there are three cores, one of them might be used as the supply earth.

I specifically questioned the earthing of the armour. That’s the bright steel stuff that is on the outside of the cable. No banjo etc at the outbuilding, just curious if this habit is the same at the house end.

AndyPRK · 17 Sep 2020

yeah its the house end that is important, as you say.

From the pic we can see ( of house cu) it looks professional entry

Taylortwocities · 17 Sep 2020

Professional

Definition =

following an occupation as a means of livelihood or for gain

not to be confused with competent!

Squiral1 · 17 Sep 2020

Thanks for the replies all. The garage feed is earthed using the grey cable see picture 4 attached.

Using the correct terminology now - the garage is fed from a 32A MCD, so I could just put the same on the garage consumer, 32A MCD? I already have the consumer unit there so just to make it neat. Does that create problems?

The additional cables in the outbuilding are for future items like having a hob and oven in there

Taylortwocities · 17 Sep 2020

Squiral1 said:
. The garage feed is earthed using the grey cable see picture 4 attached

the grey cable should be for neutral. It should be sleeved blue. the black cable should be earth, sleeved green yellow.

I was not concerned about that. My query is where is the armour of the SWA earthed? Your photo doesn’t show this clearly, or at all.

re terminology. The are RCDs and MCBs. There are no MCDs.

the future oven and hob. Just how long is your cable run??

Squiral1 said:
the garage is fed from a 32A MCD, so I could just put the same on the garage consumer, 32A MCD? I

NO! You must have discrimination between up and downstream devices. The downstream must be quite a lot smaller that the supply end, otherwise in the event of a fault the house MCB will trip.
Good installation design provides localisation of fault. You go not have good installation design.

Squiral1 · 17 Sep 2020

Oops sorry I meant MCB.
Understood re grey and black, this is how I found it, but can switch them.

Cable run is about 30 meters. I'll look at the earthing of the SWA.

Taylortwocities said:
NO! You must have discrimination between up and downstream devices. The downstream must be quite a lot smaller that the supply end, otherwise in the event of a fault the house will trip.
Good installation design provides localisation of fault. You go not have good installation design.

Right understood, so what would you recommend?

chivers67 · 17 Sep 2020

Can you clarify the Conductor size of the SWA it should be marked on the sleeving in Writing. Looks 6mm but i'm not 100%

AndyPRK · 17 Sep 2020

OP. In that small CU, you would normally have a (double width) red main switch or an RCD. Do you have either of those already? What brand is that small one?

NO! You must have discrimination between up and downstream devices. The downstream must be quite a lot smaller that the supply end, otherwise in the event of a fault the house MCB will trip.
Good installation design provides localisation of fault. You go not have good installation design.

So that's an interesting discussion point.

Lets take an example. You have 2 MCB's in this CU. 6A for lights, 32A for socket (ring) - Ok a ring maybe considered excessive, but on a 6mm supply cable and the OP have already configured it as a ring.

What is more important ?

A) Discrimination.
B) Having usable power. If you were to locally fit a 20A MCB you would be missing out on 12A of current you could have.

Ok, if its a workshop with cutting equipment, you wouldn't want the lights going out with moving machinery. (though both would stop at the same time)
And remember this is an MCB we are talking about. (not RCD). So very unlikely to trip anyway.

Personally I would want the 32A MCB, and accept in the very unlikely event of the MCB tripping, the MCB in the house would also trip.

What about fitting a C32 MCB at the house?

EFLImpudence · 17 Sep 2020

It is also on the house RCD.

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Which RCD for Garage Consumer Unit, mains coming from house

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