MCB as a Main Switch in Garage Panel?

[DPG]

I don’t like that set up at all, 6mm cable should be protected by 32A protective device especially when you consider insulation factors, so from the utility meter what csa of tails/cable is selected 6mm? And what size supply cut out fuse is there? If it’s 60A needs to be 16mm if it’s 80/100A needs to be 25mm in my opinion. That diagram isn’t a British one is it?

Tails wouldn't necessarily need upgrading to 25mm.

 

[jowjow]

I don’t like that set up at all, 6mm cable should be protected by 32A protective device especially when you consider insulation factors, so from the utility meter what csa of tails/cable is selected 6mm? And what size supply cut out fuse is there? If it’s 60A needs to be 16mm if it’s 80/100A needs to be 25mm in my opinion. That diagram isn’t a British one is it?

6mm on a 40a mcb is what virtually everyone uses in my country. note that we don't use thermal insulation and its also up to local code as long as there isn't a long distance.

I know that its debatable whether to use a 10mm or a 6mm on a 40mcb but things are what they are and most likely a 6mm is used due to cost and ease of installation. I agree with you that it would be best to opt for the 10mm just to be safe because while a 6mm is in theory enough other things can effect performance - but its not up to me to decide.

From the utility meter its also 6mm and the supply is capped at 40a. Don't know about the fuse for this particular meter because none of this is already installed.

the diagram is not British

 

[pc1966]

Looking at your diagram do you mean 300mA/100mA instead of 300ms/100ms for the RCD as you also say delay and no-delay?

A 100mA RCD without delay is not considered acceptable here for personnel protection against shock, it has to be 30mA (or less no-delay). Really you would be better with three RCBO that each provide the 30mA protection if you have concerns about total leakage being too high for a 30mA incomer.

 

[jowjow]

Looking at your diagram do you mean 300mA/100mA instead of 300ms/100ms for the RCD as you also say delay and no-delay?

A 100mA RCD without delay is not considered acceptable here for personnel protection against shock, it has to be 30mA (or less no-delay). Really you would be better with three RCBO that each provide the 30mA protection if you have concerns about total leakage being too high for a 30mA incomer.

actually the diagram was just a quick sketch without being too technically detailed.

As you know when rcd's are used in succession such as in this case one must take into account discrimination. the 100ms (<= 100ms) is the tripping time of a standard rcd which is meant to trigger the moment theirs an earth leak without any delays

The 300ms is the delay for the upstream rcd.

The above just means that the downstream RCD will trigger first, followed by the upstream RCD (if leakage is still present)

RCBO's are cost prohibitive unfortunately

 

[pc1966]

actually the diagram was just a quick sketch without being too technically detailed.

As you know when rcd's are used in succession such as in this case one must take into account discrimination. the 100ms (<= 100ms) is the tripping time of a standard rcd which is meant to trigger the moment theirs an earth leak without any delays

The 300ms is the delay for the upstream rcd.

The above just means that the downstream RCD will trigger first, followed by the upstream RCD (if leakage is still present)

For selectivity you need both a difference in current threshold and delay. The first so a small slowly rising fault trips the downstream one first, the delay so a hard fault (which exceeds both currents) is cleared by the downstream one before the upstream one decides to act.

RCBO's are cost prohibitive unfortunately

What is your definition of prohibitive? You can get RCBO for £17+VAT each in the UK, for example:
https://thomaselectricaldistributors.co.uk/product/fusebox-mini-rcbo-double-pole-type-a/

If you only have 3 or so final circuits there and delete the cost of an incomer RCD due to the RCBOs providing that, it is not such a big coast difference for higher reliability.