MCB as a Main Switch in Garage Panel?

[jowjow]

Hello and thanks for checking this out.

I have a garage with its own consumer unit and at this point I'm trying to get some feedback on the pros and cons whether its best to install a "double pole mcb" or "main switch" in the garage panel before going to the individual mcb's.

so whats your opinions?

(I have seen similar installations and they seen to prefer one or the other)


Thanks

 

[Dartlec]

I can't speak to what the code in Malta allows, or what is normal practise there.

But from a UK perspective, the issue would be that there is no over current protection at the source end in the scenario you are giving. (Other than the main incoming supplier fuse).

We can use the main incoming fuse as protection for meter tails up to a certain distance, but they are 10-25mm cables. I thought I saw 4mm being mentioned for this feed, but can't see that now looking back.

That may be why the Main switch B is often replaced with an MCB I presume, to provide some sort of overcurrent protection, albeit at the load end. If the run was short and suitably protected etc then it might be electrically safe, but I'd consider it bad practise when an MCB or RCBO could be added at the initial board.

Otherwise, what is to stop people adding new MCBs on the garage end and overloading the current feed cable beyond it's safe rating in the future?

 

[jowjow]

I can't speak to what the code in Malta allows, or what is normal practise there.

But from a UK perspective, the issue would be that there is no over current protection at the source end in the scenario you are giving. (Other than the main incoming supplier fuse).

We can use the main incoming fuse as protection for meter tails up to a certain distance, but they are 10-25mm cables. I thought I saw 4mm being mentioned for this feed, but can't see that now looking back.

That may be why the Main switch B is often replaced with an MCB I presume, to provide some sort of overcurrent protection, albeit at the load end. If the run was short and suitably protected etc then it might be electrically safe, but I'd consider it bad practise when an MCB or RCBO could be added at the initial board.

Otherwise, what is to stop people adding new MCBs on the garage end and overloading the current feed cable beyond it's safe rating in the future?

Thanks for the message and great observation.

With Regards to a DP MCB at the source. This would add another layer of safety. My concern is that in rare instances an MCB could malfunction and not disconnect even through its manually flipped off.

But to counter argument the above, the utility company meter also has an integrated switch to disconnect from (with over current protection too, and not just via a fuse)

So I guess that's why some people use a Main Switch and others a DP MCB, almost interchangeably - because any would work, yet a DP MCB at source would cut off the supply very early on.

thanks for the observation

 

[Grant1987]

thanks for the reply and details. the wire to garage will be a 4mm wire in a pvc pipe (more than enough given the relatively short distance)



almost like you said but theirs no mcb (by design) at that point. basically the flow is like below:

Utility Company Meter(Which some type of overcurrent protection too) -> Main Switch A -> Surge Protector-> Over/Under Voltage Relay/Protector -> RCD (with time delay for discrimination) -> ---------Wire to Garage Panel------------> Main Switch B -> RCD -> individual mcbs

The above is a standard setup in my country so I'm not trying to reinvent the wheel.

So as not to get out of topic, my question with reference to the above was:

"Main Switch A" and "Main Switch B" - I have seen many(the majority) installations using a DP MCB instead of a Manual Main Switch/isolator, especially for "Main Switch B"

Also, some local shops dont even stock them in all brands and would tell you to get a DP MCB instead. And obviously I'm not very keen in opting for something just because the shop happen to have only that one...

Also, just because many local electricians do that does not mean that its the best way.

Personally (and I stand to be corrected) I think the ideal setup would be like this:

Utility Company Meter(Which some type of overcurrent protection too) -> Main Switch A -> Surge Protector-> Over/Under Voltage Relay/Protector -> RCD (with time delay for discrimination) -> ---------Wire to Garage Panel------------> DP MCB -> RCD -> individual mcbs

Notes:

Main Switch A - a manual isolator is a must here because its the only sure way that the power can be switched off (a DP MCB could malfunction given that it has more components)

DP MCB - an added layer of safety for overcurrent here, because one could always used the "Main Switch A" to be 100% sure that electricity is off.

any insight would be greatly appreciated, thanks

Right ok you’ve done one of my pet hates and over complicated the situation. You’ve run a 4mm twin and earth pvc/pvc cable in plastic conduit to garage consumer. You’ve used what size protective device at source to protect 4mm cable? And what size double pole mcb at load end?
I personally do it different to this but I like to learn and see how others do it.

 

[jowjow]

Right ok you’ve done one of my pet hates and over complicated the situation. You’ve run a 4mm twin and earth pvc/pvc cable in plastic conduit to garage consumer. You’ve used what size protective device at source to protect 4mm cable? And what size double pole mcb at load end?
I personally do it different to this but I like to learn and see how others do it.

actually I got the answer already for my question (which was very simple and then kind of went out of topic)

But to keep on the conversation. The wires between panels will be 6mm (I wrote 4mm by mistake last time) and protected by a 40amps DP MCB.

The wires will be single core. The distance between panels is of less than 10 meters so a 6mm wire in a pvc pipe should be nice and cool. For longer distance would probably use 8mm just in case.

You can find a quick diagram of the setup. Note that said setup is how 99% of installations are done locally so I'm not taking the credit for it or anything.

 

[Grant1987]

actually I got the answer already for my question (which was very simple and then kind of went out of topic)

But to keep on the conversation. The wires between panels will be 6mm (I wrote 4mm by mistake last time) and protected by a 40amps DP MCB.

The wires will be single core. The distance between panels is of less than 10 meters so a 6mm wire in a pvc pipe should be nice and cool. For longer distance would probably use 8mm just in case.

You can find a quick diagram of the setup. Note that said setup is how 99% of installations are done locally so I'm not taking the credit for it or anything.

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?

 

[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.