Protecting runs between buildings from short

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Hi Guys,

Does anyone here use RCBOs/MCBs between the mains and an outbuilding CU? I've heard of people using time-delayed RCDs at the mains, but what if the run between the buildings is damaged for whatever reason and there is a short between L-N?
 
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Does anyone here use RCBOs/MCBs between the mains and an outbuilding CU? I've heard of people using time-delayed RCDs at the mains, but what if the run between the buildings is damaged for whatever reason and there is a short between L-N?
The cable has to have over-current protection provided by something - so, a fuse, MCB or RCBO.

What sort of cable are you contemplating and how will it be routed (buried underground or what)?

When you say "the mains", where are you thinking it will originate?

Kind Regards, John
 
Yes, precisely my thoughts, but I've seen people use a 100ma T/D RCD only, so no over current protection.

It'll be either 16mm^2 or 25mm^2 SWA (still to finalise max current for all machinery), and about a 25m run. The plan is to run it under the floorboards, out the 2nd floor exterior wall (drop is about 3 metres to ground), then underground through some twin wall duct and into the other building.

By mains I'm inferring the primary CU.
 
Yes, precisely my thoughts, but I've seen people use a 100ma T/D RCD only, so no over current protection.
As I said, there has to be over-current protection, even if there is an RCD. However ...
By mains I'm inferring the primary CU.
Do you mean that it will be originating from an MCB in the main CU? If so, that MCB would provide the required over-current protection. However, what sort of current are you envisaging may be required (and hence what would be the rating of that MCB)?

Kind Regards, John
 
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As I said, there has to be over-current protection, even if there is an RCD. However ...
Do you mean that it will be originating from an MCB in the main CU? If so, that MCB would provide the required over-current protection. However, what sort of current are you envisaging may be required (and hence what would be the rating of that MCB)?

Kind Regards, John
It's a bit convoluted but at the moment I have a 40 amp supply, and the supplier has offered to upgrade it to 60a for free
which should suffice in the short to medium term. Eventually I plan to upgrade to 100a.

I am anticipating about 15-20a peak at the moment. BUT, I do intend to purchase some inverted MIGs, a TIG, and possibly an air compressor so I expect that to bump it up to 50-60a.

As for the MCB, I am thinking a 63a Type B should suffice for now.
 
... BUT, I do intend to purchase some inverted MIGs, a TIG, and possibly an air compressor so I expect that to bump it up to 50-60a.
There are quite a lot of things to be said in response to what you have just told us, but one of first questions is whether you have asked for, and obtained, permission to run the loads you mention above from your supply (assuming that it is upgraded)?

Kind Regards, John
 
There are quite a lot of things to be said in response to what you have just told us, but one of first questions is whether you have asked for, and obtained, permission to run the loads you mention above from your supply (assuming that it is upgraded)?

Kind Regards, John
The 50-60a will be the case once I have upgraded to a 100a supply. I am using inverter MIGs primarily so I can cap the input current and won't need to fork out for a 3-phase supply, or annoy the neighbours. With that said, surely I won't need to ask for permission to use 60a when I have paid for a 100a supply?
 
With that said, surely I won't need to ask for permission to use 60a when I have paid for a 100a supply?
You wouldn't pay any more for 'having' a 100A supply than for 'having' a 40A one, although you might get a very hefty initial one-off bill if they have to lay in new cables.

However, regardless of what supply you have, you need to get permission to install things like welders, compressors or large machine tools, because they can have an impact on the supply to neighbouring properties. Don't forget that, although most dwellings have a '60A' or '80A' supply, the suppliers work on the assumption that the average load represented by each dwelling is about 10A.

Others may have things to say about this.

Kind Regards, John
 
You wouldn't pay any more for 'having' a 100A supply than for 'having' a 40A one, although you might get a very hefty initial one-off bill if they have to lay in new cables.

However, regardless of what supply you have, you need to get permission to install things like welders, compressors or large machine tools, because they can have an impact on the supply to neighbouring properties. Don't forget that, although most dwellings have a '60A' or '80A' supply, the suppliers work on the assumption that the average load represented by each dwelling is about 10A.

Others may have things to say about this.

Kind Regards, John
That is a fair point. I guess over provisioning is normal business practice in this day and age.
 
but what if the run between the buildings is damaged for whatever reason and there is a short between L-N?
The fuse or circuit breaker will disconnect the supply.
An RCD will not detect a L-N fault.

but I've seen people use a 100ma T/D RCD only, so no over current protection.
Those people were blunderers who had no idea what they were doing.
A L-N fault on such an arrangement might take out the main suppliers fuse if you are lucky.
Or it could continue to arc and burn until the cables have melted.
 
A L-N fault on such an arrangement might take out the main suppliers fuse if you are lucky. Or it could continue to arc and burn until the cables have melted.
That was obviously my initial concern when I started quizzing the OP. However, he has now indictated that he is thinking of feeding the outhouse supply from an MCB in his house's CU - in which case there obviously would, in fact, be over-current protection.

However, as you will have seen, he has now mentioned things which put a new complexion on his whole project - and, of course, we would be talking about notifiable work.

Kind Regards, John
 
Just for clarity's sake, I was always intent on using an RCBO type of arrangement to feed the outbuilding. I was merely curious as to whether there was a reason why some sparks were omitting MCBs, which to me seemed a bit silly and dangerous.
 
You do not really want an RCD on the supply side of the cable.

It will be inconvenient in the workshop if a small fault causes the disconnection of everything.
 

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