Protecting a 2nd consumer unit

[vwes59]

Done this loads of times before, but this situation is quite unique so bare with me.

I've been asked to cable up an out building at a country park which is about 30m from another out building which has mains supply already installed from a main building some distance away via a 10mm SWA buried cable (It is fed from a Henley block in the main house, not through the main house consumer unit). This cable terminates in a small consumer unit with an RCD 63A main switch, and has 3 MCB circuits off that. (Those 3 circuits protected by the RCD main switch)

I'm going to hook up a 4mm SWA cable into a 32A MCB in the first out building consumer unit, but I don't want a fault in the 2nd out building to trip the RCD Main Switch in the first out building.

So my thought was, to not export the earth from the first out building to the second, but to install an earth rod at the 2nd outbuilding and it having it's own independent earth arrangement (TT). This would mean I could install an RCD in the 2nd out building without causing a selectivity issue.

I'm having one of those days with loads of unusual jobs, so my brain is fried - Am I right in my plan or talking a load of rubbish ?

 

[ferg]

My first thoughts would be to:
Use a switch fuse at the house end to protect the first cable, as to my mind it is non compliant using the DNO fuse and you are arguably making it less safe by adding load to it.
Install a second CU with MCB / replace MCB's with RCBO's except for 4mm at second building.
Keep DNO earthing as it's less hassle expense and arguably more reliable.

Undoubtedly someone smarter / more awake / different approach will be along shortly to contradict me.

Edit: Obviously not keeping RCD main switch when installing RCBO's

 

[Pretty Mouth]

So my thought was, to not export the earth from the first out building to the second, but to install an earth rod at the 2nd outbuilding and it having it's own independent earth arrangement (TT). This would mean I could install an RCD in the 2nd out building without causing a selectivity issue.

This won't help the selectivity issue. Any current straying from the circuit downstream of the RCD will cause an imbalance at the RCD, so tripping it.

 

[timhoward]

With this kind of job you have to keep an eye on
-selectivity of over current protective devices (and MCBs generally won't offer selectivity)
-selectivity for additional protection so a fault in the final area doesn't trip upstream RCDs, as you said
-total Zs
I think @ferg has a good plan and I wouldn't be happy extending an installation that is already hanging off the DNO fuse.

 

[mainline]

Done this loads of times before, but this situation is quite unique so bare with me.

I've been asked to cable up an out building at a country park which is about 30m from another out building which has mains supply already installed from a main building some distance away via a 10mm SWA buried cable (It is fed from a Henley block in the main house, not through the main house consumer unit). This cable terminates in a small consumer unit with an RCD 63A main switch, and has 3 MCB circuits off that. (Those 3 circuits protected by the RCD main switch)

I'm going to hook up a 4mm SWA cable into a 32A MCB in the first out building consumer unit, but I don't want a fault in the 2nd out building to trip the RCD Main Switch in the first out building.

So my thought was, to not export the earth from the first out building to the second, but to install an earth rod at the 2nd outbuilding and it having it's own independent earth arrangement (TT). This would mean I could install an RCD in the 2nd out building without causing a selectivity issue.

I'm having one of those days with loads of unusual jobs, so my brain is fried - Am I right in my plan or talking a load of rubbish ?

What type of earthing at the origin ? And is the earth exported to the 2nd building ?

 

[davesparks]

Am I right in my plan or talking a load of rubbish ?

You are talking a load of rubbish.

Creating a seperate TT installation at the outbuilding will not affect the selectivity of the RCDs.

RCDs detect an imbalance of current flowing between live and neutral, it doesn't matter what your earthing system is.