No RCD in sub main

[KeenPensioner]

Hi, as I've said I'm not an electrician but studying C&G in my own time and have worked as a mate.
I've just had a qualified sparky upgrade my CU to a split board 10 way dual RCD. He has run a 4mm2 cable (from a 40A MCB protected by one of the RCDs) to my hut and will be coming back to wire up a sub main for a radial with a couple of double sockets and a light circuit. He says he is fitting a sub board whose main switch will have no RCD (since the circuit is already protected by the RCD form the main board?) I don't understand that. Can someone help? BTW, I couldn't find the DIY section to post this.

 

[littlespark]

If any circuit has an rcd at its origin, it doesn’t need another one.
2 rcd’s in-line like that can cause a lot of head scratching if there’s a fault....

4mm to a hut? What’s in there? Sounds like a lot of power for a potting shed

 

[buzzlightyear]

could he not put a rcd in the shed and put that circuit on the non rcd side .

 

[westward10]

40A seems oversized for a 4mm cable.

 

[littlespark]

could he not put a rcd in the shed and put that circuit on the non rcd side .

Dual rcd, Buzz. There’s rcd’s on both sides.

 

[buzzlightyear]

Dual rcd, Buzz. There’s rcd’s on both sides

yes I read that but you can move a rcd on some boards and put a mcb in for non side .

 

[Andy78]

If you're studying then performing a cable calculation for the hut feed could be a useful and enlightening experience.
A said, the RCD affords it's protection for all circuits downstream of it, it will detect imbalances in the hut distribution circuit and all the associated final circuits so another RCD in the hut is not necessary.

 

[buzzlightyear]

yes you are correct ANDY,but if a fault in the shed or their abouts the RCD will fail and what ever that is on that rcd in house will go off has well.but i did not design it .

 

[Andy78]

yes you are correct ANDY,but if a fault in the shed or their abouts the RCD will fail and what ever that is on that rcd in house will go off has well.but i did not design it .

Yeah it's not how I'd go about it either.

 

[KeenPensioner]

If any circuit has an rcd at its origin, it doesn’t need another one.
2 rcd’s in-line like that can cause a lot of head scratching if there’s a fault....

4mm to a hut? What’s in there? Sounds like a lot of power for a potting shed

Thanks for the prompt reply. Potting shed......I have great plans
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Wow, that was quick and thanks for the prompt responses. I agree that the 4mm cable (rated at 37A) is overkill. By my calculations on a load of say 4KW gives approx 18A and since there is little need to allow for derating factors (wooden framed man cave) then 2.5 mm (rated at 27A) would be enough, but if this is wrong please let me know.

 

[dmxtothemax]

it's always good practice to run a biger than needed cable,
1 - Reduces any loses
2 - Future proofed

 

[davesparks]

Thanks for the prompt reply. Potting shed......I have great plans
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Wow, that was quick and thanks for the prompt responses. I agree that the 4mm cable (rated at 37A) is overkill. By my calculations on a load of say 4KW gives approx 18A and since there is little need to allow for derating factors (wooden framed man cave) then 2.5 mm (rated at 27A) would be enough, but if this is wrong please let me know.

The problem is that if the cables maximum rating is 37A then the largest standard size ocpd it could be connected to is 32A, it is not suitable for a 40A ocpd unless overload protection can be omitted, which it can't for a submain.

I think you have some terminology confused in your OP, 'submain' is the common term for a distribution circuit. So a submain generally refers to the cable and associated ocpd which forms the circuit feeding a distribution board.

 

[KeenPensioner]

The problem is that if the cables maximum rating is 37A then the largest standard size ocpd it could be connected to is 32A, it is not suitable for a 40A ocpd unless overload protection can be omitted, which it can't for a submain.

I think you have some terminology confused in your OP, 'submain' is the common term for a distribution circuit. So a submain generally refers to the cable and associated ocpd which forms the circuit feeding a distribution board.

Thanks for taking the time to reply. After studying a bit more I can see what you mean. We have Ib = 18A, In = 40A and Iz = 37A. We need to have Ib<=In<=Iz so these numbers fail. Should have used 32A MCB - am I correct?

 

[sparks1234]

I dont have cable calc charts but I would have thought only 4 mm cable that might take 40 amp might be MICC

 

[KeenPensioner]

I dont have cable calc charts but I would have thought only 4 mm cable that might take 40 amp might be MICC

Thanks for taking the time to reply. I didn't mention that in my OP. It is 4mm MICC. I found this bit of the chart online..........4mm - 40amps method 1, 42amps method 11

 

[davesparks]

Thanks for taking the time to reply. After studying a bit more I can see what you mean. We have Ib = 18A, In = 40A and Iz = 37A. We need to have Ib<=In<=Iz so these numbers fail. Should have used 32A MCB - am I correct?

The cable should have been sized correctly for the selected OCPD.
A soloution now could be to use a 32A OCPD, but I wouldn't use an MCB if it can be helped.

 

[davesparks]

Thanks for taking the time to reply. I didn't mention that in my OP. It is 4mm MICC. I found this bit of the chart online..........4mm - 40amps method 1, 42amps method 11

Are you sure? That is a very odd cable to use for a domestic installation these days considering it costs around £10/m

 

[James]

Wow, I am surprised, not many of us out there that know how to gland off micc cable now days. Seems a surprising choice of cable for a domestic setting.

 

[KeenPensioner]

The cable should have been sized correctly for the selected OCPD.
A soloution now could be to use a 32A OCPD, but I wouldn't use an MCB if it can be helped.

Thanks again. As I've said, I'm not qualified so what alternative to an MCB would there be bearing in mind a 10 way dual RCD Hager board has just been fitted by my electrician

 

[davesparks]

Thanks again. As I've said, I'm not qualified so what alternative to an MCB would there be bearing in mind a 10 way dual RCD Hager board has just been fitted by my electrician

A switchfuse seperate from the main CU then an RCD CU in the outbuilding would be a better choice from the point of view of selectivity and minimising disruption in the event of a fault.