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.

 

[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.

 

[KeenPensioner]

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

I'm not sure whether its MICC but I attach an image of what they've run to my hut. None of the cable is exposed to the elements.

 

[westward10]

Looks like rubber flex to me. Can you show a larger section in situ.

 

[davesparks]

That looks like NYYJ to me, very different to MICC!

 

[KeenPensioner]

Looks like rubber flex to me. Can you show a larger section in situ.

Thanks for getting back so quickly. I attach another image. My sparky is coming back tomorrow to hook up the CU

 

[Pete999]

I'm not sure whether its MICC but I attach an image of what they've run to my hut. None of the cable is exposed to the elements.
View attachment 53750

Not MICC then, was this cable circular before you split it to show it's construction? or was it flat?

 

[westward10]

Are the copper cores solid, stranded or multi-stranded.

 

[KeenPensioner]

Not MICC then, was this cable circular before you split it to show it's construction? or was it flat?

Thanks for reply. The cable is circular

 

[KeenPensioner]

Are the copper cores solid, stranded or multi-stranded.

Thanks for the reply. Conductors are solid core

 

[westward10]

I go with Dave then in #23 It is NYY-J.

 

[Pete999]

Thanks for reply. The cable is circular

Solid conductors, some sort of FP Cable???