Question regarding SWA Cable

[Dave-sparks]

its pointless having them you allready have a mains isolation switch on the twin rcd board

But they are quite useful when you want to isolate the supply to the CU to work on it or do testing.

 

[brightspark]

Just to update, the electrician came round today, cracking down to earth guy, he had a look at the house side of the supply and then had a look at the workshop, asked me a few questions to gather info in what I was requiring and had a look at the install I’ve done already, I’ve 9 led strip lights all wired up through conduit and temporarily connected to a plug run off an extension cable so I can see what I’m doing.
Theres a connection box at each light with joining terminations using Wago connectors, his only criticism was that I should have used glands where each light cable goes through the 20 MM cutout at the junction boxes, that’s an easy fix.
He‘s going to add a tail from the house supply which will go into a mini CU and suggested that a 40 Amp RCD should go there that way it protects the cable and I won’t need one at the garage CU then at the garage end I can have a CU with everything I asked for as he understood that it’s just me myself and I and said that as long as I use common sense there should be no problems with tripped CB’s
So I can have a 32 amp socket , he suggested a C breaker for the welder
A 16 Amp C breaker for the car lift
32 amp ring main and 6 amp lights
He was happy with my lights wiring so he’s fine with me wiring all the sockets, told me not to connect all the faces so he can check them, he told me I can get the house and garage CU’s, glands etc and he’ll fit them, he recommended Hager CU, but as long as its metal with a 40 amp RCD.
So on looking at CU’s I see a small one which has a 100 amp switch with space for an RCBO for the house end, I’m presuming that an RCBO can be used instead of an RCD as it does the same thing but with overload protection?
I‘ll also get a good metal CU for the workshop with no CB’s and just get the cb’s separately with the C rating needed where required.

you can get a 1 way db with rcd mainswitch and the 40 amp breaker in with it i says shower but can be used for any submains https://www.screwfix.com/p/chint-nx3-3-module-1-way-populated-shower-consumer-unit/2611v

 

[GRW]

When I did my shed, I took the opportunity to bring my house CU up to the current regs with a new one (with more breaker slots in to accommodate the feed to the shed and a spare or two).

Then, I utilised the old house CU for the shed

 

[AVI_8]

Hi Dave, I understand everything you just said and it makes sense, though one question, what would protect the cable in the event of a short with no RCD I know it’s swa so a short in the cable is unlikely, I’m not sure if he intends to take the tail from the meter cable to the cu or from the cu itself, with hindsight I should have asked

 

[AVI_8]

you can get a 1 way db with rcd mainswitch and the 40 amp breaker in with it i says shower but can be used for any submains https://www.screwfix.com/p/chint-nx3-3-module-1-way-populated-shower-consumer-unit/2611v

Hi thanks for that, Does it take 20 MM glands for the SWA outlet?

 

[AVI_8]

When I did my shed, I took the opportunity to bring my house CU up to the current regs with a new one (with more breaker slots in to accommodate the feed to the shed and a spare or two).

Then, I utilised the old house CU for the shed

My house was fully rewired 6 years ago with a new CU then, it doesn’t have RCBO though

 

[brightspark]

My house was fully rewired 6 years ago with a new CU then, it doesn’t have RCBO though

it will be twin rcd db then or should be

 

[AVI_8]

it will be twin rcd db then or should be

There’s a pic of my cu in post 53 here, not sure what twin red db is

 

[brightspark]

There’s a pic of my cu in post 53 here, not sure what twin red db is

yep twin rcd

 

[Dave-sparks]

Hi Dave, I understand everything you just said and it makes sense, though one question, what would protect the cable in the event of a short with no RCD I know it’s swa so a short in the cable is unlikely, I’m not sure if he intends to take the tail from the meter cable to the cu or from the cu itself, with hindsight I should have asked

The fuse or MCB protects against faults (including short circuits) and overloads.

A 30mA RCD is used as additional protection against electric shock, it isn't normally used to protect a cable. They are required for a number of reasons, including where non-armoured cables are buried in walls (keeping it simple, there's a lot more detail to the regulation), but this does not protect the cable, it protects a person if they accidentally nail/drill into the cable.

Armoured cable doesn't need this because anything conductive which penetrates the cable will first make contact with the earthed armour befire making contact with any live conductor.

 

[brightspark]

The fuse or MCB protects against faults (including short circuits) and overloads.

A 30mA RCD is used as additional protection against electric shock, it isn't normally used to protect a cable. They are required for a number of reasons, including where non-armoured cables are buried in walls (keeping it simple, there's a lot more detail to the regulation), but this does not protect the cable, it protects a person if they accidentally nail/drill into the cable.

Armoured cable doesn't need this because anything conductive which penetrates the cable will first make contact with the earthed armour befire making contact with any live conductor.

it does need rcd protection if its fed from a tt supply .it can be fed with a 100 ma time delayed trip though if the downline db has a 30ma trip or rcbo fitted

 

[Dave-sparks]

it does need rcd protection if its fed from a tt supply

But not 30mA RCD protection. If it is a TT supply and the Ra is too high to meet disconnection times then an RCD will be required for fault protection, a time delayed RCD 100mA or more would be far more suitable than a 30mA RCD protecting a submain.

Plus of course we already know it is TN supply from the above picture.

 

[brightspark]

But not 30mA RCD protection. If it is a TT supply and the Ra is too high to meet disconnection times then an RCD will be required for fault protection, a time delayed RCD 100mA or more would be far more suitable than a 30mA RCD protecting a submain.

Plus of course we already know it is TN supply from the above picture.

look at the post . i put a tt supply with a 100ma time delayed NOT A 30

 

[Dave-sparks]

look at the post . i put a tt supply with a 100ma time delayed NOT A 30

Which you edited after I replied ‍

 

[brightspark]

Which you edited after I replied ‍

no i diodnt it would say if i did

 

[AVI_8]

Just read back to what I said earlier and remembered what the electrician said to me why the 40 amp RCD should be at the house end of the SWA, I can’t remember exactly what he said but I think he meant that’s it should be there to protect the SWA from receiving a current which exceeds the rating of the cable and he said that by having it at that end I wouldn’t need one at the workshop end. TBH I think he was trying to make the install as less expensive as possible.
Though I have no problem spending a bit more at this stage to make thinks as simple and as safe as possible.
Dave and Brightspark I’d love to have your knowledge, I thank you both for your input it’s very much appreciated

 

[dobbslc]

It's worth fitting an emergency light over the exit door just in case the power trips and you're left standing in the dark!
Even more so if the RCD is in the house!

 

[AVI_8]

Thanks that sounds like a good plan, though I already have the lighting install in place and at that end of the workshop where the switch is I think I’m near the limit of wires running through the conduit using the factor table at that junction box, there’s three strips of 3 lights plus the input from the CU which is at the other side of the workshop so that particular bit of the run is a bit too busy unless I opened it up and increased the size to 25mm which would require some surgery at this stage.
I think I’ll just rely on my sense of direction, worst case scenario it’s 9 metres to the door.

 

[awemawson]

It's worth fitting an emergency light over the exit door just in case the power trips and you're left standing in the dark!
Even more so if the RCD is in the house!

Just to underline to the o/p that needs to be a battery backed light near the door - anything else and you have nothing to power it !

They are not expensive - something like this perhaps:

LED Exit Box Maintained & Non Maintained | Knightsbridge (EMRUN)

 

[dobbslc]

I don't know many emergency lights that don't have battery backup