Seperate CPC out to garage - duct or not to duct?

[munty12]

Hello there. I'm forever looking over these forums looking for advice, and typically I always find an answer, except it seems in this one instance - which is what brings me on my merry way to here today.

I was thinking of running a new supply to the garage. Problem is it has a water pipe (only four inches of lead and the rest is poly!) so I need to earth it with a 10mm. The garage is only 1 metre away from the house. I dug a trench one metre long across the concrete path between the house and garage (about 300mm deep)

The length of the run is only 6M in total, so I thought I'd run a 10mm 3 core SWA, protected by a 1 way RCD CU, but someone else suggested running a smaller SWA supply cable with a seperate 10mm CPC for the water main - which makes obvious sense, right? This being the case, I wondered what are the recommendations for protecting a 10mm CPC in a domestic trench. From what I gather it has to provide mechanical protection and be anti-corrosive. Surely galv conduit is too extreme for this application? Is it okay to simply run the CPC in a PVC duct of some description? Is it ok to use polypipe?
Electrical Conduits | International

Which begs the quetion, if polypipe is considered to provide the correct protection, what's stopping me from doing away with SWA altogether and instead running the supply in twin and earth inside polypipe?

Much thanks for any suggestions.

 

[munty12]

I should add that I thought 10mm earth was deemed as having adequate mechanical protection, as it can be run surface on walls and such. If it is being buried in concrete, then surely all that is required is some form of protection against corrossion? Hence polypipe.

 

[telectrix]

if it's TN-S you can use 3 core SWA 6mm. use the 3rd core and the steel armour for earthing and bonding. sorted.

 

[D Skelton]

Is this a euphemism, or something! I'm scared now.

No, I literally am a qualified tank operator lol

Now I'm terrified!

Don't be, they're good guys

It's a TN-S system. The house CU is plastic, with little space (it's crammed with RCBOs!)

I'd take the garage feed from an MCB via a steel adaptable box unless it is possible to terminate at the DB (depends entirely on the brand of DB).

There is an old CU in the garage, and I found continuity between the tap and the light switch in the garage, when there is no sign whatsoever of a CPC between garage CU and said pipe (which is a little odd perhaps because there's 3m of plastic pipe between the point where the lead pipe comes in and where the tap is - I thought plastic was insulated, and that water was deemed a poor earth conductor).

So yes, the pipe is bringing a potential into the garage.

Finding continuity between the tap and a light switch is not good enough to determine whether or not the water pipe is an extraneous conductive part. This needs testing!

If there is only one pipe, what is the difference between bonding and earthing?

This question confirms my assumption that you're not an electrician. There is a vast difference, one that I'd be happy to go into at length if you really need me to, however I have explained this difference many times over the years on different threads. A forum search would help you here mate.

Yes I'm going to back fill it with a layer of sand, then solid concrete. Surely depth is decided by the need to protect it from accidental damage, such as a spade?

Yes, 600mm is recommended but if it is solid concrete going back over then depth isn't such an issue here, however future proofing is! And that is why I would personally duct the SWA

Do I really need to thru-crimp the 10mm?

If you wish to achieve a single, unbroken conductor then yes. Although as you have pointed out the system is TN-S then Teletrix is quite right in saying that you may be able to get away with a smaller csa. This does depend on your adiabatic equations though.

Can't I simply terminate it at the garage CU, and then run a seperate 10mm CPC from the earth terminal to the pipe?

No, because earthing and bonding are two different things. If this pipe is truly an extraneous conductive part (which still needs establishing) then to conform with the regs then a conductor complying with section 543 needs running from the main earthing terminal to the extraneous conductive part. You're extending an equipotential zone here, not creating a new one.

Just how good is this polypipe? If it is not considered strong enough to offer mechanical protection from a spade - what is it considered strong enough for? What's to stop someone running the stuff on the surface alongside a hedge to a shed or somesuch at the bottom of the garden? In theory, the shed could then be supplied by twin and earth.

Polypipe is used for ducting purposes only! It is only there to make it easy to pull long lenghts of SWA in underground whilst protecting it's sheath from damage or penetration by sharp stones. It is not to be used to run unarmoured mains voltage cables in underground!

Thanks again.

No worries, however I would seriously consider getting a qualified spark in to do this work. I don't mean to be disparaging but your lack of knowledge is worrying when you are talking about carrying out such a task. This is also notifyable work. You'd be in breach of the building regulations if this work wasn't notified to LABC. There are a number of things here that having a professional carry out the work will ensure; that your SWA is terminated correctly, that your design is correct and that the correct cables and overcurrent protective devices are selected, that fault protection is achieved correctly, that the correct things are bonded or earthed only if they need to be, that the work is tested, inspected and certified upon completion and that the work is signed off under Part P.

 

[munty12]

[QUOTE}This question confirms my assumption that you're not an electrician.[/QUOTE]

This is a little embarrassing perhaps, but I was qualified upto 15th edition, though I'm a bit rusty now. I've not so much as wired a plug for over five years now.

Polypipe is used for ducting purposes only! It is only there to make it easy to pull long lenghts of SWA in underground whilst protecting it's sheath from damage or penetration by sharp stones. It is not to be used to run unarmoured mains voltage cables in underground!

I was talking about an example where it was running above ground though. But at least I now know that this polypipe is not some new wonder material I've not seen before.

With my length of run it's not really an issue, but imagine if it were more like 50m, and I needed only a 20A supply and an equipotential bond - why incur the extra cost of 10mm SWA? Why not utilise 6mm SWA and then run a seperate 10 mm CPC (assuming I don't run a TT-system)? Is it then acceptable to lay the 10mm in a polypipe duct, in a trench across a lawn or somesuch?

Where the final connection to the earth electrode or earthing terminal is made there must be a clear and permanent label Safety Electrical Connection - Do not remove (see {Fig 5.17}). Where a buried earthing conductor is not protected against mechanical damage but is protected against corrosion by a sheath, its minimum size must be 16 mm² whether made of copper or coated steel. If it has no corrosion protection, minimum sizes for mechanically unprotected earthing conductors are 25 mm² for copper and 50 mm² for coated steel.

Am I getting this right? Do the regs say I can run a 16mm CPC without a duct?

In the distant past, I can remember times where I've pulled a 4 core SWA 3-phase supply cable through ducts, with a seperate CPC. Is there anything to stop me from following such an example here in my garage (2 core SWA with seperate CPC for a equipotential bond)?

No, because earthing and bonding are two different things. If this pipe is truly an extraneous conductive part (which still needs establishing) then to conform with the regs then a conductor complying with section 543 needs running from themain earthing terminal to the extraneous conductive part. You're extending an equipotential zone here, not creating a new one.

Okay. I think I got it at last. When earthing I have to run to the main earth terminal. If bonding, I only have to earth equipment to the supply, right?

Don't worry. I won't do anything that is not all above board with the LABC.

 

[D Skelton]

This is a little embarrassing perhaps, but I was qualified upto 15th edition, though I'm a bit rusty now. I've not so much as wired a plug for over five years now.

My bad, I get it though, you're out of practice and wanna get your hands dirty again. Well my advice would be to settle down for some bed time reading with the latest copy of the Big Green Book, it's quite good reading! Even I still do it occasionally! Seriously though, you do really need to get your head around earthing and bonding if you're going to be doing any regular electrical work.

With my length of run it's not really an issue, but imagine if it were more like 50m, and I needed only a 20A supply and an equipotential bond - why incur the extra cost of 10mm SWA? Why not utilise 6mm SWA and then run a seperate 10 mm CPC (assuming I don't run a TT-system)? Is it then acceptable to lay the 10mm in a polypipe duct, in a trench across a lawn or somesuch?

In theory yes, there'd be no problem doing that, but you'd need a bigger bonding conductor that 10mm to keep your resistance on it below 0.05 ohms. On a PME system you'd also have to ensure that the copper equivalent csa of the steel armour (if you were using that as the earth) was the same size as the bonding conductor. If it wasn't, you'd have to up the size of the SWA anyway

Am I getting this right? Do the regs say I can run a 16mm CPC without a duct?

In short, sheathed, yes

In the distant past, I can remember times where I've pulled a 4 core SWA 3-phase supply cable through ducts, with a seperate CPC. Is there anything to stop me from following such an example here in my garage (2 core SWA with seperate CPC for a equipotential bond)?

You wouldn't be running a CPC for a bond! We've been through this lol.

There is nothing stopping you using a seperate cable for your main bonding conductor (if it's even needed in the first place) and using the armour on the SWA as your earth, but, as you're on a TN-S, depending on your calculations you may well be able to use a 6mm bond and 6mm earth, therefore, in this situation you may as well run a three core SWA with one core as the bond and the armour as the earth.

Okay. I think I got it at last. When earthing I have to run to the main earth terminal. If bonding, I only have to earth equipment to the supply, right?

No! Earthing can be run from DB to DB, main bonding has to go from the extraneous conductive part back to the main earthing terminal at the point of origin.

Don't worry. I won't do anything that is not all above board with the LABC.

What exactly is your plan out of interest? You do realise it will cost you a small fortune to notify them personally, and you'll have to notify them before work commences.