Circuits earthed on metal box/conduit: Does it need a rewire?

[Risteard]

Yep, but I mean you would be bypassing it's use as an earth? And just using the older section purely for holding the new cable?

The conduit will still need to be Earthed. If the conduit's in bad enough condition it may not be usable.

 

[Ratmeister]

The conduit will still need to be Earthed. If the conduit's in bad enough condition it may not be usable.

Imagine a warehouse with 2 offices at one side. These have high quality conduit that the sockets and lights use as earth. The CU is at the other side of the warehouse approx 10 metres away. Could you not just run an earth cable from the CU inside plastic conduit and connect it to the metal conduit?

 

[davesparks]

. Could you not just run an earth cable from the CU inside plastic conduit and connect it to the metal conduit?

That depends on why. What is wrong/has happened to the original conduit to make this necessary?

This really needs to be looked at by a good electrician, we've got nothing more than a picture of a light switch to go on here

 

[Ratmeister]

That depends on why. What is wrong/has happened to the original conduit to make this necessary?

This really needs to be looked at by a good electrician, we've got nothing more than a picture of a light switch to go on here

Thanks Dave. I'm just being hypothetical here to see if I understand how it's supposed to work. I'll speak to the electrician again to see precisely why he thinks the earth in the metal conduit isn't acceptable.

At least I know now from your help that the wiring regulations permit it.

 

[Ratmeister]

The electrician says there is a section of older conduit coming across the warehouse that is rusted slightly and is probably reducing the earth readings. The conduit in the actual offices themselves is in good condition. I suggested we try to connect a new earth cable to the conduit inside the offices if possible but he says he can't do that as changing the CU to a new box is in effect a new installation now and using conduit for earth is an old way of doing things.

I'm really confused now.

 

[James]

Nothing wrong with using conduit as an earthing system.

 

[pc1966]

I suggested we try to connect a new earth cable to the conduit inside the offices if possible but he says he can't do that as changing the CU to a new box is in effect a new installation now and using conduit for earth is an old way of doing things.

It might be the "old way" in some installations, but for many where high strength and robustness is needed (industrial, agriculture) it is still the preferred option. A quick check on any electrical wholesale place will show plenty of conduit and related accessories for sale today!

I'm really confused now.

I think your electrician is confused, or at least not very experienced.

Conduit is capable of perfectly good use and earth continuity but, as mentioned above, the ends have to be reliably connected to the fuse box / distribution board and the can be done in many ways.

Worse case is an idiot cuts the conduit off instead of adapting the box or using some trunking to link metal parts, etc.

 

[davesparks]

The electrician says there is a section of older conduit coming across the warehouse that is rusted slightly and is probably reducing the earth readings.

what are those earth readings? have they actually tested anything?
and if they are being reduced then that is a good thing as the readings should be as low as possible. poor connections would increase the readings!

"rusted slightly" sounds to me like a bit of paint is required, not a rewire, but again I haven't actually seen it to know for sure.
if it is very rusty then the conduit may need to be replaced regardless of how the earthing is done as it would no longer be providing adequate protection to the cables.

The conduit in the actual offices themselves is in good condition. I suggested we try to connect a new earth cable to the conduit inside the offices if possible but he says he can't do that as changing the CU to a new box is in effect a new installation now

no, it doesn't become a new installation, changing a CU is an alteration to an existing installation.

and using conduit for earth is an old way of doing things.

so what if it is an old way of doing things? just because it is old it doesn't mean it cant be done now nor kept in service. Granted most of us wouldn't use steel conduit as the earth for circuits these days for new work but that doesn't mean you can't maintain an existing installation.
I look after a few installations where old conduit and trunking has been installed in this way and normally the only issues i have with earthing is fixing what some cowboys have done to modify the installations over the years

I'm really confused now.

I think your electrician is confused.

 

[Ratmeister]

what are those earth readings? have they actually tested anything?
and if they are being reduced then that is a good thing as the readings should be as low as possible. poor connections would increase the readings!

"rusted slightly" sounds to me like a bit of paint is required, not a rewire, but again I haven't actually seen it to know for sure.
if it is very rusty then the conduit may need to be replaced regardless of how the earthing is done as it would no longer be providing adequate protection to the cables.


no, it doesn't become a new installation, changing a CU is an alteration to an existing installation.

so what if it is an old way of doing things? just because it is old it doesn't mean it cant be done now nor kept in service. Granted most of us wouldn't use steel conduit as the earth for circuits these days for new work but that doesn't mean you can't maintain an existing installation.
I look after a few installations where old conduit and trunking has been installed in this way and normally the only issues i have with earthing is fixing what some cowboys have done to modify the installations over the years


I think your electrician is confused.

Thanks Dave, if you were my electrician my life would be a lot easier. I've asked them to fully test the circuits today. If the readings are too high and he insists on a rewire then I'll get someone else in for a second look.

 

[littlespark]

Get your electrical to quote for a rewire, and post it up here…. We could do with a laugh.

 

[davesparks]

If it's a point for point rewire then it should be relatively cheap as its just pulling new cable in to existing conduit.

 

[rewireIT]

Get a spark in that knows what he's doing. Suggest he read the regs book that he's supposed to keep in his van.

 

[Ratmeister]

If it's a point for point rewire then it should be relatively cheap as its just pulling new cable in to existing conduit.

There's so many sockets and circuits though, it would cost a fortune in labour I assume. There's 8 offices within the warehouse in total that are all done the same way.

 

[Ratmeister]

Get a spark in that knows what he's doing. Suggest he read the regs book that he's supposed to keep in his van.

He told me straight out that metal conduit used as earth is not allowed in new installs.
He also told me that changing the CU meant the entire install had to be treated as a new install.
I'm trying to work with it for now and I hope he'll have a read of the regs before he returns after my subtle prompting.

 

[littlespark]

The regs ask for a cpc on every circuit... whether that be a separate earth wire, steel conduit, or the steel strands around a heavy duty armoured cable. Steel conduit, properly installed, is the most substantial of any of these.

 

[James]

I have had similar discussions with clients and other electrical contractors working the same site.
sometimes it comes to a meeting with a client and contractor and myself.

I normally stay fairly quiet, just put my point across at times.
Eventually it will come down to the other guy telling me in front of the customer that I am wrong and it has to be done "this way" because it is "against the regs" to do what i suggest and that i should go away because i don't understand.

it always makes for an interesting few minutes when I place a copy of the regs book on the table and ask them to point it out.

I have found over the years that a lot of people confuse the way they were taught it should be done with an imaginary regulation specifying that it must be done that way.

p.s. To this day, I have never walked out of a meeting like that feeling embarrassed that i had got it wrong, you can normally tell you are on a winner when your opponent keeps telling you its against the regs but will never tell you which one.

 

[rewireIT]

Exactly as James says - just ask him to point out what regs it contravenes and where it specifies a DB change means existing wiring, accessories and containment has to be treated as new. He won't be able to show you them because they don't exist.

 

[davesparks]

He told me straight out that metal conduit used as earth is not allowed in new installs.
He also told me that changing the CU meant the entire install had to be treated as a new install.
I'm trying to work with it for now and I hope he'll have a read of the regs before he returns after my subtle prompting.

Someone like that is unlikely to change their mind based on a customers research from the Internet or advice from a forum.

The second point is an unfortunately common misconception which is quite prevalent in domestic work but usually less so in commercial.

It should be pretty obvious that replacing a distribution board on an existing installation does not magically change anything about the existing installation or somehow turn it into a new one.
Yes whoever does the job will need to ensure that the installation is in a safe condition for continued use, but they don't have to do everything that would be necessary to make it fully compliant as if it was installed today.

 

[Risteard]

Thanks Dave, if you were my electrician my life would be a lot easier. I've asked them to fully test the circuits today. If the readings are too high and he insists on a rewire then I'll get someone else in for a second look.

In fairness very few electrical contractors would have test equipment to perform high current continuity tests on containment now.

 

[davesparks]

In fairness very few electrical contractors would have test equipment to perform high current continuity tests on containment now.

Yes but it can be hired for a relatively small cost.