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

[Ratmeister]

No need to rewire IF the existing cables are in good condition.

It MAY need a partial rewire, in as much as running in an earth wire through the conduit.... but it all depends on how bad the older part of conduit is.... which is why we suggested testing first.

Sorry if I'm picking this up wrong, but could we bypass the older conduit with a new earth wire that just connects the CU direct to the newer conduit?

 

[davesparks]

Or is there any scenario at all where he is correct and it needs rewired?

If the existing cable is damaged or deteriorated then it may need to be rewired for that reason.

But there is no need to rewire it for not having a seperate earth in there. The worst case scenario for that would be adding an earth wire into the conduit in any part where it is required.

 

[davesparks]

Sorry if I'm picking this up wrong, but could we bypass the older conduit with a new earth wire that just connects the CU direct to the newer conduit?

You wouldn't bypass it, you would run the earth inside the conduit.

 

[Ratmeister]

You wouldn't bypass it, you would run the earth inside the conduit.

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?

 

[James]

Wouldn't that connection need to be maintenance free

Only if it is not accessible.
or at least that is my humble opinion.

 

[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.