Using metal trunking as the earth

[sparkwright]

Not something I'm intending on doing, but thought I'd raise the topic for fun and maybe learn a bit more.

I gather using metal trunking to provide the earth/cpc for a circuit is acceptable, in the same way metal conduit can be used.

Usually we don't see metal trunking used as a cpc, as it's generally easy enough to bung a green and yellow wire in the trunking.

But has anyone used the trunking where a phase or three phase wires, and a neutral wire perhaps were already in place, and running an earth wire was too much trouble due to it being a massive long run?

And would you do this if the trunking has many circuits (with separate cpcs), and again, you don't fancy running a new cpc?

 

[ericmark]

Be it tray or trunking it can be used, but the problem is the number of link wires required, in the main other way around, a large earth wire with C crimps, and all the sections of tray or trunking have a wire to main earth. The problem is these need hydraulic crimping tools, so rather time consuming.

 

[sparkwright]

So you're saying all joins need to be securely crimped and linked for this to be permissible?

And yet conlock conduit can also be used as a cpc, relying on flimsy grub screws?

 

[ericmark]

I have been told to fit earth braid section to section many times, and it does seem daft, one would think the screwing of the sections together would be enough, but a few things, one expansion and contraction can cause the trunking or tray work to move causing the galvanising to wear off and resulting bad connection, use of galv paint could cause a poor electrical connection, and it has been known where Germans have been employed to have free air between sections.

I have not seen any regulations which says you must use the earth braid links, but it is standard practice, the problem is when it does go wrong, consider in 10 years time when you do an earth loop impedance test and it fails, with 100 joints along the tray or trunking can you see the problem finding out where the problem lies.

It is the same problem with steel conduit, when fitted great, return in 10 years time, and then there are faults, and finding which joint, and curing the problem is not easy.

Each electrical supervisor seems to have different ideas, one wants copper slip grease on every nut and bolt, and copper nuts and bolts, another seems to think the copper with cause a problem and wants galvanised nuts and bolts, and painted with glav paint. Another goes for petroleum jelly. At the end of the day the guy who signs that he has designed the installation calls the tune.

In industry it is common to have three signatures on the installation certificate, and as electrician I did as I was told, as the electrical engineer I liked copper slip. But as to if I was right don't know, I don't think I have ever returned after 10 years to inspect some thing I have installed except in my own home. Never worked 10 years for same firm did do 9½ years for council, but most jobs were installation so once completed I left, and although I loved maintenance it did not pay as well as installation, so next installation job and I was off.

 

[SUNRAY]

I have been told to fit earth braid section to section many times, and it does seem daft, one would think the screwing of the sections together would be enough, but a few things, one expansion and contraction can cause the trunking or tray work to move causing the galvanising to wear off and resulting bad connection, use of galv paint could cause a poor electrical connection, and it has been known where Germans have been employed to have free air between sections.

I have not seen any regulations which says you must use the earth braid links, but it is standard practice, the problem is when it does go wrong, consider in 10 years time when you do an earth loop impedance test and it fails, with 100 joints along the tray or trunking can you see the problem finding out where the problem lies.

It is the same problem with steel conduit, when fitted great, return in 10 years time, and then there are faults, and finding which joint, and curing the problem is not easy.

Each electrical supervisor seems to have different ideas, one wants copper slip grease on every nut and bolt, and copper nuts and bolts, another seems to think the copper with cause a problem and wants galvanised nuts and bolts, and painted with glav paint. Another goes for petroleum jelly. At the end of the day the guy who signs that he has designed the installation calls the tune.

In industry it is common to have three signatures on the installation certificate, and as electrician I did as I was told, as the electrical engineer I liked copper slip. But as to if I was right don't know, I don't think I have ever returned after 10 years to inspect some thing I have installed except in my own home. Never worked 10 years for same firm did do 9½ years for council, but most jobs were installation so once completed I left, and although I loved maintenance it did not pay as well as installation, so next installation job and I was off.

The standard method for a long time was to fit a copper strap and they were included with the fittings.

 

[Jackrae]

Could it be that there is confusion between ensuring the exposed metal trays, trunking and conduit must be continuously "earthed" to ensure it cannot inadvertantly develop a potential and the requirement to run a copper CPC to provide "earthing of equipment. To my mind it simply doesn't make sense to rely upon multiple connections on a relatively poor conducting material (steel Vs copper) as a means of ensuring a proper good CPC facility.

On sites I've worked on all traywork was continuously bonded to a common 'earth' point and insulated CPC cables were run to equipment powered by cables running within the same traywork.

 

[SUNRAY]

I've worked in many, many, many older installations with steel trunking/conduit, some quite big like 12 x 12" with all sorts of combinations of 1ph & 3ph and control circuits without a single earth wire to be seen within the cable management. It did used to be very standard. More recently it's quite unusual.

 

[Jackrae]

It's called evolutionary improvement in standards. We learn and build on what went before.
I've seen open spring-lever type power switches on HV slate boards in use, but it doesn't make it right.
Unfortunately, however, a great deal of evolutionary so-called 'improvement' invariably results in poorer system operation or function

 

[SUNRAY]

It's called evolutionary improvement in standards. We learn and build on what went before.
I've seen open spring-lever type power switches on HV slate boards in use, but it doesn't make it right.
Unfortunately, however, a great deal of evolutionary so-called 'improvement' invariably results in poorer system operation or function

If that was aimed at my comment then yes I happily agree, these days I wouldn't dream of running a circuit without a proper CPC however I wouldn't be jumping up and down to change what's there if it tests OK.

 

[JohnW2]

... To my mind it simply doesn't make sense to rely upon multiple connections on a relatively poor conducting material (steel Vs copper) as a means of ensuring a proper good CPC facility.

Does that mean that you don't think it makes sense to rely on the armour of SWA to "ensure a proper good CPC facility"?

Kind Regards, John

 

[Jackrae]

Of course not, but SWA correctly terminated by the use of the correct fittings isn't the same as indeterminate lengths of sheet metal-work coupled together by not-necessarily perfect bolting being used as the only earthing system in use.
However, more to the point was that the original enquiry related to the concept of the job being "too much trouble" and "don't fancy"

 

[JohnW2]

Of course not ...

I don't think it is a matter of "of course not" (see below).

, but SWA correctly terminated by the use of the correct fittings isn't the same as indeterminate lengths of sheet metal-work coupled together by not-necessarily perfect bolting being used as the only earthing system in use.

It's not. However, I was responding to your comment when you seemed to be suggesting that the material (steel) was not suitable for a CPC, because it was a "relatively poor conducting material".

Contrary to what you seem to have thought, it was a genuine question, not a 'trick' or rhetorical, one. The potential problem with SWA armour is not so much the correct termination (which there obviously should be) but, rather, the possibility of water ingress (due to outer sheath damage) leading to corrosion of the armour. For that reason I (and some others) are reluctant to rely on it as a CPC, particularly if the SWA is buried or 'outdoors'.

Other, of course, disagree, and regard having a CPC core in SWA as being 'wasteful' and 'unnecessary'.

Kind Regards, John