well, I imagine the person who put it in has decided that it is safer to earth the tray, or RSJ or what have you than not.


It may or may not be, depending if it is outdoors, and how the main supply earth is arranged.

On a metal post set a foot or so into cement or metal trays or beams held with a handful of anchor bolts in to masonry, the "electrode resistance" it provides is likely to be pretty minimal ,hundreds or even thousands of ohms between the metal and terra-firna. So even with a full 230V fault, the current flowing to earth will not trouble the link wire.

But, at least if a live wire hits that tray, due to a vandalised light fitting or whatever,  there should now be enough of a connection to operate the ADS.


If this was a metal pipe running underground for many tens of metres the answer would be different, then it maybe a few ohms to terra-firma, and 'self earthing'

The regs are written assuming that all such metalwork is well grounded, and so requires a substantial main earth bond, as large currents may flow into the earth from the substation CPC.

This is not correct of course (quite a few assumptions in the regs are a bit iffy  - another good one is that all faults are zero resistance....), but it does err on the side of safety, by ensuring that most of the time, the bonding is over-sized. The only times problems really arise is where the same chunk of metal is earthed twice, bot not quite to the same voltage - this can happen with service pipes or with larger metal framed structures with more than one incoming supply. I'm going to ignore railways, mines, shipyards and other special cases, as these need, and have, their own rules.


Assuming that SWA is not a very large size, it is quite possible the armour is not really providing a low enough resistance to satisfy the regs in terms of the cross-section needed for a main bonding conductor, but I suspect that it is not really a problem either  - if the box had been metal the same connection would have been made but less obviously, and no-one would even notice.

Equally, a light switch on a metal post rarely really needs a 10mm2 cable, so there would need to be a lot of upgrading to meet the letter of the regulations.

So maybe not compliant, unlikely to be an issue in practice.


Mike

well, I imagine the person who put it in has decided that it is safer to earth the tray, or RSJ or what have you than not.


It may or may not be, depending if it is outdoors, and how the main supply earth is arranged.

On a metal post set a foot or so into cement or metal trays or beams held with a handful of anchor bolts in to masonry, the "electrode resistance" it provides is likely to be pretty minimal ,hundreds or even thousands of ohms between the metal and terra-firna. So even with a full 230V fault, the current flowing to earth will not trouble the link wire.

But, at least if a live wire hits that tray, due to a vandalised light fitting or whatever,  there should now be enough of a connection to operate the ADS.


If this was a metal pipe running underground for many tens of metres the answer would be different, then it maybe a few ohms to terra-firma, and 'self earthing'

The regs are written assuming that all such metalwork is well grounded, and so requires a substantial main earth bond, as large currents may flow into the earth from the substation CPC.

This is not correct of course (quite a few assumptions in the regs are a bit iffy  - another good one is that all faults are zero resistance....), but it does err on the side of safety, by ensuring that most of the time, the bonding is over-sized. The only times problems really arise is where the same chunk of metal is earthed twice, bot not quite to the same voltage - this can happen with service pipes or with larger metal framed structures with more than one incoming supply. I'm going to ignore railways, mines, shipyards and other special cases, as these need, and have, their own rules.


Assuming that SWA is not a very large size, it is quite possible the armour is not really providing a low enough resistance to satisfy the regs in terms of the cross-section needed for a main bonding conductor, but I suspect that it is not really a problem either  - if the box had been metal the same connection would have been made but less obviously, and no-one would even notice.

Equally, a light switch on a metal post rarely really needs a 10mm2 cable, so there would need to be a lot of upgrading to meet the letter of the regulations.

So maybe not compliant, unlikely to be an issue in practice.


Mike