I've been ponding this one and realized I don't really understand what a functional bonding conductor - as distinct from a functional earthing conductor - would do. Having grown up in the days of RS-232 and earth break recall on PBX phones, I think I understand the idea of signal lines using earth as a return (or 0V reference) conductor - so those "GND" lines would be functional earths I guess. So what would a functional bonding conductor do that would be different to a functional earth conductor? Back in LV land we use bonding conductors to reduce potential differences for shock protection purposes, especially during faults, but functional systems don't have to worry about shock protection (the PE system takes care of that). I know we sometimes reinforce or shunt thin functional earth conductors with large parallel (by-pass) conductors - are those thought of as bonding conductors, or just parallel/low-impedance FE conductors?

  - Andy.

I've been ponding this one and realized I don't really understand what a functional bonding conductor - as distinct from a functional earthing conductor - would do. Having grown up in the days of RS-232 and earth break recall on PBX phones, I think I understand the idea of signal lines using earth as a return (or 0V reference) conductor - so those "GND" lines would be functional earths I guess. So what would a functional bonding conductor do that would be different to a functional earth conductor? Back in LV land we use bonding conductors to reduce potential differences for shock protection purposes, especially during faults, but functional systems don't have to worry about shock protection (the PE system takes care of that). I know we sometimes reinforce or shunt thin functional earth conductors with large parallel (by-pass) conductors - are those thought of as bonding conductors, or just parallel/low-impedance FE conductors?

  - Andy.

My understanding is as you suggested that the shunt conductors serve to provide parallel, low-impedance paths for functional earthing conductors. Are the bonding conductors intended to ensure that different parts of the system maintain the same potential, thereby preventing interference throughout ?

The problem is much weirder than that - for cables carrying high speed/ high frequency signals, the currents that represent the start of the 'message' have not actually come out the far end, while the next cycle(s) of the message is/ are being stuffed in, so if you had many little volt-meters along the line at intervals, and could magically take a snapshot of them all at once, they would all be reading different things at any moment.   The same applies to return paths, and very odd and unpleasant things happen if the difference in time between the 'out' and 'return' paths for the current are offset in time by more than a small fraction of a symbol period/cycle. (100m cable of 100base T is a good example, where a very short packet may be completely 'in flight' while transmission at the start has already finished, and the reception at the far end has not yet begun..)
An earth wire that is too long is actually worse than none at all, especially at frequencies where it is more than a quarter cycle long.
Earthing for such systems may contain RF traps and ferrite beads and things to encourage signal currents to go the right way whilel leaving the DC/50Hz path intact (those bumps on USB leads and video cables do that)
To the outsider this often looks not dissimilar to magic, though it is fully logical, it is far from simple and full of exceptions.
Mike.