Earthing Metal Parts

I am looking at a building with a car park in Sydney. Electrical circuits are run on perforated cable trays at high level.

I understand that the trays require earthing, however, I'm trying to determine the extent.

The trays are made from galvanized sheet and come in 3 meter lengths and are butted to each other. Two short straps (around 100mm long), one on each side, are used to join the lengths together. 10mm bolts and nuts are used for the connection.

Should the trays be individually earthed or only one at each run end or some thing in between? I have been told by one contractor that one earth per tray is adequate. I've been told by an electrical engineer that each 3m length of tray should be individually earthed.

Advice please ..

Thanks

First check with the manufacturer of the tray! It may be that they have supporting data on the type of ground they require. They may also have short jumpers that connect the sections electrically. What follows is USA data so it will be similar there but the exact rule may vary. Rule of thumb for structure is .05 ohms or less, so if you can measure between sections and see that, or less, you are probablly ok, unless the manufacturers data indicates otherwise. From tray to earth #6 AWG solid copper is minimum size. A complete wire run is expected for each ground, no splices allowed! A separate run is required for each continuous section of tray, ie: each floor may qualify for that tray to be considered 1 electrically continuous section but a separate ground from each floor may still need to go to earth. I cant imagine a earth wire from each 3 meter section of tray, That could be hundreds of earth wires, over kill! No engineer has ever done that before (smirk).