Bonding Jumper in Cable Trays

Why are these trays powder-coated? If it is to withstand atmospheric corrosion, then scratching through the powder coat to obtain electrical contact nullifies the corrosion resistance!

• Why not use a different tray, such as glassfibre, that doesn't need corrosion protection?
• What can be done about the containment problem that requires corrosion protection for equipment outside the process lines?

Electrical continuity is important for personnel protection, so the answer is "both".

Hi,

The benefit of the bonding of the cable tray besides providing the physical support to the cables is the prevention of power induced emfs into sensitive circuitry in close proximity to the power cables.

It is an approximation to the principle of the Faraday Cage which creates zones of minimal electromagnetic interference.

Therefore in a busy plant room environment it is permissible to put control cables on the some tray as say motor drive circuits without jeopardising the reliability of the more sensitive circuits. I prefer to use fairly wide tray 300mm+ and put the dissimilar ciruits as far apart as possible. This is over and above using cables spec'd as per manufacturers requirements.

The bonded tray should also be taken back,if possible, to a 'clean earth' terminal; this will also help to ensure that the Faraday principle is applied in practice.

It all depends on how sensitive and potentially interactive the respective circuits are thought to be.

Good luck,

Massey.

Good luck

It is absolutely essential that unless you can maintain equivalent to double insulation that your cable tray must have good reliable earth bonds beteen each and every section back to the main earth. Under our rules, all metal in an install is normally held at earth potetial. This includes such things as water pipes, pool reinforcing, building frames etc. This would depend of course on the wiring rules in your country, but in Australia, I believe that bonding must be maintained. This should be able to be done by bonding straps of a suitable size or simply by the screws or material making the joins. ELectrical continuity is of equal importance for all parts of an installation, for both safety and for reliability

Regards

Bill Pilgrim, Queensland, Austalia

Please refer to the NEC Article 392.7 metallic cable trays must be grounded and electrically continuous