A cable tray or a cable basket where used as a support and cable management system has to be considered in the context of earthing and bonding. In other words, are such systems where consisting of metal and plastic-coated metal, exposed-conductive-parts or extraneous-conductive-parts and consequently do they require earthing or bonding?
Addressing first, the question of earthing and whether the cable tray or basket should be earthed, electrical equipment such as cables mounted on a metallic support system will normally be equivalent either a Class l construction (for example copper sheathed, mineral insulated cables without an overall PVC covering) or a Class ll equivalent construction (for example PVC insulated and sheathed cable).
Exposed-conductive-parts of cables, such as the copper sheath of a mineral insulated cable are required to be connected to the MET of the installation by a CPC designed to conduct earth fault currents. The cable tray or basket which the mineral insulated cable is attached to, or may be in contact with, is not itself an exposed-conductive-part and therefore it does not require earthing. To do so would only serve to distribute further any touch voltage resulting from an earth fault on an item of equipment to which the cable is connected.
A cable complying with the appropriate standard having a non-metallic sheath or a non-metallic enclosure is deemed to provide satisfactory protection against both direct and indirect contact, as does an item of Class ll equipment (Regulation 471-09-04 refers). Class ll equipment is constructed such that any insulation fault in the cable cannot result in a fault current flowing into any conductive parts with which the equipment may be in contact. Hence, the cable tray or basket need not be earthed.
Generally, the conductive parts of a metal cable tray or basket system need not be puropsely earthed. Some conductive parts of a metal cable support system may be earthed, however, by virtue of fortuitous contact with exposed-conductive-parts.
Should the cable tray or basket be equipotentially bonded? Unless the metal cable support system introduces a potential that does not already exist in the location which the system is installed, it will not meet the definition of an extraneous-conductive-part. In normal circumstances therefore there is no need to arrange for the conductive parts of the support system to be connected to either a main bonding conductor or any supplementary bonding conductor.
However, should the cable tray be installed in such a manner that it is lekely to introduce a potential from outside the location, thereby meeting the definition of an extraneous-conductive-part, then main equipotential bonding will be required (Regulatiuon 413-02-02 refers). For example, consider a run of cable tray carrying services into a particular building. The cable tray may be in contact with the earth potential ouside a building and upon entering into the building would be likely to introduce the earth potential into that building. In such a case the tray would warrant the defintion of an extraneous-conductive-part and consequently main equipotenial bonding would be required.
Where, for example, the installation designer has selected a cable tray for use as a protective conductor, which is permitted under Regulation 543-02-02(vi) where it is described as an electrically continous support systerm for conductors, the cable tray would be required to meet the requirements for a protective conductor given in Regulation 543-02-04 and would need to be connected with earth.
