Earthing clamps not to be used on cables

Under no circumstance should an earthing clamp (complying with BS 951 or any other type) be attached to the lead sheath of any cable. In the case of a supply cable this practice is wrong for two reasons:

• The supply cable is the property of the electricity distributor and
• The securing of the clamp to the lead sheath is likely to damage the conductor insulation of the supply cable and, with the cold-flow of the lead, the connection is liable to loosen over time. BS 951: 1999 states in Note 3 (second sentence) to its scope that 'such clamps are not intended for connection to the armour or sheath of a cable'.

In the worse cases, this practice may lead to increased risks from the hazards of fire or electric shock.

Solution

Many installations are supplied by paper-insulated/lead sheathed incoming service cables, and earthing facilities are often provided (by the distribution) by means of a wiped soldered joint on to the lead sheath, with an attached earth tail to an earthing block. However, there are many paper-insulated/lead sheathed supply cables that are not suitable for providing a connection to a means of earthing via the network distribution cables.

Where an adequate earthing facility has not been provided, electrical contractors are advised always to enquire of the cable owner (such as the electricity distributor) whether or not such a facility can be made available. It must never be assumed that, because a supply cable has a lead or other metallic sheath or armour, it is capable of providing an effective connection to Earth, or that is adequate for carrying the prospective fault current.

The compressive forces exerted by tightening a clamp onto most types of armoured or metal sheathed cables (sufficient to provide a low resistance joint for fault or other currents to flow) are liable to cause damage to the conductor insulation and bedding. Such misuse of a clamp is a departure from Regulation 512.1.5.

One solution open to the installation designer is to make the installation part of a TT System.

Regulation 512.1.5

Every item of equipment shall be selected and erected so that it will neither cause harmful effects to other equipment nor impair the supply during normal service including switching operations.

This article is extracted from Snags and Solutions Part 3 published by the: NICEIC Group