BS 951 speciﬁes the performance and important mechanical requirements for metal clamps that are used to provide mechanically and electrically sound earthing and bonding connections to metal tubes. The information in this article relates to the correct selection and installation of such clamps within electrical installations.
The scope of BS 951: 2009 states that the clamps are primarily intended for use in electrical installations for:
- connecting earthing conductors having a cross-sectional area in the range 2.5 mm2 to 70 mm2 to earth electrode rods or other means of earthing
- connecting bonding conductors to metal tubes (including conduits and pipes) or rods of not less than 6 mm diameter for round tubes
- electrically bonding other conductive parts where at least one is a tube of circular cross-section
- The clamps manufactured to BS 951 are intended to be used singly when encircling a tube at a given position along the tube. Regulation 134.1.4 of BS 7671 requires that all joints and connections are of proper construction as regards conductance as well as mechanical strength
The termination on a BS 951 clamp for the protective conductor is separate from the means of tightening and locking the clamp for making electrical contact with the tube. A screw termination is capable of accepting one of the following:
(i) a conductor clamped under a screw head provided with a captive washer so that the screw head does not act directly on the conductor; or
(ii) a single conductor clamped directly by a screw-threaded arrangement, the cross-sectional area of the conductor being within the range speciﬁed in Table 1 of BS 951 (which has been reproduced in this article for ease of reference); or
(iii) a bolted-on cable socket from a range of sockets that can accommodate conductors having cross-sectional areas covering the whole range speciﬁed in Table 1 of BS 951.
Clamps having terminations in accordance with (i) or (ii) above are marked with the appropriate termination reference in accordance with Table 1 of BS 951 . The limits of the range are presented in the form ‘X-Y’, where X is the reference of the smallest acceptable conductor and Y is the reference of the largest acceptable conductor. In addition, for clamps having provision for terminations in accordance with (i) or (ii), it must be possible to loop in and out an unbroken conductor.
Resistance to corrosion
Clamps manufactured to BS 951 are made in a choice of different metals, to suit differing levels of corrosion that are likely to be encountered in service. In order to comply with the requirements of Section 522 of BS 7671, it is important to select a clamp which is suitable for the expected conditions in the particular location where it is to be installed.
Earth clamps come in three common types and conform to BS 951. E14 clamps are for dry conditions only and are often coloured red. E15 clamps are for dry or damp conditions and are often coloured blue. E16 clamps are for dry, damp or corrosive conditions and are often coloured green. The E16 clamps allow for a larger conductor (usually 16mm² or greater) to be connected than for E14 and E15 clamps.
Misuse of BS 951 clamps
BS 951 states that clamps to this standard are not intended for connection to the armour or sheath of a cable and requires the packaging of the clamp to include the warning “Not intended for use on the sheath or armour of a cable”. This is because the action of tightening the clamp is liable to damage the cable, for example by crushing the bedding or insulation. Such damage may result in high resistance and unreliability in the electrical connection between the clamp and the armouring or metallic sheath, and may even result in an electrical fault within the cable, with the possibility of serious injury to persons. Fitting a BS 951 clamp to the armour or metallic sheath of a cable is therefore unacceptable.
A BS 951 clamp must be correctly selected for both the size of pipe or tube to which it is to be ﬁtted and the size of cable. Fitting a clamp which is unsuitable in either of these respects is liable to be a departure from the requirements of Regulations 526.1 and/or 526.2 of BS 7671 relating to durable electrical continuity, adequate mechanical strength, and appropriate selection of a means of connection.
Installing a clamp in an inaccessible position
Like all types of electrical connection that are not excused by Regulation 526.3 of BS 7671, the electrical connections between a BS 951 clamp, the pipe (or other tube) to which it is attached, and the protective conductor connected to its terminal must be accessible for inspection, testing and maintenance (Regulation 543.3.2 refers).
For other guidance and publications please see the ELECSA website : www.elecsa.co.uk