Cable joints and repairs
Health and Safety Executive/Local Authorities Enforcement Liaison Committee (HELA)
Local Authority Circular
- Subject: Electrical Equipment
- Open Government Status: Fully Open
- LAC Number: 18/1 (rev1)
- Publication Date: 01/02/2011
- Review Date: 01/02/2014
To: Health and Safety Enforcing Authorities
For the attention of: Local Authority Health and Safety Enforcement Managers, Health and Safety Regulators and others
This Local Authority Circular (LAC) provides technical guidance to enforcement officers and others on health and safety issues associated with electrical safety
Cable joints and repairs
1 This circular gives guidance for enforcement officers on joints and repairs in electric cables. Ever since electricity cables were first used the problem has arisen of how to joint them together. In order to achieve the degree of insulation, tensile and crushing strengths, conductivity and accessibility required in practice the traditional solution has been some form of junction box. The junction box typically incorporates: a method of securing the cable conductors (usually by soldering, screw-clamps or compressed ferrules); a method of insulation, which may be air, oil, bitumen or insulation applied in the form of tapes; and a method of enclosure and protection applicable to the environment.
2. The Electricity at Work Regulations 1989 require in Regulation 10, that every joint and connection shall be mechanically and electrically suitable for use. In this respect the joint or connection should be of proper construction as regards conductivity, insulation, mechanical strength and protection.
3. British Standard BS7671 (also known as the IEE Wiring Regulations – 17th Edition) gives the requirements for electrical installations; the requirements for joints and terminations are contained in Section 526. The performance of mechanical and compression joints is the subject of BS EN 61238 (or BS 4579 for old joints); the tensile strength of such joints is specified as a percentage of that of the cables which are being joined.
4. The following terms used in the circular reflect the general usage in industry although in conversation some interchangeability of terms does arise.
- "joint" is a term usually applied where two (or more) cables are joined together, in a way which is intended to reflect the construction of the cable itself;
- a "connector" is usually a mechanical device for connecting conductors. Whilst it may provide the degree of electrical conductivity required by the Electricity at Work Regulations Regulation 10 there is no implication that it provides, on its own, adequate insulation or mechanical protection;
- "junction box" implies an enclosure which may, or may not, incorporate a connector. Junction boxes often provide mechanical protection but frequently do not give any stress relief to the conductors. Junction boxes are not generally suitable for use with flexible cables unless special cable glands are used;
- a "cable coupler" consists of a plug and socket, the components of which are designed to take the axial load which may be imposed by flexible cables. The 2 halves of the coupler are often secured by a screwed ring to prevent the pulling apart. The 2 sections of cable can be readily disconnected by unplugging the coupler halves;
- "flexible cables" have conductors which consist of a relatively large number of thin strands. The insulation and any outer protective sheath or covering is also chosen to give the required degree of flexibility. Small size flexible cables are sometimes known in the trade as flexible cords. Flexible cables are self-supporting over short spans; where long lengths are unsupported special cables are necessary;
- "non-flexible cables" for fixed wiring have conductors which are solid or have a few thick strands. They are designed to be secured or supported at regular intervals along their length and should not be used where any appreciable degree of flexing occurs; and
- "strain relief" is the provision in the joint construction for tensile strain to be taken by the joint without stressing the conductors.
Joints in non-flexible cables
5. Underground cables are joined by ferrules (sweated or crimped) and the outer protection enclosure or box is usually filled with a plastic or bituminous compound. Such joints are often used above-ground for non-flexible cables and provided they are adequately protected and supported, are quite satisfactory. Modern versions of these joints use themo-shrink sleeving as the insulating and/or sheathing material but the principle remains the same.
6. Other cables in fixed wiring installations are generally joined by making them off in some form of enclosed junction box which, in many cases, does not incorporate any method of securing the cable against strain.
Joints in flexible cables
7. Home-made joints in flexible cables are not usually satisfactory because:
- stranded conductors do not lend themselves to certain methods of jointing;
- mechanical tensile strength and resistance to crushing are difficult to maintain; and
- fatigue damage may occur where the flexible cable enters a rigid joint.
8. In most cases the quality of a joint can only be assessed by dismantling, a process that usually destroys it.
9. Some proprietary joints and cable connectors are much more acceptable; these incorporate terminals or compression fittings suitable for stranded conductors, cable clamps of a design similar to those used for plugs, and sleeving to reduce the flexing of the cable where it enters the connector. Where these features are present and the cables are properly terminated it would be difficult to show that the joint does not meet the requirements of Regulation 10 in respect of conductivity, insulation and mechanical strength. The adequacy of the mechanical protection afforded by the enclosure depends on the environment in which it is issued. Heat-shrinkable or pre-stretched sleeving may be adequate in some cases but other circumstances may demand additional protection.
10. Regulation 10 refers to the prevention of danger, additionally enforcement officers will need to consider the requirements of Regulation 4(1) of the Electricity at Work Regulations – all systems shall at all times be of such construction as to prevent, so far as is reasonably practicable, danger. eg An uninsulated joint or connector at less than 50 volts will not under normal circumstances present a shock risk although if completely unprotected it may come into contact with another conductor and cause a short circuit leading to burns or fire.
11. In non-flexible fixed wiring, joints which are clearly properly protected or enclosed in polymeric sleeving will not normally require action on the part of enforcement officers. Joints in fixed wiring which are finished with adhesive plastic tape may be satisfactory but it is unlikely that this can be determined with certainty unless the joint is dismantled.
12. A taped joint in which the tape is becoming loose or which shows other signs of distress should be viewed with suspicion and the occupier should be advised to replace it. "Home made" joints are not usually satisfactory (see para 7) and it is suggested that enforcement officers recommend the use of proprietary joints.
13. A flexible cable having a proprietary cable connector would generally be satisfactory provided the cable is properly made off and that the mechanical protection afforded by the enclosure is suitable for the environment.
14. In most cases occupiers should be advised to replace jointed flexible cables by a longer length of sound cable or to use properly designed cable couplers to join adjacent sections.
15. Before taking enforcement action in relation to apparently defective cable joints Enforcement Officers should seek advice from an HSE Electrical Specialist through their Enforcement Liaison Officer (ELO).