4.4.1 Cable supports and protection

Cables must be fixed securely at intervals which are close enough to ensure that there will be no excessive strain on the cable or on its joints and terminations, and to prevent cable loops appearing which could lead to mechanical damage. {Table 4.10} indicates minimum acceptable spacings of fixings for some common types of cables.

Table 4.10 Maximum spacing for cable supports

Overall cable	p.v.c. sheathed		Mineral insulated
diameter	Horizontal	Vertical	Horizontal	Vertical
(mm)	(mm)	(mm)	(mm)	(mm)
up to 9	250	400	600	800
10 to 15	300	400	900	1200
16 to 20	350	450	1500	2000
21 to 40	400	550	2000	3000

Where cable runs are neither vertical nor horizontal, the spacing depends on the angle as shown in {Fig 4.11}.

Where a cable is flat in cross-section as in the case of a p.v.c. insulated and sheathed type, the overall diameter is taken as the major axis as shown in {Fig 4.12}.

Fig 4.11 Spacing of support clips on angled runs

 

Fig 4.12 Effective diameter of a flat cable

The Regulations are concerned to protect hidden cables from damage. Thus, where cables are run beneath boarded floors, they must pass through holes drilled in the joists which are at least 50 mm below the top surface of the joist. This is to prevent accidental damage due to nails being driven into the joists. The hole diameters must not exceed one quarter of the depth of the joist and they must be drilled at the joist centre (the neutral axis). Hole centres must be at least three diameters apart, and the holes must only be drilled in a zone which extends 25% to 40% of the beam length from both ends.

An alternative is to protect the cable in steel conduit. It is not practicable to thread rigid conduit through holes in the joists, so the steel conduit may be laid in slots cut in the upper or lower edges as shown in {Fig 4.13}. The depth of the slot must be no greater than one eighth of the joist depth and notches must be in a zone extending from 10% to 25% of the beam length from both ends.

 

Fig 4.13 Support and protection for cables run under floors

Where cable runs are concealed behind plaster they must he installed in 'acceptable zones' which are intended to reduce the danger to the cables and to people who drill holes or knock nails into walls. Cable runs must only follow paths which are horizontal or vertical from an outlet, or be within 150 mm of the top (but not the bottom) of the wall, or within 150 mm of the angle formed by two adjoining {Fig 4.14} Acceptable installation zones for concealed cables. The diagonal cable must be enclosed in earthed metal walls. Where a cable run has to be diagonal, it must be protected by being enclosed in steel conduit, or must be a cable with an earthed metal sheath (such as mineral insulated cable), or an insulated concentric cable. In this latter case, the phase conductor will be surrounded by the neutral, so that if a nail or a screw penetrates the cable it will be impossible for it to become live. The possible zones are shown in (Fig 4.14). The internal partition walls of some modern buildings are very thin, and where cables complying with the requirements above are within 50 mm of the surface on the other side, they will require protection.

 

Fig 4.14 Acceptable installation zones for concealed cables. The diagonal cable must be enclosed in earthed metal.

There are cases where cables are enclosed in long vertical runs of trunking or conduit. The weight of the cable run, which effectively is hanging onto the top support, can easily cause damage by compressing the insulation where it is pulled against the support. In trunking there must be effective supports no more than 5 m apart, examples of which are shown in Fig 4.15, whilst for conduit the run must be provided with adaptable boxes at similar intervals which can accommodate the necessary supports.

 

Fig 4.15 Support for vertical cables in trunking

The top of a vertical conduit or trunking run must have a rounded support to reduce compression of insulation. The diameters required will be the same as those for cable bends given in {4.4.2}.

Support for overhead conductors is considered in {7.13}.