Jake Green, Head of Technical Engagement with Scolmore Group, takes a look at some of the ways to avoid undue strain on cables, as well as delving deeper into the requirements of BS7671.

Wiring systems must be selected and erected to avoid during installation, use and maintenance, damage to the sheath or insulation of cables and their terminations (522.8.1). To ensure the requirements of this regulation are met, cables not installed in conduit or trunking are typically supported using a range of clips, saddles, band, glands and ties.

Requirements

All cables and conductors must be supported in such a way that the level of mechanical strain which naturally exists will not cause either the cable/conductor or the terminations to be compromised (522.8.5). Furthermore, all cable supports and enclosures shall not have sharp edges liable to damage the wiring system, and cable/conductors are not to be damaged by the means of fixing (522.8.11 & 522.8.12). It is important, therefore, that care is taken when selecting for use supports that are appropriate for the cable/conductors.

Types of support

There is a wide range of options available for the contractor when selecting cable supports. These include amongst other things:

• Cable ties
• Cable cleats
• Cable clips
• Cable tie accessories
• Cable glands

The type of fixing will depend on the nature of the surface to which the support is fixed. For example, cable ties would be suitable for fixing cables installed on basket tray or cable tray, whereas cable clips would be suitable for wooden surfaces. Similarly, the type of support will depend on the type of cable being installed. For example, cable cleats will be suitable for steel-wire armoured cables.

Guidance

The guidance issued within the On-Site Guide (OSG) published by the IET is helpful in deciding on the nature of cable support and the distances recommended between clips. Appendix D covers cables generally, specific applications such as caravans and the like, overhead wiring as well as conduit and trunking support.

Table D1 details spacings of supports for cables in accessible positions. Fig 1 gives an example of the recommended distance between clips for a cable having a diameter not exceeding 9mm based on Table D1. As the cable diameter increases the distance between clips is permitted to grow.

Image: Fig 1 Cable supports example – image courtesy of T.J. Bratley

Whilst the support distances are important, it remains the case that the purpose of cable supports is to ensure there is no undue strain on the cable or its terminations.

Similarly, when cables are bent it is important that there is no undue strain on the internal conductors caused when the bend is too tight (522.8.3). Fig 2 shows an example of the recommended bending radius for 1.5 mm² insulated and sheathed ‘twin and earth’ (Table D5 OSG).

Image: Fig 2 Bending radius example – image courtesy of T.J. Bratley 

Cable entry

Two issues must be addressed when cables enter an accessory: the risk of damage to the sheath/insulation at the point of entry, and any potential strain on connections.

Where a cable enters a metallic accessory box having no protection from sharp edges, there is a risk that the insulation surrounding conductors may become damaged leading to arc and shock risk. Where cables enter a metallic accessory box, it is important that protection is provided, such as with rubber grommets or similar.

Even where cables entering an accessory box or other metallic enclosure are protected from damage, terminations may still be impacted by strain as cables are left unsupported. In such circumstances it is necessary for suitable support to be provided and compression glands provided to ensure the electrical and mechanical strength of terminations (522.8.5 and Note).

Conclusion

All cables and conductors must be suitably supported to ensure that no undue strain exists on the cable or the terminations (522.8). Care should be taken to select appropriate support (clips, glands and the like) for cables and Unicrimp has a range of products to help the contractor in carrying out their duties.

Find out more here

Find more industry feature articles here

The post How to avoid placing undue strain on cables appeared first on Professional Electricians Wholesaler.

Jake Green, Head of Technical Engagement with Scolmore Group, considers the necessity of maintaining signal integrity for life safety services such as fire alarm systems and emergency lighting systems.

In any electrical installation, it is necessary to give adequate support to cables. The reasons for this are twofold:

• Maintenance of the signal pathway
• Reduce the risk of premature collapse to protect firefighters accessing a premises during fire.

Three specific life safety standards to be considered are:

• BS5266-1: 2016 Emergency lighting – Part 1: Code of practice for the emergency lighting of premises;
• BS5839-1: 2017 Fire detection and fire alarm systems for buildings Part 1: Code of practice for design, installation, commissioning and maintenance of systems in non-domestic premises; and
• BS8519: 2020 Selection and installation of fire-resistant power and control cable systems for life safety, fire-fighting and other critical applications – Code of practice.

Other relevant references will be made to the requirements of BS7671: 2018 Amd 2: Requirements for Electrical Installations.

Emergency lighting

Emergency lighting systems require installed cables to be able to withstand the effects of fire as well as remain in-situ for a period necessary to ensure safe egress of persons. To address these issues, Clause 8 of BS5266-1 covers wiring systems and circuits in general.

For self-contained emergency luminaires, there is no recommendation for fire-protected cables (clause 8.1), and the general requirements of BS7671 will apply. These include the need to support the wiring system in such a way that it is not liable to premature collapse (521.10.202).

However, where the emergency lighting system consists of a central battery, it is of critical importance that the quality of the installation, the fire-integrity of the cables and their supports is maintained. The risk of a loss of signal, and hence illuminance, is increased where poor installation practices are carried out (Note 1 Clause 8.2.1).

Clause 8.22 of BS5266-1 details the nature of cable types recognised as being acceptable. This clause details minimum cable survival times ranging from 60 minutes to 120 minutes.

Without suitable survival times for the support of such cables, it will be impossible to ensure circuit integrity. The methods of cable support and fixings should be non-combustible and such that circuit integrity will not be reduced below that afforded by the cable used (Clause 8.2.3). This means, for example, that where a cable is rated as ‘enhanced emergency lighting cables with inherently high resistance to attach by fire’, the support means should also be capable of performing its function for 120 minutes.

Furthermore, where cable management system support is provided by drop rods, the drop rod size should be calculated in accordance with the recommendations of BS8519.

Fire detection and fire alarm systems

The commentary to clause 26 of BS5839-1 recognises that all interconnections must operate correctly at the time of a fire. Clause 26.2 recommends that cables forming part of the critical path conform to certain standards. These include mineral insulated copper sheathed cables, cables conforming to BS7629-1, and the like.

Like the recommendations of BS5266-1, clause 26.2f) of BS5839-1 recommends that methods of cable support should be non-combustible and such that circuit integrity is not reduced below that afforded by the cable used, and that they should be able to withstand a similar temperature to that of the cable being supported. This recommendation, in effect, precludes the use of plastic cable clips, ties and trunking as means of cable support.

Fire-resistant power and control cable systems

As with both emergency lighting systems and fire detection and fire alarm systems, BS8519: 2020 addresses the need to ensure cable support systems are adequate to meet the fire survival times of the cables they support.

The recommendations found in BS8519 are like those detailed in both BS5266-1 and BS5839-1. However, additional guidance is provided where a bracket and rod system is used to support safety critical cable systems. Clause 16 of BS8519 recognises that the strength of steel is adversely affected by fire such that great care should be taken in determining the cross-sectional area of the steel rod drops.

Conclusion 

It is essential that great care is taken when installing cables supplying life safety systems, such as fire detection and fire alarm systems, emergency lighting systems and fire-fighting lifts. The installer must use appropriate cables and support systems to ensure that critical systems continue to function during fire.

Find out more here

The post The importance of maintaining signal integrity for life safety systems | Scolmore appeared first on Professional Electricians Wholesaler.