GETTING TO GRIPS WITH SOFT BEDDED CABLES

The increasing popularity of fire resistant and flame retardant cables particularly for safety focussed application such as oil & gas installations, refineries and chemical plants has led to a proliferation of what are commonly described as soft bedded cables. A general description of a soft-bedded cable is one which employs materials, which exhibit, ‘significant cold flow’ characteristics, i.e. thermoplastic materials, which flow when subject to pressure at ambient temperature. This bedding is the cable inner bedding, which in the case of an armoured cable, is the part of the cable which enters the equipment.

Particular care should generally be taken when installing this type of cable, so as to avoid cable damage due to over tightening of cable gland seals on the cable inner bedding. The selection of suitable cable glands to complement this type of cable construction is vital, as the traditional compression seal cable gland is most likely to cause excessive pressure on the part of the cable which is most vulnerable, leading to cable damage and potential long term di-electric problems.

An increased level of attention in this area may be required for installations in Explosive Atmospheres for obvious reasons which are subsequently explained on the Cold Flow page of the site, click here for more details.

A common method of addressing this problem is to use a displacement seal, whereby the seal is gradually displaced as the cable gland is tightened, virtually eliminating any damage to cable bedding. The purpose of this design enables the user to control the level of displacement of the seal, ensuring a functional and more importantly, safe, seal is achieved without exerting any undue pressure onto the cable. An example of a cable gland utilising a displacement seal on the cable inner bedding is illustrated below.

The other important fact to note is that this also utilises a separate tightening action for the inner seal and the armour termination compartment, which is the differentiating feature compared with other cable gland designs.

Naturally, many factors influence the final method of installation including, environment, ambient temperatures, and the specific construction of the cable being used. CMP Products are always willing to provide an expert assessment of a given application, drawing on many years experience of installation procedures, providing assurances that a safe solution is achieved every time.