OVERVIEW OF CABLE GLAND CONSTRUCTION TYPES
It should be recognised that cables are produced in various parts of the world to numerous different national and international standards, and the physical characteristics of one cable made by a certain manufacturer may differ considerably from another maker’s product, even when the two cables have the same theoretical electrical parameters or ratings.
Close attention should be paid to this selection process taking into account the cable construction and any vulnerability it may present.
The reason for this is because some cable gland constructions are more compatible with certain cable types than others, and this is often due to the sealing method incorporated.
Cable Gland Construction utilising Compression Seal
A typical cable gland utilising a compression seal within its design, as the example shown below, has been known to cause cable damage due to over tightening as it has no inbuilt functionality to prevent the user from over tightening. More to the point is that when this type of cable gland is installed, the action of tightening the inner sealing ring is combined with the termination of the metallic cable armour. This leads the user to feel the need to tighten the assembly as far as he can to ensure that he has a good reliable earth continuity path, and often the cable inner bedding suffers as a result. In effect there is no real control over how tight the inner sealing ring is applied to the cable inner bedding, when the tightening of the cable gland body performs two actions in one step.
The sealing on the cable inner bedding and the armour termination are effected in one simultaneous action, which typically involves the armour cone being tightened until it reaches a defined mechanical stop. This can lead to the compression seal being over compressed, especially on the larger cable diameters for each cable gland size.
This can be loosely described as "over tightening", and is a common problem with certain cable gland types.
It should be noted that none of the cable glands supplied by CMP Products utilise compression seals on the cable inner bedding.
Cable Gland Construction utilising Displacement Seal
This type of seal is usually employed in a cable gland that utilises a 3 stage ‘Step by Step’ sequential make off, where the tightening of the armour termination is separated from the activation of the inner seal. This enables the effectiveness of the seal on the cable inner bedding to be tested before the armour is terminated. This means that unlike other designs the user is able to determine that the engagement of the seal on the cable inner bedding is not too light and not too heavy, affording some level of control over the installation.
It can be noted that this displacement seal arrangement is the standard method of sealing on the cable inner bedding adopted by CMP Products.
Cable Gland Construction utilising Compensating Displacement Seal (CDS) System
The Compensating Displacement Seal system is designed to allow users to fully tighten the cable gland components together every time, without damaging the cable. The use of an internal compensator enables the CDS arrangement to be fully tightened regardless of the cable diameter. This unique approach offers a solution that allows the installation personnel to carry out their objectives to schedule without the likelihood of re-work potentially arising from over tightening.
The unique Compensating Displacement Seal (CDS) system is a design patented by CMP Products.
Industry Standard Cable Gland Designations
Cable Glands may be designed, tested and manufactured to various standards around the world, but since CMP Products is a long established UK based manufacturer, it just so happens that part of its history includes close participation in the development of British Standards for these products.
The British Standard BS 6121, and its predecessor BS 4121, have more than 30 years of exposure in meeting real life industry situations, not just in the UK but also around the world.
In fact BS 6121 also established the industry standard cable gland type designations that still prevail today for simple cable gland identification. It should be noted however that the type designation for a given cable gland product does not determine the precise method of sealing adopted.