CRUCIAL CABLE CARE

When terminating cables and wires during the process of installation of electrical equipment this should be done by qualified and competent personnel in line with good engineering practice, observing safe electrical practices. It is also important that the correct cable entry device or cable gland is selected to suit the cable being used. For instances using normal unarmoured cables, the cable would usually be fed through a cable entry device that has a sealing ring activated onto the cable outer sheath. The choice of cable glands to suit this unarmoured cable may need to take into account any vulnerability of the cable, which may be especially critical if the cable is an instrument cable. Two factors which could affect the long term cable performance are the type and design of the cable gland sealing function, and the possibility of this being inadvertently, or otherwise, over tightened onto the cable sheath.

In some applications it may be necessary to provide some mechanical protection to prevent the cables from being damaged or completely severed by accidental encroachment of machinery or other major impact. This mechanical protection may be provided in the shape of a metallic conduit that can house individual insulated cable conductors or a normal unarmoured sheathed cable. Alternatively a metallic sheath or armour included in the cable construction during its manufacturing process may be used.

When cables with a protective metallic sheath or armour are chosen, these may be constructed with or without an extruded inner bedding, underneath the layer of armouring. In some cases this extruded bedding may be substituted by a polymeric covering or tube that contains the insulated conductors.

Cable glands for armoured cables, with a single outer seal should be selected for cables without an inner bedding or covering under the armour. Cable glands for armoured cables, with a double, inner & outer, seal configuration would normally be selected for cables with an inner bedding or covering under the armour.

Overview of Different Seal Types

In general there are five different types of sealing methods used on the cable inner bedding, which are:-
  1. Compression Seal
  2. Displacement Seal
  3. Diaphragm Seal
  4. Compensating Displacement Seal (CDS) System
  5. Compound Barrier Seal.

These are better described as follows :-
  1. Compression Seal (Sealing Ring)
    The Compression Seal is an elastomeric sealing ring that has a V groove or weak back in its design that is intended to be closed in order to create a downward seal on the cable inner bedding, when the same compressive force is equally applied to both sides of the seal.
  2. Displacement Seal (Sealing Ring)
    The Displacement Seal does not employ a weak back design. Instead the Displacement Seal is gradually pushed down a taper until it makes an effective seal on the cable.
  3. Diaphragm Seal (Sealing Ring)
    The Diaphragm Seal comprises of a flexible elastomeric membrane that is usually attached to the armour cone, and it is designed to fit snugly after stretching over the cable inner bedding. This is intended to make no impression on the cable inner bedding.
  4. Compensating Displacement Seal (CDS) System (Sealing Ring)
    The Compensating Displacement Seal system utilises the principal of the Displacement Seal but with the addition of a compensator that limits or controls the force applied to the cable inner bedding.
  5. Compound Barrier Seal (Epoxy Resin Compound)
    The compound barrier seal is made on site by the technician completing the installation and is used primarily in Explosive Atmospheres where the inner cable bedding must be removed and a hard setting resin barrier seal that has been specially tested for use in potentially explosive atmospheres is applied around the conductors.

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