Cable Insulation and Ingress Protection

Class A Insulation

-These are materials or combinations of materials such as cotton, silk, and paper when I suitably impregnated* or coated or when immersed in a dielectric liquid such as oil. Other materials or combinations of materials may be included in this class if experience or accepted tests can reveal their capability to operate at 105°C (221°F).

Class E Insulation

There are materials or combinations of materials which, by experience or accepted tests, can reveal their capability to operate at 120°C (248°F) (materials possessing a degree of thermal stability allowing them to be operated at a temperature 15°C (27°F) higher than Class A material).

Class B Insulation

These are materials or combinations of materials such as mica, glass, fibre, etc, with suitable bonding substances. Other materials or combination of materials, not necessarily imorganic, may be included in this class, if experience or accepted tests can reveal then capability to operate at 130°C (266°F).

Class F Insulation

These are materials or combinations of materials such as mica, glass, fibre, etc, with table bonding substances. Other materials or combination of materials, not necessarily imorganic, may be included in this class if, experience or accepted tests can show them shown can be capable of operation at 155°C (311°F).

Class H Insulation

These are materials or combinations of materials such as silicone, elastomer, mica, glass, fibre, etc, with suitable bonding substances such as appropriate silicone resins. Other materials or combination of materials, not necessarily inorganic, may be included in this class , if experience or accepted tests can reveal their capability to operate at 180°C (356°F).

Conductor Insulation

To be useful and safe, electric current must be forced to flow only where it is needed. It must be channelled from the power source to a useful load. In general, current-carrying conductors must not be allowed to come in contact with one another, their supporting hardware, or personnel working near them.

Cable Gland

Cables are insulated, mechanically protected and watertight. They may be armoured and suitable for installation in a hazardous / explosive area. A cable gland maintains these properties where the cable is terminated at an appliance, e.g., at a motor terminal box. It prevents ingress of any foreign material, liquids even at pre-determined pressures and sometimes gases too.

The cable gland comes in three separate parts i.e., the gland body which has a standard electrical thread, usually 20 mm for the smaller cables and 25mm for the larger, an olive or compression ring, which tightens on to the cable thus affording good continuity and the lacking nut which tightens down on the olive when the gland has been positioned correctly to the appliance terminal box. Nuts on the gland while compressing sealing rings maintain watertight seals on the inner and outer sheaths and clamp the armour braiding. The gland must suit the type and size of cable (Refer Figure 15.5). MICC cables require special sealing points to prevent ingress of moisture into its hygroscopic insulation. A simple gland clamps the copper outer sheath to earth it and make a watertight seal. A non-combustible, pressure tested scaling compound may also be used in addition. Sometimes rubber-based compounds are used. The standard seals for MICC cables are designed for continuous operating temperatures up to 105°C. High temperature sealing kits can be used to allow operation up to 210°C.

Figure 15.5 - Cable Glands

In most cases earthing of the cable is done on the cable gland armouring. Where cables piss through watertight bulkheads and fire-stop barriers they must be especially "glanded" to maintain the integrity of such bulkheads.

Cable terminal sockets can be soldered to the conductors but are more frequently crimped onto the cable by a compression tool. Cable sockets must be securely attached to the appliance terminal screw by nuts and shakeproof (or split) washers. A loose terminal will invariably become a source of localised overheating and may also lead to severing of He I cable at that point due to undue stress. Periodic maintenance should always include checking the tightness of terminal connections. Small cables are terminated in terminal blocks of various designs. (Refer Figure 15.6).