Page 261 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 245

burnt cable, the better is the flame retardant property. IEC60332 Part 3 defines three categories of
the volume of combustible non-metallic material present in the test before the flame is applied,

Class A specifies 7 litres of material per metre length.
Class B specifies 3.5 litres of material per metre length.
Class C specifies 1.5 litres of material per metre length.

Good flame retardance can be achieved by PVC, PCP, PTFE and compounds rich in EPR and
containing CSP. However, some of these materials may cause the cable to have a poor performance
when the emission of toxic gases and smoke are considered.
Reference 10 provides full descriptions of the IEC60331 and IEC60332 tests, together with
practical aspects of cable choice and installation.
Reference 11 also describes the testing of cables and the materials that are available.

9.5.2 Fire Resistance

Fire resistance is a much more demanding requirement than fire retardance, and is more difficult
to achieve in the manufacturing processes. The fire resistance tests of IEC60331 impose a severe
duty on the cable sample. A 1.2 metre sample is mounted horizontally and subjected to a ribbon
flame from below for a given period of time at a pre-described temperature. The cable is energised
at its rated voltage so that a fault current can be detected. The general requirement is that the cable
remains in tact, albeit in a fragile state, throughout the test and that no fault current passes across
the insulation material.

In a practical situation a fire could otherwise destroy the cable, but it should still perform as
a cable for a period of time sufficient to provide a necessary emergency or shut down service.
Fire resistance is primarily a function of the insulation material. In addition fire resistant mica
tapes are often wound round the conductors in the form of a continuous helix. The mica is a good
electrical insulator as well as being very resistant to directly applied flames and heat. Fire resistance
requires the material to be self-extinguishing after the flame is removed.
It should be noted that fire resistance performance is not normally designed into high voltage
cables. This is because the time required to burn down a live high voltage cable, to the point where
it fails electrically, is usually much greater than the time required to shut-down and control the
emergency. It is also unusual to have high voltage power supplies involved directly in shutdown and
emergency services.

9.5.3 Emission of Toxic Gases and Smoke

When some elastomers are burned they evolve what is known as ‘acid gas’ or ‘halogen gas’. These
gases are typically composed of hydrochloric or hydrofluoric acid. They are toxic even in relatively
small volumes and can cause serious damage to the human respiratory system. Fatal results can occur
from bad fire situations.