4.1 - Cable insulation materials
For many years wiring cables were insulated with vulcanised
natural rubber (VIR). Much cable of this type is still in
service, although it is many years since it was last manufactured.
Since the insulation is organic, it is subject to the normal
ageing process, becoming hard and brittle. In this condition
it will continue to give satisfactory service unless it
is disturbed, when the rubber cracks and loses its insulating
properties. It is advisable that wiring of this type which
is still in service should be replaced by a more modern
cable. Synthetic rubber compounds are used widely for insulation
and sheathing of cables for flexible and for heavy duty
applications. Many variations are possible, with conductor
temperature ratings from 60°C to 180°C, as well as resistance
to oil, ozone and ultra-violet radiation depending on the
Dry paper is an excellent insulator but loses its
insulating properties if it becomes wet. Dry paper is hygroscopic,
that is, it absorbs moisture from the air. It must be sealed
to ensure that there is no contact with the air. Because
of this, paper insulated cables are sheathed with impervious
materials, lead being the most common. PILC (paper insulated
lead covered) is traditionally used for heavy power work.
The paper insulation is impregnated with oil or non-draining
compound to improve its long-term performance. Cables of
this kind need special jointing methods to ensure that the
insulation remains sealed. This difficulty, as well as the
weight of the cable, has led to the widespread use of p.v.c.
and XLPE (thermosetting) insulated cables in place of paper
Polyvinyl chloride (p.v.c.) is now the most usual low voltage
cable insulation. It is clean to handle and is reasonably
resistant to oils and other chemicals. When p.v.c. burns,
it emits dense smoke and corrosive hydrogen chloride gas.
The physical characteristics of the material change with
temperature: when cold it becomes hard and difficult to
strip, and so BS 7671 specifies that it should not be worked
at temperatures below 5°C. However a special p.v.c. is available
which remains flexible at temperatures down to -20°C.
At high temperatures the material becomes
soft so that conductors which are
the insulation (eg at bends) will 'migrate' through it,
sometimes moving to the edge of the insulation. Because
of this property the temperature of general purpose P.V.C.
must not be allowed to exceed 70°C, although versions which
will operate safely at temperatures up to 85°C are also
available. If p.v.c. is exposed to sunlight it may be degraded
by ultra-violet radiation. If it is in contact with absorbent
materials, the plasticiser may be 'leached out' making the
p.v.c. hard and brittle.
LSF (Low smoke and fume)
Materials which have reduced smoke and corrosive gas
emissions in fire compared with p.v.c. have been available
for some years. They are normally used as sheathing compounds
over XLPE or LSF insulation, and can give considerable safety
advantages in situations where numbers of people may have
to be evacuated in the event of fire.
Gross-linked polyethylene (XLPE) is a thermosetting
compound which has better electrical properties than p.v.c.
and is therefore used for medium- and high-voltage applications.
It has more resistance to deformation at higher temperatures
than p.v.c., which it is gradually replacing. It is also
replacing PILC in some applications. Thermosetting insulation
may be used safely with conductor temperatures up to 90°C
thus increasing the useful current rating, especially when
ambient temperature is high. A LSF (low smoke and fume)
type of thermosetting cable is available.
Provided that it is kept dry, a mineral insulation
such as magnesium oxide is an excellent insulator. Since
it is hygroscopic (it absorbs moisture from the air) this
insulation is kept sealed within a copper sheath. The resulting
cable is totally fireproof and will operate at temperatures
of up to 250°C. It is also entirely inorganic and thus non-ageing.
These cables have small diameters compared with alternatives,
great mechanical strength, are waterproof, resistant to
radiation and electromagnetic pulses, are pliable and corrosion
resistant. In cases where the copper sheath may corrode,
the cable is used with an overall LSF covering, which reduces
the temperature at which the cable may be allowed to operate.
Since it is necessary to prevent the ingress of moisture,
special seals are used to terminate cables. Special mineral-insulated
cables with twisted cores to reduce the effect of electromagnetic
interference are available.