Page 557 - Industrial Power Engineering and Applications Handbook

Captive (emergency) power generation 16/531
Appendix 1 : Selection of power The selection process of power cables is almost the
cables same as that of a bus system discussed in Section 28.3.
For simplicity we consider only the basic data for selection
which would suffice the majority of applications. For
A16.1 Introduction accurate calculations a similar approach will be essential
as for the bus systems (Chapter 28). For site conditions
To provide a reference for those working on power projects and laying arrangements which may influence the basic
or at sites, we provide some important data on different rating of a cable, corresponding derating factors have
types of LT and HT power cables in this appendix. The also been provided. The information covered here will
cables described here are in use for all kinds of power be useful to users to meet their cable requirements,
distribution applications. Of these, XLPE cables are also although the data may vary marginally for different
used for power transmission applications. To help a user manufacturers. For more data on cables, not covered
to select the most appropriate types of cables, we also here, reference may be made to the cable manufacturers.
provide a brief comparative chart of the various types of The choice of any of the cables mentioned in Table
cables being manufactured. Tables giving the technical A16.1 will depend upon the site conditions, fault level
particulars of such cables in all voltage ratings have also and the voltage rating of the system. A brief comparison
been provided. of all these insulating systems is given in Table A16.2

Table A16.1 Insulating systems for cables
SI: no. Insulating system Constituents

1 Polyvinyl chloride (PVC) A thermoplastic compound
2 Paper insulated (PI)
(Figure A16.1) Impregnated paper
3 Unfilled or filled crosslinked
polyethylene (XLPE) A thermoplastic compound
insulated
(Figure A16.2)
4 Polyethylene (PE) A thermoplastic compound. These are basically polyethylene compounds only, with
a little crosslinking to save on cost. LT cables and those below 6.6 kV are costly to
produce and hence not in great use
5 Ethylene propylene (EP) rubber A synthetic rubber (butyl rubber)
6 Flame retardant low smoke cables (FRLS) The outer sheathing of such cables is made with the base insulation of chlorinated
polymers (e.g. PVC, polythene, CSP, PCP, XLPE or EP rubber) and hence they can
be manufactured for all system voltages. All polymers are self-extinguishing and
fire retardant. But in the event of fire, they propagate fine and release large volumes
of dense smoke, toxic gases and HC1. When combined with water, such as during
firefighting, they produce corrosive acids which are highly dangerous for human
inhalation. A good FRLS cable must therefore possess the following properties:
- Ability to restrict the propagation of flame
- Emit low smoke,
- The smoke emitted should not obscure visibility
- Emit low acid (HCl) gas
- Emit low toxic gases
To make these polymers have the desired properties, certain additives (chemicals),
as noted below, are added to the sheathing compound in specific ratios. The additives
act like flame retardants and diminish the ignitability of the insulation by lowering
the temperature of the cable, delaying ignition and resisting the spread of fire in the
insulation and the polymeric compounds
- Alumina trihydrate - to achieve reduction of heat by cooling through an endothermic
process that decomposes the flame
- Molybdenum trihydrate - reduces smoke
- Antimony trioxide - also provides a flame retardant effect
- Zinc borate-forms a protective coating of a glass-like film, retards the burning
process and protects the insulation
- Calcium carbonate - emits non-flammable gases and helps to reduce the supply
of oxygen to the burning surfaces. The FRLS cables thus produced would possess
the required properties
7 Fire survival cables (FS) These are silicon rubber, glass tape or glass mica tape sheathed, with an elastomer,
having fire retardant and low smoke properties