Page 214 - Handbook of Electrical Engineering

198 HANDBOOK OF ELECTRICAL ENGINEERING

9.3.8 Armouring

Most cables used in oil industry plants are installed for at least part of their length in exposed
machinery areas or on trays or ladders. This exposure can permit mechanical damage to occur to the
cables e.g. objects falling on to them or impacting into them. Even buried cables are at risk from
excavation mistakes, e.g. digging machines.

In order to minimise the possible damage to a cable, and to provide a safe path for electrical
earth return currents, it is necessary to specify a metallic armouring. There are several types of
metallic armouring used in the oil industry, i.e. galvanised steel wires (GSWA), aluminium wires
(AWA), tinned copper wire braid (TCUWB), phosphor bronze wire braid (PBWB).
Various standards specify the diameter, number and design of the wires and braids, e.g.
BSEN10257 part 1 and IEC60502 for steel armouring, BS2873 for phosphor bronze and BS4109
for copper braid armouring. GSWA and PBWB are the most commonly used armouring. Armouring
in the form of metallic tape is not normally used for oil industry installations.

9.3.9 Outer Sheath

Oil industry cables are usually finished with an extruded heavy-duty polymeric sheath such as PVC,
PE or CSP. For situations where resistance to heat, oil and flames is necessary it is the practice to use
special elastomerics that are identified as HOFR types. These compounds include EVA, EMA, CPE,
and EPR together with suitable fillers that are used during their curing processes. BS7655 details the
requirements for HOFR cables, and IEC60332 for their fire retardance.

9.4 CURRENT RATINGS OF POWER CABLES

The choice of cross-sectional area of cable conductors depends upon several factors, the main fac-
tors being:-

• Continuous load current.
• Continuous rated current of cable.
• Volt-drop developed across the cable under steady state and transient conditions.

• Dissipation of heat from the conductors during short-circuit conditions.
• Earth loop impedance.

9.4.1 Continuous Load Current

Cables are used to supply power to individual loads such as motors, and to groups of loads as in the
case of a feeder to a switchboard. Selecting the load current for the first case is reasonably simple.
Most individual loads have a manufacturer’s nameplate that gives details such as rated power, voltage,
current and power factor. If the current is not given on the plate then it should be calculated from
the output power, power factor and efficiency of the load.