Page 212 - Handbook of Electrical Engineering

196 HANDBOOK OF ELECTRICAL ENGINEERING

Table 9.10. (continued)

Actual Nominal Circular mils American
2
2
CSA (mm ) CSA (mm ) wire gauge
— 150 — —
152.0 (150) 300,000 300 MCM
161.0 — 318,000 —
177.0 (185) 350,000 350 MCM
— 185 — —
194.0 — 382,000 —
203.0 — 400,000 400 MCM
— 240 — —
253.0 (240) 500,000 500 MCM
258.0 — 509,000 —
— 300 — —
304.0 (300) 600,000 600 MCM

It can be seen that the preferred choices give an accuracy of better than 2.1% in the calculated
cross-sectional area if the wire diameter is as shown. Different combinations of the wire diameter
and the number of layers can in several cases give almost the same cross-sectional areas. The pre-
ferred choices are the most economical in terms of stocking wire sizes in a factory. The largest wire
diameter is usually 3.199 mm.
Where the insulation is a rubber-based elastomer it is common practice to tin coat the copper
wires, to protect against chemical attack from the elastomer.

9.3.3 Conductor Semiconducting Screen

A semiconducting screen of tape or extruded compound is normally specified for cables that have
a rated line voltage of 3000 V and above. IEC60502 applies to solid extrusions of insulation, and
requires PE and XLPE compounds to have the screen for 3000 V and above. Likewise the standard
requires the screen for 6000 V and above for PVC and EPR compounds. BS6622 calls for screens
for all cables for voltages between 6600 and 33,000 volts.

The purpose of the screen is to reduce the voltage gradient (electric stress) at the surface of
the conductor where it interfaces with the insulation. Otherwise irregularities in the interface could
initiate failure of the insulation in the longer term.

9.3.4 Insulation

The most frequently used insulating compounds are PVC, XLPE and EPR. For most onshore appli-
cations PVC and XLPE are preferred because of economic reasons, and XLPE is becoming more
popular than PVC. Marine and offshore applications tend to prefer XLPE and EPR. EPR is usually
more expensive than XLPE. Both compounds have the advantage that they permit the conductors
◦
◦
to operate at higher temperatures (85 to 90 C) than those of PVC (70 C). PVC compounds can be
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specially manufactured to tolerate conductor temperatures up to 85 C. Silicon rubber can be speci-
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fied if high conductor temperatures (up to 180 C), and for even higher temperatures (up to 260 C)