Page 112 - Offshore Electrical Engineering Manual

Cables for Intrinsically Safe Circuits 99

CABLES FOR INTRINSICALLY SAFE CIRCUITS

1. Intrinsically safe (IS) cables should have a light-blue outer sheath and be
screened (shielded).
2. Requirements for armouring vary between countries and standards, but
mechanical protection should be provided, particularly in cables containing
multiple IS loops. Such multicore IS cables are acceptable, provided each
IS loop pair is individually screened. If the IS cables are subject to frequent
flexing, a separate cable for each loop, incorporated into a purpose-designed
umbilical with hydraulic or pneumatic hoses (if any), is recommended.
3. It must be possible to complete a valid loop safety calculation for every loop,
so it is essential that comprehensive cable electrical parameters, including
inductances and reactances, are available for the cable used.
4. All IS cables should be segregated from non-IS cables by using separate cable-
racking or earthed-metal partitions.
5. In the United Kingdom, cable screens are earthed at the nonhazardous end, but
earth or ground loops must always be avoided.
6. IS circuits should also be separated from non-IS circuits in enclosures (junction
boxes, etc.). At bare live connections (terminals) a minimum distance of 50 mm
should be allowed between IS and non-IS circuits. Between IS circuits, the
distance should be a minimum of 6 mm. IS and non-IS wiring may be contained
in the same enclosure, provided the same segregation is maintained. IS and
non-IS earth bars should be located at opposite ends of the enclosure. Good
labelling and strict adherence to light-blue sheathing for IS cables/wiring is
essential.

Please refer to
PART 4 Chapter 5 Cable Sizing Calculations
PART 6 Any Variations for Floating Installations and
Ships