Page 240 - Offshore Electrical Engineering Manual

CHAPTER

Feeder Protection, 5

Conductor Sizing, Load

Flow and Fault Calculation

FUSES
Low-voltage (LV) distribution systems in offshore installations are protected by
fuses in the same way as their onshore counterparts. Although the Institution of
Electrical Engineers (IEE) Wiring Regulations specifically excludes offshore instal-
lations from its scope (Part 1 paragraph II-3(vii)), the document that it refers to (IEE
Recommendations for the Electrical and Electronic Equipment of Mobile and Fixed
Offshore Installations) has been replaced by BS IEC 61892, Mobile and fixed off-
shore units. Electrical installations. The methods of calculation given in the IEE
Wiring Regulations are normally adopted when designing lighting and small power
distribution systems, particularly for accommodation modules.
The use and limitations of high rupturing capacity fuses are discussed in PART 2
Chapter 5.

MINIATURE CIRCUIT BREAKERS

In most situations, miniature circuit breakers (MCBs) may be used as an alternative
to fuses where the fault rating of the MCB is sufficiently high. However, because
of the different shape of the MCB tripping characteristic compared with that of a
fuse (see Fig. 4.5.1), it is not advisable to mix fuses with MCBs in the same circuit
if discrimination is required. The degree of discrimination between one MCB and
another, and between MCBs and fuses, varies with the BS EN 60898-1 MCB type,
but generally a better discrimination can be obtained using fuses.
MCBs with fault current ratings more than 16 kA are now available and are in
common use offshore, where they save weight and reduce the quantity and type of
spare fuses stocked. MCBs used offshore must provide positive indication of contact
clearance and must be padlockable when in the open position.

OVERCURRENT AND EARTH FAULT PROTECTION
Most of the conventional forms of short cable feeder protection may be used off-
shore. The use of inverse definite minimum time (IDMT) overcurrent and residu-
ally connected earth fault relays may give discrimination problems if a large

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