Section
6 System design – Protection
6.1 General
6.1.1 The general requirements for protection are given in IEC 61892-2:2012,
Mobile and fixed offshore units – Electrical installations – Part 2: System
design, Section 10 and Pt 6, Ch 2, 6.1 General of the Rules for Ships, which are to be complied
with.
6.2 Protection against short-circuit
6.2.1 The general requirements for protection against short-circuit are given
in Pt 6, Ch 2, 6.2 Protection against short-circuit of the Rules for Ships, which are to be complied
with. Additions or amendments to these requirements are given in the following
paragraph(s) of this sub-Section.
6.2.2 The rated short-circuit making and breaking capacity of every protective
device is to be adequate for the prospective fault level at its point of
installation; the requirements for circuit-breakers and fuses are detailed in Pt 6, Ch 2, 6.5 Circuit-breakers and Pt 6, Ch 2, 6.6 Fuses respectively.
6.3 Protection against overload
6.4 Protection against earth faults
6.4.2 Where any circuit, other than an intrinsically safe circuit, passes into
or through any Zone 0 area, the circuit is to be disconnected automatically and/or
is to be prevented from being energised in the event of an abnormally low level of
insulation resistance and/or high level of leakage current.
6.4.3 Where a circuit passes into any zone 0 area, the protective systems
shall be arranged so that manual intervention is necessary for the reconnection of
the circuit after disconnection as the result of a short-circuit, overload or
earth-fault condition.
6.5 Circuit-breakers
6.6 Fuses
6.6.1 The requirements for fuses are given in Pt 6, Ch 2, 6.6 Fuses of the Rules for Ships, which are to be complied
with. Additions or amendments to these requirements are given in the following
paragraph(s) of this sub-Section.
6.6.2 The use of fuses for overload protection is permitted up to 320A,
provided they have suitable characteristics, but the use of circuit-breakers or
similar devices is recommended above 200A. For high voltage a.c. systems (above 1 kV
a.c.), the use of fuses for overload protection is not acceptable.
Is Limiters
The use of Is Limiters is permitted in situations where circuit-breakers
cannot provide any protection against unduly high peak short-circuit currents, as
circuit-breakers are too slow.
NOTE
Only the Is Limiter is capable of detecting and limiting a short-circuit
current at the first rise, i.e. in less than 1 ms. The maximum instantaneous current
occurring remains well below the level of the peak short-circuit current.
6.7 Circuit-breakers requiring back-up by fuse
or other device
6.8 Protection of generators
6.8.1 The requirements for the protection of generators are given in Pt 6, Ch 2, 6.8 Protection of generators of the Rules for Ships and IEC 61892-2:2012,
Mobile and fixed offshore units – Electrical installations – Part 2: System
design, Section 10.4.2, which are to be complied with. Additions or
amendments to these requirements are given in the following paragraph(s) of this
sub-Section.
6.8.2 Generators not arranged to run in parallel are to be provided with a
circuit-breaker arranged to open simultaneously, in the event of short-circuit,
overload or under-voltage, all insulated poles. In the case of generators rated at
less than 50 kW, a multipole linked switch with a fuse, complying with Pt 6, Ch 2, 5.3 Isolation and switching 5.3.2, in each insulated pole will be acceptable.
6.8.3 Where generators are intended to operate in parallel:
- Generators are to be equipped with a protective device which, in
the event of a short-circuit in the generator or in the cables between the
generator and its circuit-breaker, will instantaneously open the
circuit-breaker and de-excite the generator.
- Under-voltage protection shall be provided to prevent the
generator circuit-breaker from closing if the generator is not generating,
in accordance with Section 10.5.1 of IEC 61892-2:2012, Mobile and fixed
offshore units – Electrical installations – Part 2: System design.
6.9 Load management
6.9.1 The requirements for load management are given in IEC 61892-5:2010,
Mobile and fixed offshore units – Electrical installations – Part 5: Mobile
units, Section 9.9.2, IEC 60092-504, Electrical installations in ships –
Part 504: Special features – Control and instrumentation and Pt 6, Ch 2, 6.9 Load management of the Rules for Ships,
which are to be complied with where applicable. Additions or amendments to these
requirements are given in the following paragraph(s) of this sub-Section.
6.9.2 Arrangements are to be made to disconnect automatically, after an
appropriate time delay, circuits of the categories noted below, when the
generator(s) is/are overloaded, sufficient to ensure the connected generating set(s)
is/are not overloaded:
- non-essential circuits;
- circuits feeding services for habitability, see
Pt 6, Ch 2, 1.6 Definitions 1.6.2 of the Rules for Ships; and
- circuits for other essential services, when it can be
established that safe operation can be maintained during the temporary loss
of such services.
6.10 Feeder circuits
6.10.1 The requirements for feeder circuits are given in IEC 61892-2:2012,
Mobile and fixed offshore units – Electrical installations – Part 2: System
design, Section 10.4.5 and Pt 6, Ch 2, 6.10 Feeder circuits of the Rules for Ships, which are to be complied
with.
6.11 Motor circuits
6.11.1 The requirements for motor circuits are given in IEC 61892-2:2012,
Mobile and fixed offshore units – Electrical installations – Part 2: System
design, Section 10.4.6 and Pt 6, Ch 2, 6.11 Motor circuits of the Rules for Ships, which are to be complied
with.
6.12 Protection of transformers
6.12.1 The requirements for protection of transformers are given in IEC
61892-2:2012, Mobile and fixed offshore units – Electrical installations –
Part 2: System design, Section 10.4.4 and Pt 6, Ch 2, 6.12 Protection of transformers of the Rules for Ships, which are to be complied
with where applicable.
6.13 Harmonic filters
|