Section
5 Supply and distribution
5.1 Systems of supply and distribution
5.1.1 The requirements for systems of supply and distribution are given in
Pt 6, Ch 2, 5.1 Systems of supply and distribution 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.
5.1.2 The following systems of generation and distribution are acceptable:
- d.c., two-wire, insulated;
- a.c., single-phase, two-wire, insulated;
- a.c., three-phase; three-wire, insulated;
- earthed systems, a.c. or d.c.
The following neutral earthing methods are permitted:
- Directly earthed TN System.
- Impedance earthed IT System.
- Isolated IT System.
Earthing systems complying with IEC 61892-2:2012, Mobile and fixed
offshore units Electrical installations Part 2: System design, Section 5
and IEC 60092-502:1999, Electrical installations in ships Part 502: Tankers
Special features, Section 5 are acceptable.
While both insulated and earthed distribution systems (TN-S) are
permitted, systems which may result in the presence of electrical currents within
the hull or unit structure return (TN-C and TN-C-S) are not permitted, with the
exception of:
- limited and locally earthed systems outside any hazardous
area;
- intrinsically safe systems;
- impressed current cathodic protection systems.
NOTES
- IEC 61892-2:2012, Mobile and fixed offshore units
Electrical installations Part 2: System design, Table 2 summarises
the principal features of the neutral earthing methods.
- Systems installed in hazardous areas shall comply with the
requirements of Pt 7, Ch 2, 8 Electrical equipment for use in explosive gas atmospheres, Pt 7, Ch 2, 9 Additional requirements for electrical equipment on oil storage units for the storage of oil in bulk having
a flash point not exceeding 60°C (closed-cup test), Pt 7, Ch 2, 10 Additional requirements for electrical equipment on units for the storage of liquefied gases in bulk and Pt 7, Ch 2, 11 Additional requirements for electrical equipment on units intended for the storage in bulk of other flammable
liquid cargoes .
- In hazardous areas (where inflammable gas may be present as
defined in IEC 60092-502:1999, Electrical installations in ships Part
502: Tankers Special features, Section 4) a.c. systems are to be
earthed to comply with IEC 60079-14, Explosive atmospheres Part 14:
Electrical installations design, selection and erection, in
particular Section 6 Protection from dangerous (incentive) sparking, and
be arranged so that no current arising from an earth fault in any part of
the system could pass through extraneous metalwork located in a hazardous
area. Earthed intrinsically safe circuits are permitted to pass into and
through hazardous areas.
5.2 Essential services
5.2.1 The requirements for essential services are given in Pt 6, Ch 2, 5.2 Essential services 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.
5.2.2 Essential services that are required by Pt 5 Main and Auxiliary Machinery to be duplicated are to be served by individual circuits,
separated in their switchboard or section board and throughout their length as
widely as is practicable without the use of common feeders, protective devices,
control circuits or control gear assemblies, so that any single fault will not cause
the loss of both services.
5.3 Isolation and switching
5.3.1 The requirements for isolation and switching are given in Pt 6, Ch 2, 5.3 Isolation and switching 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.
5.3.2 Isolation and switching is to be by means of a circuit-breaker or switch
arranged to open and close simultaneously all insulated poles. Where a switch is
used as the means of isolation and switching, it is to be capable of:
- switching off the circuit on load;
- withstanding, without damage, the overcurrents which may arise
during overloads and short-circuit;
In addition, these requirements do not preclude the provision of single
pole control switches in final sub-circuits, for example light switches. For
circuit-breakers, see
Pt 6, Ch 2, 6.5 Circuit-breakers and Pt 6, Ch 2, 7.3 Circuit-breakers.
5.3.3 Devices selected for isolation of circuits up to and including 1000V a.c
or 1500V d.c. shall comply with the relevant International or National
Standards.
5.4 Insulated distribution systems (IT
systems)
5.4.1 The requirements for insulated distribution systems are given in Pt 6, Ch 2, 5.4 Insulated distribution systems 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.
5.4.2 A device(s) is to be installed for every insulated distribution system,
whether primary or secondary, for power, heating and lighting circuits, to monitor
continuously the insulation level to earth and to operate an alarm in the engine
control room, or equivalent attended position, in the event of an abnormally low
level of insulation resistance and/or high level of leakage current, see also
Pt 6, Ch 1, 4.2 Alarm system for machinery.
5.4.3 IT systems (neutral isolated from earth or earthed through a high
impedance) shall meet the requirements of IEC 61892-2, Mobile and fixed offshore
units Electrical installations Part 2: System design, IEC 60092-502,
Electrical installations in ships Part 502: Tankers Special features
and IEC 60079-14, Mobile and fixed offshore units Electrical installations
Part 2: System design.
5.5 Earthed distribution systems (TN
systems)
5.5.2 TN systems (directly earthed) shall meet the requirements of IEC
61892-2, Mobile and fixed offshore units Electrical installations Part 2:
System design. TN-C and TN-C-S systems are only permitted for the
applications listed in Pt 6, Ch 2, 5.1 Systems of supply and distribution 5.1.2.
5.5.3 A device(s) is to be installed for every earthed distribution system,
whether primary or secondary, for power, heating and lighting circuits, to monitor
continuously the insulation level to earth and to operate an alarm in the engine
control room, or equivalent attended position, in the event of an abnormally low
level of insulation resistance and/or high level of leakage current, see also
Pt 6, Ch 1, 4.2 Alarm system for machinery. This does not apply to the following
systems:
- limited and locally earthed systems outside any hazardous
area;
- intrinsically safe systems;
- impressed current cathodic protection systems.
5.6 High voltage distribution systems
5.6.1 For systems with nominal voltage 15 kV a.c., or greater, the neutral
(star point) shall be earthed by one of the following methods:
- High impedance resistor.
- Earthing transformer.
The earth fault current should be as low as is reasonably practicable to
allow the earth fault protection to operate in a time specified by the protection
coordination study and to minimise touch voltages. Systems with resonant earthing
(Petersen coil) are not permitted.
5.6.2 Earthing systems serving high voltage systems, including systems with
nominal voltage 15 kV a.c. or greater, shall be solidly interconnected with the LV
system earthing network to minimise touch voltages. The touch voltages and fault
durations shall not exceed the values given in Annex B Touch Voltage and Body
Current of BS EN 50522:2012, Earthing of power installations exceeding 1 kV
a.c.
NOTE
Although offshore units are outside the scope of BS EN 50522, earthing of
power installations exceeding 1 kV a.c. guidance on dangerous touch voltages and
earthing system design may be obtained from this standard.
5.7 Diversity factor
5.7.1 The requirements for the diversity factor are given in IEC 61892-2:2012,
Mobile and fixed offshore units Electrical installations Part 2: System
design, Section 8 and Pt 6, Ch 2, 5.6 Diversity factor of the Rules for Ships, which are to
be complied with.
5.8 Lighting circuits
5.8.1 The requirements for lighting circuits are given in IEC 61892-2:2012,
Mobile and fixed offshore units Electrical installations Part 2: System
design, Section 11 and Pt 6, Ch 2, 5.7 Lighting circuits 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.
5.8.2 Escape lighting fittings supplied by a central battery system or UPS are
to be connected to the power source using fire-resistant cables and comply with the
relevant International or National Standards.
5.8.3 Lighting for enclosed hazardous spaces is to be supplied from at least
two final sub-circuits to permit light from one circuit to be retained while
maintenance is carried out on the other. One of these circuits may be an emergency
circuit, provided it is normally energised, in which case the arrangements are to
comply with Pt 6, Ch 2, 3 Emergency source of electrical power.
5.8.5 Where lighting circuits in a stored oil pump-room adjacent to a storage
tank are also used for emergency lighting, and have been interlocked with
ventilation, the interlocking arrangements are:
- not to cause the lighting to go out following a failure of the
ventilation system; and
- not to prevent operation of the emergency lighting following the
loss of the main source of electrical power.
5.9 Motor circuits
5.10 Motor control
5.10.1 The requirements for motor control are given in IEC 61892-2:2012,
Mobile and fixed offshore units Electrical installations Part 2: System
design, Section 7.8 and Pt 6, Ch 2, 5.9 Motor control 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.
5.10.2 Means for automatic disconnection of the supply in the event of excess
current due to mechanical overloading of the motor are to be provided, see
also
Pt 6, Ch 2, 6.10 Feeder circuits.
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