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)

5.5.1 The requirements for earthed distribution systems are given in Pt 6, Ch 2, 5.5 Earthed distribution systems of the Rules for Ships, which are to be complied with where applicable.

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.

5.8.4 Emergency lighting is to be fitted in accordance with Pt 6, Ch 2, 3 Emergency source of electrical power, see also Pt 7, Ch 1, 4 Emergency lighting.

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

5.9.1 A separate final sub-circuit is to be provided for every motor for essential services, see Pt 6, Ch 2, 1.6 Definitions 1.6.2.

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.