Section 4 Supply and distribution

4.1.1 The following systems of generation and distribution are acceptable:

1. d.c., two-wire;

2. a.c., single-phase, two-wire;

3. a.c., three-phase:

   1. three-wire insulated;

   2. three-wire with neutral solidly earthed or earthed through an impedance;

   3. four-wire with neutral solidly earthed but without hull return.

4.1.2 For oil supply ships intended for the carriage in bulk of oil and other hazardous liquids having a flash point not exceeding 60°C (closed-cup test) only the following systems of generation and distribution are acceptable:

1. d.c., two-wire insulated;

2. a.c., single-phase, two-wire, insulated;

3. a.c., three-phase, three-wire, insulated;

4. earthed systems, a.c. or d.c., limited to areas outside any dangerous space or zone, and arranged so that no current arising from an earth-fault in any part of the system could pass through a dangerous space or zone. Earthed intrinsically safe circuits are permitted to pass into and through dangerous spaces and zones.

Voltages above these will be the subject of special consideration.

4.1.4 The arrangement of the main system of supply is to be such that a fire or other casualty in a space containing any of the main sources of electrical power will not render inoperable any of the other main sources of electrical power, or any emergency supply system if fitted.

4.1.5 Main switchboards, lighting distribution boards and any converting equipment are to be so placed relative to their associated generator(s) so that, as far as is practicable, the integrity of the main system(s) of supply will be affected only by a fire or other casualty in one space. Switchboards are to be located as close as practicable to their associated generators.

4.1.6 The arrangement of the emergency system of supply, where fitted, is to be such that a fire or other casualty in spaces containing the emergency source of electrical power, associated converting equipment, if any, the emergency switchboard and the emergency lighting switchboard, will not cause loss of services required to maintain the propulsion and safety of the ship.

4.1.7 Distribution systems are to be so arranged that a fire in any one main fire zone will not interfere with either the main or emergency distribution systems and services in any other such zone.

4.2.1 Mobility or Ship Type systems that are required 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 controlgear assemblies, so that any single fault will not cause the loss of both services.

4.2.2 Where Vol 2, Pt 9, Ch 3, 4.2 Essential services 4.2.1 is applicable the main busbars of the switchboard, or section boards, are to be capable of being split, by a multi-pole linked circuit breaker, disconnector or switch-disconnector, into at least two independent sections, each supplied by at least one generator, either directly or through a converter. The Mobility and/or Ship Type systems are to be equally divided, as far as is practicable, between the independent sections.

4.2.3 Where Vol 2, Pt 9, Ch 3, 4.2 Essential services 4.2.2 is applicable provision is to be made to transfer to a temporary circuit those Mobility and/or Ship Type systems which are not required to be, and have not been, duplicated in the event of loss of their normal section of switchboard or section-board.

4.2.4 A Risk Based Assessment (RA) is to be carried out in accordance with the requirements of Vol 2, Pt 1, Ch 3, 18 Risk Assessment (RA). The RA is to verify the availability of electrical power to Mobility and/or Ship Type systems (see Vol 2, Pt 1, Ch 1, 3.1 Categories 3.1.5) in the event of a failure in the power supply and distribution system. The RA is to address the different system operating modes and configurations. The RA should include relevant mechanical failures which may have significant effects on the electrical power and distribution system, i.e. prime mover failures causing loss of electrical power supplies to Mobility and/or Ship Type systems. An analysis carried out by applying the relevant generic failure modes listed in IEC 60812:1985, Analysis techniques for system reliability - Procedure for failure mode and effects analysis (FMEA) to the system and equipment identified in the single line diagram of the electrical system (see Vol 2, Pt 9, Ch 1, 1.4 Documentation required for design review 1.4.4) would generally be acceptable.

4.3.1 The incoming and outgoing circuits from every switchboard or section board are to be provided with a means of isolation and switching to permit each circuit to be switched off:

1. on load;

2. for mechanical maintenance;

3. in an emergency to prevent or remove danger.

In addition, the requirements of Vol 2, Pt 9, Ch 3, 4.3 Isolation and switching 4.3.2 and Vol 2, Pt 9, Ch 3, 4.3 Isolation and switching 4.3.3 are to be complied with.

4.3.2 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:

1. switching off the circuit on load;

2. 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 Vol 2, Pt 9, Ch 3, 5.3 Circuit-breakers and Vol 2, Pt 9, Ch 4, 4.5 Circuit-breakers.

4.3.3 Provision is to be made, in accordance with one of the following, to prevent any circuit being inadvertently energised:

1. the circuit breaker or switch can be withdrawn, or locked in the open position;

2. the operating handle of the circuit breaker or switch can be removed;

3. the circuit fuses, where fitted, can be readily removed and retained by authorised personnel.

4.3.4 Where a section board, distribution board or item of equipment can be supplied by more than one circuit, a switching device is to be provided to permit each incoming circuit to be isolated and the supply transferred to the alternative circuit. In addition, the requirements of Vol 2, Pt 9, Ch 3, 4.3 Isolation and switching 4.3.5 and Vol 2, Pt 9, Ch 3, 4.3 Isolation and switching 4.3.6 are to be complied with.

4.3.5 The switching device required by Vol 2, Pt 9, Ch 3, 4.3 Isolation and switching 4.3.4 is to be situated within or adjacent to the section board, distribution board or item of equipment. Where necessary, interlocking arrangements are to be provided to prevent circuits being inadvertently energised.

4.3.6 A notice is to be fixed to any section board, distribution board or item of equipment to which Vol 2, Pt 9, Ch 3, 4.3 Isolation and switching 4.3.4 applies warning personnel before gaining access to live parts of the need to open the appropriate circuit breakers or switches, unless an interlocking arrangement is provided so that all circuits concerned are isolated before access is gained.

4.3.7 Where high voltage equipment is contained in a room, or protected area, which also forms its enclosure, the access door(s) is (are) to be so interlocked that it (they) cannot be opened until the high voltage supply(ies) to the equipment is (are) switched off. Provision is also to be made to enable the equipment and its cable(s) to be earthed down and any stored energy dissipated, sufficient to ensure personnel safety.

4.3.8 The access to the space(s) described in Vol 2, Pt 9, Ch 3, 4.3 Isolation and switching 4.3.7 is to be suitably marked to indicate the danger of high voltage.

4.3.9 High voltage switchgear and controlgear assemblies that use liquids or gasses other than ambient air as an insulating medium are to be installed in spaces that are adequately protected according to the following requirements:

• The space is to be provided with ventilation to ensure that gasses cannot accumulate unnoticed.
• Leakages of liquid are to be properly collected and contained.
• An alarm is to be initiated at a manned control station to indicate when the insulating medium has reduced to an abnormally low level. Interlocks are to be provided to ensure that the switchgear or controlgear cannot be closed when this alarm is active. The interlocks are to ensure that services essential for the propulsion and safety of the ship are not disconnected in the event of this alarm being activated.

4.4.1 A device(s) is (are) to be installed for every insulated distribution system, whether primary or secondary, for power, heating and lighting circuits, to continuously monitor 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.

4.4.2 Where any insulated lower voltage system is supplied through transformers from a high voltage system, adequate precautions are to be taken to prevent the low voltage system being charged by capacitive leakage from the high voltage system.

4.4.3 Where filters are fitted, for example to reduce EMC susceptibility, these are not to cause distribution systems to be unintentionally connected to earth.

4.5.1 No fuse, non-linked switch or non-linked circuitbreaker is to be inserted in an earthed conductor. Any switch or circuit-breaker fitted is to operate simultaneously in the earthed conductor and the insulated conductors. These requirements do not preclude the provision (for test purposes) of an isolating link to be used only when the other conductors are isolated.

4.5.2 For high voltage systems, where the earthed neutral system of generation and primary distribution is used, earthing is to be through an impedance in order to limit the total earth fault current to a magnitude which does not exceed that of the three phase short-circuit current for which the generators are designed.

4.5.3 Generator neutrals may be connected in common, provided that the third harmonic content of the voltage waveform of each generator does not exceed five per cent.

4.5.4 Where a switchboard is split into sections operated independently or where there are separate switchboards, neutral earthing is to be provided for each section or for each switchboard. Means are to be provided to ensure that the earth connection is not removed when generators are isolated.

4.5.5 A means of isolation is to be fitted in the earthing connection of each generator so that generators can be completely isolated for maintenance.

4.5.6 All earthing impedances are to be connected to the hull. The connections to the hull are to be so arranged that any circulating currents in the earth connections do not interfere with radio, radar, communication and control equipment circuits.

4.5.7 Where the ELS notation is to be assigned, earthed systems are not permitted unless they are isolated from the main generation and distribution system, e.g. through transformers and/or motor generator sets. See also Vol 2, Pt 9, Ch 1, 3.2 General requirements 3.2.3.

4.6.1 Circuits supplying two or more final sub-circuits are to be rated in accordance with the total connected load subject, where justified, to the application of a diversity factor. Where spare ways are provided on a section or distribution board, an allowance for future increase of load is to be added to the total connected load before application of any diversity factor.

4.6.2 A diversity factor may be applied to the calculation for size of cable and rating of switchgear and fusegear, taking into account the duty cycle of the connected loads and the frequency and duration of any motor starting loads.

4.7.1 Lighting circuits are to comply with the requirements of Vol 2, Pt 9, Ch 6 Lighting.

4.8.1 A separate final sub-circuit is to be provided for every motor for Mobility and/or Ship Type systems, see Vol 2, Pt 1, Ch 1, 3.1 Categories 3.1.5

4.9.1 Every electric motor is to be provided with efficient means for starting and stopping so placed as to be easily operated by the person controlling the motor. Every motor above 0,5 kW is to be provided with control apparatus as given in Vol 2, Pt 9, Ch 3, 4.9 Motor control 4.9.2 to Vol 2, Pt 9, Ch 3, 4.9 Motor control 4.9.4.

4.9.2 Means to prevent undesired restarting after a stoppage due to low volts or complete loss of volts are to be provided. This does not apply to motors where a dangerous condition might result from the failure to restart automatically, e.g. steering gear motor.

4.9.3 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 Vol 2, Pt 9, Ch 4, 4.11 Motor circuits

4.9.4 Motor controlgear is to be suitable for the starting current and for the full load rated current of the motor.

4.10.1 The requirements of Vol 2, Pt 9, Ch 3, 4.10 Harmonic distortion measurement apply to electrical distribution systems that include harmonic filters. This requirement applies both to high voltage and low voltage busbars. See also Vol 2, Pt 9, Ch 3, 4.11 Harmonic filtering. Harmonic filters associated with frequency drives for individual applications (i.e. pump motors) are excluded from the following requirements.

4.10.2 Means are to be provided to continuously monitor the levels of harmonic distortion experienced on the main busbars and to operate an alarm in the engine control room, or equivalent attended position, in the event that the harmonic distortion exceeds the acceptable limits, see also Vol 2, Pt 9, Ch 1, 4.2 Alarm systems for machinery

4.10.3 Where the engine room is provided with automation systems to continuously monitor the levels of harmonic distortion experienced on the main busbar(s), this reading is to be logged electronically; otherwise it is to be measured annually and after any modification to the ship electrical distribution system or associated consumers, and is to be recorded in the engine log book for future inspection by the Surveyor.

4.11.1 The requirements in this Section apply to systems provided with harmonic filters. They apply in particular to, but are not limited to, electrical propulsion systems and are in addition to the requirements for harmonic filters in Vol 2, Pt 9, Ch 4, 4.13 Harmonic filters

4.11.2 Filters used to control harmonic distortion are to keep the distortion within acceptable limits at the main supply. See also Vol 2, Pt 9, Ch 1, 2.1 Quality of power supplies (QPS)

4.11.3 The service life of the harmonic filter is to be declared, and details are to be included in the harmonic calculation report.

4.11.4 The temperature rating of the harmonic filter is to allow for the increased heating effect of the harmonic distortion.

4.11.5 The construction of cabinets for harmonic filters shall be in accordance with the standards for main switchboards, where applicable. See also Vol 2, Pt 9, Ch 3, 5 Switchgear and controlgear assemblies.

4.11.6 The modes of operation of the electrical distribution system for which harmonic distortion levels at the main switchboard busbars are maintained within the acceptable limits during normal operation are to be defined by the system integrator.

4.11.7 Harmonic distortion calculations are to include levels of harmonic distortion expected in normal operation and in the event of a failure of a harmonic filter or the failure of any combination of harmonic filters. See also Vol 2, Pt 9, Ch 12, 1.2 Trials