Section 5 Supply and distribution

5.1.1 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;

   three-wire;

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

5.1.3 The arrangement of the main system of supply is to be such that a fire or other casualty in any space containing the main source of electrical power, associated converting equipment, if any, the main switchboard or the main lighting switchboard will not render inoperable any emergency service, other than those located within the space where the fire or casualty has occurred.

5.1.4 The main switchboard is to be so placed relative to the main source of power that, as far as is practicable, the integrity of the main system of supply will be affected only by a fire or other casualty in one space.

5.1.5 The arrangement of the emergency system of supply 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 craft.

5.1.6 Distribution systems required in an emergency are to be so arranged that a fire in any main vertical zone will not interfere with the emergency distribution in any other such zone.

5.1.7 Feeders from the main and the emergency sources of electrical power are to be separated both vertically and horizontally as widely as is practicable.

5.1.8 For Passenger (A) or Passenger (B) Craft or cargo craft of 500 tons gross tonnage and over, and in any case where the total installed electrical power of the main generating sets is in excess of 3 MW or is supplied at high voltage, arrangements are to be made so that it is possible to split the switchboard, by a multipole linked circuit-breaker, disconnector or switch-disconnector, into at least two independent sections, each supplied by at least one generator.

5.1.9 Where Pt 16, Ch 2, 5.1 Systems of supply and distribution 5.1.8 is applicable and the essential services which are duplicated are supplied from a section-board, arrangements are to be made so that it is possible to split the section-board into at least two independent sections each supplied by an independent section of the main switchboard either directly or through a transformer.

5.1.10 For Passenger (B) high speed craft, each part of the main busbars with its associated generators is to be arranged in separate compartments.

5.2.1 Essential services 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.

5.2.2 Where Pt 16, Ch 2, 5.2 Essential services 5.2.1 is applicable the main busbars of the switchboard, or section boards, are to be capable of being split, by removable links or other means, into at least two independent sections, each supplied by at least one generator, either directly or through a converter. The essential services are to be equally divided, as far as is practicable, between the independent sections.

5.2.3 Where Pt 16, Ch 2, 5.1 Systems of supply and distribution 5.1.8 is applicable provision is to be made to transfer to a temporary circuit those essential services 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.

5.2.4 Where the loss of the electrical supply to a particular essential service which is not duplicated would cause serious risk to the craft, it is to be fed by two independent supplies complying with Pt 16, Ch 2, 5.2 Essential services 5.2.1. Such circuits are to be provided with short circuit protection and an overload and phase-failure alarm. Failure of either supply is not to cause risk to the craft during switching to the alternative supply.

5.3.1 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.

Precautions are to be taken to minimise the risk of inadvertent or accidental switching.

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:

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 Pt 16, Ch 2, 6.5 Circuit-breakers and Pt 16, Ch 2, 7.3 Circuit-breakers.

5.3.3 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.

5.3.4 All lighting and power circuits installed in unattended spaces are to be controlled by multipole linked switches situated outside such spaces. Provision is to be made for the complete isolation of these circuits and locking the means of control in the off position.

5.3.5 Where arrangements are in place for automatic changeover between two or more supplies of electrical power in the event of failure of one supply, the arrangements are to be such that a fault in one feeder does not result in the loss of all supplies to the automatic changeover switch.

5.3.6 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.

5.3.7 The switching device required by Pt 16, Ch 2, 5.3 Isolation and switching 5.3.6 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.

5.3.8 A notice is to be fixed to any section board, distribution board or item of equipment to which Pt 16, Ch 2, 5.3 Isolation and switching 5.3.4 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.

5.3.9 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.

5.4.1 A device(s) is 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.

5.4.2 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.

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

5.5.1 No fuse, non-linked switch or non-linked circuit-breaker 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.

5.5.2 For high speed craft, earthed electrical distribution systems are not to be used, with the exception of earthed intrinsically safe circuits, in areas where an explosive gas atmosphere may arise from the presence of fuel with a flash point below 43ºC, see Pt 15, Ch 3, 3.1 Flash point.

5.5.3 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.

5.5.4 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.

5.5.5 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.

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

5.5.7 All earthing impedances are to be connected to a common earth connection/bar. The connections to the common earth connection/bar are to be so arranged that any circulating currents in the earth connections do not interfere with radio, radar, communication and control equipment circuits.

5.6.1 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 connection load before application of any diversity factor.

5.6.2 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.

5.6.3 For winches and crane motors the diversity factor is to be calculated and submitted when required.

5.7.1 Lighting circuits are to be supplied by final sub-circuits separate from those for heating and power. This does not preclude the supply from a lighting circuit supplying a single fixed appliance, such as a cabin fan, a dry shaver, a wardrobe or anti-condensation heater, taking a maximum current of 2 A. (This does not apply to cabin and wardrobe heaters).

5.7.2 Lighting for machinery spaces, control stations, normal working spaces, large galleys, corridors, stairways leading to boat decks and in public rooms is to be supplied from at least two final sub-circuits in such a way that failure of any one of the circuits does not leave the space in darkness. One of these circuits may be the emergency circuit, provided it is normally energised.

5.7.3 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.

5.7.4 Emergency lighting is to be fitted in accordance with Pt 16, Ch 2, 3 Emergency source of electrical power, see also Pt 16, Ch 2, 18 Crew and passenger emergency safety systems.

5.8.1 A separate final sub-circuit is to be provided for every motor for essential services, see Pt 16, Ch 2, 1.6 Definitions 1.6.1.

5.9.1 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 Pt 16, Ch 2, 5.9 Motor control 5.9.2.

5.9.2 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.

5.9.3 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 Pt 16, Ch 2, 6.9 Load management.

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

5.10.1 The requirements of Pt 16, Ch 2, 5.10 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 Pt 16, Ch 2, 6.13 Harmonic filters. Harmonic filters associated with frequency drives for individual applications (i.e. pump motors) are excluded from the following requirements.

5.10.2 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 Pt 6, Ch 1, 4.2 Alarm system for machinery.

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

5.11.1 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 Pt 16, Ch 2, 6.13 Harmonic filters.

5.11.2 Filters used to control harmonic distortion are to keep the distortion within acceptable limits at the main supply. See also Pt 16, Ch 2, 1.8 Quality of power supplies.

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

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

5.11.5 The construction of cabinets for harmonic filters shall be in accordance with the standards for main switchboards, where applicable. See also Pt 16, Ch 2, 7 Switchgear and controlgear assemblies.

5.11.6 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.

5.11.7  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 Pt 16, Ch 2, 21.2 Trials.