Section 2 System Design
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Construction & Classification of Submersibles & Diving Systems, July 2022 - Part 6 Electrical Installations and Control Engineering Systems - Chapter 1 Electrical Installations - Section 2 System Design

Section 2 System Design

Parent topic: Chapter 1 Electrical Installations

2.1 Systems of supply design and distribution

2.1.1 The following systems of supply and distribution are acceptable provided that adequate safeguards are taken to minimize shock hazard to divers:
  1. d.c., two-wire, insulated from earth IT System;
  2. d.c., two-wire earthed system TN-S system;
  3. a.c., single-phase, two-wire, insulated from earth IT system;
  4. a.c., three-phase,
    1. Three-wire insulated from earth IT system;
    2. Four-wire with neutral solidly earthed but without hull return. TN-S system.
      Note Refer to IMCA D 045 “Code of Practice for The Safe Use of Electricity Under Water” for additional guidance.

2.1.2 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.

2.2 Sources of power

2.2.1 Where electric power is used for services essential for the safety of the unit and life-support systems, the arrangements of the electrical power sources and any associated transforming, storage or rectifying equipment are to be such that failure of any item of such electrical equipment will not prevent the operation of life-support systems and any equipment necessary for recovery of the unit.

2.2.2 The electrical power available from main and standby sources is, in each case, to be sufficient to ensure the operation of electrical services for essential equipment and habitable conditions, due regard being paid to such services as may have to be operated simultaneously.

2.2.3 Each source is to have sufficient reserve capacity to permit the starting of the largest motor without causing any motor to stall or any device to fail due to excessive voltage drop on the system.

2.2.4 Where the source of power is totally dependent on batteries, a separate battery is to be provided for emergency life-support systems. This separate battery is to be automatically connected upon loss of the main source of power.

2.2.5 Batteries are to maintain their system voltage within the limits detailed under Pt 6, Ch 1, 1.5 Design and construction 1.5.12. The duration of autonomy required of each battery is to be determined by consideration of the most onerous operating conditions for which the unit is designed.

2.2.6 For a.c. systems, isolating transformers are to be provided between the main power system and the supply circuits to the unit.

2.2.7 The power supply to welding equipment is to be from an independent circuit with a clear means of isolation at a manned control station.

2.3 Power Distribution

2.3.1 Electrical distribution systems are to be so designed that a fault or failure in one circuit cannot impair the operation of other circuits or the power supply.

2.3.2 The following consumers at least are to be supplied via individual circuits equipped with all necessary safety devices and switchgear from a distribution panel supplied direct from the main switchboard of the support vessel:
  1. The diving system handling equipment on the support vessel
  2. The compression chamber and diving bell lighting system
  3. The electrical consumers of the life support systems
  4. The communication systems

2.3.3 In normal operation the emergency power distribution system may be supplied via a transfer line from the main power distribution system.

2.3.4 Distribution boards with their own individual feed circuits may not be mounted in a shared casing, i.e. each of these switchgear units must have its own enclosure.

2.3.5 Effective measures are to be taken to prevent the occurrence of vagabond voltages inside switchgear. Circuits at protective low voltage may not be routed with circuits at higher voltage in a joint conductor bundle or cable duct. Terminals for different voltage levels are to be arranged separately and are to be clearly identified.

2.3.6 Switchgear units for a connected load of 100 kW and over are to be tested at the manufacturer's works in the presence of a LR Surveyor. The test shall be performed in accordance with the Rules for Classification and Construction– Ships.

2.3.7 Switchgear units for a connected load of less than 100 kW are to undergo an internal works test of the same scope. These tests are to be certified by a Works Test Certificate issued by the manufacturer. Test certificates are to be submitted to LR not later than the trial of the diving system. For voltage ratings below 60 V, the voltage test is to be performed at a power-frequency withstand voltage of 500 V plus twice the rated voltage.

2.4 Emergency power

2.4.1 Where equipment requires an emergency source of power in order to satisfy the requirements of Pt 6, Ch 1, 2.2 Sources of power 2.2.1 the distribution arrangements are to be such that no single failure within any switchboard or distribution board, and no single cable fault, can cause the failure of both normal and emergency supplies to the equipment.

2.4.2 Where practicable, the switchboards, distribution boards and cable routes for normal and emergency power distribution are to be physically separated so as to minimise the risk of fire or mechanical damage affecting both systems of distribution.

2.4.3 The emergency power supply must be able to simultaneously meet the requirements of at least the following items of equipment:
  1. Emergency lighting systems in compression chambers and diving bells;
  2. Emergency communication systems;
  3. Emergency life support systems;
  4. Emergency diving system handling equipment;
  5. Emergency surveillance and alarm systems.

2.4.4 In the design of the emergency power supply system, appropriate reserve capacity is to be provided to meet peak loads (e.g. caused by the starting of electric motors). In determining the necessary battery capacity, allowance is also to be made for the cut-off voltage and voltage drop of battery.

2.4.5 Diving bells are to be equipped with their own independent emergency power supply capable of meeting the power requirements of the autonomous life support system of the diving bell for at least 24 hours.

2.5 System design – Protection

2.5.1 Installations are to be protected against overcurrent’s including short-circuits, and other electrical faults. The tripping/fault clearance times of the protective devices are to provide complete and co-ordinated protection to ensure:
  1. Availability of essential and emergency services under fault conditions through discriminative action of the protective devices; as far as practicable the arrangements are to secure the availability of other services.
  2. Elimination of the fault to reduce damage to the system and the hazard of fire.

2.5.2 Each circuit is to be protected against overload and short-circuit. Refer to IMCA D 045 “Code of Practice for The Safe Use of Electricity Under Water” for additional guidance.

2.5.3 Short-circuit and overload protection are to be provided in each non-earthed line of each system of supply and distribution, unless exempted under the provisions of any paragraph in this section.

2.5.4 The protection of circuits is to be such that a fault in a circuit does not cause the interruption of supplies used to provide emergency or essential services other than those dependent on the circuit where the fault occurred. For circuits used to provide essential services which need not necessarily be in continuous operation to maintain propulsion and steering but which are necessary for maintaining the vessel's safety, arrangements that ensure that a fault in a circuit does not cause the sustained interruption of supply to healthy circuits may be accepted. Such arrangements are to ensure the supply to healthy circuits is automatically re-established in sufficient time after a fault in a circuit.

2.5.5 Protection systems are to be developed using a systematic design procedure incorporating verification and validation methods to ensure successful implementation of the requirements above. Details of the procedures used are to be submitted when requested.

2.5.6 Short circuit protection is to be provided for each source of power and at each point at which a distribution circuit branches into two or more subsidiary circuits.

2.5.7 Protection for battery circuits is, where practicable, to be provided at a position external and adjacent to the battery compartment.

2.5.8 Consideration may be given to the provision of short circuit protection within the compartment where, either, the protective device is an encapsulated unit, certified as having type of protection ‘m’, or, where the batteries are of the sealed valve-regulated type and the arrangements are such that standard marine or industrial equipment is permitted within the compartment.

2.5.9 Protection may be omitted from circuits for which it can be shown that the risk resulting from spurious operation of the protective device may be greater than that resulting from a fault.

2.5.10 Short circuit protection may be omitted from cabling or wiring to items of equipment internally protected against short-circuit or where it can be shown that they are unlikely to fail to a short-circuit condition and where the cabling or wiring is installed in a manner such as to minimize the risk of short-circuit.

2.5.11 Overload protection may be omitted from the following:
  1. One line of circuits of the insulated type;
  2. Circuits supplying equipment incapable of being overloaded, or overloading the associated supply cable, under normal conditions, and unlikely to fail to an overload condition.

2.5.12 Diving bells are to be equipped with an earthing and potential equalizing system. Connections for external earthing are to be provided in all chambers at diagonally opposite corners.

2.5.13 The connections between the earthing conductor and the chamber and to the ship's earth are to be made with corrosion-resistant screw unions effectively safeguarded against accidental loosening. The dimensions of the screw unions are to be compatible with the requisite cross-sections of the earth conductor to be connected and may not be used for other purposes.

2.5.14 All metal parts of electrical installations – with the exception of live components – are to be earthed. The casings of electrical equipment mounted directly against the inside wall of compression chambers and diving bells are considered to be effectively earthed only if the contact surfaces are permanently free from rust, scale and paint and the casings are fastened with at least two corrosion-resistant screws secured to prevent accidental loosening. If these conditions are not met, earthing must be effected by separate earthing conductors.

2.5.15 Earth connections must be accessible for maintenance and inspection. Wherever possible, they are to be marked. Earthing conductors in multi-core cables are to be marked green and yellow, at least at the terminals.

2.5.16 Earthing conductors are to be provided with corrosion protection compatible with their place of installation.

2.5.17 Copper earthing conductors are subject to the following minimum cross-sections:
  1. External connections on ship and water: 10 mm2;
  2. External connections inside chambers and living compartments: 6 mm2;
  3. Separate earthing conductors inside switchgear and casings: 4 mm2;
  4. Earthing conductors in multi-core cables up to a conductor cross-section of 16 mm2 must correspond to the cross-section of the main conductor subject to a minimum of 1 mm2;
  5. Earthing conductors in multi-core cables with a conductor cross-section of more than 16 mm2 equal to at least half that of the main conductor.

2.5.18 If other materials are used, the minimum cross-section is to be determined by the ratio of the electrical conductivity of these materials to the electrical conductivity of copper.

2.5.19 Cable sheaths and armouring may not be used as earthing conductors.

2.5.20 Electrical switches for circuits with a current rating above 0,5 A are permitted inside compression chambers and diving bells only subject to the use of additional safety features (such as pressurized enclosure in protective gas).

2.5.21 Electrical fuses may not be located inside compression chambers and diving bells. Wherever possible, fuses for the independent emergency power supply to the diving bell are to be located outside the chamber. If installed inside the diving bell, special protective measures are necessary. The fuses shall in any case be protected against intervention by the occupants of the chamber.

2.5.22 Electric motors installed inside chambers are to be fitted with an overcurrent alarm. The alarm must be activated in good time before the motor protection responds. This does not apply to those electric motors which cannot be damaged by overcurrent. For motors in the diving bell, the alarm may take place in the diving bell.

2.5.23 Devices are to be fitted which, in the event of danger, enable the power supply to all the electrical consumers in the compression chamber to be quickly disconnected. The switches needed for this purpose are to be mounted at the Central Control Position. Means must be provided to enable the disconnection separately for each chamber.

2.5.24 All unearthed distribution systems, including the groups of consumers and individual consumers supplied via isolating transformers, safety transformers, rectifiers and inverters, are to be equipped with a continuously operating insulation monitoring system. For systems using protective low voltage, an alarm must be actuated at the Central Control Position if the insulation value drops below a pre-set limit. For higher voltage systems, the insulation monitor must trip an alarm at the Central Control Position when a predetermined fault current is reached, and the system concerned must be automatically disconnected. For the electrical equipment of the diving bell, the alarm actuated by the insulation monitoring system may take place in the diving bell.
Note The current / time characteristics of insulation monitoring systems directly concerned with personnel safety must meet the requirements of diver protection. In assessing the time characteristics, account is to be taken of the response time of the insulation monitoring system and of the tripping time of the switching devices which it actuates.

2.6 Protection against short-circuit

2.6.1 Protection against short-circuit currents is to be provided by circuit-breakers or fuses.

2.6.2 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.

2.6.3 The prospective fault current is to be calculated for the following set of conditions:
  1. All generators, motors and, where applicable, all transformers, of the vessel from which the supply to the unit is taken, connected as far as permitted by any interlocking arrangements;
  2. A fault of negligible impedance close up to the load side of the protective device.
Consideration should be given to the impedance of the cable connections between the vessel switchboard and the unit and the possible current limiting action of the protective device at the vessel switchboard or distribution board from which the supply to the unit is taken.
2.6.4 In the absence of precise data, the prospective fault current may be taken to be:
  1. For alternating current systems at the main switchboard: 10 x f.l.c. (rated full load current) for each generator that may be connected, or, if the subtransient direct axis reactance, X”d, of each generator is known, or each generator, and 3 x f.l.c. for motors simultaneously in service. The value derived from the above is an approximation to the r.m.s. symmetrical fault current; the peak asymmetrical fault current may be estimated to be 2,5 times this figure (corresponding to a fault power factor of approximately 0,1).
  2. Battery-fed direct current systems at the battery terminals:
    1. 15 times ampere hour rating of the battery for vented lead-acid cells, or of alkaline type intended for discharge at low rates corresponding to a battery duration exceeding three hours, or
    2. 30 times ampere hour rating of the battery for sealed lead-acid cells having a capacity of 100 ampere hours or more, or of alkaline type intended for discharge at high rates corresponding to a battery duration not exceeding three hours and,
    3. 6 x f.l.c. for motors simultaneously in service (if applicable).

2.6.5 Protection against overload is to be provided by circuit breaker, overcurrent trip relay or fuse, having characteristics ensuring that cabling and electrical machinery is protected against overheating resulting from mechanical or electrical overload. Fuses of a type intended for short-circuit protection only (e.g. fuse links complying with IEC 60269-1, of type ‘a’) are not to be used for overload protection.

2.6.6 The requirements for circuit breakers are given in the Rules and Regulations for the Classification of Ships, July 2022, Pt 6, Ch 2, 6.5 Circuit-breakers, which are to be complied with where applicable.

2.6.7 The requirements for fuses are given in the Rules and Regulations for the Classification of Ships, July 2022, Pt 6, Ch 2, 6.6 Fuses, which are to be complied with where applicable.

2.6.8 The requirements for circuit breakers requiring back-up by fuse or other device are given in the Rules and Regulations for the Classification of Ships, July 2022, Pt 6, Ch 2, 6.7 Circuit-breakers requiring back-up by fuse or other device, which are to be complied with where applicable.

2.6.9 The requirements for feeder circuits are given in the Rules and Regulations for the Classification of Ships, July 2022, Pt 6, Ch 2, 6.10 Feeder circuits, which are to be complied with where applicable.

2.6.10 The requirements for motor circuits are given in the Rules and Regulations for the Classification of Ships, July 2022, Pt 6, Ch 2, 6.11 Motor circuits, which are to be complied with where applicable.

2.6.11 The requirements for the protection of transformers are given in the Rules and Regulations for the Classification of Ships, July 2022, Pt 6, Ch 2, 6.12 Protection of transformers, which are to be complied with where applicable.

2.6.12 Mains units for automation equipment must contain at least one short-circuit protection and one overload protection device.

2.6.13 The reference conductor system is to be designed to preclude circuit breaks as far as possible. This may, for example, be achieved by duplicating exposed reference conductor joints and connections.

2.7 Earth leakage protection

2.7.1 For all insulated systems, a device(s) is to be installed to continuously monitor the insulation level to earth and to operate an alarm, at a recognized control position, in the event of an abnormally low level of insulation. Refer to IMCA D 045 “Code of Practice for The Safe Use of Electricity Under Water” for additional guidance.

2.7.2 Where there is a danger of current flow through a diver’s body which will exceed 10 mA a.c. or 40 mA d.c., earth leakage trip devices are to be provided and arranged to disconnect the power supply to the protected circuit in the event of the earth leakage reaching a dangerous level.

Note Guidance on assessing the magnitude of current flow and its effects is given in International Electro-technical Commission Publications IEC 60479-1:2005, Effects of current on human beings and livestock – Part 1: General Aspects and IEC 60479- 1:2007 Part 2: Special aspects.

In the absence of detailed information, the current route resistance of a diver’s body should be assumed to be 750 Ω for voltages up to 50 V and 500 Ω for voltages above 50 V for all applications other than electrically heated diving suits, where a current route resistance of 100 Ω should be assumed.

2.8 Earth leakage trip override facility

2.8.1 A facility to override an earth leakage trip may be provided to permit the diving operator to restore power if he considers the danger to the diver as a result of loss of power to be greater than the possible electrical hazard.

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