Section 4 System design - protection

4.1.1 Installations are to be protected against overcurrents 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 Mobility systems, Ship Type systems, and emergency services under fault conditions through discriminative action of the protective devices; as far as practicable the arrangements are also to secure the availability of other services;

2. elimination of the fault to reduce damage to the system and hazard of fire;

3. uninterrupted electrical supply during normal operation of the system including motor starting and similar transient over-current conditions.

4.1.2 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.

4.1.3 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, Mobility systems or Ship Type systems other than those dependent on the circuit where the fault occurred. For circuits used to provide Mobility or Ship Type systems 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.

4.1.4 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. An approved copy of the details required by Vol 2, Pt 9, Ch 1, 1.4 Documentation required for design review 1.4.6 and Vol 2, Pt 9, Ch 1, 1.4 Documentation required for design review 1.4.7 is to be retained on board and made available to the LR Surveyor on request. Access to protection relays is to be restricted, such that they will generally only be adjusted by authorised personnel to avoid incorrect operation.

4.1.5 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.

4.1.6 Where protection for generator power circuits is provided at the associated switchboard, the cabling between generator and switchboard is to be of a type, and installed in a manner such as to minimise the risk of short-circuit.

4.1.7 Protection for battery circuits is to be provided at a position external and adjacent to the battery compartments. Where arrangements comply with Vol 2, Pt 9, Ch 2, 7.3 Location 7.3.5, the protection may be installed at a suitable location in the battery compartment.

4.1.8 Protection may be omitted from the following:

1. Engine-starting battery circuits.

2. 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.

4.1.9 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 minimise the risk of short circuit.

4.1.10 Overload protection may be omitted from the following:

1. one line of circuits of the insulated type;

2. circuits supplying equipment incapable of being over-loaded, or overloading the associated supply cable, under normal conditions, and unlikely to fail to an overload condition.

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

4.2.2 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; the requirements for circuit breakers and fuses are detailed in Vol 2, Pt 9, Ch 4, 4.5 Circuit-breakers and Vol 2, Pt 9, Ch 4, 4.6 Fuses respectively.

4.2.3 The prospective fault current is to be calculated for the following set of conditions:

1. all generators, motors and, where applicable, all transformers, 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.

4.2.4 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,

   

   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).

4.3.1 The characteristics of protective devices provided for overload protection are to ensure that cabling and electrical machinery is protected against overheating resulting from mechanical or electrical overload.

4.3.2 Fuses of a type intended for short-circuit protection only (e.g. high-voltage fuses or fuses complying with IEC 60269-1: Low-voltage fuses – Part 1: General requirements, of type ‘a’) are not to be used for overload protection.

4.4.1 Every distribution system that has an intentional connection to earth, by way of an impedance, is to be provided with a means to continuously monitor and indicate the current flowing in the earth connection.

4.4.2 If the current in the earth connection exceeds 5 A there is to be an alarm and the fault current is to be automatically interrupted or limited to a safe value.

4.4.3 The rated short circuit capacity of any device used for interrupting earth fault currents is to be not less than the prospective earth fault current at its point of installation.

4.4.4 Insulated neutral systems with harmonic distortion of the voltage waveform, which may result in earth fault currents exceeding the level given in Vol 2, Pt 9, Ch 4, 4.4 Protection against earth faults 4.4.2 because of capacitive effects, are to be provided with arrangements to isolate the faulty circuit(s).

4.5.1 Circuit-breakers for alternating current systems are to satisfy the following conditions:

1. the r.m.s. symmetrical breaking current for which the device is rated is to be not less than the r.m.s. value of the a.c. component of the prospective fault current, at the instant of contact separation (i.e. first half cycle, or time of interruption where an intentional time delay is provided to ensure suitability);

2. the peak asymmetrical making current for which the device is rated is not to be less than the peak value of the prospective fault current at the first half cycle, allowing for maximum asymmetry;

3. the power factor at which the device short circuit ratings are assigned is to be no greater than that of the prospective fault current; alternatively for high voltage, the rated percentage d.c. component of the short-circuit breaking current of the device is to be not less than that of the prospective fault current.

4.5.2 Circuit-breakers for d.c. systems are to have a breaking current not less than the initial prospective fault current. The time constant of the fault current is not to be greater than that for which the circuit-breaker was tested.

4.5.3 The fault ratings considered in Vol 2, Pt 9, Ch 4, 4.5 Circuit-breakers 4.5.1 and Vol 2, Pt 9, Ch 4, 4.5 Circuit-breakers 4.5.2, are to be assigned on the basis that the device is suitable for further use after fault clearance.

4.5.4 Circuit-breakers selection is, and ratings are, to be in accordance with the relevant requirements of IEC 60092- 202: Electrical installations in ships – Part 202: System design Alternative methods acceptable to LR of selecting suitable circuit-breakers may be considered.

4.6.1 Fuses for a.c.systems are to have a breaking current rating not less than the initial r.m.s. value of the a.c. component of the prospective fault current.

4.6.2 Fuses for d.c. systems are to have a d.c. breaking current rating not less than the initial value of the prospective fault current.

4.7.1 The use of a circuit-breaker having a short-circuit current capacity less than the prospective short-circuit current at the point of installation is permitted, provided that it is preceded by a device having at least the necessary short-circuit capacity. The generator circuit breakers are not to be used for this purpose.

4.7.2 The same device may back-up more than one circuit-breaker provided that no Mobility systems, Ship Type systems, or emergency service is supplied from there, or that any such service is duplicated by arrangements unaffected by tripping of the device.

4.7.3 The combination of back-up device and circuitbreaker is to have a short-circuit performance at least equal to that of a single circuit-breaker satisfying the requirements of Vol 2, Pt 9, Ch 4, 4.5 Circuit-breakers

4.7.4 Evidence of testing of the combination is to be submitted for consideration; alternatively, consideration may be given to arrangements where it can be shown that:

1. the takeover current, above which the back-up device would clear a fault, is not greater than the rated shortcircuit breaking capacity of the circuit-breaker; and

2. the characteristics of the back-up device, and the prospective fault level, are such that the peak fault current rating of the circuit-breaker cannot be exceeded; and

3. the Joule integral of the let-through current of the back-up device does not exceed that corresponding to the rated breaking current and opening time of the circuit-breaker.

4.8.1 The protective gear required by Vol 2, Pt 9, Ch 4, 4.8 Protection of generators 4.8.2 and Vol 2, Pt 9, Ch 4, 4.8 Protection of generators 4.8.3 is to be provided as a minimum.

4.8.2 Generators not arranged to run in parallel are to be provided with a circuit-breaker arranged to open simultaneously, in the event of a short-circuit, an overload or an under-voltage, all insulated poles. In the case of generators rated at less than 50 kW, a multipole linked switch with a fuse, complying with Vol 2, Pt 9, Ch 4, 4.3 Protection against overload 4.3.2, in each insulated pole will be acceptable.

4.8.3 Generators arranged to operate in parallel are to be provided with a circuit-breaker arranged to open simultaneously, in the event of a short-circuit, an overload or an under-voltage, all insulated poles. This circuit-breaker is to be provided with reverse power protection with time delay, selected or set within the limits of 2 per cent to 15 per cent of full load to a value fixed in accordance with the characteristics of the prime mover. A fall of 50 per cent in the applied voltage is not to render the reverse power mechanism inoperative, although it may alter the amount of reverse power required to open the breakers.

4.8.4 The generator circuit-breaker short circuit and overload tripping arrangements, or fuse characteristics, are to be such that the machine’s thermal withstand capability is not exceeded.

4.8.5 All high-voltage generators and low-voltage generators having a capacity of 1500 kVA or above are to be equipped with a protective device which, in the event of a short-circuit in the generator or in the cables between the generator and its circuit-breaker, will instantaneously open the circuit breaker and de-excite the generator.

4.8.6 The voltage and time delay settings of the undervoltage release mechanism(s) required byVol 2, Pt 9, Ch 4, 4.8 Protection of generators 4.8.2 and Vol 2, Pt 9, Ch 4, 4.8 Protection of generators 4.8.3 are to be chosen to ensure that the discriminative action required by Vol 2, Pt 9, Ch 4, 4.1 General 4.1.1is maintained.

4.8.7 The protection of electrical power generation and distribution systems is to be so arranged that, in the event of failure of a protection device, including integrated multifunction relays, sufficient power can be supplied to all Mobility and/or Ship Type systems, see also Vol 2, Pt 9, Ch 3, 4.2 Essential services 4.2.4.

4.8.8 It is to be possible to control the generator prime mover in the event of failure of an electrical system protection device.

4.9.1 Arrangements are to be made to disconnect automatically sufficient load to reduce the load when the generator(s) is/are overloaded. Appropriate time delays are to be provided in these load reduction arrangements.

4.9.2 Arrangements are also to be made such that, where applicable, the maximum load that can be applied to generators driven by prime movers is not in excess of that determined by Vol 2, Pt 2, Ch 1, 7.3 Auxiliary and emergency engine governors 7.3.4 and Vol 2, Pt 2, Ch 1, 7.3 Auxiliary and emergency engine governors 7.3.5.

4.9.3 If required, this load switching may be carried out in one or more stages, in which case circuits other than Mobility or Ship Type systems are to be included in the first group to be disconnected.

4.9.4 The load management of power systems supplying electric propulsion motors is to satisfy the requirements of Vol 2, Pt 4, Ch 5, 4.3 Power requirements.

4.9.5 Consideration is to be given to providing means to inhibit automatically the starting of large motors, or the connection of other large loads, until sufficient generating capacity is available to supply them.

4.10.1 Isolation and protection of each feeder circuit is to be ensured by a multiple circuit-breaker or linked switch with a fuse in each insulated conductor. Protection is to be in accordance with Vol 2, Pt 9, Ch 4, 4.2 Protection against short-circuit and Vol 2, Pt 9, Ch 4, 4.3 Protection against overload. The protective devices are to allow excess current to pass during the normal accelerating period of motors.

4.11.1 Motors of rating exceeding 0,5 kW and all motors for Mobility or Ship Type systems are to be protected individually against overload and short circuit. Motors for Mobility or Ship Type systems for the safety and propulsion of the ship and which are duplicated are to be provided with arrangements to start the standby motor and disconnect the faulty motor. Such motors may have an overload alarm instead of overload protection. The actions to start the standby motor and disconnect the faulty motor may be carried out by suitably trained personnel but in ships with UMS notation the arrangements are to be automatic.

4.11.2 Protection for both the motor and its supply cable may be provided by the same device, provided that due account is taken of any differences between ratings of cable and motor.

4.11.3 Where operation of an item of equipment is dependent upon a number of motors, consideration may be given to the provision of a common means of short circuit protection.

4.11.4 The characteristics of the arrangements for overload protection of motors are to be selected in relation to both the starting and normal rated conditions that are to include any load factors of intermittent service motors.

4.11.5 Where fuses are used to protect polyphase motor circuits, means are to be provided to protect the motor from unacceptable overcurrent in the case of single phasing.

4.12.1 Short circuit protection for transformers is to be provided by circuit breakers or fuses in the primary circuit and in addition, overload protection is to be provided either in the primary or secondary circuit.

4.12.2 Arrangements are to be made to prevent the primary windings of transformers being inadvertently energised from their secondary side when disconnected from their source of supply.

4.13.1 Harmonic filters’ final sub-circuits are to be protected individually and individually on each phase against overload and short-circuit. The activation of the protection arrangement in a single phase shall result in automatic disconnection of the complete filter.

4.13.2 A current imbalance detection system is to be installed; it is to be independent from the protection specified in Vol 2, Pt 9, Ch 4, 4.13 Harmonic filters 4.13.1.

4.13.3 An alarm is to be initiated in the event of protective device operation or current unbalance that could lead to failure of harmonic filter.

4.13.4 Current imbalance circuits are to be ‘fail safe’. The characteristics of the ‘fail safe’ operation are to be evaluated not only on the basis of the system and its associated machinery, but also the complete installation, as well as of the ship.

4.13.5 The reconnection of harmonic filters is to require manual intervention.

4.13.6 Individual harmonic filter capacitors are to be provided with a pressure relief valve or overpressure disconnector to protect against damage from rupture where pressure build-up within hermetically sealed capacitors may occur.