Section 3 System design - Protection

3.1.1 Installations are to be protected against accidental overcurrents including short-circuit. The protective devices are to provide complete and co-ordinated protection to ensure:

1. Continuity of service under fault conditions through discriminative action of the protective devices.

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

3.2.1 Circuit-breakers and automatic switches provided for overload protection are to have tripping characteristics appropriate to the system. Fuses above 320 A are not to be used for overload protection, but may be used for short-circuit protection.

3.3.1 Protection against short-circuit currents is to be provided.

3.3.2 The breaking capacity of every protective device is to be not less than the maximum value of the short-circuit current which can flow at the point of installation at the instant of contact separation.

3.3.3 The making capacity of every switching device intended to be capable of being closed, if necessary, on short-circuit, is to be not less than the maximum value of the short-circuit current at the point of installation. On alternating current, this maximum value corresponds to the peak value allowing for maximum asymmetry.

3.3.4 Every protective device or contactor not intended for short-circuit interruption is to be adequate for the maximum short-circuit current which can occur at the point of installation having regard to the time required for the short-circuit to be removed.

3.3.5 In the absence of precise data, the following short-circuit currents at the machine terminals are to be assumed:

1. Direct current systems

   10 times full load current for generators normally connected (including spare). Six times full load current for motors simultaneously in service.

2. Alternating current systems

   10 times full load current for generators normally connected (including spare) – symmetrical r.m.s. Three times full load current for motors simultaneously in service.

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

3.4.2 Fused circuit-breakers with fuses connected to the load side may be used where operation of the circuit-breaker and fuses is co-ordinated.

3.4.3 The characteristics of the arrangement shall be such that:

1. When the short-circuit current is broken, the circuit-breaker on the load side shall not be damaged and is to be capable of further service.

2. When the circuit-breaker is closed on the short-circuit current, the remainder of the installation shall not be damaged. However, it is admissible that the circuit-breaker on the load side may require servicing after the fault has been cleared.

3.5.1 Short-circuit protection is to be provided in each live pole of a direct current system and in each phase of an alternating current system.

3.5.2 Overload protection is to be provided in:

1. Two-wire direct current or single-phase alternating current system – at least one line or phase.

2. Three-wire direct current system – both outer lines.

3. Insulated three-phase alternating current system – at least two phases.

4. Earthed three-phase alternating current system – all three phases.

3.5.3 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.

3.5.4 These requirements do not preclude the provision (for test purposes) of an isolating link to be used only when the other conductors are isolated.

3.6.1 In addition to the protection required by Pt 6, Ch 2, 3.5 Protection of circuits 3.5.1 and Pt 6, Ch 2, 3.5 Protection of circuits 3.5.2, protective gear required by Pt 6, Ch 2, 3.6 Protection of generators 3.6.2, is to be provided as a minimum.

3.6.2 For generators not arranged to run in parallel: A circuit-breaker or contactor arranged to open simultaneously all insulated poles or in the case of generators rated at less than 50 kW a multi-pole linked switch with a fuse in each insulated pole.

3.6.3 For generators arranged to operate in parallel: A circuit-breaker or contactor arranged to open simultaneously all insulated poles and provided with:

1. For direct current generators, instantaneous reverse-current protection operating at not more than 15 per cent rated current.

2. For alternating current generators, a reverse-power protection, with time delay, selected and set within the limits of two per cent to 15 per cent of full load to a value fixed in accordance with the characteristics of the prime mover.

3.6.4 The reverse-current protection is to be adequate to deal with the reverse-current conditions emanating from the ship’s network, e.g. winches.

3.7.1 Isolation and protection of each main distribution circuit is to be ensured by a multi-pole circuit breaker or switch and fuses. Protection is to be in accordance with Pt 6, Ch 2, 3.2 Protection against overload, Pt 6, Ch 2, 3.3 Protection against short-circuitand Pt 6, Ch 2, 3.5 Protection of circuits. The protective devices are to allow excess current to pass during the normal accelerating period of motors.

3.7.2 Circuits which supply motors fitted with overload protection may be provided with short-circuit protection only.

3.7.3 Motors of rating exceeding 0,5 kW are to be protected individually against overload and short-circuit. The short-circuit protection can be provided by the same protective device for the motor and its supply cable. The overload protection may be replaced by an overload alarm if desired by the Owner.

3.8.1 The primary circuits of power transformers are to be protected against short-circuit by circuit-breakers or fuses.

3.8.2 When transformers are arranged to operate in parallel, means of isolation are to be provided on the secondary windings. Switches and circuit-breakers are to be capable of withstanding surge currents.

3.9.1 Lighting circuits are to be provided with overload and short-circuit protection.

3.9.2 Where more than one light is installed in a space, the lighting is to be supplied from at least two final sub-circuits in such a way that failure of one of the circuits does not leave the space in darkness.

3.10.1 Protection is to be provided for voltmeters, voltage coils of measuring instruments, earth indicating devices and pilot lamps, together with their connecting leads.

3.10.2 A pilot lamp installed as an integral part of another item of equipment need not be individually protected, provided it is fitted in the same enclosure.

3.11.1 Batteries, except starter batteries, are to be protected against short-circuit by a fuse in each insulated conductor or a multi-pole circuit-breaker at a position adjacent to the battery compartment.