Transport Publication TP 127 E
10.1 Generator power circuit breakers shall be equipped with adjustable inverse time over-current and instantaneous trip units; the inverse time over-current trip units shall be capable of being set to trip the circuit breaker at 150% generator full-load in 20-30 seconds; other proposed trip settings may be considered provided it is proven that the generator time/current characteristic is such that the over-current can be sustained for the delayed period without impairing the life or effectiveness of the generator insulation or the generator main feeder cables; the pickup setting of the long time over-current trip shall not exceed 115 percent of the generator rating for continuous rated machines and shall not exceed 15% above the overload rating for special rated machines; instantaneous trips shall be set at the lowest value of time and current which will co-ordinate with the trip settings of other system protective devices and shall function at the generator sustained short-circuit current; short-time delays may be incorporated in instantaneous trips for discrimination purposes; where two or more generators are operated in parallel the generator air circuit breakers shall be equipped with undervoltage protection.
10.2 Subject to subsection (1), generators rated below 25 kW (not arranged for parallel operation may be protected by molded case circuit breakers having non-adjustable long time over-current trip units. Generators rated between 25 kW and 50 kW may be protected by molded case circuit breakers used in conjunction with long time over-current protection devices. Generators rated between 50 kW and 250 kW may be protected by the use of molded case circuit breakers provided each such circuit breaker is equipped with long time delay over-current protection that has inverse time characteristics. The pickup setting of the long time over-current trip shall not exceed 115 percent of the generator rating for continuous rated machines and shall not exceed 15% above the overload for special rated machines; the setting of the long time delay overcurrent protection should offer maximum protection for the generator, while maintaining selective tripping co-ordination with the distribution system; where the value of generator fault current is such that the instantaneous magnetic trip in the molded case breaker cannot be set at a point low enough to detect the minimum value of fault current available, suitably rated fuses shall be provided in series with the breaker and, in addition, single-phase protection shall be incorporated; generator molded case circuit breakers shall be equipped with undervoltage protection where generators are operated in parallel; the use of molded case circuit breakers for generators intended for parallel operation shall be provided with a device or be designed to ensure the circuit breaker closes in the time necessary to achieve synchronization.;
10.3 The rating or setting of over-current devices shall not exceed the current carrying capacity of the conductors which they protect, as specified in Section 12.9 and 12.16, except:
- where the starting current for a motor considerably exceeds the full-load current, such protection shall be designed to allow the excess current to pass during the normal accelerating period of the motor,
- motor-running over-current protection may be shunted or cut out of the circuit during the starting period of a motor, provided the device by which the over-current protection is shunted or cut-out is not left in the starting position and the motor shall be considered as protected against overcurrent during the starting period if fuses or time-delay circuit breakers are so located in the circuit as to be operative during the starting period of the motor; and
- short-circuit protection and an overload alarm shall be provided for each steering gear circuit:
- for dc motors, the feeder circuit breaker shall be set to trip instantaneously at not less than 300% and not more than 375% of the rated full-load current of the steering gear motor, except that the feeder circuit breaker on the emergency switchboard may be set to trip at not less than 200%;
- for ac motors, the instantaneous trip setting of the feeder circuit breaker shall be 200% of the steady state locked-rotor current of one steering gear motor plus all other steering gear related loads that may be on this feeder.
10.4 Circuit breakers, contactors and switches in outgoing circuits shall be so installed that, as far as practicable, their moving parts and associated relays are not live when in the “off” position so that inspection, adjustment and replacement of parts is facilitated.
10.5 Every fuse shall be of the non-renewable type and provided with a suitable non-combustible insulating fuse carrier of such form as to protect a person handling it from shock and burns; effectual provision shall be made for retaining in position all fuses subject to vibration or shock and mechanical damage.
10.6 Bus material shall be of copper.
10.7 A separate power source shall not be used for the direct control of circuit breakers. ( i.e. opening, closing and tripping).