Protective relays Coordination

Ground fault protection, including circuit breakers or relays, should be set as low as possible without causing nuisance tripping. The ground fault pickup and time delay for the main overcurrent protection should coordinate with... 

Equation (9.7) can be used for plotting the time-current characteristic of the cable when this needs to be coordinated with those of the protective relays and fuses in the circuit. 

High-voltage contactor-type motor controls depend on power fuses for short-circuit protection. The fuses are coordinated with the overload relays to protect the motor circuit over the full range of fault conditions from overload conditions to solid maximum-current short circmts. 

Coordination of fuses with an overcurrent relay or any other overcurrcnt protective device The selection of the fuses should be such that ... 

The dominant protection scheme for generators and motors is the differential relay. Access to all enti y points of the protected zone is usually readily available, no coordination with the protection of other... 

Fuses should not be used to protect secondary voltage feeders. The time current characteristics of fuses above 100 A will not coordinate with the groimd fault pickup currents and time delays of the main overcurrent protection (circuit breaker or fused disconnect switch) ground fault protection. A main load break disconnect switch can be equipped with current-limiting fuses to reduce the available short-circuit current from the utility and should have a three-phase voltage relay for single-phase protection. 

It is not normally necessary to provide overcurrent protection in the bus-section circuit because the presence of overcurrent, not caused by an in-zone fault, would be detected by an outgoing circuit relay. For the busbar to be overloaded the outgoing system must also be overloaded. Introducing an overcurrent relay in the bus-section circuit will add comphcation to the coordination of the incoming and outgoing relays, since a time margin is necessary between each relay. In systems where there are large induction motors the coordination can already be awkward to achieve. 

Short circuits often develop from faults of a leakage nature. It is therefore advisable to provide each sub-circuit with an earth leakage current relay or alarm unit, which has a sensitivity that adequately coordinates with other devices. Indeed this is a necessary requirement for sub-circuits that feed power to hazardous area equipment. The use of these earth leakage current relays and detectors will greatly increase the confidence that can be placed on the overall performance of the system of protective devices in the UPS. 

An unrestricted earth fault relay that is connected in the star-point earth circuit of the equipment being protected. The characteristic is time dependent so that time coordination is achieved with the 50 N devices downstream. 

Switchboard An integrated, factory-coordinated combination of circuit protective devices, control devices, meters, relays, busbars, and wireways enclosed in a single, preplanned unit designed to be a self-contained center. Protective devices may be individually compartment-mounted (switchgear) or group-mounted in barriered compartments. The entire structure is designed and built to operate as a coordinated unit. 

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