Handbook of Electrical Engineering For Practitioners in the Oil, Gas ...

154 HANDBOOK OF ELECTRICAL ENGINEERINGFigure 7.3 Functions pertaining to the calculation of the critical switching time of circuit breaker that disconnectsa generator from a switchboard. The generator has high values of sub-transient and transient reactances.The sensitivity of the time constant T a is shown.extra heat is generated and the copper conductors are under considerable mechanical stress. In orderto account for any cumulative effects it is necessary to rate the equipment for the long duration ofa fault. The time duration usually adopted in European practice is one second, but occasional upto three seconds are applied for particularly severe service conditions. IEC60439 part 1, clause 4.3,and IEC60964 clause 4.7 give more details for the root-mean-square and peak values and duty. Thewithstand current duty is usually proportional to the making duty for a given voltage rating.7.3 SWITCHING DEVICESSwitching devices for the power circuits that need to be operated frequently are usually circuit breakersand contactors. Manually operated load break switches, fuse-switch combinations and molded casecircuit breakers are used for feeder circuits that are infrequently operated.7.3.1 Outgoing Switching Device for SwitchgearThe outgoing switching device in a high current, high fault level, switchboard will usually be a powercircuit breaker if it feeds more that about 400 amps to the load. Below 400 amps the circuit couldhave a fuse-contactor combination, see sub-section 7.3.2 for comments on contactors and Chapter 8on fuses. Therefore if the outgoing device must be a circuit breaker then the comments and discussionin sub-section 7.2.3 above apply. Low voltage switchboards often use moulded case circuit breakers