Circuit breakers operating conditions

For oil-filled control equipment such as autotransformer starters or oil circuit breakers (BOCBs or MOCBs, Chapter 19), insulating oil should be checked periodically for its insulating properties. Leading manufacturers of this equipment indicate the number of switching operations under different conditions of load and fault, after which the oil must be replaced and these must be followed. 

Where a bus-section circuit breaker is used to divide the busbars (during abnormal operating conditions) each set of busbars is protected as a separate zone. Each zone consists of the incomers, the outgoing circuits and the bus-section circuit breaker(s). An accurate current transformer is connected in each line of each circuit. All the current leaving the zone must be balanced by current from the incomer circuits. A fault in the zone will be detected by the (87) relay. Rapid operation is required... 

The DBA fire scenario was an electrical wiring short or overheats that caused an electrical fire. Then the fire spread to combustible packaging material and spread to involve all of the radioactive material in storage. Power isolation could prevent a fire of combustible material other than the electrical wiring if the short or overheat condition were detected promptly by an operator or isolated by an automatic circuit protection device (circuit breaker). To investigate fire termination by power isolation, the event tree analysis for this DBA considered three scenario variations ... 

Sequence C Safe condition and no further fire of combustible material. The automatic circuit breaker does not detect the short or overheat and remove power. The operators present in the facility promptly detect the electrical short or overheat and remove power to prevent a combustible material fire that is assumed to involve stored material and release the radiological material. 

Circuits are controlled by switchgear which is assembled so that the circuit may be operated safely under normal conditions, isolated automatically under fault conditions, or isolated manually for safe maintenance. These requirements are met by good workmanship carried out by electrically skilled persons and the installation of approved British Standard materials such as switches, isolators, fuses or circuit-breakers (IET Regulation 131.1.1). The switchgear must also be non-combustible or housed in a non-combustible cabinet or enclosure (IET Regulation 421.1.200). The equipment belonging to the supply authority is sealed to prevent unauthorized entry, because if connection were made to... 

A modular power center, commonly found in computer-room installations, provides a comprehensive solution to AC power-distribution and ground-noise considerations. Such equipment is available from several manufacturers, with various options and features. A computer power-distribution center generally includes an isolation transformer designed for noise suppression, distribution circuit breakers, power-supply cables, and a status-monitoring unit. The system concept is shown in Fig. 10.268. Input power is fed to an isolation transformer with primary taps to match the AC voltage required at the facility. A bank of circuit breakers is included in the chassis, and individual preassembled and terminated cables supply AC power to the various loads. A status-monitoring circuit signals the operator if any condition is detected outside normal parameters. 

Fuses and circuit breakers are intended primarily for the protection of conductors and equipment. They prevent overheating of wires and components that might otherwise create hazards for operators. They also open the circuit under certain hazardous ground fault conditions. 

Circuit breaker coordination is a good engineering practice that can limit the degradation of safety related systems during a fault condition on a system component. Without coordination between breakers, an electrical fault on one component, such as a motor or motor-operated valve, could result in the loss of an entire motor control center or switchgear bus instead of just the single faulted component. 

Circuit breakers and fuses are installed in circuits to operate in the event of excess current arising from overload conditions and faults. The most common type of fault is an earth fault, but it is frequently the case that the current flowing due to earth faults is too low to operate the overcurrent protection devices. In addition, the overcurrent protective devices will not operate in the event of somebody making direct contact with a live conductor the current which flows through the body to earth will be too low to operate the devices but will often be high enough to cause fatal electric shocks. These two problems can be obviated by the use of earth leakage protection devices. 

The equipment should meet the criteria outlined in the materials electric code NFPA 70. Faulty electrics would be an unwanted ignition source. There have been occasions where operators were so against shutting down processes that they would modify the circuit breakers to a point where they would not deactivate the equipment even under the most hazardous conditions. De-energizing process equipment could fully destroy a multimillion dollar machine. Many of these machines could take years to replace. If all product needs to pass through this equipment, operations could cease until this equipment can finally be replaced. 

Attempts have been made to develop true, high-current arc resistance tests. A standardized German test employs carbon electrodes with 5 A at short circuit and 250-V dc (open circuit). The test is so severe that it destroys most organic materials and is useful only for inorganic materials. A United Kingdom test uses the test geometry of a small circuit breaker with a plastic arc chute. This practical approach evaluates plastics under actual operating conditions. However, the results depend on the size and shape of the arc chute standardization is difficult. 

A CT is provided in series with the spark gap to sense its operation during a line fault. As soon as there is arcing, it provides an instantaneous command to a short-circuit relay. The relay, in turn, closes the bypass breaker, within 3 to 5 cycles, leaving only the natural line impedance in the faulty eircuit. Now = 0, which limits the fault current to the natural level of the system, as if the capacitors were not connected. The shorting device is restored to its original status as soon as the fault condition is cleared. The device must be capable of interrupting the line fault... 

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