专业外语-第7章

Chap.7 Circuit-Interrupting Devices
Part 5 Circuit Breaker Characteristics
1. At a given voltage each insulation class requires minimum clearance and creepage paths.
Chap.7 Circuit-Interrupting Devices
2) A circuit breaker is a device for interrupting a circuit between separable contacts under normal or abnormal conditions.
Chap.7 Circuit-Interrupting Devices
2. Three basic types of circuit-interrupting devices are recognized.
1) A switch is a device for making, breaking, or changing the connections in an electric circuit. This definition implies that the switch is used in a system in its normal operating condition.
The abnormal conditions during which a circuit breaker may be asked to operate are primarily those which exist during short circuits.
Chap.7 Circuit-Interrupting Devices
Chap.7 Circuit-Interrupting Devices
4. The inertia of the heavy operating mechanisms limits the rate at which breaker contacts may part, with a consequent time delay in the opening of the arc.
Fuses are used in many applications on distribution primary circuits and to a limited extent on bulkpower high-voltage equipment.
Chap.7 Circuit-Interrupting Devices
Chap.7 Circuit-Interrupting Devices
Part 4 Vacuum Circuit Breaker
2. For all conventional arc quenching methods this means that the arc has to be cooled even before the contact pieces have reached the minimum quenching distance and before the next current zero, a fact which automatically increases the arc energy.
Chap.7 Circuit-Interrupting Devices
Part 4 Vacuum Circuit Breaker
Notes
1. The purpose of the arc quenching device of AC breakers is to deionize the breaker immediately after nt aero.
Part 2 Air Circuit Breaker
Notes
1. The longer arc is readily cooled by convection air currents and by radiation, and the longer positive column requires a higher voltage.
Chap.7 Circuit-Interrupting Devices
Part 5 Circuit Breaker Characteristics
5. The rated short-circuit current of a breaker is the highest symmetrical current that it is capable of interrupting when used in a system carrying rated maximum voltage.
Chap.7 Circuit-Interrupting Devices
Part 5 Circuit Breaker Characteristics
2. The minimum clearance path is the minimum distance that must be provided between conducting surfaces which differ in voltage. It is primarily dependent on the dielectric strength of the insulating material.
Chap.7 Circuit-Interrupting Devices
Part 4 Vacuum Circuit Breaker
4. For this reason and because of the short arcing times, the arc energy developed in the breaker is very small. This also accounts for the long services life of vacuum breakers.
Chap.7 Circuit-Interrupting Devices
Part 5 Circuit Breaker Characteristics
4. Besides clearance and creepage restrictions, the insulation classes impose diverse temperature limitations which must be respected at all times by the circuit breaker during its operations.
3. In breakers for operation in low-voltage ac circuits, advantages is taken of the voltage required to establish a cathode spot.
Chap.7 Circuit-Interrupting Devices
Chap.7 Circuit-Interrupting Devices
Part 5 Circuit Breaker Characteristics
3. Creepage denotes the undesirable flow of current between conductors, along the insulation surface. The minimum creepage paths are thus dependent on the environmental contamination.
Chap.7 Circuit-Interrupting Devices
Part 4 Vacuum Circuit Breaker
3. The arc drawn in the vacuum breaker, on the other hand, is not cooled since the metal vapour plasma is highly conductive and the resulting arc voltage only attains values between 20V and 200V.
3) A fuse is an overcurrent protective device with a circuit-opening fusible member directly heated and destroyed by the passage of overcurrent through it.
Chap.7 Circuit-Interrupting Devices
Part 1 Types
Notes
1. Circuit-interrupting devices in power systems are of many forms, ranging from the small key switches to the enormous circuit breakers that control the flow of power in high-voltage networks.
Chap.7 Circuit-Interrupting Devices
Part 5 Circuit Breaker Characteristics
6. Under no circumstance does the currentinterrupting capability exceed K times rated short-circuit current no matter how low the system voltage is.
Chap.7 Circuit-Interrupting Devices
3. The portion of the arc within the chamber generates gas which forces oil out through the throat, where it is in intimate contact with the arc to cool it, encourage recombination, and physically carry away the free electrons and positive ions.
Chap.7 Circuit-Interrupting Devices
Part 3 Oil Circuit Breaker
Notes
1. An insulating chamber filled with oil surrounds the closed contacts.
2. When the moving contact is lowered, the arc is drawn out through the throat of insulating chamber.