Breakers, Relays and Disconnects Flashcards
What are five basic categories of circuit interruption devices found in the power system?
Disconnect switches Air break switches Load break switches Power fuses, and Circuit breakers
What is the purpose of a Disconnect Switch?
The are unable to interrupt any current flow, and must only be operated when current flow is zero. Disconnect
switches are essential to personnel safety when performing maintenance work because they provide positive visual evidence that it is open.
What is the purpose of Air Break Switches?
They are normally used in high voltage systems, and can be used to interrupt the exciting currents of transformers or the moderate capacitive currents of unloaded transmission lines. They cannot interrupt normal load
currents. The name “air break” denotes that the operation of the switch and quenching of the arc are done in the open air.
What is the purpose of Load Break Switches?
They are used to stop and/or transfer current flow. Load pickup is not as severe as load interruption because the arc is reduced when the switch contacts are closed. Due to the potentially large current flows, the load break switch construction is more rugged, with additional components to prevent main contact damage.
What is the purpose of Power Fuses?
Fuses disconnect the protected circuit from the power source. The disadvantage of this type of interrupter is that
replacement is required after each actuation (failure).
What is the purpose of Circuit Breakers?
They are used to connect or disconnect a power circuit to a power source and can perform the protective function of interrupting fault current of the power source.
What are three common ways of classifying circuit interrupting devices?
By dielectric material, operating voltage range, and type of operating mechanism.
Describe the use of dielectric materials in circuit interrupters.
The purpose of a dielectric medium is to surround the breaker contacts and help cool and extinguish (quench)
the arc during operation. It insulates (prevents current flow) the space between the contacts when they are open. A number of different mediums for quenching an arc: ambient air, oil, high-pressure air, or special gases such as sulfur hexafluoride (SF6) are available.
Define dielectric strength.
It is the ability of a material to withstand voltages without conducting current.
Define Voltage Rating in terms of circuit breakers.
Three common voltage classifications are low (15kv). This voltage classification may not always be the rated voltage. For example, a breaker rated at 7.25 kV may be called a 6.9 kV breaker because that is the operating voltage of the system in which it is installed.
Define Frame Rating in terms of circuit breakers.
The maximum continuous current ratings in amps of all parts except the overcurrent device. Some breakers have a mechanical interlock to prohibit insertion of a different rated breaker into a cabinet.
Define Continuous Current Rating in terms of circuit breakers.
A current value equal to or less than the frame rating. This value of current is the rating of the overcurrent device. It is possible to have a 600 amp frame circuit breaker that has a continuous current rating of only 40 amps.
Define Interrupting Rating in terms of circuit breakers.
The highest current at rated voltage the device can interrupt without external damage. Typical ratings range from 22,000 to 200,000 amps symmetrical. Interrupt ratings vary between manufacturer, voltage, frame rating of breaker, and whether high-fault protectors are installed.
Define Time Delay Rating in terms of circuit breakers.
Circuit breakers, like fuses, are rated by the amount of time delay they provide. In circuit breakers, the ratings are: instantaneous, short time delay, and long time delay. The delay times of circuit breakers can be used to provide selective tripping. Selective tripping allows breakers to trip in sequence. Normally the breaker nearest the load should trip first.
Define Control Power in terms of circuit breakers.
If a circuit breaker can be electrically opened, closed, or has a motor to charge the closing springs, there will be a control power requirement. Often this control power
is further broken down into control and trip power. This separation allows the protective circuits to be fused differently than the control power circuits.
Define Main Stabs in terms of circuit breakers.
Plug-in connection points between the circuit breaker and switchgear cabinet. Circuit load current flows through them.
Define Auxiliary Stabs in terms of circuit breakers.
Plug-in connection points between the circuit breaker and switchgear cabinet. These stabs can be used for spring-charging motor, motor power, trip circuits, control circuits, instrumentation circuits, or any other electrical interface with the breaker.
Define Main Contacts in terms of circuit breakers.
These contacts are designed to carry the normal operating current.
Define Arcing Contacts in terms of circuit breakers.
Close first and open last during breaker operation. These contacts are designed and used as sacrificial elements so that only minimal damage to the main contacts occurs during breaker operation.
What are the four general classifications of circuit breakers.
- Air Circuit Breakers (ACBs)
- Oil Circuit Breakers (OCBs)
- Vacuum Circuit Breakers (VCBs)
- Gas Circuit Breakers (GCBs)
Describe an Air Circuit Breaker.
They simply elongate the arc in air until it is extinguished. ACBs can be made to interrupt currents of 50,000 amps in AC circuits operating at up to 10,000 volts. The breakers typically have a piston that puffs air across the contacts as they operate. This puff of air assists in extinguishing the arc.
