Circuit Breakers: Breaking Down the Basics
Circuit Breaker Frame
Contacts and Operating Mechanism
Mounting a Circuit Breaker
Circuit Breaker Video Overview
- Metal Frame - Metal Frame circuit breakers are assembled from precise metal pieces that are bolted and welded together to form the frame. Older low voltage power circuit breakers and current medium voltage power circuit breakers are of the metal frame design. Historically, all power circuit breakers, both above and below 600 volts, have been referred to as metal frame circuit breakers. The metal frame design is still being used for higher voltages.
- Molded Insulating Material - Molded insulated material frames are made from a strong insulating material, such as glass-polyester or thermoset composite resins. Sizes vary according to the Ampere Rating size of the circuit breaker. Molded insulated material frames are primarily associated with low voltage molded case circuit breakers and insulated case circuit breakers. Because of advances in materials and technology, we are now seeing molded insulated case power circuit breakers at 600 volts and higher.
- Open and close the contacts manually
- Open and close the contacts on demand
- Open the contacts automatically
- Over Toggle Mechanism - A manual handle on the circuit breaker is operated to set the mechanism in motion. The handle is moved, whether opening or closing the circuit breaker, until a point is reached where the handle goes over-toggle (past the point of no return), and the spring-assisted mechanism automatically opens or closes the circuit breaker. This toggle mechanism is called the Quick-Make, Quick-Break type, which means that the speed with which the contacts open or close is independent of how fast the handle is moved. A motor operator can be used to operate the handle automatically in lieu of manual operation. The design is such that the circuit breaker would trip open when required, even if the manual handle was held in the ON (closed) position.
- Two Step Stored Energy Mechanism - The two-step stored energy mechanism is used when a lot of energy is required to close the circuit breaker and when it needs to close rapidly. The two-step stored energy process is designed to charge the closing spring and release energy to close the breaker. It uses separate opening and closing springs. This is important because it permits the closing spring to be changed independently of the opening process. This allows for an open-close-open duty cycle. The motor can be operated remotely, allowing maximum safety for the operator. The major advantages of the two-step stored energy mechanism are rapid reclosing and safety. Rapid reclosing is achieved by storing charged energy in a separate closing spring. Safety is achieved by providing remote charging of the spring.
- Thermal Overload
- Short Circuit
- Ground Fault
- Electromechanical - This type of trip unit is generally used in low voltage circuit breakers. It is integrally mounted into the circuit breaker and is temperature sensitive. Thermal magnetic trip units act to protect the conductor (wire), safeguarding equipment under high ambient conditions and permitting higher safe loading under low ambient conditions. This trip unit utilizes bimetals and electromagnets to provide overload and short circuit protection, which is referred to as thermal magnetic. The thermal trip portion is used for overload protection. Its action is achieved using a bimetal heated by the load current. On a sustained overload, the bimetal will deflect, causing the operating mechanism to trip. The magnetic trip portion is used for short circuit (instantaneous) protection. Its action is achieved with an electromagnet whose winding is in series with the load current. When a short circuit occurs, the current passing through the conductor causes the electromagnet's magnetic field to rapidly increase, attracting the armature and causing the circuit breaker to trip.
- Electronic - In general, electronic trip units are composed of three components, which are internal to the trip unit. These components include the current transformer, circuit board and flux transfer shunt trip. The current transformer is used in each current phase to monitor and reduce the current to the proper input level. The circuit board is the brains of the system. It interprets input current and makes a decision based on predetermined parameters. A decision to trip sends an output to the flux-transfer shunt trip. The flux-transfer shunt trip is the component that trips the circuit breaker.
- Fixed Mounted - A circuit breaker that is bolted in its enclosure or assembly and hard-wired on to the frame is considered a fixed mounted circuit breaker. This method has to lowest purchase cost, is very reliable and front mountable. It is appropriate for 600V and below. Power feeding the circuit breaker must be turned off in order to remove and replace this unit.
- Removable - A removable circuit breaker has two parts: a base, which is bolted and hard-wired to the frame, and the actual breaker, which is plugged into the base. These allow the unit to be replaced without rewiring. This method has a moderate purchase cost, good reliability and is front-mountable. It is appropriate for 600V and below. Power feeing the circuit breaker must be turned off in order to remove and replace the circuit breaker.
- Drawout Mounted - A drawout circuit breaker also has two parts: a base, which is bolted and hard wired to the frame and the actual breaker, which slides into the base. This allows the unit to be replaced without having to remove power feeding the circuit breaker. Movement of the circuit breaker in or out could be manual or it could be accomplished by using some type of racking mechanism. This method has the highest purchase cost, is very reliable, allows for power ON testing, and is near mounted. It is appropriate for all voltages. The load must be turned off in order to test, remove or replace the breaker. As a safety feature, it is interlocked to automatically turn the power off during removal. A racking mechanism permits a circuit breaker to be moved, usually by turning or ratcheting a handle. By design, only the circuit breakers load must be turned off to rack the circuit breaker from the "Connected" position. This is accomplished by built-in interlocks, which automatically open the circuit breaker before racking out begins. The drawout feature is quite beneficial because the power to the entire assembly does not have to be turned off to service on circuit breaker.
This diagram above represents the different components of a circuit breaker, including the upper and lower terminals, trip lever, and operating mechanism.
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