Circuit Breakers

Circuit Breakers: Learn About Circuit Breakers Here > Circuit Breakers are reusable overcurrent protection devices. After tripping to break the circuit, the breaker can be reset to protect the circuit again. There are two accepted definitions for circuit breakers. The National Electrical Manufacturers Association (NEMA) defines a circuit breaker as a device designed to open and close a circuit, by non-automatic means, and to open the circuit automatically on a predetermined overcurrent without injury to itself when properly applied within its rating. The American National Standards Institute (ANSI) states that a circuit breaker is a mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions. Also, it is capable of making and carrying a current for a specified time, and breaking currents under specified abnormal circuit conditions such as those of a short circuit. All circuit breakers have the following common design and functional characteristics:

Circuit Breaker Frame
Contacts and Operating Mechanism
Trip Unit
Arc Extinguishers
Mounting a Circuit Breaker

The circuit breakers frame provides a method by which all the required components can be mounted and kept in place, ensuring the proper operation of the circuit breaker. The circuit breaker frame provides the rigidity and strength required to successfully deal with the interruption process and achieve the desired Interrupting Ratings. The frame's mechanical strength must be sufficient to withstand the forces created by the square of the current (I2), which could be quite large and potentially destructive. The frame provides for insulation and isolation of the current path, offering personnel protection near the equipment during operation. The frame also plays a critical role in the circuit breakers ability to comply with standards. There are two types of frames:

• 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.

Contacts in a circuit breaker provide a method for connecting the circuit with the system. They also provide a method for isolating a part of a circuit from the rest of the system. A contact set contains a fixed and movable contact. As a circuit breaker opens or closes, the fixed contact maintains its position while the movable contact moves to close (make) or open (break) the circuit. When all is said and done, contacts perform a simple function; they open and close. Circuit breakers require some type of operating mechanism to open and close the contacts. This operating mechanism can be mechanical or a combination of mechanical and power. Depending upon the type of circuit breaker being considered, the operating mechanism could be called upon to:

1. Open and close the contacts manually
2. Open and close the contacts on demand
3. Open the contacts automatically

Circuit breakers by virtue of their size and/or some standards requirements need additional assistance to set the mechanism in motion to open or close the contacts. The additional assistance takes the form of springs. Springs play a big role in the precise functioning of circuit breaker mechanisms. Springs are stretched or compressed to provide the energy necessary to assist with the proper opening or closing of the contacts. There are two types of spring-assisted mechanisms:

• 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.

For a circuit breaker to be effective, it needs to have some intelligence to enable it to perform automatically or respond to a command. Without this capability, a circuit breaker would just be a fancy switch. A trip unit is the circuit breakers intelligence. Its function is to trip the operating mechanism (open the circuit) in the event of these overcurrent conditions:

1. Thermal Overload
2. Short Circuit
3. Ground Fault

There are two types of Trip Units:

• 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.

An Arc Extinguisher is the component of the circuit breaker that extinguishes an arc when the contacts are opened. An arc is a discharge of electric current crossing a gap between two contacts. Circuit breakers must be designed to control them because arcs cannot be prevented. There are four techniques to extinguish an arc and there are several arc control methods. Arcs are formed when the contacts of a circuit breaker are opened under a load. Arcs can be very destructive and vary greatly in size and intensity. The size of the arc depends on the amount of current present when the contacts are pulled apart. The heat associated with an arc creates an ionized gas environment. The more ionization, the better the conditions are for an arc to be maintained and grow. The bigger the arc, the more heat creates an ionized gas environment. Arcing is a condition that must be dealt with quickly and effectively by a circuit breaker. The important thing to remember here is that the ability of the circuit breaker to control the arc is the key to its short circuit interrupting capability. This is a critical factor for selecting circuit breakers. A short circuit is the most devastating overcurrent condition.

Methods for Mounting Circuit Breakers refer to the way a circuit breaker is used in its individual enclosure or assembly. Ease of replacement and unit cost are two factors to take into account when choosing a method for mounting circuit breakers. Circuit breakers, depending on the type and/or the particular application, are mounted for use in one of three basic ways:

• 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.