Internal Accessories
Auxiliary Contacts: Contacts that are mechanically interlinked with the opening and closing mechanism of the circuit breaker's main circuit. They are primarily used to indicate the open or closed status of the circuit breaker. Connected within the circuit breaker's control circuit, they enable the control or interlocking of associated electrical devices based on the circuit breaker's switching operations-for instance, by outputting signals to indicator lights, relays, and similar components. For molded-case circuit breakers with a frame size rated current of 100A, the auxiliary contacts utilize a single-break changeover contact design; for frame sizes of 225A and above, a bridge-type contact structure is employed. The conventional thermal current rating for these contacts is 3A. For frame sizes with a rated current of 400A and above, configurations featuring two normally open (NO) and two normally closed (NC) contacts are available, with a conventional thermal current rating of 6A. The operational endurance (number of switching cycles) of the auxiliary contacts is identical to the total operational endurance of the circuit breaker itself.
Alarm Contacts: Contacts dedicated to signaling faults within the circuit breaker. These contacts actuate *only* when the circuit breaker has tripped open. They are primarily utilized to indicate instances where the circuit breaker has undergone a "free trip" (automatic disconnection) due to load-side faults-such as overload, short circuit, or undervoltage conditions. Upon such a trip, the alarm contact switches from its original normally open position to a closed position, thereby energizing auxiliary circuit components-such as indicator lights, electric bells, or buzzers-to visually display or audibly alert personnel to the circuit breaker's tripped status. Since the probability of a circuit breaker undergoing a free trip due to load faults is relatively low, the service life of the alarm contacts is typically rated at one-tenth (1/10) of the circuit breaker's overall service life. The operating current flowing through the alarm contacts generally does not exceed 1A.
Shunt Trip Unit: A shunt trip unit is a type of tripping device actuated by an external voltage source; the voltage supplied to this unit is independent of the voltage within the circuit breaker's main circuit. The shunt trip unit serves as an accessory that enables the remote-controlled opening (tripping) of the circuit breaker. The unit reliably trips the circuit breaker whenever the supply voltage applied to it falls within the range of 70% to 110% of its rated control voltage. The shunt trip unit is designed for intermittent duty operation; consequently, the coil energization time must generally not exceed 1 second, otherwise, the coil risks burning out. To prevent coil burnout in molded-case circuit breakers, a microswitch is connected in series with the shunt trip coil. When the shunt trip mechanism is actuated-causing the armature to pull in-the microswitch switches from its normally closed state to a normally open state. Because the control circuit powering the shunt trip mechanism is thereby interrupted, the shunt coil remains de-energized even if the manual trip button is held down; this effectively prevents the coil from overheating and burning out. Once the circuit breaker is reset and re-closed, the microswitch returns to its normally closed position.
Undervoltage Trip Unit: An undervoltage trip unit is a tripping device designed to open a circuit breaker-either instantaneously or with a time delay-when the voltage across its terminals drops below a specified threshold. The undervoltage trip unit is required to actuate when the supply voltage falls (even gradually) to a level within the range of 35% to 70% of the rated operating voltage. Furthermore, when the supply voltage is at 35% of the trip unit's rated operating voltage, the unit must prevent the circuit breaker from closing. Conversely, when the supply voltage is equal to or greater than 85% of the trip unit's rated operating voltage-under thermal steady-state conditions-the unit must ensure that the circuit breaker can close reliably.
Consequently, should a significant voltage drop occur within the protected circuit, the device automatically trips the circuit breaker to cut off the power supply, thereby safeguarding downstream electrical loads and equipment from potential damage caused by undervoltage conditions. In practical application, the coil of the undervoltage trip unit is connected to the power supply side of the circuit breaker; the circuit breaker can only be closed once the undervoltage trip unit has been energized.
External Accessories
Electric Operating Mechanism: This accessory is used for the remote, automatic opening and closing of circuit breakers. Electric operating mechanisms fall into two categories: motor-driven mechanisms and electromagnetic mechanisms. Motor-driven mechanisms are designed for molded-case circuit breakers with a frame size rating of 400A or higher, while electromagnetic mechanisms are suitable for molded-case circuit breakers with a frame size rating of 225A or lower. Regardless of whether an electromagnetic or motor-driven mechanism is used, their actuation and rotational directions remain consistent; the opening and closing operations are achieved solely through the positioning of a cam located within the electric operating mechanism. When a circuit breaker is operated via an electric mechanism, it must be capable of reliably closing at any control voltage ranging from 85% to 110% of its rated control voltage.
Rotary Operating Handle: Designed for use with molded-case circuit breakers, this mechanism is mounted directly onto the circuit breaker's cover. The handle's spindle is seated within a corresponding mounting hole in the mechanism, and the opposite end of the spindle passes through an aperture in the door of the drawer-type switchgear cabinet. The handle grip is then attached to the exposed end of the spindle protruding through the cabinet door; the circular or square base of the handle is secured to the door using screws. This installation configuration allows an operator standing outside the cabinet to rotate the handle grip-either clockwise or counter-clockwise-to reliably open or close the circuit breaker. Additionally, the rotary handle mechanism ensures that the cabinet door cannot be opened while the circuit breaker is in the "closed" position; the door can only be opened when the rotary handle is in the "open" or "reset" position. In emergency situations-specifically when the circuit breaker is in the "closed" position but the cabinet door must be opened-a red release button located adjacent to the rotary handle's base may be pressed.
Extended Handle: This is an external handle extension designed to be mounted directly onto the circuit breaker's standard handle. It is typically used with high-capacity circuit breakers (rated at 600A or higher) to facilitate manual opening and closing operations.
Handle Locking Device: This device consists of a locking tab installed on the handle frame, paired with a corresponding hole drilled into the handle itself, allowing the mechanism to be secured using a padlock. It is primarily utilized to lock the circuit breaker in the "closed" position. During operational periods-specifically when unauthorized tripping by others could result in a power outage, or when the load-side circuit of a circuit breaker requires maintenance or must remain de-energized-this measure serves to prevent the circuit breaker from being inadvertently closed.
