| Electronic Trip Unit | Protection | Measures current and initiates circuit interruption when an overload, short circuit, or ground-fault condition exceeds the configured protection limits. | May provide long-time, short-time, instantaneous, and ground-fault functions, commonly identified as L, S, I, and G. | Low-voltage molded-case and insulated-case circuit breakers in commercial, industrial, and data-center distribution systems. | IEC 60947-2; UL 489 where applicable |
| Thermal-Magnetic Trip Mechanism | Protection | Provides automatic overcurrent protection through a thermal element for sustained overloads and a magnetic element for high fault currents. | The thermal element responds with an inverse-time action; the magnetic element responds rapidly to severe short circuits. | General-purpose branch and feeder protection where electronic monitoring is not required. | IEC 60947-2; UL 489 where applicable |
| Shunt Trip Release | Control | Opens the circuit breaker remotely when an external electrical signal is applied. | Often used with emergency-stop circuits, fire-alarm interfaces, smoke-control systems, and remote shutdown controls. | Facilities requiring supervised or emergency remote disconnection of electrical loads. | IEC 60947-2 and applicable installation requirements |
| Undervoltage Release | Safety | Prevents closing or causes opening when the control voltage falls below the release operating threshold. | Helps prevent automatic restart after a supply interruption and supports low-voltage protection schemes. | Motor control panels, safety-critical machinery, and installations requiring controlled restart. | IEC 60947-2; machine-control requirements may also apply |
| Auxiliary Contact | Signaling | Reports the main contact position or circuit-breaker status to a control, monitoring, or building-management system. | Normally open and normally closed contacts change state when the breaker opens or closes. | Remote status indication, electrical interlocking, PLC inputs, and energy-management systems. | IEC 60947-5-1 for control-circuit contact devices |
| Alarm Contact | Signaling | Indicates that the breaker has tripped because of a fault rather than being manually switched off. | Changes state only after a qualifying trip event and normally resets when the breaker mechanism is reset. | Fault annunciation, remote alarm panels, supervisory systems, and maintenance notifications. | IEC 60947-5-1 and circuit-breaker accessory requirements |
| Motor Operator | Automation | Charges or operates the circuit-breaker mechanism electrically for remote opening, closing, or automatic transfer functions. | Can support remote switching and integration with automatic transfer or power-management equipment, subject to breaker design. | Unmanned electrical rooms, automatic transfer systems, and remotely managed distribution boards. | IEC 60947-2; control-circuit requirements depend on the installation |
| Closing Release | Control | Closes the circuit breaker electrically after the mechanism has been charged and all permissive conditions are satisfied. | Usually works with interlocks, undervoltage protection, and anti-pumping logic to prevent unsafe repeated closing. | Remote switching, automated substations, industrial control systems, and transfer equipment. | IEC 60947-2 |
| Arc Chute Assembly | Safety | Splits, cools, and extinguishes the electrical arc produced when the breaker interrupts current. | Uses insulating barriers and arc plates to increase arc voltage and dissipate energy inside the interruption chamber. | Low-voltage air circuit breakers and other breakers designed for interrupting load or fault current. | IEC 60947-2 test and performance requirements |
| Mechanical Interlock | Safety | Prevents incompatible circuit breakers or switching devices from being closed at the same time. | Creates a physical dependency between operating mechanisms, reducing the risk of unintended paralleling or backfeed. | Source-transfer systems, dual-incomer switchboards, and generator utility changeover arrangements. | IEC 60947-6-1 and applicable switchboard requirements |
| Terminal Lugs and Line/Load Connectors | Connection | Provide the electrical and mechanical connection between the circuit breaker and field conductors or busbars. | Must match conductor material, cross-sectional area, torque requirements, temperature rating, and available fault-duty requirements. | Panelboards, switchboards, motor-control centers, and distribution assemblies. | IEC 60947-1; applicable wiring and product certification requirements |
| Padlocking and Lockout Attachment | Safety | Restricts operation of the circuit breaker during inspection, maintenance, or de-energization procedures. | Can hold the handle in the OFF position or prevent unauthorized operation, depending on the breaker design. | Electrical maintenance, lockout/tagout procedures, industrial facilities, and service-disconnect applications. | Applicable occupational safety and installation requirements |
| Ground-Fault Sensor or Residual-Current Sensor | Protection | Detects current imbalance or leakage to ground and sends a signal to the trip system. | Works with a trip unit or protection relay to disconnect the circuit when the configured residual-current threshold is exceeded. | Personnel protection, equipment protection, industrial feeders, and systems with increased leakage-risk exposure. | IEC 60947-2 and applicable residual-current protection requirements |
| Arc-Fault Detection Module | Fire Safety | Identifies electrical signatures associated with dangerous series or parallel arcing and initiates disconnection. | Uses electronic signal processing and is normally applied in accordance with the requirements of the installation jurisdiction. | Selected residential, commercial, and special-use circuits where arc-fault protection is required or specified. | IEC 62606 or applicable national arc-fault requirements |