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Alarm Switch/Signal Contact in a circuit breaker - Working Principle
An alarm switch (or signal contact) in a circuit breaker is a specialized accessory that changes state only when the breaker trips due to a fault (overload, short circuit, or shunt trip). It distinguishes fault trips from manual ON/OFF operations, sending a signal to remote indicators or SCADA systems.
Key Aspects of Alarm Switches
- Function: Activates an alarm, light, or sends a signal to a control system to indicate a, potentially, unattended breaker trip.
- Trip Detection Only: Unlike an auxiliary switch (which shows ON/OFF status), the alarm switch does not change state if the breaker is simply turned OFF manually.
- Reset Requirement: When tripped, the switch often remains in the alarm position until the breaker is physically reset (sometimes via a button on the breaker).
- Installation: Typically, it is an internal accessory, mounted inside the circuit breaker case.
- Types: Usually Form C (SPDT - Single Pole Double Throw) contacts.
Difference from Auxiliary Contact (AX)
- Alarm Switch (AL/Signal Contact): Trips only.
- Auxiliary Switch (AX): Follows the breaker main contact position (ON or OFF
In a circuit breaker, an alarm switch (also known as a signal or fault contact) is an auxiliary device that provides a remote electrical signal only when the breaker trips due to an electrical fault.
Function and Operation
The primary purpose of an alarm switch is to distinguish between a manual operation and a fault condition.
- Trigger Mechanism: The contact changes state only during a trip event caused by an overload, short circuit, or through accessories like a shunt trip or undervoltage release.
- Manual Override: If a user manually switches the breaker handle to the "OFF" position, the alarm contact does not change state.
- Resetting: Once tripped, the alarm contact often remains in the "fault" state until the breaker is manually reset, sometimes requiring a specific reset button on the accessory itself.
Comparison: Alarm Switch vs. Auxiliary Switch
| Feature |
Alarm Switch (SD/AL) |
Auxiliary Switch (OF/AX) |
| Monitors |
Trip status (faults) |
ON/OFF status (contact position) |
| Manual Operation |
No change in state |
Changes state every time handle moves |
| Fault Operation |
Changes state when tripped |
Changes state when tripped |
| Common Use |
Remote fault signaling (PLC, sirens) |
Status indication (indicator lights) |
Practical Applications
- Remote Signaling: Used in automated plants to notify operators via a PLC or panel light that a specific circuit has failed due to a fault.
- System Interlocking: Can be wired to prevent other equipment from starting if a critical breaker has tripped.
- Visual Indicators: Many units include a mechanical indicator (often a red flag or pop-out button) that provides a local visual confirmation of the trip.
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