Understanding Auxiliary (AX) and Alarm (AL) Contacts for Miniature Circuit Breakers (MCBs)
Understanding Auxiliary (AX) and Alarm (AL) Contacts for Miniature Circuit Breakers (MCBs)
Overview
Auxiliary (AX) and Alarm (AL) contacts are optional accessories available for Miniature Circuit Breakers (MCBs) that provide remote indication of breaker status and fault conditions.
Although these accessories may appear similar, they serve different purposes and operate under different conditions.
Auxiliary Contact (AX-H)
Purpose
The Auxiliary Contact (AX) is used to indicate the position of the breaker's main contacts.
The AX contact changes state whenever the breaker main contacts open or close, allowing remote monitoring of the breaker's operating status.
Contact Configuration
Contact Type | Terminal Numbers |
Normally Closed (NC) | 11 – 12 |
Normally Open (NO) | 11 - 14 |
Operation
Main Breaker Contacts Open
| Contact | Status |
| 11-12 (NC) | Closed |
| 11-14 (NO) | Open |
Main Breaker Contacts Closed
| Contact | Status |
| 11-12 (NC) | Open |
| 11-14 (NO) | Closed |
Key Point
The AX contact follows the position of the main breaker contacts and changes state whenever the breaker is opened or closed.
Alarm Contact (AL-H)
Purpose
The Alarm Contact (AL) is used to indicate that the breaker has tripped due to a fault or abnormal operating condition.
Unlike the AX contact, the AL contact does not change state during normal opening or closing of the breaker. It only changes state when a trip event occurs.
Contact Configuration
| Contact Type | Terminal Numbers |
| Normally Closed (NC) | 91 – 92 |
| Normally Open (NO) | 91 – 94 |
Operation
Normal Condition (No Trip Present)
| Contact | Status |
| 91-92 (NC) | Closed |
| 91-94 (NO) | Open |
Breaker Trip Condition
| Contact | Status |
| 91-92 (NC) | Open |
| 91-94 (NO) | Closed |
Key Point
The AL contact only changes state when the breaker trips due to a fault condition. Normal opening and closing of the breaker will not affect the AL contact.
Summary
- AX Contacts provide remote indication of the main breaker contact position (open or closed).
- AL Contacts provide remote indication of a breaker trip or fault condition.
- Both accessories include NO and NC contact outputs for integration into monitoring and control systems.
- Using both AX and AL contacts together provides complete visibility of breaker status and trip conditions.