What Are the Three Most Common Relay Outputs on a Fire Alarm Control Panel?

Fire Alarm Control Panels (FACPs) typically include three key relay outputs:

1. Alarm

2. Supervisory

3. Fault

Each relay operates electrically isolated from the rest of the FACP’s electronics. When a specific condition is detected (like fire, maintenance issue, or system fault), the relay changes state to send a signal to external systems.

How Relays Work in an FACP?

A relay inside an FACP is like a remote-controlled switch. It has three terminals: Common (C)

Normally Open (NO)

Normally Closed (NC)

A small electromagnet activates the relay to switch the connection between terminals:

At rest: C is connected to NC

When energised: C switches to NO

These contacts are used to control other equipment or send signals out to other systems like monitoring stations.

Alarm Relay Output

Purpose: Sends a fire alarm signal to external systems (like monitoring panels or emergency response systems).

Behaviour:

Alarm light turns on → Relay activates → C → NO

Stays active until alarm resets

Common uses:

1. Notifying central stations

2. Closing fire doors

3. Shutting down HVAC

4. Releasing magnetic door locks

5. Activating the AOV system

6. Grounding the lift

Supervisory Relay Output:

Purpose:

Sends a signal for non-emergency conditions like valve tamper or pump running.

Behavior:

Normal: C → NC

Supervisory event active:

Amber light turns on → Relay activates → C → NO Supervisory alarms are informational. They typically don’t cause building evacuation or fire department dispatch.

Trouble Relay Output

Purpose:

Indicates faults in the FACP such as open circuits, power loss, or component failures.

Behaviour:

Normal (no trouble): Relay is active → C → NO

Trouble condition: Yellow trouble light → Relay deactivates → C → NC Why this relay is “weird”: It is designed to be normally active so that if the panel loses power or dies completely, the relay drops out — signaling a problem even when the panel can’t.

Always Check the Manual

Some older or proprietary panels may use relays differently. Always refer to the FACP manual to verify relay behavior before installation or troubleshooting.

Here’s a diagram that visually explains how the three relays work:

Always Check the Manual

While most modern Fire Alarm Control Panels follow standard relay behaviours, older or specialised panels may differ. Always consult the FACP’s instruction manual before wiring or programming relay outputs.

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