Skip to main content

How To...

Wiring HVAC Shutdown Using Fire System Relay With High Current Relay

When a fire alarm occurs it is common to shut down HVAC equipment to control the spread of smoke and fire. In some cases, positive pressure fans are turned on to create a high-pressure area in stairwells which keeps smoke out. Smoke and Fire Dampers are very common in modern construction and they must close in case of fire.

In either case, these fans/damper motors can draw more current than a fire system relay is rated to handle.  500mA  is commonly the accepted maximum current for fire system relays.  This is enough for a few magnetic door holders and maybe even a smaller damper motor, larger loads need to be switched by a relay rated for the expected continuous load.

The drawing below shows a Fire Relay and a Higher Load Current capable relay wired without out power supplied (Breaker off).

Fire relay wired to higher current relay with power off
Relays Circuits Shown De-Energized


The neutral is common to both the coil side of the relay and the Load Side Neutral. 

When the supply voltage is applied (Breaker On), the drawing below shows the different currents on each circuit. The blue line shows the lower current circuit and the red line shows the higher load current.

Supply current on
Energized Circuit in Nomal Operational State


Notice the Load Current relay has switched to the Normally Open Position. Relay connections s are usually labeled in their de-energized state. Below you can see the effect of the Fire Relay when the fire alarm activates.

Power is removed from the coil of the Load Current relay, resulting in the loss of power to the load.

It is important to note that the power is now present on the Normally Closed terminal or wire of the Load Current relay. If using a RIB relay, make sure you isolate that wire lead. 

Fire Relay activated removes power from the high current relay, powering down the HVAC equipment
Energized Circuit with Fire Relay De-Energizing the Load Current Relay

Relays used for Fire Alarm Systems to control high current loads

A RIB (Relay in a box) is often used, or some similar relay, to switch ON or OFF an HVAC system, remove power from a smoke damper circuit (dampers close automatically on power loss), or turn on a pressurization system.

RIB brand relays are commonly used for their ease of installation. The RIB attaches to a box through a 1/2 KO. The LED light provides an indication of the status for easy verification of the operation of the system. 

RIB Relay

There are dozens of models to accommodate almost any load requirement.

Functional Devices Relay Web Site

An MR-101 Relay is another common product type. Its ease of use with its built-in KO's and multiple voltages to activate the coil makes it a favorite.

Image of MR-101 Relay
MR-101 C/R Relay

Space Age Electronics MR Series WebSite