When a fire alarm occurs it is common to shut down HVAC equipment to reduce the spread of smoke and air to feed a fire. In some cases, positive pressure fans are turned on to create a high pressure area in stairwells which keeps smoke out. In either case, these fans draw more current than a fire system relay is rated to handle. A RIB (Relay in a box) is often used, or some similar relay, to switch ON or OFF a HVAC system. A fire system relay rated for 120 VAC .5 amp is used to control a high current relay. The drawing below illustrates such an install.
RIB brand relays are commonly used for there ease of installation. The RIB attaches to a box through a 1/2 KO. The LED light provides indication of status for easy verification of the systems operation.