How does a Relay Work?

Turned on and off by an electromagnet, a relay is a mechanical switch.

There are many places in a fire alarm system that relays are used. For instance, on a printed circuit board for a Fire Alarm Control Panel:

----- Every Notification Appliance Circuit (NAC) has at least one relay

----- Every Plain Old Telephone System (POTS) communications port has one relay

----- Every output showing C (Comm or Common), NC (Normally Closed), and NO (Normally Open) have relays

----- Every time you hear a quiet click each minute or so from inside a fire alarm control panel or NAC power supply, the click is produced by one or more relays

Most of the time, the relays themselves are Form C relays, having Common, Normally Closed, and Normally Open contacts.

You see the relays on the circuit board, and you hear the relays clicking, but how do the relays work?

In order to operate, a relay:

----- Uses electricity to energize an electromagnet

----- Uses the energized electromagnet to move a mechanical arm (an armature)

----- Uses the moving armature to transfer the Common contacts from the Normally Closed contacts to the Normally Open contacts

----- Uses the contacts to turn on or off electricity

----- Uses a spring to return the armature to a relaxed state when the energizing electrical current stops

A single relay coil can move:

----- One set of contacts for a Single Pole relay

----- Two sets of contacts for a Double Pole relay

----- Up to 24 sets of contacts for an (up to) 24 Pole relay

Usually, fire alarm relays have Single Poles or Double Poles.

Mechanical Relays versus Electronic Relays

One of the main uses for relays is to isolate, electrically, the circuit being controlled (like NAC circuits, the alarm output signals, the supervisory output signals, the trouble output signals, the door holder control circuits, phone lines, etc.) from the electronics of the fire alarm system.

Electrical isolation prevents ground faults on other equipment from being transferred to the fire alarm system.

The relays used for the NAC circuits that power the horns and strobes are also there to handle the higher current levels.

The bottom line, though, as it turned on and off by an electromagnet, a relay is a mechanical switch.