Veteran installers will undoubtedly remember when fire alarm panels worked almost exclusively by relay technology. As technicians, we encounter relays every day in the field on alarm panel motherboards, smoke detectors, addressable heat sensors and a host of other devices.
Opening up a panel that’s been in the field a while will reveal that the predominant technology is the electromechanical relay. For each zone there’s a corresponding relay, and when a detector or sensor goes into alarm it’s a relay that signals the event. One or more relays are then used to power the remote notification appliance circuit(s) (NAC).
Relays remain an important part of what we do on a daily basis, especially where subsystem integration is involved. In this regard, two of the most important aspects of what we do involve the isolation of dissimilar electrical circuits and the control of multiple loads when only one relay output is available. Let’s take a closer look at the essential, ubiquitous and reliable relay.
Relay Integration and Isolation
Modern fire alarm panels rely on multiple relays for a variety of purposes, such as isolation between dissimilar electrical circuits. Common subsystems to which fire alarm panels must interface include heating, ventilation and air conditioning (HVAC) systems; elevator controls; access controlled doors; fire suppression systems; and more.
For example, remote relay modules are commonly used to control HVAC dampers throughout a building, thus hindering the spread of smoke. In this case when an alarm condition occurs, the fire panel will signal remote relay modules via the signaling line circuit (SLC). These relay modules then shut off operating power to each HVAC damper motor causing them to close. This is true whether the controlling relay is on the fire alarm motherboard or a smoke detector.
In a similar fashion, per Section 21.7 of NFPA 72, 2010 Edition, remote relay modules can be used to control HVAC system fans as well as other functions. Relays provide isolation between a fire alarm motherboard and HVAC systems, such as 120VAC/240VAC-operated motors and 16VAC/24VAC thermostat control systems.
Electrically locked doors in electronic access control systems, per Section 21.9, NFPA 72, 2010, are also controlled via relays. In this case, when there’s a fire alarm condition the motherboard will automatically unlock all electrically locked doors for the duration of the condition, until the fire alarm system is manually reset.
Duct-type smoke detectors in HVAC duct work also make use of relays. If you look closely at one of these detectors you’ll see a number of form ‘A’ relays, one of them being an alarm output. You’ll also see a form ‘B’ supervisory relay and a form ‘C’ relay, listed for auxiliary functions. The form ‘C’ is often used for integration with an LED annunciator, graphic annunciator or some other type of subsystem.
Electrically, form ‘A’ relays provide a single pole, single throw (SPST), normally-open (NO) interface while form ‘B’ provides a SPST, normally-closed (NC) connection. Form ‘C’ offers the best of both ‘A’ and ‘B’ by providing a single pole double throw NO/NC connection (see diagram on the right).
Controlling Multiple Loads
Just as they can be used to signal more than one subsystem, relays can also be used to control more than one load. We often encounter this when an application calls for the signaling of an alarm condition to two or more locations within the facility.
Because most fire alarm control panels come with a form ‘A’ alarm output relay, only one subsystem can be connected to it. Many alarm panels also come with a programmable form ‘C’ relay, which is usually designated for auxiliary purposes.
Form ‘C’ relays essentially provide two or more outputs for multiple loads or signal paths. At a minimum this means the presence of one DPDT, NO/NC output.
In some cases there may be more than one form ‘A’ or ‘C’ relay available. But where there’s not, a fire alarm technician must be resourceful and knowledgeable enough to add a form ‘A’ or ‘C’ relay to the mix (see diagram on the left). As you can see in this drawing, using the 24VDC auxiliary power on the motherboard we can use a single form ‘A’ output to switch one or more form ‘C’ relay modules. When this is done, however, you must calculate the overall load and compare it to the specifications assigned to the form ‘A’ relay contacts.
Sciene of Coil Mechanics
The relays we use in fire alarm work are electromechanical devices. No matter what kind of relay or what the application may be, they are made up of three basic components: 1) electric coil, 2) moveable contacts, and 3) mechanical frame.
One of the characteristics that make the modern relay so powerful in our industry is that electrically there is no connection between coil and contacts. Because these two items are naturally isolated by distance, we’re able to control dissimilar voltages from dissimilar sources.
Relay coils are available in every conceivable AC or DC voltage. Available coil voltages vary from 1V to many hundred or even thousands, depending on the application. For the type of work fire alarm technicians do, the most common coil voltage is 24VDC.
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