Relays are one of the most important commodities in the fire alarm business but not every fire technician truly understands how they work. Relays, for example, usually are found at the very heart of fire safety functions, such as smoke control and access control locking devices. They’re also used to interface with elevator control systems, smoke dampers and door-holding devices.
The use of relays for these and other purposes is common and, in many cases, required, and yet many fire alarm installers never realize it. Even more alarming is the fact many fire technicians cannot effectively work with them when it comes to replacement or choosing new relays for new functions. In this installment of “Fire Side Chat,” we’ll cover relay basics in fire alarm systems. In addition, we’ll look at fire code and examine a situation or two where the common, ordinary relay rules.
Code Is Specific on Relay Use
Fire technicians are able to do a lot of neat things using relays. Because our work involves life safety, however, it’s important to keep in mind that when we replace an old relay or 
engineer the use of a new one, it must be listed for the purpose it is to be used.
According to Section 4.3.1 of NFPA 72, 2002 Edition, “Equipment constructed and installed in conformity with this Code shall be listed for the purpose for which it is used. Fire alarm system components shall be installed in accordance with the manufacturer’s installation instructions.”
That’s right; you should take a few minutes to read the manufacturer’s instructions. Also, there are times when you may be faced with engineering the use of a new relay — one that may not be specifically listed with the fire alarm panel you use.
“The only time you would be allowed to use nonlisted relays, I would think, would be with door-holder release circuits. These relay circuits are designed to be de-energized when the alarm activates,” says Nick Markowitz, owner of Markowitz Electric Protection of Verona, Pa. “A wire or relay failure would not be a worry in this case since everything is designed to unlock the doors.”
In other words, you must know how to choose the right relay with the voltage and current characteristics of the circuit in mind. Another consideration related to relay use is the distance between the device to be controlled and the relay itself. In the “NTC Brown Book for Fire Alarm Systems,” authored by Charles Aulner and Bryan Mclane, it says, “The device used to control the other system must be located within three feet of the device it is controlling.”
Observe Specifications
In most cases, fire alarm panels contain one or more relays that interface with an assortment of building subsystems. Thus, there will be times when the internal relay built into a fire alarm panel cannot handle the external current load or voltage.
“Here is where you will need a heavier relay than the one internal to the fire alarm panel. This is usually due to the amount of power being pulled, such as when you control multiple door-holding devices,” says Markowitz. Relays inherently carry several ratings that you must know when putting them to specific uses. In most fire applications, the only two ratings to consider are voltage and current.
“When sizing a relay you must take into consideration the voltage being applied to the coil. First, is it alternating current [AC] or direct current [DC]. Second, is it 24VDC or 120/240VAC?,” Markowitz advises. “The wrong coil can either cause a relay not to operate or it might cause it to operate erratically. The coil might also burn up when you apply voltage.”
A relay’s contacts also come with their own voltage and current specifications. In this regard, it is common to see ratings such as 12VDC @ 5A or 30VAC @ 8A. In this manner, most relay outputs will carry two ratings, one for DC and the other for AC. You must assure the load you intend to place on the selected relay does not exceed these ratings.
Engineering and Selection
“The main reason for observing contact ratings is to ensure the relay functions as required without interfering with the fire alarm panel’s internal functions,” says Markowitz. “If these specifications are followed, the relay coil will not burn up a fuse or interrupt any portion of the panel’s internal operations.” Also, according to Markowitz, undersized output contacts may work once or twice when overloaded, but they will eventually fail. Whatever a relay’s ratings are, you must assure that you do not exceed the specifications.
When engineering and attaching a relay to a fire alarm panel, it’s your job to ensure it bears a third-party testing facility’s label and that it is rated for the intended use. In some cases, it must also be listed with the panel you intend to use.
Life-Safety Functions In the area of smoke control, electromechanical or electromagnetic door-holder devices are fed voltage/current by one or more relays. During normal operating conditions the relay will remain engaged, thus holding the doors open.
When the building fire alarm system or local smoke detector (listed for door-holding application) goes into alarm, one or more relays are made to de-power. This severs the connection between power supply and the door-holder circuit. In this case, all the doors in the affected area will close. The purpose of door holders in this example is to contain smoke to only the affected area. Relays also play an important role in the control of air-handling equipment, which is part of the smoke control function. In this case relays are used to interface the alarm system to a central ventilation or HVAC system. When a fire is detected, these relays de-energize, causing the air handlers or HVAC system to shut down.
EOL Relays and 4-Wire IDCs Another area where fire technicians commonly use relays is in the supervision of operating power in initiating device circuits (IDCs) used with four-wire smoke detectors (see drawing).
Unlike their two-wire cousin, four-wire smoke detectors 
require a separate set of wires for operating power. The end-of-line (EOL) resistor merely supervises the physical connection between the IDC input terminals and the last smoke detector in the circuit.
Operating power cannot be supervised using an EOL resistor unless an EOL relay is also used (see photo). As long as power remains intact, the relay coil in the EOL relay remains engaged, thus maintaining closure between the relay’s normally closed and common relay contacts.
By running the IDC’s EOL resistor through these contacts, we’re able to monitor power integrity. For example, if power disappears at the end of the IDC shown in the drawing, the relay coil will de-power, allowing the normally closed and common contacts to open. This will sever the connection between the EOL resistor and IDC input on the fire alarm panel. It will also signal both the central station and the end user that something is wrong with the IDC. For more information on relays and their application in fire alarm systems, refer to NFPA 70 and 72 or visit www.firenetonline.com.
