Controlling Devices the Relay Way
Last month we talked about the many ways fire and burglar alarm technicians use relays. We covered some of the code compliance issues associated with their use as well as how to select the right relay for the job.
There’s little doubt many of today’s technicians would rather wrestle a hungry bear than install a Form C relay. Yet for those who worked their way up through the ranks in the early years of fire protection, relays are seen as an asset to their efforts. For them, relays are how things get done.
For example, a relay built into a four-wire smoke detector has many uses, one of which is smoke control. Two others include elevator control and heating, ventilation and air conditioning (HVAC) shutdown.
This month, we’ll explore some additional ways fire technicians use relays, including code references.
Form A & B Contact Mechanics
The best place to begin with relay mechanics is at the beginning with a brief discussion on the various types of relay connections commonly used in fire and burglar alarm work.
The most fundamental contact types in use by fire alarm technicians as well as burglar alarm installers are Form A, normally open (NO); Form B, normally closed (NC); and Form C.
Form A relay outputs are encountered in fire alarm work where the use of NO devices is commonplace. Supervision is carried out in this case by placing end-of-line (EOL) resistors over the NO contacts.
Form B, or NC, are more common to tamper switches, such as sprinkler systems. Supervision in this case is assured by placing an EOL in series with the NC contacts.
Form A and B contacts also are used in burglar alarm systems, but the most common is B. This is because devices used in burglar alarm work are generally based on the NC 
concept. Like the NC tamper switch in a fire alarm system, the NC contact in a burglar alarm detector is supervised by placing the EOL in series with the contacts.
Form C relay contacts are actually two sets in one (see drawing below). In this case, we have a Form A and a Form B contained in the same relay, only the two common (C) contacts are shared. This type of relay output is common to both fire and burglar alarm systems, and offers the most flexibility to the installer. Another way alarm technicians often refer to relay contacts relates to their functionality. For example, both Form A and B are commonly referred to as single pole, single throw (SPST). A simple Form C contact, however, is called a single pole, double throw (SPDT). When two ganged Form C relay contacts are used, however, we call them double pole, double throw (DPDT).
Smoke Detectors Release Doors
Probably one of the most important areas where relays come into play in life safety is in door releasing service. This is where a smoke detector(s) is used to release a door(s), thus controlling the flow of smoke from affected areas into unaffected ones.
“Door hold-open devices when activated will cause the door to close, thus controlling smoke and the spread of fire in hallways and stairwells,” says Tim Creenan, fire engineer and founder of Amherst Alarm Inc. of Amherst, N.Y. According to Section 5.16.6.1 of NFPA 72, National Fire Alarm Code, 2007 Edition, “Smoke detectors that are part of an open area protection system covering the room, corridor, or enclosed space on each side of the smoke door and that are located and spaced as required by 5.7.3 shall be permitted to accomplish smoke door release service.”
Door releasing service can be accomplished by two basic methods using a spot-type smoke detector. The first relies on one or more relays in a fire alarm panel to disconnect power feeding one or more door holder devices. The second method relies on specific 
smoke detectors releasing specific doors. The latter method, although not as prevalent, entails the use of a spot-type smoke detector equipped with an auxiliary Form C relay output in addition to the Form A commonly used to activate the fire alarm panel.
In this case one or two smoke detectors are placed on one or both sides of the header of a specific door. The Form C relay outputs are used to channel power from a centralized power supply to the door holder device on the nearby door. When smoke is encountered, the Form C changes states, thus shutting off power to the door holder device.
Probably the most common method of door control, however, relates to the use of ordinary spot-type smoke detectors and a common relay output at the fire alarm panel that drops power to all the door holder devices throughout a facility. In some cases relay modules are used to affect door holding devices by zone or some other type of configuration.
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