In today’s article we explain the how, what and why on fire alarm control relays. Only output functions will be covered. We will give an overview of input relays on another article in the coming weeks.
Many of our readers are not only fire alarm technicians, but are electricians, air conditioning technicians and building maintenance engineers. If you are reading this, there is a good chance you fall into one of these categories.
While performing routine tasks throughout commercial buildings, you have seen or have worked with various output relays and you may have often wondered how these powerful components work with a fire alarm system.
There are two commons types of fire alarm relays, one is called a conventional relay and the other is called an intelligent (A.K.A., programmable or addressable) relay.
The Conventional Relay
The conventional relay can be used on both conventional fire alarm systems or intelligent fire alarm systems. On one side of the relay, you will find contacts that require energized power to trigger an electromagnetic coil.
On the other end of the relay, you will find a set of “dry” contacts, which are commonly called “normally open” and “normally closed” contacts. When power is applied or taken away from the input side (the electromagnetic/coil side) of the relay, the secondary side or contact side of the relay either opens or closes a set of contacts.
The contact side can be used to energize or de-energize power to turn on lights, shut down air conditioning units, close (or open) doors, or a number of other building automation functions.
In simple terms, a relay is used as leverage. On the input side, you may only have 12 to 24 volts DC triggering the relay. On the secondary side, you might be energizing 12- 24 volts DC, but you can also have 120 volts or 240 volts AC.
Not only is a relay used as a leverage tool, but it also serves as a way to separate power sources and different types of power as well. For example, you may want DC (direct current) driving an AC (alternating current) source and visa versa.
One can even use one relay to control another relay or multiple relays. There are so many things that can be done with conventional relays.
Depending on the size of the relay, the sky is the limit on how much power is applied to the primary and secondary parts of a relay to get the results needed to perform a specific function.
On most commercial fire alarm systems you may be required to shut down an air-handling unit. On the primary side, you would more than likely use 24 volts DC from a fire alarm power supply.
On the secondary side you would either break the low-voltage power from a thermostat or break a jumper on a sophisticated PC board inside the unit or VSD (variable speed drive).
How To Supervise a Conventional Relay Without An End Of Line Resistor
In most circumstances, when a fire alarm system is triggered, the power to the relay will turn off, which in turn changes the state of the relay contacts. By keeping the relay in a “powered up” state, you are, in affect, supervising the relay.
In other words, if the power terminals or wires get cut, the relay will act as though it had been triggered by the alarm system. This form of supervision isn’t necessarily required, but it is a part of good installation practice and should be implemented during the fire alarm installation process.
If this style of supervision is not acceptable by your AHJ, fire codes, or specifications for a project, then you might be interested in …
How To Supervise A Conventional Relay Using a NAC Power Supply
Firstly, a NAC power supply is a power supply typically used to provide power to N.A.C.’s or Notification Appliance Circuits.
Each NAC circuit has various outputs that lead out to strobes and horn strobes on any given fire alarm system. Each circuit has an end-of-line resistor (aka., “EOL”) which supervises the strobes on each circuit.
To supervise a conventional relay on a NAC circuit you would simply send the circuit wiring out to the relay and put the end of line resistor on the power input terminals along with the appropriate EOL that comes with the NAC power supply. Now, if the circuit opens up, the power supply will no longer “see” the EOL and go into trouble, which, if wired correctly, will create a trouble on the main fire alarm panel.
The Intelligent Fire Alarm Relay
The intelligent fire alarm relay can only be used on intelligent fire alarm systems and is made specifically to work with particular brands of fire alarm systems. Gamewell/FCI, Silent Knight, Firelite, Notifier, Siemens, and EST (Edwards) are just a few name-brand examples.
The intelligent relay performs the same function as a convention relay on the contact side. The difference between a conventional and intelligent relay is that the input side (on an intelligent relay) is triggered by the SLC (signaling line circuit).
Not to get too far off topic, Let me explain a little about what an intelligent fire alarm system is so you get a stronger grasp on how intelligent relays function…
What Is a Signaling Line Circuit?
In case you don’t know, the SLC circuit is the intelligent circuit that runs devices on the fire alarm system. Each intelligent device has it’s own mini circuit board and is much like peripherals on a computer system, such as a printer or mouse.
Similar to a computer system, each intelligent device requires a main-frame or PC to function, in the case of fire alarm systems, it would be the main fire alarm control panel. All logic and commands are sent via data, just like a personal computer.
When an intelligent relay is triggered it is done so via a signal sent by the main fire alarm system’s operating system (software).
What Makes Intelligent Fire Alarm Systems Different From One Another?
As an example, you can simply take two operating systems, such as PC and Mac. You see, it’s not the hardware that makes up the intelligent system as much as it is the “software” that runs these systems.
The same can be applied to commercial-grade fire alarm systems as well. Each system is made up of similar components, such as fire alarm smoke detectors, intelligent relays, annnunciators (keypads), etc., but they will be required to be of particular brand that works as part of a complete fire alarm system or they will not function properly.
Something Else You Should Know About When Choosing A Relay Type (Very Important!)
When deciding which relay is best for your fire alarm system’s specific application, always consider how much power is being controlled by the relay. Although many intelligent relays can handle 120 volts AC, always check the ampere ratings on both the devices you are controlling as well as the ratings listed for your intended relay.
On variable speed drives, many times you will find that the ampere ratings and voltage are very minimal, but there will also be times where you will be tied directly into transformers, contactors, and controls with a lot higher ratings. In cases where your amp ratings are high, a conventional relay may be a better choice.
Single or Dual Contacts
Both, intelligent and conventional relays give you the option of having one or more contact relays on the output side (depending on the manufacturer). Although there might be a slightly higher cost in using dual contact relays, it might be beneficial in utilizing dual or multiple outputs for future additions to your fire alarm system. Consider this options when choosing your relay.
Stand-Alone Or Built Right In?
Most fire alarm manufacturers sell equipment that have relay contacts built right in to the actual devices. A couple of examples might be duct smoke detectors and smoke detector relay bases.
Relays built into duct smoke detectors are normally intended for controlling the AHU (air handling unit) it is mounted on.
Smoke detector relay bases can be used for a number of functions. One might use a relay base to close fire doors, activate dampers, activate elevator controls, or even shut down AHU’s.
When purchasing these types of devices, evaluate the cost differences between having relays built in or simply using stand-alone relays. Consider not just the cost of the devices, but the labor involved as well.
To stay competitive and to keep costs down during the bidding process, it is always a wise to weigh these differences.
If you aren’t sure of what you need in way of relay controls or need assistance with anything related to your Houston area, commercial fire alarm system, please feel free to contact our someone by filling out our contact form or giving us a call at 281-409-9524. We will be happy to put you in direct contact with one of our professional fire alarm consultants or technicians to get you on your way.
Thanks for reading this article.
Filed Under: Building Automation
Tagged: air handler, airconditioning shutdown relay, control relay, conventional relay, door release relay, dry contacts, End-of-line resistor, EOL, fire alarm relay, intelligent fire alarm system, intelligent relay, NAC circuits, output relay, shut down relay, signaling line circuit, SLC, strobe circuits, variable speed drive, VFD