Fire Alarm System Basics

FIRE ALARM SYSTEM COMPONENTS

Selection of Fire Alarm System

There are many devices that are available for installing Fire alarm system, but those devices needs to be selected as per required criteria. Some of the features that had to be considered are as follows.

1.     The Purpose of system.

2.     Amount of protection needed.

3.     Number of occupants present.

4.     Contents present inside and outside the premises.

5.     Required response time.

6.     The basic function of system.

7.     The other fire protection systems that must be interfaced.

The purpose of the system is generally to notify the occupants and the fire department of a fire condition in the building. The system may also be used to actuate suppression systems or shut down equipment and manufacturing processes.  A fire alarm system designed shall typically achieve the following goals such as Life safety, property protection.

1.     Basic Fire Alarm System

Fire Alarm systems perform several important functions to reduce life and property losses dur­ing fires. They can provide fire detection, early warning for evacuation, and Emergency Response Team or public fire de­partment notification.

1.1       Fire Alarm System Components

The following is a list of the basic components that can be installed together to make up a typical fire alarm system:

1.1.1 Alarm Initiation Devices

v Manual Fire Alarm Boxes

v Waterflow Initiating Devices

v Heat Detectors

v Smoke Detectors 

v Other Fire Detectors

1.1.2 Notification Appliances

v Bells

v Horns

v Speakers

v Sirens 

v Strobes

v Combination units

1.1.3 Fire Alarm Control Units

v Conventional fire alarm systems

v Addressable fire alarm systems

v Analog-addressable fire alarm systems

1.1.4 Remote On-Site Annunciation

v Alphanumeric

v Liquid Crystal Displays (LCD’s)

2.     Alarm Initiating Devices

2.1Manual Fire Alarm Boxes

Manual fire alarm boxes should be installed at unobstructed, readily accessible locations throughout the protected area with at least one box on each floor. Travel distance to a box should not exceed 200 ft. from any point in the area. The operable part of each manual fire alarm box shall be not less than 42 in. (1.07 m) and not more than 48 in. (1.22 m) above floor level and, the box location should be positioned in the normal path of exit from the area. The mounting surface shall be of a contrasting color.

  Types of Manual Fire alarm boxes:

1.     Non Coded

2.     Coded

3.     Break glass

4.     Non Break Glass

5.     Single Action

6.     Double Action

Manual alarm boxes may be used for following types of systems:

1.     General Alarm: When activated, the fire alarm evacuation signals sound immediately throughout the premises.

2.     Pre – signal: Initial fire alarm signals only sound at designated areas. The subsequent actuation of a key switch on the box (or control panel) causes an evacuation signal to sound throughout the premises.

2.2Waterflow-Actuated Fire Alarm initiating devices

The fire alarm system should monitor the opera­tion of the automatic sprinkler system and other fire extinguishing and suppression systems by means of listed fire alarm initiating devices. When the automatic sprinkler system operates, the waterflow-actuated fire alarm initiating device will initiate a fire alarm signal.

The fire alarm system should also monitor the normal standby condition of these extinguish­ing or suppression systems by means of listed supervisory initiating devices. If someone closes a sprinkler system control valve or otherwise impairs the protective system, the supervisory initiating device will cause the fire alarm system control unit to indicate a “supervisory off-nor­mal condition.” When the valve is reopened or the other impairment is cleared, the supervisory initiating device will cause the fire alarm system control unit to indicate a “supervisory restora­tion to normal signal.”

The waterflow alarm and the supervisory ini­tiating devices must be monitored separately to enable the differentiation between waterflow, trouble and supervisory conditions. Generally with conventional hard-wiring systems this requires a separate zone for waterflow alarm and one for supervisory initiating devices with the supervisory device being a “normally-open” device.

There are four basic types of automatic sprinkler systems that an alarm system may be connected to:

v Wet Pipe

v Dry Pipe

v Pre-Action

v Deluge

While a vane flow switch may be used on a wet pipe system, a pressure flow switch is required for the supervision of dry pipe, pre-action and deluge systems.

 2.3 Heat Detectors

1.     Fixed Temperature and Spot- type enclosed area (Rooms, Closets, Etc.):

This the most common types of heat detector. The fixed temperature point for electrically connected heat detectors is 136.4°F (58°C). Technological developments have enabled the perfection of detectors that activate at a temperature of 117°F (47°C), increasing the available reaction time and margin of safety. Primarily for property protection not considered an early warning device.

2.     Rate of rise, Spot-type enclosed area (rooms, closets, etc.):

Primarily used for property protection where design goals require more sensitive heat detection and response to developing fires. Aviod in areas of fluctuating ambient temperature. NOt considered an early device, rise of rate heat detectors operate on a rapid rise in element temperature of 12° to 15°F (6.7° to 8.3°C)  increase per minute, irrespective of the starting temperature. This type of heat detector can operate at a lower temperature fire condition than would be possible if the threshold were fixed.

2.4  Flame Detectors

          1. Infrared / ultraviolet:

 Special applications such as oil refineries, aircraft hangers, explosion or special hazard protection. Avoid use in areas where detectors are exposed to sunlight or welding unless the detector is listed for this environment. Must have an unobstructed view of the protected area.

2.5 CO and Gas Detectors

            1. Co Monitor:

Signals from carbon monoxide detectors and carbon monoxide detection systems transmitted to a fire alarm system shall be permitted to be supervisory signals.

2.     Gas Detector:

     Gas detection equipment shall be listed for detection of the specific gas or vapor to be encountered.

 3 Notification Appliances

3.1 Speakers

Speakers are frequently used as fire alarm signal­ing appliances. Since they reproduce electronic signals, they can be made to sound like any mechanical signaling device and have the capa­bility of reproducing unique sounds that are not practical on mechanical appliances. In addition, they may be used to give live or recorded voice instructions. Speakers are either direct radiating cone type, or of the compression driver and horn type.

Speakers are generally operated from audio am­plifiers delivering standard output line levels of 70.7 or 25 volt AC rms. The speakers are driven by an electronic tone generator, microphone, or voice synthesizer and an electronic amplifier. Two types are in wide use:

Integral – that type in which the tone generator amplifier, and speaker are enclosed in a common housing.

Remote – that type in which the speaker is energized from a remotely located tone generator, microphone and/or voice synthesizer and amplifier.

3.2 Sirens

Sirens usually are limited to outdoor applica­tions but are sometimes used in extremely noisy indoor areas. Sirens are motor-driven or elec­tronic appliances and may be either alternating or direct current operated. They are not very practical for use as coded audible signals.

3.3 Strobes

Strobe lights operate on the energy discharge principle to produce a high intensity flash of short duration. These lights are very efficient. The short bright flash is not only attention get­ting but is effective when general visibility is low. Strobe appliances come in a wide range of light intensities and operating voltages. Repeti­tion rates are not allowed to exceed two flashes per second nor less the one flash every second throughout the listed voltage range of the appliance.

3.4 Combination units

The audible and visible functions can be com­bined in one unit to produce both sound and light from a single appliance. For example, the sounder can be a horn, bell, or speaker.

Advantages of the combined signals are:

v The visible signal localizes the particular audible alarm appliance that is operating.

v The visible signal produces a recognizable alarm when an ambient noise level may affect the audible signal.

v Persons having impaired hearing can see the visible portion of the alarm signals.

The combined signals are available in all voltages up to line voltage. Twenty-four volt dc units are the most prevalent. Polarized versions facilitate line monitoring. Two or four-wire connected types permit application of either a common or separate power supply. Combination appliances are not required at every location throughout a building.

4.     Fire Alarm Control Units

The primary purpose of the fire alarm control unit is to process signal received form initiat­ing devices and to output appropriate signals to notification appliances and the off-premises supervising station. Typically the control unit is a microprocessor (similar to a computer) and can be programmed for many additional functions. Where ambient conditions prohibit instal­lation of automatic smoke detection, automatic heat detection shall be permitted.

In addition, where connected to a supervising station, fire alarm systems employing automatic fire detectors or waterflow detection devices shall include a manual fire alarm box to initiate a signal to the supervising station. This is intended to provide a backup means to manually activate the fire alarm system when the automatic fire detection system or waterflow devices are out of service due to maintenance or testing, or where human discovery of the fire precedes automatic sprinkler system or automatic detec­tion system activation.

Fire alarm system can have multiple operating configurations:

v Conventional

v Addressable

v Analog/Addressable (sometimes called “intelligent” system)

Conventional systems are normally used in small buildings applications where point identi­fication of the device in alarm is not considered necessary. Addressable fire alarm systems pro­vide detail as to the device in alarm or trouble.

Addressable and Analog/Addressable fire alarm systems are designed to identify the device that has been actuated. Analog/Addressable detec­tors may be adjusted for different sensitivity levels, depending on the environment that the detector is to be installed within.

These systems will also indicate when a device is approaching an alarm state due to contamination and will allow the sensitivity of analog smoke detectors to be individually set at the fire alarm control unit. The more complex systems require programming and specialized maintenance but once properly installed have proven to be more stable.

 5. Fire Alarm Loop

In a fire alarm system, a loop is a pair of wires. It carries power and signals between the circuit boards inside the control panel and the off-panel devices in the field. 

The loop:

v  As an Initiating Device Circuit (IDC), carries signals from the input devices to the panel.

v  As a Notification Appliance Circuit (NAC), carries signals and power from the panel to the fire horns, speakers, and strobes, causing the devices to make noise and flash lights, telling the occupants of the alarm.

v  As a Signaling Line Circuit (SLC), carries signals in the form of data between the panel and the input and output devices.

v  As a Power or Control Circuit, carries signals or power to specific fire safety controls and devices.

5.1 IDC (Initiating Device Circuit)

The IDC is a pair of wires without t-taps, connecting the panel to the input sensing devices. 
     5.1.1 Fire Sensing Input Devices (Alarm)

v  Waterflow

v  Smoke Detectors

v  Heat Detectors

v  Manual Pull Stations

v  Fire Suppression System Alarms

     5.1.2 Non-Fire Input Sensing Devices (Supervising other systems)

v  Duct Detectors (in older systems these were alarm)

v  Fire Suppression System Supervisory

v  Fire Pump Supervisory

v  Gate valve Tamper Supervisory

To avoid signal confusion, Fire Sensing (Alarm) and Non-Fire Sensing (Supervisory) devices are not mixed on the same IDC loop.

The panel supervises the fire alarm wiring using an end-of-line resistor. This resistor allows small electrical current to pass through the wires of the loop, making sure the wires and connections are always complete. For a Class B loop, the end-of-line resistor may be in a distant part of the building, or for a Class A loop, the end-of-line resistor is part of the circuitry of the panel. 

5.2 NAC (Notification Appliance circuit)

Wired similar to the IDC Class A or Class B loop, the NAC carries power to operate the "Notification Appliances":

v  Fire horns

v  Strobes

v  Bells

v  Speakers

In alarm, the voltage on the loop is forward biased; the electrical current provided by the panel goes through the notification appliances, letting people know about the alarm. They are notified.

During normal times, when there are no alarms, the voltage polarity is reversed, allowing the loop to be supervised as a Class A or Class B loop. Because the polarity is reversed, no current flows through the notification appliances.

When the fire panel goes into alarm, the panel stops supervising the loop and changes the voltage polarity to normal. Electrical current then goes through the devices, causing the devices to notify the occupants of the building. 

5.3 SLC (Signalling line circuit)

The Signalling Line Circuit (SLC) carries signals both to and from the panel; it uses data to carry the signals out from the panel to field devices, and data to carry the signals back from the field devices to the panel.

The SLC also provides a small amount of power to operate the field devices. 
5.3.1 Field Devices Connected to an SLC

1.     Input

v  Smoke Detectors

v  Heat Detectors

v  Combination Smoke/Heat Detectors

v  Pull Stations

v  Alarm and Supervisory Input Modules

2.     Output

v  Control Relays

v  NAC Riser Modules

Ø  Audible

Ø  Visible

Ø  Combination Audible/Visible

Ø  Audio for Speakers

To supervise the SLC and to make sure all devices are always connected, the panel sends data to each input or output device checking weather all the devices are connected or not.

With the constant checking of the field devices, the need to supervise the wiring itself isn't necessary; the "Class B" SLC may be t-tapped. 

As well as the polling of the devices, Class A SLC loops are not t-tapped and are still supervised for wiring issues. 

5.3.2 Power or control circuit

The control circuit, usually from an on-board relay in the panel, turns on or off devices and systems.

The on-board relays can also be used as part of another panel's IDC loop, sending alarm, supervisory, and control signals to that other panel. This is particularly useful when the fire alarm panel itself doesn't have an on-board communicator, to provide for off-site monitoring.

The actual wiring may be a little different from the standard IDC, NAC, and SLC loops, but the control loops still have to be supervised for integrity. 
Some of the uses for the on-board relays:

v  Fire Door Holders

v  Door Unlock

v  Elevator Capture

v  Off-Site Monitoring

v  Smoke Control

Ø  Stair Pressurization Fans

Ø  Fan and Damper Shutdowns

Ø  Smoke Curtains

Ø  Smoke Evacuation

Ø  Makeup Air Fans and Doors

5.3.3 Supervisory Circuit

Fire alarm systems save lives and protect property. Fire alarm systems also break down because they're electrical, hence we need fail safe design and protection. Class A or Class B wiring loops help the fire alarm panel to find these breakdowns (faults) before a fire, while there is time for repairs.

Class B Loops

Normal Class B wiring - All devices are supervised and working.

In conventional Class B Loops, all devices are daisy-chained together. By watching a small electrical current passing through the wires, the panel supervises them, and to limit this supervising current, at the end of the daisy-chain is an end-of-line resistor. The panel constantly watches for this current.

Open Fault in the Class B wiring. Supervision tells the panel that the wiring does not go through, but also the devices further from the panel don't work.

If the supervising current stops flowing, the panel assumes a wire is broken (an open fault), and displays a trouble. When a wire breaks in Class B, the devices closest to the panel will still work, but because of the wire break, the devices further from the panel are cut off. 

Class A Loops

Normal Class A wiring - All devices are supervised and working.

Under normal conditions, Class A Loops are similar to Class B Loops, but with an important difference. 

Class A wiring takes error detection further than Class B. If a wire breaks, the panel uses a redundant wire path to maintain communication with devices beyond the break. Here even though a wire is broken, all devices work.

To keep more devices working, Class A uses a second path from the fire alarm panel; a redundant wire loop goes around the broken wire. A fire can still be detected, because, using this redundant path, most, if not all, devices on the loop remain connected to the panel. 

Basically, when the fire alarm panel detects an open wire in the Class A Loop, it automatically switches tousling two separate un-supervised Class B loops. The first one is the original Class A loop, and second one back-feeds on the separate pair of wires to make the second Class B loop. Most of the devices on the original Class A loop will be on either the first or the second Class B loop.

Note: For more information refer to NFPA 72 Fire Alarm Installation, selection and maintenance.