Crash Course Fire alarm – Detection devices

Introduction

 

Your boss puts you on an fire alarm design job, but your knowledge is limited to your smoke detector at home? Is it true that a pull station contains an inkjet to incriminate you if you pull it for no other reason than to shorten your Friday? These are the questions I will try to help you with in this series of posts on building fire alarm systems.

 

These posts are intended, but not limited to:

 

  • Electrical engineers from consulting engineering firms
  • Subcontractors at fire alarm companies
  • Junior engineers who have no idea what it is (trust me, i’ve been there once!)

 

The fire alarm scope is the responsibility of the electrical engineer. The fire alarm system is not intended to suppress the fire, but rather to detect the fire, facilitate the evacuation of personnel and alert the fire department.

 

I will introduce today the detection devices that are part of fire alarm system.


1- Standards in force

Code de Construction du Québec

 

For current articles of the Quebec Construction Code, please refer to:

 

  • Requirements for manual pull stations (ref 3.2.4.16)
  • Requirements for fire detectors (ref 3.2.4.10)
  • Requirements for smoke detectors (ref 3.2.4.11)

 

Please note that while Quebec has its own Construction Code, you should also learn from the american National Fire Protection Agency or NFPA as an additional reference.


2- Manual pull station

 

Pull station

 

A manual pull station is a device designed to trigger the fire alarm system when operated manually.

A manual pull station is a set of contacts where a switch has the following characteristics:

 

  • A normally open contact, which closes when actuated
  • When triggered, the contact will hold the alarm on until it is rearmed. The rearming is done by the building maintenance team or the firemen with a ordinary key, a screwdriver or an Allen key.

 

The alarm is raised throughout the building or in an area only. Generally, the manual station is close to the emergency exits. Some models are installed in a glass enclosure or with a glass strip. This rod is simply intended to deter those who are tempted to trigger false alarms.

About false alarms, there is no inkjet to spray and incriminate a person for triggering a false alarm! However, with the advent of the Internet of Things, it would be very possible to have a smart manual pull station.

For example, it would have its IP address and would be able to communicate with the fire alarm panel. Once the manual pull station is pulled, the fire alarm panel could identify which station was pulled. Better yet, it would be possible to have a model with digital camera to photograph who pulled the station!

 

Trustworthy Manufacturers: SimplexGrinnell


3- Heat detector

Heat detector

 

A heat detector is used where a rapid increase in temperature is anticipated, sometimes without smoke or when a smoke detector can not be used for environmental reasons.

Industrial processes causing vapors or fumes, sterilizers, laundries and kitchens are examples. A smoke detector could be triggered repeatedly! In order to tell them apart from smoke detectors, it is good practice to identify them as such with lettering.

 

There are two types of heat detectors

 

A) Fixed temperature

The most common type. If the temperature of the central disk reaches the nominal threshold of the detector, the thermostatic element is triggered. The sensor contacts close to transmit an alarm to the control panel.

The thermostatic element can not be reused: when it has triggered, the detector must be replaced. The reason is that the heat causes the melting of the alloy that triggers the alarm, much like a fuse. Separation of the disc from the main body indicates that the detector needs to be replaced. The most common fixed temperature point for electrically connected heat detectors is 58.4 ° C (136.4 ° F).

 

B) Rate of Rise (ROR):

This detector is triggered when the temperature at the sensor increases at a rate of 6.7 to 8.3C per minute, regardless of the starting temperature. It has two thermocouples: the first thermocouple monitors the heat transferred by convection or radiation while the other reacts to the ambient temperature. The detector responds when the temperature of the first sensing element increases relative to the other. The sensor contacts close to transmit an alarm to the control panel. This type of detector is able to rearm itself.


4- Room smoke detector

 

Ionization type smoke detector

 

A room smoke detector is used to detect the presence of visible or invisible smoke produced by combustion and thus automatically raise an alarm. Thus there is the ionization type for invisible smoke and the electric photo type for visible smoke

 

A) Ionization type (invisible smoke)

There is very low radioactivity inside the detector that ionizes the air. The negative ions of the air molecules are attracted to the positive plate, circulating a current inside the chamber.

Since the smoke particles being larger than air molecules, when it enters the chamber, they provide greater resistance to current flow and trigger the alarm.

The reason this type of smoke detector is used is to spot fires known to produce little smoke. For example, the burning of ethanol. At home and on small-scale use, combustion makes it a smokeless fireplace or fire pit. However, in an industrial production of ethanol, a fire could occur in the tanks, be undetectable by the absence of smoke, then quickly become uncontrollable.

 

B) Photoelectric type (visible smoke)

The most common. The photocell detector has a dark chamber constructed so that smoke can penetrate without external light entering.

Inside the darkroom, there is a light source and a receiver element built in such a way that the light beam does not strike the photocell directly.

The visible smoke particles entering the darkroom reflect the light on the photocell, causing a change in conductivity and triggering the alarm.

This type of smoke detector is the most common because it can detect both paper fires and grease fires.

 

For more information between the two types, here is a link I found with the National Fire Protection Agency or NFPA.

https://www.nfpa.org/Public-Education/By-topic/Smoke-alarms/Ionization-vs-photoelectric


5- Smoke alarms

Smoke alarm

 

Mainly residential, it contains a fire detection circuit that controls a piezo electrical buzzer for local sound signaling. It is not connected to a fire alarm panel and can be powered by battery or 120V AC.

The difference between the smoke alarm shown here and the smoke detector above is that the alarm detects and produces an audible signal when there is smoke. On the other hand, the smoke detector sends an electric signal to an alarm device, for example a bell and it is thas bell that produces an audible signal.


6- Linear smoke detector

Linear smoke detector

 

The reflected linear beam detector consists of a transmitter and a receiver. The transmitter emits an invisible infrared light beam that is reflected by a prism installed directly in alignment, with an unobstructed line of sight. The receiver detects and analyzes the infrared light reflected by the prism.

The presence of smoke in the light beam reduces the intensity of the infrared ray received proportionally to the density of the smoke. The detector analyzes the weakening of the beam and triggers the alarm.

These detectors are the ideal solution for protecting large open areas such as atriums, gymnasiums, warehouses, concert halls, sports centers, amphitheatres, etc.

 

Trustworthy manufacturers: Siemens, Honeywell, Bosch


7- Ventilation duct smoke detector

Ventilation duct smoke detector

 

The ventilation duct smoke detector continually samples and analyzes the air circulating in a ventilation duct and triggers an alarm as soon as it detects the presence of smoke.

The main purpose is to prevent smoke from spreading to the entire building. It consists of a sampling tube, an exhaust tube and a photoelectric smoke detector.

Another reason is to avoid contaminating the ducts with dust or combustion particles.

When installing, it is of course necessary to ensure that the holes in the sampling tube are aligned with the direction of the air flow. In addition, avoid installation after an elbow, because after a bend, the smoke tends to accumulate against the wall of the duct, thus defeating the sampling purpose.

 

Trustworthy manufacturers: Honeywell, Bosch


8- Flame detector

Flame detector

 

Flame detectors react directly to the presence of flame. They detect the ultraviolet rays emitted as soon as sparks appear that can ignite any combustible material nearby. Although capable of detecting fires and explosions in 3-4 milliseconds, a 2-3 second delay is often included to minimize false alarms that can be triggered by other UV sources such as lightning, arc welding, radiation and sunlight.

Other models work on infrared rays, on the same principle as thermography cameras. It becomes possible to save these images and send them back to the security station. These videos are very useful for determining the origin of the fire and claiming insurance.

These are visual range devices, so they must be installed at the location providing the most direct visual line with the anticipated fire source.

These detectors are designed to protect dangerous areas where a fire could develop quickly and where ignition is almost instantaneous (eg flammable liquids, natural gas, propane, petrochemicals, etc.)

 

Trustworthy: Honeywell, Emerson, Omicron


Conclusion

 

The fire alarm is a vast subject for which the electrical engineer is responsible. You learned today which articles were in force concerning the detection devices where to look for them in the Quebec Construction Code. You have also learned about manual stations, smoke detectors for rooms and ventilation ducts, heat detectors and flame detectors.

The next article will focus on fire alarm signaling devices.

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