Working of Fire Alarm Wiring

Working of Fire Alarm Wiring

Fire alarm systems save lives and protect property. Fire alarm systems also break down because they're electrical.

During fire, if a wire breaks, Class A Wiring provides an alternate route for signals to pass between field devices and the fire alarm panel.

A Signaling Line Circuit (SLC) carries information in the form of data to and from the field devices for the fire alarm system, and also carries power from the control panel to the devices.

Class A or Class B wiring loops/zone help the fire alarm panel to find and fix 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 / non-addressable Class B Loops / Zone Wiring, 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. 

The current, as it leaves the panel, goes out one of a pair of wires, goes through a current limiting resistor called an End-of-Line resistor (EOL), and returns on the other wire. This pair, along with the EOL, makes up a fire alarm loop. Devices are connected to this loop so their connections, also, receive the supervision 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 B sometime called Style 4 loop.

Class A Loops

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

Under normal conditions, Class A Loops  sometimes called Style 7 loop 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. So loop makes addressable type, now the word came SLC.

Basically, when the fire alarm panel detects an open wire in the Class A Loop, it automatically switches to using 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.

Separation on Class A Wiring Routes

The question is asked: In Class A wiring, how close to the feed wires are we allowed to get the return wires?

The answer is: It depends. During a fire, as the fire damages the wiring, how badly does one want the fire alarm system to continue to work?

1.      The feed and return wires can be in the same bundle or conduit if there's no concern.

2.      The feed will be in one part of the building and the return will be in the opposite part of the building if one is concerned about life safety.

True Class A wiring schemes make sure to protect the redundant return path by routing it through the building on a separate route.

The concern here is that whatever breaks a wire in the first part of the loop might break all the wires in the same bundle. An example: A forklift tears through all the wires in a bundle at once. If both feed and return wiring routes use the same wire bundle, and the whole bundle of wires is broken, and all the devices beyond the break will not communicate with the panel.

In that case, Class A wiring will not be any better than Class B.

The NFPA Code does allow for some exceptions, but mostly the code says the outgoing wiring path and the incoming wiring path should be separated by some distance.

Resetting Class A Troubles

Most fire alarm panels automatically restore trouble messages when the trouble is repaired. However, because the Class A Loop isn't supervised the same way as Class B Loops, the fire alarm panel can't detect corrections. 

With Class A faults, after correcting the open fault, resetting the panel will clear the trouble message.

Bottom Line for Class A

Class A Loop wiring uses both a primary wire path, and a redundant secondary wire path.

When a wire breaks, by using both paths, devices are still able to communicate with the fire panel. 

Smoke Detector vs Smoke Alarm

Smoke Detector vs Smoke Alarm

Google Trends is a service provided by Google to where you can compare the world's interest in your favorite topics. Google Trends also shows how frequently your topics have appeared in Google News stories, and in which geographic regions people have searched for them most.

Google Trends works by analyzing a portion of Google web searches to compute how many searches have been done for the terms you enter, relative to the total number of searches done on Google over time. Then Google displays the search results as a graph plotted on a linear scale.

So your intrepid blogger decided to see what was more popular a Smoke Detector or a Smoke Alarm....

The results are interesting....

As you can see from this graph, generally a the term Smoke Detector is a more popular search phrase, with both terms fluctuating roughly in line with each other. Since the start of 2008 however, the phrase Smoke Alarm is gaining.

Of particular interest is the regional trends in the search terms, with the term Smoke Detector being overwhelmingly more popular in the Unites States, Canada and India. Alternatively in Australia and the United Kingdom the search term "Smoke Alarm is more popular.

Whats the difference between a Smoke Detector and a Smoke Alarm?

Generally, a Smoke Detector is a sensing device that is used in a commercial application. Smoke Detectors don't normally have an inbuilt sounder and are interconnected together via a collective or addressable loop configuration, back to a main control panel.

A Smoke Alarm however is a sensing device, often found in a residential application and is fitted with an alarm sounder. Some Smoke Alarms can be interconnected however they are most likely found as a stand alone device.

The starting price of a smoke detector is approximately Rs. 1500.00/- (INR)

Both devices are manufactured to recognised Standards such as AS7240.7 for Smoke Detectors and AS3786-1993 for Smoke Alarms.

Duct Smoke Detector vs. Duct Detector

In Duct Smoke Detector vs. Duct Detector for FSD

"For fire smoke dampers, should I use an in-duct spot type smoke detector or a duct detector?"

Sampling tube style duct detectors are listed for a minimum air velocity of 100 fpm (feet per minute). If you are to ensure the rating of the wall at duct penetrations (dampers), you must provide coverage per The International Building Code IBC 2012 Edition Section 717.3.3.2.  

717.3.3.2 Smoke damper actuation. The smoke fire damper shall close upon actuation of a listed smoke detector or detectors installed in accordance with Section 907.3 and one of the following methods, as applicable:

1. Where a smoke damper is installed within a duct, a smoke detector shall be installed in the duct within 5 feet (1524 mm) of the damper with no air outlets or inlets between the detector and the damper. The detector shall be listed for the air velocity, temperature and humidity anticipated at the point where it is installed. Other than in mechanical smoke control systems, dampers shall be closed upon fan shutdown where local smoke detectors require a minimum velocity to operate.

2. Where a smoke damper is installed above smoke barrier doors in a smoke barrier, a spot-type detector listed for releasing service shall be installed on either side of the smoke barrier door opening.

3. Where a smoke damper is installed within an air transfer opening in a wall, a spot-type detector listed for releasing service shall be installed within 5 feet (1524 mm) horizontally of the damper.

4. Where a smoke damper is installed in a corridor wall or ceiling, the damper shall be permitted to be controlled by a smoke detection system installed in the corridor.

5. Where a total-coverage smoke detector system is provided within areas served by a heating, ventilation and air-conditioning (HVAC) system, smoke dampers shall be permitted to be controlled by the smoke detection system.

The 1st method above states that you must provide a smoke detector installed in the duct within 5’ of the fire smoke damper, and that the smoke detector shall be listed for the air velocity.  If you shut down the fan, there will not be a minimum air velocity of 100 fpm to achieve the proper listing of a sampling tube style duct detector (System Sensor D4120 for example).  Notice that this same paragraph states that “Other than in mechanical smoke control systems, dampers shall be closed upon fan shutdown where local smoke detectors require a minimum velocity to operate”.

What if we decide to use sample tube duct smoke detectors?

If we are using duct detectors for damper control, then whenever the fan is off, the dampers must be shut since the duct detectors such as the System Sensor D4120 require a minimum velocity of 100 fpm to operate.  Does this mean we then have to monitor fan status, and shut dampers upon fan shutoff, or shall the mechanical contractor have to inter-tie with all the dampers associated with that fan?  If we decide to monitor the fan, that creates another issue—per the IMC (International Mechanical Code), if a damper has automatically been activated to close, the associated fan must be shutdown.  Picture this scenario—AHU1 turns off, so we shut down the dampers.  The damper circuit we are controlling also serves dampers associated with AHU2, therefore we must shut that one down as well.  Since AHU2 is serving other areas of the building, we must shut those circuits of dampers as well, and so on until the whole building has typically been shut down for HAVC equipment including dampers.

What if we use spot type In-Duct Smoke Detectors?

On the other hand, if we are protecting the fire smoke dampers with an in-duct detector, you can have the fan shut down upon duct detector for the fan, and the dampers throughout the rest of the building may remain open and the other fans may remain on.  The damper is still protected with the in-duct smoke detector to ensure the rating of the wall to shut the damper upon detection of smoke, and if the in-duct smoke detector activates, then we go through the shut-down process of the fans and dampers throughout the building as exampled above.  Note that in-duct detectors have a listed minimum air velocity of 0 fpm, meaning that it does not “require a minimum velocity to operate“, therefore not requiring shutdown of the building upon a single fan being shut down.

Building owners really don’t like when their building heats up because they are servicing one of their fans, or one of the many fans in their building turns off due to a number of circumstances that are not necessarily indicative of a fire, and our fire alarm system shuts down all HVAC movement in the building.  Mechanical contractors don’t like it when they are forced to interface with individual dampers to close upon fan shutdown.  And finally, electrical/mechanical contractors really don’t like it when we force them to give us a true status of the fan via a Current or Differential Pressure switch when the fan is not a part of a smoke control system and it was not shown on their drawings or bid docs.

NFPA Codes for Maintaining Fire Alarm

NFPA Codes for Maintaining Fire Alarm Systems

Fire alarm system can employ the best design and have been installed by a top-notch installation crew. 

However, if the system is not maintained in accordance with Chapter 14, Inspection Testing and Maintenance asfound within NPFA 72, National Fire Alarm and Signaling Code, after it has been accepted by the authority having jurisdiction (AHJ) during the final acceptance test, it will decay over time.

Mr. Meghnad Is on the Fire Alarm System Owner

I own a library of photos that I have taken over the years featuring various fire alarm systems that were not maintained in accordance with NFPA 72.

I have seen everything from dead batteries, smashed manual pull stations, smoke detectors hanging by their connecting conductors, painted heat detectors and many more offenses.

In some of these cases, the harm done resulted in the alarm system not operating when there was an actual fire and was a contributor to extended property damage. It is the owner of the building who has the ultimate responsibility for maintaining the system.

Let’s take a look at how the industry’s various standards and codes literature addresses this matter.  NFPA 72 includes the following requirements in Chapter 14:

14.2.3 Responsibilities.

14.2.3.1  The property or building or system owner or the owner’s designated representative shall be responsible for inspection, testing, and maintenance of the system and for alterations or additions to this system.

Similar language is also found in NPFA 1, Fire Code. Within Chapter 13, Fire Protection Systems, the following is stated:

13.1.2 The property owner shall be responsible for the proper testing and maintenance of the equipment and systems.

The language offered within NFPA 101, Life Safety Code, requires that fire alarm systems be maintained in accordance with NFPA 72, but has no direct reference to the owner’s responsibility:

9.6.1.3  Fire alarm systems required by this Code shall be installed, tested, and maintained in accordance with the applicable requirements of NFPA 70, National Electrical Code, and NFPA 72, unless it is an approved existing installation, which shall be permitted to be continued in use.

9.6.1.4  To ensure operational integrity, the fire alarm system shall have an approved maintenance and testing program complying with the applicable requirements of NFPA 70 and NFPA 72.

The International Fire Code uses the following verbiage, which like NFPA 72 and NFPA 1, places the responsibility of inspection and testing upon the building owner:

907.8.5 Maintenance, inspection and testing. The building owner shall be responsible to maintain the fire and life safety systems in an operable condition at all times. Service personnel shall meet the qualification requirements of NFPA 72 for maintaining, inspecting and testing such systems. A written record shall be maintained and shall be made available to the fire code official.

If your business involves the installation of fire alarm systems, then your services portfolio should include the inspection and testing of these systems. The requirements for the same are found in both of the model fire codes that are used in the United States as well as NFPA 72.

A lot of building owners would prefer not to think about the system that was required to be installed within their building, until it does not operate during a fire. Installed systems also need to be maintained on a regular basis so as to prevent unwanted alarms.

Cable Tray Selection for Fire Cable

Cable Tray Selection for Fire Cable

In electrical Systems for laying power cables, cable trays are preferable mode for laying cables over a long distance where digging can’t be feasible or digging cost is very high. Cable trays are also used where space is low. Also cable trays gives aesthetic looks to electrical systems.

There are following recommendations for designing cable trays:-

1.     Always take 10-20% as Spare capacity.

2.     Distances between cables should be from 5 mm to 10 mm.

Now let’s illustrate the same by taking an example:-

Here we will design cable tray for aluminum cables:-

(i)     10 no’s 2.5 mmsq X 4 Core cables

(ii)    25 mmsq X 4 Core 10 no’s cables

Calculation:

For calculations following Table is used:-

Total Outer Diameter:

Diameter of 2.5 mmSq.mm Cable =No of Cable X Outer Diameter of Each Cable

                                                       =10 X 16 = 160 mm

Similarly for 25 mmsq cables = 25X10 =250 mm

Total Diameter of All Cables laying in Tray = (160+250) mm= 410 mm

Now Weight of Cables will be:

Weight of 2.5 Sq.mm Cables =No of Cable X Weight of Each Cable

  =10 X 0.56 = 5.6 Kg/Meter

Similarly for 25 mmsq cable = 10X1.11= 11.1 Kg/ Meter

Total Weight of All Cables laying in Tray = (5.6+11.1) Kg/Meter= 16.7 Kg/ Meter

Width of all Cables:

Total Width of all Cables = (Total Cables X min. Distance between Cable) + Cable Outer Diameter in total

Total Width of all Cables = (20 X 5) + 410 = 510 mm

Now after taking 10% spare Capacity of Cable Tray

Final Width of all Cables = 1.1X510 =561 mm

Now total area of cables

Total Area of Cable = width of all Cables X Maximum Height Cable

Total Area of Cable = 561 X 25 =14025 Sq.mm

Taking 10% Spare Capacity of Cable Tray

Final Area of all Cables = 1.1X14025 = 15427.5 mmsq.

 

Fire Cable Tray

Now cable tray selected from above calculations is that either we can install 1 no. 600 mm X 50 mm cable tray having weight carrying capacity of 50 kg/ meter, this cable tray has overall area of 30000 mmsq , Or we can install 300 mm X 50 mmsq 2 no. s cable trays with area = 30000 mmsq. 

We can even use single 300 mmsq X 50 mmsq cable tray and placing one cable over another. 

Usually perforated cable trays are used in electrical systems.

Smoke Detector Placing for Magnetic Door Holders

Smoke Detector Placeing for Magnetic Door Holders

Smoke detector placement is important when designing a fire life safety system for obvious reasons.  In this article, we are going to focus on the smoke detector placement requirements per NFPA 72 2010 edition for smoke doors with magnetic door holders.  If you are using smoke detectors for the purpose of door releasing service than the following code shall apply:

NFPA 72 2010 edition:     

17.7.5.6.5.1 (A)
If the depth of the wall section above the door is 24 inches (610 mm) or less, one ceiling mounted smoke detector shall be required on one side of the doorway only, or two wall mounted smoke detectors shall be required, one on each side of the doorway.

17.7.5.6.5.1 (B)
If the depth of the wall section above the door is greater than 24 inches (610 mm) on one side only, one ceiling mounted smoke detector shall be required on the higher side of the doorway only, or two wall mounted smoke detectors shall be required, one on each side of the doorway.

17.7.5.6.5.1 (C)
If the depth of the wall section above the door is greater than 24 inches (610 mm) on both sides, two ceiling mounted smoke detectors or two wall mounted smoke detectors shall be required.  One on each side of the doorway.

In the above picture, you can clearly see that the header or space above the doorway with magnetic door holders is less than 24 inches (610 mm).  However, the installing fire life safety company installed a smoke detector on the ceiling on both sides of the doorway.  This smoke detector placement is not required but it never hurts to have extra coverage.  Just thought I would point it out.

Conventional or Addressable fire alarm system

Which is better, conventional or addressable fire alarm system

A fire alarm system has a number of devices working together to detect and warn people through visual and audio appliances when smoke, fire, carbon monoxide or other emergencies are present.

Choosing a fire alarm system isn’t just about adhering to legal requirements; it’s also about saving time, money and effort.

A Fire Alarm Control Panel (FACP), or Fire Alarm Control Unit (FACU), is the controlling component of a Fire Alarm System.

Regardless of the type of system installed, all have to meet the National Fire Protection Association (NFPA) codes, local regulations, and the NBC Part 4 Life Safety Code enforced by the authority having jurisdiction (AHJ).  All commercial buildings are required to have a code compliant fire alarm system, the best way to ensure proper installation is to hire a licensed integrator with knowledge and experience.

The most common types of alarms that businesses use are conventional and addressable alarm systems.

Both types of alarm link devices (such as call points and smoke detectors) to a main control panel. The main difference between the two is that with addressable fire alarm systems, you can pinpoint exactly which device has been activated.

Conventional Fire Alarm

The conventional fire alarm system is based on analog electrical system. It is an old system usually installed in small offices or homes.

The main difference is the configuration or the connection of the devices with the main control panel. There is a separate circuit or wired connection for each device.

Since there are separate wires for each device, the cost for cables and the installation cost is relatively higher than addressable fire alarm. But the main control panel is quite simple and cheap which reduces the overall cost of the conventional fire alarm. Therefore they are used in business having smaller building sizes and lower budget.

Each circuit connected with the main panel is used for monitoring a specified area known as zones. For example, each floor of a multi-storied building can be divided into zones. Each floor has multiple sensors and detectors connected with the main control panel through a single circuit.

Since it cannot pinpoint the exact location of the fire in the building, it is difficult for fire fighter to find the exact location and extinguish the fire before it cause further damage. In case the device connected in a circuit stops working or if the wire of the circuit breaks, it is quite difficult to spot the fault. You need to check each device in the circuit to spot the faulty one.

Addressable Fire Alarm

As the name suggest, the addressable fire alarm system assigned each individual devices a unique address. It is a digital updated version of the conventional fire alarm where each detectors or sensors have unique identity. Therefore, each device has its own identity which makes it easier to identify which device has been triggered and spot the location of fire. The system quickly notifies the local responders to take the necessary action to stop further damage.

The wiring of addressable fire alarm is very simple. They are connected in a single loop thus saving the cost of wiring and installation. However, the equipment used is quite expensive and the initial cost of the system is quite high. Therefore, such systems are suitable for large buildings having many rooms and have high budget.

It is a digital system with a digitally programmable main control panel that allows to program the threshold of the detectors. This feature allows it to be used in any kind of environment and prevent any false alarm that could end up wasting first responder’s resources and affect the business.

Apart from that, the control panel can detect any faulty device and its exact location in order to replace or troubleshoot the said device. The smart programmable control panel can also be programmed to detect any potential fire hazard (such as gas leaks) and notify the personnel to remove it before the buzzer is triggered. Since the detectors are connected to the main panel through both ends of the wire, break at one end of the wire will not affect its operation as it can send the signal through the other end. This feature makes it more reliable than conventional fire alarm.

How do addressable and conventional alarms differ? 

Every device connected to the addressable system has its own unique address. When a fire is detected, the device’s address shows up on the main control panel, telling you exactly which device has been activated. This will enable you to find the exact location of a fire and extinguish them quickly.

With a conventional system, there is no way of pinpointing the exact location of the fire. However, by wiring your building into different zones, you can get a general idea of where the fire is. For instance, if you have two floors, you could wire the first as ‘zone 1’ and the second as zone 2. So if a fire occurs in zone 1, you know that the fire is somewhere on the first floor.

Wiring differences

Addressable alarm systems connect devices using a Loop. This is where one wire connects all devices to the control panel. Both ends of the wire loop connect to the control panel.

With a conventional alarm, each device will be connected to the control panel via its own wire, rather than a shared one. One end of the wire will be touching the device, and another touching the control panel. 

Which is the cheaper option for you? 

Conventional alarm panels cost a lot less to buy but are more expensive to install. This is because each device that is being connected needs its own wire. With addressable systems, one wire loop will connect several devices. This means conventional systems require more wire and more man hours during the installation phase.

Additionally, addressable systems have a range of other facilities that can help save money. For instance, addressable alarm panels monitor the air flow through smoke detectors to prevent the occurrence of false alarms, which can be costly to a business.

Conventional fire alarm India cost a lot less to buy but are more expensive to install. Because all the devices connected to the control panel needs its own wire. With addressable systems, several devices can be connected to the control panel with one wire loop. This means conventional systems require more wire and more man hours during the installation phase.

Following comparison table shows the key differences between addressable and conventional fire alarm systems.

Conventional Fire Alarm	Addressable Fire Alarm
The fire alarm system where the each detector is connected using separate parallel circuit to the control panel.	The fire alarm system where the detectors are connected using a single loop of wire to the main control panel.
The detectors inside a circuit cannot be distinguished from each other.	The detectors are assigned with unique binary address.
There are 1 or more than 1 circuits.	There is only one circuit.
It requires more wires to connect each device.	It require less wires as it uses only one circuit.
It cannot pin point the exact location of fire in a building.	It can pinpoint the exact location of fire.
The main control panel is not programmable.	The main control panel is programmable.
The alarm signal is sent using an analog electrical signal.	The alarm signal is sent in a digital binary form.
The detector’s threshold cannot be programmed.	The detector’s threshold can be programed according to the environment.
It is more affected by false alarms.	it nullifies any false alarms due to its programmable features.
There is no such warning feature for potential fire hazard.	It carefully monitors and based on smart decision send warning for potential fire hazard in a specific location.
There is no RTC (real time clock) chip.	The RTC chip offer a. event log for analysis.
The Conventional alarm system is less reliable.	The addressable system is far more reliable than conventional system.
The installation takes many labor hours and cost more than addressable fire alarm.	The installation is easier and can be quickly installed with minimum investment.
The overall equipment used in conventional fire alarm is very cheap.	The overall equipment used is very expensive.
It is used for small offices or building where such smart system is not necessary.	It is used for large buildings with business where they can’t afford to lose any time.

Overall, the conventional fire alarm is inexpensive and used in small building. however, the addressable fire alarm is very smart and sophisticated system which is very expensive but it is more reliable. Bottom line, conventional fire alarm should be  installed in building where it is easier to spot the fire (such as in building having fewer rooms) While the addressable fire alarm should be installed in large buildings.

Which is more reliable?
The addressable alarm panel is also the more reliable of the two. This is because the wire connects to the control panel at both ends (see the diagram above). If one end of the loop becomes severed, signals can still be sent to the control panel via the other end of the loop. Loop isolation modules are also used to separate devices on the loop. This means that if one device becomes disconnected, it won’t disable the circuit. With a conventional system, if a wire has become severed, the device will become disconnected.

Overall

The most obvious difference between the two, in terms of installation, is cabling requirements and labor costs.  Due to the fact that addressable systems require less cabling they make for less complex installations.  Functionally, the addressable fire alarm unit is superior, which can help prevent costly activities and save time when detecting a fire. It’s also cheaper and easier to install. But in terms of buying price, a conventional system is cheaper, and will meet the functional needs of small premises where a sophisticated system is not necessary.