Douglas Krantz - Technical Writer - Describing How It Works

Do You Use Normally Open Contacts for Alarm Circuits?

Do You Use Normally Open Contacts for Alarm Circuits?

Greetings Douglas,

This may be a silly one. but.. I have been provided a fire system. I'm just a PM, but I have done a ton of Automation and Electrical work so I was wondering...

System: Honeywell Notifier panel RB-2002C I have 2 detectors that I want to tell me if there is a fire A Flame Detector - Emerson 975-MR - so a quite versatile device with both NO and NC contacts A Heat Detector - Fenwall Controls Detect-o-Fire - a very simple device that I think is NO and only closes I believe when a fire activates it.

My Question is:

In automation I would tend to use a safety mentality that all devices are NO [normally open contacts] and must be energized to close during normal operation and go OPEN in the presence of a fire or a fault. Thus, any broken wires would look like a fire. Is this a good approach in a Fire system?

Given that the Heat Detector is a mechanical device and is NO does this work for fire systems? It would seem that you are at risk.

Thank you, KA

Fire alarm systems are kinda weird. All fire alarm systems, including the Notifier brand, use a "special fire alarm method" of detecting that the wires are broken, or a wire has come loose from a connector.

Inputs to Automation Systems versus Inputs to Fire Alarm Systems

The Inputs to Automation Systems have two conditions: Normal and Alarm

Normal: When the automation system input circuit is normal, that is the automation system is operating as it should, there is electrical current passing through all of the wires.

Alarm: When the automaton system input circuit is in alarm, the circuit opens up. It will open up when the device opens its switch (actual alarm), and it will open up when a forklift accidently breaks a wire (false alarm).

Bottom Line: No matter what the cause, whenever the wires open up, the automation system circuit is in alarm. Except in a few life safety circumstances, an alarm (or false alarm) stops production, and not much more.

The Inputs to Fire Alarm Systems have three conditions: Normal, Alarm, and Trouble

Normal: When a fire alarm input circuit is normal, that is no fire is detected, there is a small electrical current passing through all the wires. The small electrical current is not passing through any of the fire detection devices like switches and relays.

Used to complete the circuit so the small electrical current can pass through all the wires is an end of line resistor. The control panel (like the Notifier panel) senses this small current and considers the circuit is normal.

This is how the panel supervises the wires. It uses the small electrical current to check continuity of the wires.

Alarm: All fire alarm devices, including the flame detector and the heat detector, are across the two wires of the circuit. When there is no fire, all fire alarm input devices do not conduct electricity; their contacts are open.

When there is a fire, fire alarm devices close the circuit; when there is a fire, the devices short out the wires of the circuit.

That is how a fire alarm input device sends an alarm.

Trouble: When a wire breaks, or comes loose from a connector, the circuit has opened up. But just because a wire came loose from a connector in a fire alarm system, it is not a good idea to sound a fire alarm (in this case it would be a false alarm).

A false alarm will evacuate a building, and call the fire department. After a few false alarms, people ignore all fire alarms, and the fire department gets really annoyed. False alarms are not good.

Instead of sounding the fire alarms because a wire came loose or broke, the panel turns on its trouble light and buzzer. In essence, the fire alarm panel is saying "Fix It". All fire alarm panels, including the Notifier panel, have that feature.

Class B Wiring System

In the installation manuals for the Notifier panel and Flame Detector is the exact wiring diagrams that need to be used. I follow these diagrams, you should to.

Flame Detector and Notifier Panel Connections

The flame detector has trouble contacts that are meant to send a trouble indication to the fire panel. Also, the Notifier fire alarm control panel looks like it is meant to be used with some sort of releasing agent.

Whereas it's a simple doorbell wiring for the system (flame detector, heat detector, fire panel, and releasing agent), the consequences of not following the exact wiring, along with the proper testing procedures, can be very costly. The costs are both in terms of money and in terms of injury and life safety.

Douglas Krantz
Life Safety
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