Important Circuitry

Notification appliance circuits

Voltage standards

Since a fire alarm is an electrical appliance, it of course needs power, and there are several things to note here:

• In the early days of Fire Alarm: High Voltage AC was used to power Notification Appliances, typically bells.
• When there was a shift to horns, lower voltage levels came to be the standard. 24VAC was the standard for a little while.
• Eventually, more modern systems came out to be powered by DC. Some were 12VDC, and others were 24VDC. Certain horns needed to be put on accordingly.
• Currently: Standard voltage is 24VDC for commercial fire, and 12VDC for burglary panels that allow for fire detection.
• Though we are seeing it go away in some areas, it's very common for high voltage bells to be in place for water-flow applications.

Circuit supervision

Supervision of a Class B NAC is relatively simple: there is a resistor in the circuit known as the end-of-line resistor. The device outputting the NAC is always looking for the end-of-line resistor by outputting a current that is blocked by the notification appliances; for DC circuits, this is accomplished by reversing the polarity (blocked by diodes), and for AC circuits, this is accomplished by supplying a DC current (blocked by capacitors). If there is a break in the circuit, this test current cannot flow, and that will put the control panel into trouble on the circuit. In the event of a short circuit, the panel will see this as an increase in current, which will also be reported as a trouble. Since each panel and brand requires different resistor values, a user should refer to the manual for their device to determine what resistor value is needed.

Class A NACs are supervised similarly, but the end-of-line resistor is typically located inside the device supplying the NAC. Under normal conditions, the test current will flow from the out terminals toward the resistor, and during an alarm condition, both the out and return terminals will be energized.

Initiating device circuits

This again ties into our current supervision mentioned above. A pull station, for example, will cause a short circuit that the panel or intelligent module will see. This causes the panel to go into alarm. But what about a smoke detector? Conventional smoke detectors work a bit differently, but share similar concepts. Upon detecting smoke, a detector will cause an increase of current, but still have enough voltage to operate and latch in its alarm state. Usually, when a conventional detector is in alarm, it drops the entire circuit to about 6VDC, but this will vary.

In whatever situation, the panel sees the increase of current and uses that to determine if it should be in alarm.

Common voltage readings

So with the basics aside, it's important to note what kind of voltage readings we see in different areas of a modern Fire Alarm System. Below is a breakdown of the typical voltage readings for different applications:

• Auxiliary Power: 24VDC
• NAC (Not Active): Generally Less than 1VDC, but some older panels do full reverse polarity
• NAC (Active): 24VDC
• Initiating: 24VDC
• Active 2-Wire Smoke: ~6VDC
• Addressable SLC: ~16VDC

Keep in mind, this is very generalized, and may not be accurate in all situations. Check panel specs for more accurate readings. These readings don't apply to 12V panels, including most security panels.