Douglas Krantz - Technical Writer - Describing How It Works

How do I Fix an Open Fault?

How do I Fix an Open Fault?

Greetings Douglas,

My fire alarm system is showing "Open Fault" for all the zones, but the entire system system seems to be fine. It's an eight zone conventional fire alarm panel.

Can you help me solve the problem?

Thank you, K R

Is this a newly installed fire alarm system? If so, then it sounds like the fire alarm system outside the control panel isn't finished.

The fire alarm system is supposed to consist of the wiring in the building, all of the input devices (smoke detectors, pull stations, waterflow switches, etc.) and all of the output devices (fire horns and strobes, etc.). Between all of the input zones and output zones is a control panel. There should be an end-of-line resistor at the end of each loop (the pair of wires connected to a zone and connected to all of the devices on the zone). The Fire Alarm Control Panel (FACP) isn't very smart. It can't tell how many devices are on the system, it can only tell if the wires are OK.

It checks the wiring by checking to see if electricity will pass through all of the wires. Basically, just like a person would check continuity using an ohmmeter, it runs a continuity check of the wires. If electricity doesn't pass through the wires, then the panel assumes that, blocking the electricity, there's an open connection or a broken wire somewhere. The end-of-line resistor is needed at the end of the line to allow the panel to check continuity. If there's not an end-of-line resistor, the wires are open, and the fire alarm panel will show that there's an open connection.

To get more of a description of how the building wiring should be installed, including the end-of-line resistor, read the article at

Then, to get a better understanding, get the free diagram on Class B Wiring by clicking on the arrow at the bottom of the webpage. That will show you how to wire a system using Class B Wiring.

If this system has been working normally for months or years (no troubles showing), and suddenly has started showing open faults on all zones, then possibly the problem is in the control panel. The control panel is not where I'd be looking first, though. The wiring is where I'd start looking for the problem.

Douglas Krantz
Life Safety
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