When troubleshooting an intermittent problem, assume that the problem is there, just not always detected as a problem by the fire alarm system. Also, the trouble on the system is a cluster of possibilities; as with any troubleshooting problem, there are many possibilities that could cause the same trouble indication.
Battery Trouble
Many fire alarm power supplies supervise the batteries to see how well the batteries hold a charge. This includes the Fire Alarm Control Panel, and all the Supplemental NAC Booster Power Supplies.
In essence, every 50 to 60 seconds, the power supply checks the status of the battery. To check the status, for about half of a second, the power supply quits charging the battery. During this short time, the power supply starts draining the battery.
The power supply then watches to see how far the voltage on the battery drops while being discharged for half of a second. If during this testing time, the voltage doesn't go down very far, the power supply determines that the battery is good. If during this testing time, the voltage drops too far, the power supply determines the battery is bad.
When the power supply has determined the battery didn't pass the voltage drop test, the power supply goes into trouble until the next test in 50 to 60 seconds. Then if the battery if the battery is replaced, the new battery usually passes the voltage drop test, and the power supply goes out of trouble.
Batteries, though, usually don't suddenly quit, batteries usually fade out over months. If the batteries are on the edge of failing the voltage drop test, the batteries will sometimes pass the test, sometimes fail the test.
That gives you 50 to 60 seconds of trouble, or sometimes multiples of 50 to 60 seconds of trouble.
Keep in mind that the power supply is a terrible tester of batteries. If the batteries fail the voltage drop test performed by the panel, the batteries are way past the bad point and should be replaced immediately.
Remember, always replace the batteries in pairs: they need to be matched.
Open Notification Appliance Circuit (NAC)
The panel supervises a building's NAC circuit by performing a continuity test of the wires. The current path for the continuity test is through one wire, all the way to the end of the circuit, through the end of line resistor, through the other wire, all the way back to the panel.
If the current is reduced, or completely stopped, the continuity test is a failure, and the panel goes into trouble. The panel stays in trouble until the current goes back to normal.
One possibility is that somewhere, a wire is broken and sometimes comes apart, or a screw terminal or wire nut is loose so the circuit stops the current, and then the connection passes current again. This would be an intermittent problem.
The other possibility is that a screw terminal or wire nut is holding the wires together, but not tight enough. When the wire isn't being held tight enough, the connection is not "gas-tight"; oxygen can get into the connection and the wire becomes tarnished.
The tarnish on the wire has a "non-linear resistance". As the voltage goes down, the resistance itself increases in the connection. This is a poor connection, and the poor connection sometimes gets worse, sometimes the connection gets better.
Looking at it from a different direction, sometimes the connection doesn't pass the panel's continuity test, sometimes the connection does pass the panel's continuity test.
When testing for this, use your voltmeter. Compare the voltage, on a working NAC circuit from the other NAC booster power supply, to each of the NAC circuits on the NAC booster power supply that is having a problem.
If the voltage is very much higher on a NAC circuit on the problem booster supply, that is the circuit that is having problems. Even if the voltage is close, but unstable, that NAC circuit is having problems.
Watch out for improperly installed end of line resistors. If the wires are different sizes, like shown in the diagram, the connection is not gas-tight, and could be the source of an intermittent problem.
One time, to fix an intermittent trouble on a NAC booster power supply, I had to go around the entire NAC circuit, and tighten every screw on every horn/strobe on the entire circuit. I never found the intermittent problem, but the intermittent problem went away.
Ground Fault
The power supply has a threshold for the detection of a ground fault. If the ground fault has a higher resistance (lower current) than the threshold, the panel determines there is no ground fault; if the ground fault has a lower resistance (higher current) than the threshold, the panel determines there is a ground fault.
When the intermittent trouble is a "hard ground fault", the intermittent trouble is sometimes shorting a copper conductor in the fire alarm system to the building's ground system, sometimes it's not shorting the copper conductor to ground.
That kind of intermittent dead short doesn't happen very often.
Water has a non-linear resistance. At lower voltages, water has a very high actual resistance; at higher voltages, water has a very low actual resistance. This non-linear resistance is affected by impurities in the water.
Then again, pinched insulation or worn insulation could be the cause of non-linear resistance.
Usually, intermittent ground faults are "soft ground faults". See:
How Does One Find a Soft Ground Fault?
With EST panels, the panel is testing for a ground fault at the same time it is testing the batteries: every 50 to 60 seconds. If there is an intermittent ground fault, the power supply will be in trouble for 50 to 60 seconds, or multiples of 50 to 60 seconds.
Word of Caution: Do not disconnect the ground fault detection circuitry in the booster power supply. The detection circuitry is there to warn you about a possible future shorting out of the fire alarm system.
If there's a ground fault, fix the ground fault, keep the warning trouble indication.
Power Fluctuation
There always a possibility that power fluctuations are causing intermittent troubles in the booster power supply. However, because the main fire alarm control panel and the other booster power supply aren't having the same troubles, power fluctuations probably aren't causing the intermittent troubles.
Douglas Krantz
