Douglas Krantz - Technical Writer - Describing How It Works

What is the Best Way to Troubleshoot a Lightning Struck SLC?

By Douglas Krantz | Maintenance

What is the Best Way to Troubleshoot a Lightning Struck SLC?

What is the Best Way to Troubleshoot a Lightning Struck SLC?

Greetings Douglas,

The building is a senior living apartment that was hit by lightning about a month ago. Another technician and I were sent to troubleshoot and evaluate the damage. We've determined one power supply was latched in alarm, and there was an SLC [Signaling Line Circuit] short present, as well as 60 devices "missing".

While investigating, I discovered seven taps to the SLC. Five of the taps truly were shorted, while two of them were not shorted. I'm not sure if lighting can displace plenum jacket and "smelt the conductors together", but I did understand when I took the smoke above the panel, disconnected the SLC field wiring, and pigtailed the smoke detector directly to the SLC terminals the same smoke detector, surged the panel up to 1000 troubles.

I've spoke with my service managers; we decided the most economical choice for the customer was to replace every single initiating device, as opposed to coming back to troubleshoot every SLC, tap and device one for one. If the motherboard truly is fried, then at least they still make the control panels and we can replace this. My only regret was not having called Tech Support before calling my Service managers.

My question is, do you think this was the right way to troubleshoot a lightning-struck SLC?

Thank You, TB

When trying to troubleshoot, keep in mind that all troubles are caused by shorts (or partial shorts) or opens (or partial opens). With the lightning hit, it sounds like there are many shorts and many opens.

Both the shorts and the opens can be in the cable of the SLC (Signaling Line Circuit) itself, but because the addressable devices have delicate electronics in them, lightning damages the devices. The devices may look OK, but inside the electronics, the devices are often damaged by lightning.

Also keep in mind that the trouble lights and trouble display on the fire alarm control panel are great to determine that there is a trouble, but won't be any real help determining where the trouble is located. Until there's voltage on the entire SLC building-wide circuit, use your voltmeter on the wires of the circuit.

When troubleshooting, use a voltmeter. An ohmmeter won't work with most troubles on the SLC. OK, an ohmmeter can tell you if there's a metal-to-metal short, but most other shorts on the SLC can't be detected by an ohmmeter because most shorts on the SLC, when there's been a lightning strike, are non-linear shorts. And the ohmmeter is totally useless when trying to see if an addressable device is shorted; the electronics inside the device is by far too complicated to be tested with an ohmmeter.

The SLC circuit carries signals between the panel and the addressable devices connected to the SLC, but one of the main purposes of the panel's SLC output is to supply power to the addressable portion of the devices. When troubleshooting the SLC, this power supply voltage is what you are looking for.

It's important to note that the SLC power supply in the control panel is very, very weak. It can be shorted out with a wet finger.

Troubleshooting a Lightning Struck, Building-Wide Fire Alarm System

Start by dividing the system. Separate the fire alarm panel from the rest of the building's SLC circuit. Sometimes, loose terminal screws on the panel prevent accurate readings on the voltmeter, so, for now, tighten the panel's SLC output screws. Because both ends of a Class A circuit connects to the panel, if the SLC is a Class A circuit, disconnect the Class A return for the SLC.

Also, some panels have a built-in delay between the time the short is removed from the SLC and the time the SLC power supply turns on its power. When there's zero voltage on the SLC while testing for voltage on the SLC, wait :30 seconds to a minute before assuming that the SLC is shorted out.

Check the Voltage on the SLC Output of the Panel

The first test is whether the control panel will even power the circuit. Disconnect the Class B wires, and the Class A wires (if they're there). Because sometimes loose screws give false results, tighten the SLC Class B and Class A screws on the panel.

Measure to see if there's voltage. If there's voltage, like 10 volts to 28 volts, the SLC power supply probably is working.

A properly working, two-wire SLC circuit will have extremely unstable voltage. Don't worry, it's supposed to be that way. The unsteady voltage means that data is being transferred between the panel and the devices.

If, after a minute, the voltage appears on then SLC, then at least the panel is providing power on the SLC.

From your point of view, though, getting the voltage from the panel to the devices is the important part.

Reconnect the SLC circuit to the panel, even if it shorts out the SLC. Then, starting near the panel, measure the voltage on the SLC circuit; look for voltage.

Remember, above all else, most of the lightning damage on the SLC is in the electronics of the devices. Not very often will lightning damage the wires of the SLC, only often enough to keep you on your toes.

Most of the rest of the troubleshooting will be powering-up the circuit: one section at a time, sometimes a single device at a time.

At any device location, if there's no voltage, split the circuit. Assume anything can be bad: the input wires, the device, the output wires. With lightning damage, just keep plodding along. Eventually, you'll get voltage to every device, all along the circuit.

Don't Mix Up the Devices

Each device is addressed so the panel know the location of each device. For the firefighters when the come to put out the fire, make sure each device stays in place so the panel can display the correct location.

Full Test of the Fire Alarm System

Once you've found and fixed the entire lightning damaged system, perform a full test, just like you would when first installing a fire alarm system. You want to know everything is working again, test everything because lightning can destroy so much.

Douglas Krantz
Life Safety
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