Excellent PLC
Author: Daniel H. – Field Service Engineer, rotating machinery nerd, caffeine-powered human.
I spent the better part of this week in a turbine hall babysitting a Bently Nevada 120M8155-01 panel that kept rebooting itself for no obvious reason. If you’ve been around vibration protection systems long enough, you know this kind of thing always happens at the worst possible moment—usually right when the operators need to acknowledge alarms and pretend they’re still in control of the situation.
Anyway, here’s the story, in case it saves someone else some hair.
The Symptom (a.k.a. the Panic Button Moment)
The touchscreen panel would run fine for anywhere between 3 minutes and an hour, then poof—black screen, reboot logo, and back to normal like nothing happened. No freezing, no lag, just clean restarts like it had somewhere better to be.
The 3500 rack downstream didn’t care at all. Machine protection kept doing its job. It was just us humans who were blind for a few moments.
My First Bad Guess
My first thought was overheating. Because turbine halls are basically saunas with expensive metal.
But… the cabinet was actually pretty cool. Internal temperature never got past 39°C, which is practically air-conditioned luxury for this environment.
Strike one.
The Actual Root Cause (Turns Out It Was the Power Source)
Long story short: the panel wasn’t rebooting because it was hot, but because it was being fed garbage power.
The 120M8155-01 was sharing a 24VDC line with a couple of contactors and a solenoid valve. Every time those coils fired, there was a nice little voltage dip. Most equipment shrugs this off. The HMI? Not so much.
Here’s what we measured on the line:
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Nominal: 24.1V DC
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During coil events: 19–21V DC (for 100–300ms)
The reboot behavior lined up perfectly with those dips.
What We Did to Fix It
Was it elegant? Not really. Did it work? Absolutely.
We added:
✔ A dedicated DC-DC stabilizer for the HMI
✔ A handful of decoupling caps upstream
✔ Ferrites to clean up the worst of the transients
After that, the voltage dips barely punched through to 23.3V and the panel hasn’t rebooted since. I monitored it for two days and got zero spontaneous resets.
A Few Lessons for Anyone Who Has to Deal With These Panels
I’ll summarize what this experience reminded me:
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These displays hate bad power more than they hate heat.
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Don’t trust “24VDC” labels until you measure under load.
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Solenoid coils and electronics are not friends.
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Operators get very cranky when they can’t see vibration data.
Would I Replace the Panel?
No. The thing was fine. It just needed clean power like any civilized device.
If you’re having reboots, check your power before you start throwing parts at the problem.
Okay, that’s enough turbinehall therapy for today.
If you’re fighting with one of these panels and want to swap horror stories, I’m always up for it.
— Daniel