Electrical Troubleshooting Quiz — April 7, 2026

In one of its three buildings, the plant has a custom-built machine that dates back through several sales and acquisitions of the plant. The machine builder went out of business in 2022, and even though there’s a record of receiving the complete set of documentation for the machine, nobody has any idea where the documentation is. The plant got a new plant manager and a new plant engineer with each acquisition, and it appears that the drawings got packed up along with the personal belongings of one of those plant engineers. Nobody knew about this problem until the machine suddenly refused to start one morning.

The machine is a multi-stage dryer. Extruded parts enter via a conveyor. They are briefly fan-cooled, then pass to an enameling chamber where they are spray-painted. They are dried under heat and filtered air for eighteen minutes, then they are flipped over and painted on the other side. They are again dried for eighteen minutes, before exiting.

Because control lights and power lights are on, it’s known that there is power to the system and to the PLC that controls it. The start switch has been tested and it operates. There’s no visible damage, and there were no previous reports of problems. You’ve been brought over from one of the other buildings to figure out what is going on and how to get this thing running again. Where would you start?

Answer to Quiz

With extruded plastic and two paint sprays, this machine has a lot of VOCs. Remember, the V stands for volatile, and that means a fire hazard. The accelerated drying aggravates the fire risk, which means this machine has a fire suppression system. If it had not come with one, the plant’s insurer would have forced the plant to install one by now. Remember all those sales and acquisitions? Each time one of those happened, there should have been some question about this machine and VOCs.

The typical suppression system for an application like this uses CO2. That system would likely have one or more permissives, for example manifold pressure. If there’s not enough pressure, the machine can’t start.

But with no drawings, how can you figure this out? Even if the PLC is old, you should be able to pull up its programming on a laptop or workstation so you can view the control logic. You’ll find the permissive(s) and interlock(s).

Also, check the individual CO2 tanks. When were they last inspected, and if a pressure gage is present, what does it say? If the inspection tags are more than a few months old and/or there is no pressure gage, contact the CO2 provider for a complete bottle changeout. Ensure your CMMS is set up to schedule the required bottle maintenance (e.g., pressure checks or refills) at the recommended intervals. If your provider can handle this for a fee, it’s probably worth paying the fee because they will do more than just read a few gages.

The exhaust system should also provide permissives and interlocks to the PLC. These might include damper position after X seconds and fan running (perhaps verified with a pitot tube sensor that is now clogged with varnish).

So go through the PLC logic and examine every permissive. Then examine every interlock. You should be able to identify the problem and fix it.

Once the machine is able to start, make a copy of the PLC program and keep it on a removable disk or in the plant’s cloud-based backup. Then go about recreating all of the missing documentation, now that you have become familiar with the system. You want to do that because other problems will invariably occur. The documentation needs to include:

• Short description of the major system components.
• Short description of the theory of operation.
• Complete set of PLC logic diagrams.
• List of permissives, along with short description.
• List of interlocks, along with short description
• Information on the fire suppression system, including the contact information from your provider. Ask them also if they have any kind of write-up that you can include with your documentation. If they want to charge you for a custom write-up, convince the plant manager to pay for that because it will be in the correct lingo and probably include fairly important things you would not have thought of.

If this machine is a big revenue maker, it would be good to discuss with a custom machine builder the possibility of their conducting an engineering evaluation. Given its age, some things may be due for replacement while others can be upgraded for greater reliability and improved quality. No, this won’t be cheap. But neither will three days of downtime while your plant engineer scrambles to figure out what’s wrong, how to fix it, and how to get any needed replacement parts expedited.