Key Takeaways
- Carbon Tracking: Electrical arcing from contactors creates carbon buildup that can cause shorts and blown fuses
- Environmental Factors: Moisture and dust accelerate carbon tracking, especially in outdoor units
- Diagnosis Methods: Visual inspection for black carbon deposits and megohmmeter testing (under 20 megohms indicates a problem)
- Prevention: Regular visual inspections, contactor replacement, and electrical cabinet cleaning prevent costly service calls
Carbon Tracking Explained: A Hard-Learned HVAC Lesson
I was a second or third-year apprentice working for a very knowledgeable boss and business owner. At the time, he just happened to be the dispatcher, which was bittersweet.
The bitter part? Well, he was one tough instructor who didn’t hesitate to correct mistakes firmly. The sweet part? After the harsh lessons, he was the best tech support I’ve ever had.
The Service Call That Changed Everything
I arrived at a call at a sewage treatment plant. From what I remember, it was a ten-ton Carrier rooftop unit with no cooling. The fuses were blown, and after I replaced them and turned on the power, they instantly blew again.
I called my boss and let him know what was happening. He asked me one question: “Are the contactors all black and covered in carbon?”
Yes, yes they were. How did he know that?
He explained that what I was looking at was carbon tracking. From then on, I never forgot this phenomenon and declared war on it.
Let me finish the rest of the service call before explaining the technical details. I went out and picked up two new contactors, replaced them, cleaned up all wiring, and cleaned the electrical cabinet of remaining carbon. Replaced the fuses once again, and all was good in the world.
So how did my boss know? Experience from being in the same position, that’s how. A position I have now been in many times myself—which I am now sharing with you.
What is Carbon Tracking and How Does It Happen?
When a contactor is de-energized and pulls away, an electrical arc is created. A byproduct from this hot electrical arc is carbon. Carbon collects and builds on the contactor over time.
If enough carbon is allowed to build, it can create a conductive path across power legs or to the panel that the contactor is mounted to (which is grounded). This path is considered to be a short circuit. What happens when power is shorted to ground or across power legs?
The fuses blow instantly—that’s what happens.
Why Outdoor Units Are More Vulnerable
Something important to keep in mind: I have never encountered blown fuses from carbon tracking in a unit installed indoors. Why? It doesn’t rain inside.
Outdoor units are exposed to the elements. On a damp day, when the moisture content is high in the air, that moisture embeds itself within the carbon track, creating a conductive path. This is similar to how proper maintenance scheduling can prevent weather-related failures before they happen.
Dust can also contribute to this issue. For instance, if a unit’s electrical panel is under negative pressure due to the blower fan, dust can be pulled into the cabinet, creating the same result.
Diagnosing Carbon Tracking: A Step-by-Step Approach
Here’s what I learned to do when encountering a blown main fuse:
- Shut off the local disconnect immediately
- Visually inspect the line side of all contactors within the machine
- Check for visible signs of carbon tracks (black deposits)
- Inspect the top terminals on each contactor for signs of melting
The heat generated by the short will sometimes slightly burn up the line side of the contactor. See the images below for examples.
Using a Megohmmeter for Advanced Diagnosis
I also use a megohmmeter to check for carbon tracking if a visual check doesn’t reveal anything. This is one of those essential HVAC diagnostic tools every technician should master.
Here’s the procedure:
- Disconnect all wiring from the contactor
- Place one lead of the megger on the contactor line side terminal
- Place the other lead to ground
- A reading of anything under 20 megohms should be considered a path to ground
In this image you can see the melted terminal that was caused by dust build-up over time that created a path to ground:
This image is a close-up of the above photo showing the back side of the contactor:
Prevention: Your Best Defense Against Carbon Tracking
To avoid carbon tracking, visual inspections are key. Just like how BMS control systems can alert you to electrical anomalies before they become critical, regular manual inspections catch carbon buildup early.
Replacing contactors and cleaning up wiring and cabinet base plates can go a long way in avoiding service calls and keeping you busy with productive maintenance rather than emergency repairs.
Maintenance Best Practices
- Schedule regular electrical cabinet inspections during routine maintenance
- Clean carbon deposits before they create conductive paths
- Replace contactors showing excessive carbon buildup
- Ensure proper cabinet sealing to minimize dust infiltration
- Document carbon tracking issues to identify problem units
Building strong diagnostic skills takes time and experience. Property.com’s ‘Know Before You Go‘ tool helps you arrive at service calls better prepared, with complete permit history and home insights that let you focus on mastering technical challenges like carbon tracking. Stand out as the technician who not only fixes problems but prevents them. Learn how Property.com helps HVAC pros deliver premium service at every call.
Finally
The HVAC Know It All Podcast is the perfect way to stay up to date on the latest industry news and information. Hear from experienced professionals to help keep you sharp and give you an edge over the competition regarding knowledge and understanding of the trade.
For more troubleshooting tips on blown fuses and contactors, check out our comprehensive video guides. Remember, every experienced technician has their carbon tracking war story—now you’re prepared to handle yours.
