Control Voltage Overamp – The Less Usual Cause


In many cases when a low-voltage or control circuit blows a fuse, it's because one of the circuits is shorted to ground or common. That may happen because of rubbed-out wires, shorted components or boards, etc. Routing wires sensibly and using proper grommets to avoid rubouts and shorts will help prevent those problems.

Less commonly, you will see the low-voltage circuit draw high amperage. That may happen because a magnetic solenoid is energized but the mechanical pin, stem, or armature is stuck.

A common example is a stuck-open contactor. That condition results in high amperage because the solenoid is energized without the magnetic resistance (reactance) provided by the induced magnetic field.

Another example is a reversing valve solenoid that is improperly mounted on the reversing valve stem. As you can see below, the reversing valve solenoid responds to the low-voltage signal from the thermostat and acts as a smaller reversing valve; that's what ultimately causes the reversing valve to shift. You can see the same effect in any magnetic switchgear, such as relays, pump-down solenoids, etc.

What's happening?

Problems like the ones I just mentioned occur because the magnetic field in the coil isn't reacting with the load; there isn't enough inductive resistance (“inductive reactance“). It's essentially the same thing as locked rotor amps on a motor. If you keep a motor from spinning, the electrical resistance in the windings remains too low. So, the windings overheat and go out on thermal overload.

When that happens in low voltage circuits, it often won't blow a fuse or trip right away. A good way to catch it is to put an amp clamp on the low-voltage wires feeding different components. Keep doing that until you find the one pulling very high amperage compared to the other low-voltage components.

So, check for short circuits first; make sure that's under control when you have overamp conditions in the low-voltage circuit. But you'll still want to keep your eyes open for stuck or improperly mounted solenoids.

—Bryan

Related Tech Tips

Understand Dew Point and Absolute Moisture, The Right Side of the Psych Chart
Let's first state the obvious. Most techs are intimidated by psychrometric charts and Mollier diagrams. We JUST ARE. While there are some pretty complicated formulas that back up these diagrams, using them isn't that big of a deal once you understand the different elements and then focus on one at a time. BUT WHY DO YOU […]
Read more
Flame Sensing - The Basics
Proving flame is an important part of the gas firing sequence. Without proof of flame, you risk dumping unspent gas into the heat exchanger, resulting in an explosion. There are many ways to “prove flame,” but we are focusing on the flame sensing rod (flame rectification) method here. Here are the facts: Flame sensing rods, […]
Read more
Mean Radiant Temperature: What It Is and Why We Should Care
Our industry puts a lot of emphasis on the “AC” (air conditioning) part of HVAC. We specialize in maintaining and servicing a wide variety of simple to complex air distribution systems. But we’re really in the business of comfort—human comfort and health, to be precise. So, just how much does the human body rely on […]
Read more

One response to “Control Voltage Overamp – The Less Usual Cause”

  1. So what ur saying is the check amp draw of those components? Ive never been infront of a low voltage short before.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from