Diagnosing a Tripped Circuit Breaker
Finding the cause can make these ‘easy’ service calls a bit tricky
Have you ever come across a system that has tripped its circuit breaker and thought, “Great, a nice, easy service call. I’ll just need to reset the breaker and be on my way.”
Well, it rarely turns out to be that simple. You always have to ask yourself, “What caused the circuit breaker to trip?” Rarely, if ever, is it just a random or nuisance trip. Something caused the circuit breaker to trip. So that easy service call likely becomes a little more complex. The cause of the problem can be a little difficult to determine, and that easy service call can become a little more complex.
Generally, a circuit breaker will trip due to either a circuit shorted to ground or an overloaded circuit. If the circuit breaker trips as soon as the breaker is reset or when the system is turned on, the likely cause is a grounded circuit. If the system runs for a period of time and then trips, the cause is likely an overloaded circuit. However, the opposite scenario can also apply. A grounded circuit can trip the breaker after the system has run for a period of time, and an overloaded circuit can cause the breaker to trip quickly.
A shorted circuit generally results from an electrical conductor, such as a wire or winding inside a motor, touching ground. This is generally easy to determine, but at times, can be a little tricky. The first step in finding this type of problem is to do a very detailed examination of all the electrical connections and wiring. Look for any darkened or burnt areas around all of the electrical components and wiring. Be sure to open all the components’ cover plates, such as the compressor terminal box or any temperature or pressures switches. If this is the problem, you will normally find it during this visual inspection.
If the short is within a component, such as the winding of a motor, you will need to get out an ohmmeter and test each load to determine which one is grounded. There are several different ways to find the grounded load, but perhaps the best method is to test each load independently. This means electrically disconnecting the load and testing each electrical connection to the frame of the motor with an ohmmeter.
An overloaded circuit is one that draws current in excess of the circuit design limits. Finding this problem may be simple, but, at times, it can be frustrating if the overload does not occur while you are examining the system. Normally, the first step in discovering this problem is to measure the amperage draw of the circuit and confirm it is overloaded. This is done by comparing the measured amperage to the circuit breaker trip value and the current limits of the equipment. You need to measure the current draw of each load to see which one is causing the overload. On refrigeration systems, you generally will find the compressor is the cause, but as with all troubleshooting procedures, you need to verify this is accurate.
Recently, I was working on a dairy cooler for a supermarket that had been tripping the condensing unit’s circuit breaker. At first, it happened occasionally — maybe every two weeks — and the dairy manager just kept resetting the breaker and not informing the owners of the issue, which is not a good idea. Then, it started happening daily, so he told the owners, and they called us in to solve the problem.
First, I examined all of the electrical controls and wiring. Initially, I didn’t find any darkened or discolored wires or terminals, but I have to admit I did not do a great initial investigation. While I was examining the wiring, the breaker tripped, so I reset it. It was a three-phase system, so I measured the amperage of L1, L2, and L3. It was a 60-A circuit breaker, and L1 was drawing about 58 A; L2, about 59 A; and L3, close to zero — obviously not good values for the system.
What would cause such strange values? After pondering the problem for a while, I decided to shut down the system and open the compressor cover plate to examine its terminals. As soon as I removed the cover plate, the problem was easily observed. One of the compressor terminals was much darker than the others, and when I tugged on the wire, it was very loose. After retightening the connection, the problem was solved. The next day, I called the dairy manager, and the case was still running and had not tripped the circuit breaker since I had left. Problem solved.
Remember, circuit breakers usually trip for a reason. Taking a methodical approach to determine the cause is the best way to provide a solution.
Publication date: 2/6/2017