Return to Electrical Article List
How To Tell If A Circuit Breaker Is Bad - Circuit Breaker Basics
by Michael Chotiner
Is there a circuit breaker in your home service panel that frequently trips? If your answer is yes, do you know why? There are a number of possible causes, from an appliance with a short circuit to a loose terminal connection. But to understand what's wrong with your circuit breaker, you should understand what circuit breakers are, what they're supposed to do, and how they work.
Circuit breakers both distribute and divide electrical power
Breakers, which are found in service panels that distribute power to multiple circuits within a building, are essentially safety switches that control the flow of electric power to an individual circuit. Each circuit is designed to serve some number of outlets, lights and/or appliances. They also enable us to cut off power to individual circuits, so that wiring and appliances connected to them can be worked on safely.
Circuit breakers are specified by their amperage rating, which is the approximate total amount of electricity that can be expected to be drawn by appliances on the circuit at any one time. The amperage rating is an approximate number because breakers are actually designed to turn their circuit off if the amps drawn exceed 80% of their rating.
Breakers have an internal mechanism that heats up as electricity flows through them. The more amps drawn, the hotter the breaker gets, until the internal mechanism trips and the circuit is broken. They prevent fires this way.
What Causes Breakers to Trip?
Now back to the original question: If one or more breakers in your home's service panel trip frequently, what's the cause? As with most home maintenance mysteries, it's best to start with the most likely causes, test to validate or eliminate each in order, and work up to the less obvious explanations. Here are a few typical causes:
Overloaded circuit
An "overloaded circuit" is the most common cause for tripped breakers. Circuit overloading is often the result of a circuit design that didn't anticipate, for example, that an appliance with an electric motor and another with a heating coil might be used simultaneously. In my home, which has 200-amp service and a kitchen that was upgraded in the 1990s with a dedicated circuit for the electric range, we can't operate the microwave and toaster oven at the same time without tripping the breaker that protects the circuit servicing the outlets above the counters.
What am I going to do about it? I should add a breaker to the panel and put the outlet for the microwave on a new circuit—maybe when I remodel the kitchen again. But in the meantime, I'm going to finish making toast before I reheat the coffee!
What I'm not going to do is install a breaker with a higher rating to control the circuit. That would be dangerous, since the existing wiring could overheat without the breaker recognizing it as excessive. The breaker wouldn't be triggered to trip, potentially causing a fire.
It could be a faulty appliance
If any appliance on a circuit has a short or otherwise faulty wiring, it could cause the breaker to trip. Your service panel should be labeled to indicate which breaker controls each circuit. When a breaker trips and you suspect that a faulty appliance might be the issue, unplug everything on that circuit and reset the breaker. Plug in one appliance at a time and turn it on. Wait 15 minutes or so, and if the breaker doesn't trip, unplug the appliance and test the next one. If you find the culprit, have it repaired or replaced.
Or it could be a bad breaker!
After eliminating the possible causes mentioned, consider that the problem may be the breaker itself. To test breakers, an electrician will use a digital multimeter. They'll plug the black lead into the common (COM) input and the red lead into the input labeled 250mA/250V, setting the dial to 200 on the ACV range scale. They'll also use screwdrivers with insulated handles to prevent shock from any errant circuit.
Your electrician will follow some safety precautions before working on an open, energized service panel, including:
- Make sure the floor is clean and dry.
- Wear rubber-soled shoes.
- Don't touch anything made of metal inside the panel box with bare fingers—they will likely wear insulated electrician's gloves for an extra measure of safety. It's safe to touch plastic parts—like the plastic toggle switches—if they need to reset them.
The electrician will then test the circuit breaker using a multimeter, testing both your single-pole breakers (rated at 15, 20 or 30 amps, to protect 120-volt circuits), and your double-pole breakers, which are used to protect 240-volt circuits serving major appliances like electric ranges, air conditioning equipment and electric dryers. Double-pole breakers look like two single-pole breakers joined together.
On a single-pole breaker, they will test to see if the meter reads 120 volts. A low reading (or zero) indicates a bad breaker. Likewise, on the double-pole breaker, they'll test for a reading of 240 volts.
If the electrician identifies a bad breaker, they'll replace it and to determine what led to its failure. In some cases, it may just have been a loose connection at the breaker terminal. In others, the entire service panel may need to be replaced.
About the author: Michael Chotiner is a former carpenter and construction manager who provides advice to homeowners on a variety of DIY topics. To find out more info on Home Depot's selection of load centers and circuit breakers, click here.