When your air conditioning system comes on, it is powered by electricity that is routed through a circuit breaker. This breaker has the ability to break the circuit and stop the flow of power to the air conditioner by flipping a switch. Typically, the circuit breaker box produces little or no noise when it works properly. But if you are hearing a buzzing or humming noise in the breaker box when the A/C is on or even when it is off, you should investigate it because something is not right.
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Loose Wire Connection
One problem that could cause a buzzing sound in your breaker box, when the A/C is on, is a loose wire connection. If the wire is not attached to the breaker securely or if there is a loose connection or damaged wire somewhere between the breaker and the power source or the air conditioning unit, it could buzz and fizzle and potentially even spark. A loose connection makes noise and can be dangerous. This wire connection could cause the breaker to trip as well, cutting power to prevent a fire hazard. The fizzle and buzz sound may also indicate a bad breaker.
Failing To Trip
A circuit breaker prevents overheating and other dangerous current load situations by "tripping." A breaker that trips shuts off on its own. This is part of the design of the component to protect the wiring, your home, the air conditioner or other appliance it is assigned to. A bad breaker may fail to trip even when it is overloaded. This may cause a buzzing sound in the breaker box and could lead to a dangerous situation, such as a fire hazard. The breaker must be replaced if it will not trip when needed.
Normal Vibration or Problem?
A circuit breaker is hooked up to an alternating current. At 60 cycles per second, the energy is moving in and out at a very rapid pace. The current is very likely to create a vibration in your circuit breaker that can be heard inside the box. The noise is a very low hum or buzz, and it is completely normal. You won't always be able to hear this buzz; but if you do, it may not be a reason for concern. An especially loud buzzing could be the normal vibration shaking a loose component in the box.
Replacing a Breaker
For most DIYers, replacing a circuit breaker is a fairly simple task and a way to stop the buzzing if you can pinpoint the issue as a bad breaker. Make sure you have turned off all the power at the box. This may require you to shut off the electrical main to your home. After the power is turned off, you should stand on a rubber mat or a piece of plywood for extra protection against electrocution.
Use a voltage tester to verify there is no current to the box. When it's safe to do so, pull up on the bad breaker on the side toward the center of the box. Pull the outermost side of the breaker toward the center and it should break loose. Now you can slide it out easily. Use a flat blade, insulated screwdriver to remove the wires and then replace the breaker. This should stop any buzzing related to a malfunctioning breaker.