How Do Circuit Breakers Work? Common Reasons for Tripped Circuits

The electrical system in every home features a design of circuits controlled and protected either by circuit breakers or fuses. Most of today's homes now use circuit breakers to offer this control and protection to individual circuits, but older homes that have not upgraded their electrical systems may use fuses. The circuit breakers or fuses are typically found in a central main service panel or fuse box. Locate your main service panel and note if your home uses circuit breakers or fuses. Circuit breakers look like switches, while fuses are round and screw into sockets.

Circuit breakers and fuses are safety measures that protect homes and people from fires, electrical shock, and appliance and property damage.

Read on for more information on the different types of circuit breakers, what happens when a breaker trips, and the most common causes of electrical problems.

Types of Circuits

• Standard: Simple breakers come in two types: single-pole (most common) and double-pole. Both stop the current from overloading and short circuits, preventing wires from overheating. A single-pole breaker protects one wire (120 volts, 15-20 amps). A double-pole breaker protects two wires (120 to 240 volts, 15-200 amps).
  
• GFCI (Ground Fault Circuit Interrupter): These circuit breakers protect the entire circuit, including all the wiring and devices. This breaker panel offers complete protection against ground faults. These are essential to use in places with water and dampness, like kitchens, bathrooms, laundry rooms, outdoor spaces, and basements.
• AFCI (Arc Fault Circuit Interrupter): Arc fault breakers sense an abnormal path or electric jump in the current from one circuit to another (parallel arc); this breaker stops power before a spark can turn into a fire. These breakers are best for homes or electrical systems with old, fraying wiring.
• CAFCI (Combination Arc Fault Circuit Interrupter): An arc fault breaker also protects against a parallel arc with the bonus of defending against a series arc. A series arc is a jump in the current along the same circuit—for example, one part of the wire to another.
  

What Happens When a Circuit Trips

When all the lights and fixtures in only a portion of the house go dark or dead simultaneously, it's because one of those circuit breakers has "tripped" or one of those fuses has blown. These devices automatically shut off power to the circuit when problems occur.

In the case of circuit breakers, the immediate answer is to find the breaker that has tripped and reset the lever to the ON position. When a fuse blows, a metal filament inside the fuse has burned through, meaning you'll need to replace the fuse with a new one.  

To avoid the problem from happening again, you should aim to understand why the breaker has tripped, or the fuse has blown. In rare cases, the breaker may be damaged and need to be replaced by a professional. But in most cases, the breaker or fuse is just doing its job when it pops. Circuit breakers are designed to trip, and fuses are designed to blow and turn off the power when four dangerous situations occur: an overloaded circuit, short circuit, ground fault, or arc fault.

Overloaded Circuit

An overloaded electrical circuit is the most common reason for a circuit breaker tripping. It occurs when a circuit attempts to draw a greater electrical load than it is intended to carry. When too many appliances or light fixtures are operating simultaneously, the internal sensing mechanism in the circuit breaker heats up, and the breaker "trips," usually by means of a spring-loaded component within the breaker.

Tripping breaks the continuous pathway of the breaker and renders the circuit inactive. The circuit remains dead until the breaker lever is reset to the ON position, which also re-arms the internal spring mechanism. 

The circuit breaker or fuse is sized to match the load-carrying capacity of the wires in that circuit. Hence, the breaker or fuse is intended to trip or blow before the circuit wires can heat to a dangerous level.

When a circuit breaker regularly trips or a fuse repeatedly blows, it is a sign that you are making excessive demands on the circuit and need to move some appliances and devices to other circuits. Or, it may indicate that your house has too few circuits and is in need of a service upgrade. Usually, when a circuit is overloaded, the breaker takes 10 seconds or so to trip due to a time delay feature internal to the breaker.

Short Circuit

A short circuit is a more serious reason for a breaker tripping. A "hard short" is caused when the hot wire (black) touches a neutral wire (white), the bare ground or bond wire, or the case of a metal box.

In terms of the physics involved, a short circuit allows for a sudden unimpeded flow of electricity due to lowered resistance. This sudden increase in current flow within the breaker causes the tripping mechanism to activate.

But sometimes, a short circuit occurs not because of the circuit wiring but because of a wiring problem in an appliance or device plugged into an outlet along the circuit. Short circuits are sometimes tricky to diagnose and fix and may require the help of a professional electrician.

When a circuit breaker trips again instantly after you reset it, you might have a short circuit. 

Ground Fault

A particular type of short circuit, a "ground-fault," occurs if a hot wire comes in contact with a ground wire or a metal wall box or touches metal framing members. Ground faults can be especially dangerous in areas with high moisture levels, such as kitchens or bathrooms or in outdoor locations. A ground fault can cause electrical shock.

There are steps you can take to identify and fix a ground fault, but also essential steps you should take to prevent one from occurring in the first place. For example, in areas where direct contact with the ground or water is possible, NEC regulations may require that outlets be protected with GFCIs (ground-fault circuit interrupters).

As with hard shorts, a ground fault causes an instant reduction in resistance and an immediate increase in electrical flow. This causes the internal mechanism of the circuit breaker to heat up and trip. As with hard shorts, if a ground fault is present, the circuit breaker may trip again immediately after you reset it.

Arc Fault

In recent years, the National Electrical Code, the model code on which most local electrical codes are based, has gradually increased requirements for a special circuit breaker, known as an arc-fault circuit interrupter (AFCI).

AFCI breakers, in addition to tripping due to overloads, short circuits, and ground faults, also sense the power fluctuations that occur when sparking ("arcing") occurs between contact points in a wire connection. Arcing can happen when there are loose screw terminal connections in a switch or outlet.

These breakers sense early wiring problems before they can lead to short circuits or ground faults. Neither ordinary circuit breakers nor fuses offer any protection against arc faults. Arc fault protection is an important safeguard against fires caused by arcing. 

AFCI breakers are reset in the same manner as ordinary breakers. Repeated tripping usually indicates loose wire connections along the circuit, causing repeated arcing. 

Safety Tips for Working With Circuit Breakers

Shock prevention is essential when working with circuits and electricity. If you are not a professional electrician, follow the same safety rules that the professionals do:

• Before working on any electrical projects, always shut off the main breaker in the circuit breaker box.
• Check load requirements on your wiring and breaker panel and ensure it's sufficient before installing anything new.
• Match amp requirements; never replace a circuit breaker with a higher-amp one.
• Do not add another breaker to your panel if you do not have an open slot.
• Even if the main breaker is off, practice the strictest safety measure and act as if the circuit is live.
• Never touch the feeder wires leading into the electrical panel; they are always energized.
• Test AFCI, GFCI, and CGFCI breakers once a month.
• Have all new electrical work inspected by a licensed electrician or inspector to ensure it's safely installed and meets code requirements.