A number of electrical problems can cause the same apparent symptom: a circuit that suddenly goes dead and causes lights and appliances to stop working. Two closely related situations that can cause this problem are short circuits and ground faults. There is a good deal of confusion over the precise difference between these conditions, and even professional electricians sometimes disagree on precise definitions.
Defining a Short Circuit
A short circuit is any electrical flow that strays outside its intended circuit with little or no resistance to that flow. The usual cause is bare wires touching one another or wire connections that have come loose. The immediate impact is that a large amount of current suddenly begins to flow. This in turn causes the circuit breaker to trip, instantly stopping all current flow. This condition is known as a "short" circuit because the current is bypassing the full circuit wiring and flowing back immediately to the source by a shorter pathway.
For electricians, a short circuit is usually defined as a situation in which a hot wire makes contact with a neutral wire, such as when a hot wire loosens from its connection and makes contact with the neutral wire.
Short circuits can occur when insulation on wires melts and exposes bare wires. The principal danger of a short circuit is arcing or sparking that may occur as electrical current jumps from a hot wire to a neutral. This situation can easily cause fires. Short circuits can also occur within the wiring of individual devices, such as lamps or other plug-in appliances. Frayed or otherwise damaged electrical extension cords or appliance cords can also cause short circuits.
Protection against short circuits is provided mostly by circuit breakers, which trip and shut the circuit off when current begins to flow in an uncontrolled fashion. A special type of circuit breaker, an arc-fault circuit interrupter (AFCI) is now commonly used. It senses arcing, or sparking, and shuts off the current even before the current flow overloads the breaker.
Defining a Ground Fault
An electrical system can experience a number of different types of faults—defined as any abnormal flow of electricity. A ground fault is a type of fault in which the unintentional pathway of the straying electrical current flows directly to the earth (to ground). Here, too, the circuit is "short," in that it has bypassed the circuit wiring, so a ground fault can technically be defined as one type of short circuit. And, as with any short circuit, the immediate impact is a sudden reduction in resistance that causes current to flow in an unimpeded fashion. Like other types of short circuit, a ground fault causes the circuit breaker to trip due to the uncontrolled flow.
But for an electrician, a ground fault is generally defined as the situation when a hot wire makes contact with either the grounding wire or a grounded portion of the system, such as a metal electrical box. Electricians, therefore, think of a ground fault as being different than a short circuit, although an electrical engineer would see it somewhat differently.
The main danger of ground faults comes in the likelihood of shock if a person happens to be in contact with the path of least resistance to ground. This is why the danger of shock is much more pronounced in situations where a person is standing on the ground or in a damp location.
Protection against ground faults is offered by circuit breakers that trip if the flow of electricity suddenly increases, and by a system of grounding wires in the circuits that provide a direct pathway back to ground should current stray outside its established circuit wiring. There are also ground-fault circuit interrupter outlets that can be used in situations where ground faults are particularly likely, such as in outdoor locations, near plumbing fixtures, and in below-grade locations.
Common Causes of Short Circuits
- A loose connection on one of two wires in a junction box or outlet box may cause a short circuit.
- A short circuit can occur when a wire slips off of a terminal on an electrical device, such as an outlet. When it touches another wire, a short circuit ensues.
- An appliance may encounter an internal wiring problem, causing a hot wire and neutral wire to accidentally touch.
- Insects or rodents may chew the wire insulation and cause a short circuit between two wires within a cable bundle.
Common Causes of Ground Faults
- Water leaking into an electrical box can cause a ground fault, since water is a conductor of electricity.
- Worn hot wires or hot wires that are not completely seated into their terminals may come into contact with ground wires or grounding devices or boxes.
- Power tools or appliances without proper insulation can cause a ground fault if faulty wiring causes current to flow directly to ground. When working outdoors or below grade, always plug tools into GFCI outlets or use GFCI-protected extension cords.
Avoiding Short Circuits and Ground Faults During Repairs
Both short circuits and ground faults can happen if you fail to turn off power to the circuit before working on it. Bare wires can inevitably touch the wrong places: hot wire to neutral wire means a short circuit that causes sparks to fly; hot wire to grounding wire or to grounded metal box means ground fault and possible shock. To avoid these serious problems, always turn off the circuit before you begin working on any section of it.