What Causes an Electrical Short Circuit?

Pair of hands holding exposed wire
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The term short circuit is often used somewhat incorrectly to refer to any wiring problem in an electrical circuit. Officially, though, a short circuit refers to a specific condition in which electricity strays outside the established pathway of an electrical circuit.

Definition of a Short Circuit

As the name implies, a short circuit is a condition where electrical flow completes its circuit journey via a shorter distance than is present in the established wiring. By nature, electricity seeks to return to ground, and in a properly functioning circuit, this means that the current flows through the established wiring circuit back to the service panel, and onward back through the utility wires. However, if the connections within the wiring loosen or break, electrical current may "leak." In this instance, the electrical current instantly seeks to flow back to ground by a shorter pathway. That pathway may very well be through flammable materials or even through a human being, which is why a short circuit presents the danger of fire or lethal shock.

The reason this happens is that these other materials offer a pathway of lesser resistance than is present in the copper wiring of a circuit. For example, in a light switch with faulty wiring or a loose wire connection, if the bare copper hot wire touches the metal electrical box or a metal faceplate on the switch, the current will leap toward whatever path offers the least resistance—which could well be through the finger, hand, and body of whoever is touching the switch.

2 Types of Short Circuit

In general terms, a short circuit is any condition where the establish wiring circuit is interrupted by a flaw in the wiring or wiring connections. Actually, though, there are two situations that both qualify as short circuits, although they carry different names.

Short Circuit

The term short circuit is most commonly used by electricians to refer to the situation in which a hot wire carrying live current touches a neutral wire. When this happens, resistance lessens instantly and a large volume of current flows through an unexpected pathway. When this classic short circuit occurs, sparks sometimes fly, you may hear crackling, and sometimes smoke and flames ensue.

Ground Fault

ground fault is a type of short circuit that occurs when the hot wire carrying current comes into contact with some grounded portion of the system, such as a bare copper ground wire, a grounded metal wall box, or a grounded portion of an appliance. As with the classic short circuit, a ground-fault causes resistance to instantly lessen, which allows a large amount of unimpeded current to flow through the unexpected pathway. Here, there is less chance of flame and fire, but a notable chance of shock.

3 Causes of Short Circuits

There are several causes for short circuits, including three that are most often to blame.

Faulty Circuit Wire Insulation

Old or damaged insulation may allow neutral and hot wires to touch, which can cause a short circuit. Nail and screw punctures, as well as age, can cause wire casings or insulation to deteriorate and create short circuits. Or, if animal pests such as mice, rats, or squirrels gnaw on circuit wiring, the inner wire conductors can be exposed to cause short circuits.

Loose Wire Connections

Attachments can loosen, sometimes allowing neutral and live wires to touch. Fixing faulty wire connections is tricky and is best handled by those thoroughly familiar with wiring work.

Faulty Appliance Wiring

When an appliance is plugged into a wall outlet, its wiring effectively becomes an extension of the circuit, and any problems in the appliance wiring become circuit problems. Old or broken appliances can develop inner short circuits over time. Short circuits in appliances can occur in the plugs, in the power cords, or inside the device itself. It’s best to have a technician look at shorts in larger appliances such as ovens and dishwashers. Smaller appliances such as lamps often can be rewired yourself.

3 Means of Protection Against Short Circuits

Because both classic short circuits and ground faults pose a danger of shock and fire, your wiring system has various means of safeguarding against those dangers.

Circuit Breakers or Fuses

Since the 1960s, virtually all new or updated wiring systems are protected by a main service panel that houses individual circuit breakers that control individual circuits in the home. Older wiring installations provide similar protection through fuses. Circuit breakers use an internal system of springs or compressed air to sense changes in current flow and break the circuit connection when irregularities occur, such as the sudden unimpeded flow of current that occurs during a short circuit.

Ground-Fault Circuit Interrupters (GFCIs)

Beginning in 1971, Electrical Codes began requiring ground-fault protection, either through special GFCI circuit breakers or GFCI outlet receptacles. These devices provide a similar function to circuit breakers, in that they sense changes in current flow, but they are much more sensitive than circuit breakers and shut off the flow of current when they sense very minute fluctuations in current. GFCIs are most valuable in protecting against shocks that can occur in ground-fault type short circuits.

Arc-Fault Circuit Interrupters (AFCIs)

Beginning in 1999, Electrical Codes began requiring a new type of protection against arcing—the sparking that occurs when electricity jumps between metal contacts, such as may happen when a wire connection is loose but not fully separated. You can think of an AFCI as a device that anticipates short circuits and shuts off the power before it can reach the short circuit condition. Unlike GFCIs, which are designed to protect against shock, AFCIs are most useful for preventing fires caused by arcing. AFCI protection can be provided both by AFCI circuit breakers or AFCI receptacles.

Dealing with Short Circuits

The most common sign of a short circuit is when a circuit breaker trips and causes the circuit to shut off. However, there are other conditions that can cause a circuit breaker to trip, such as power overloads, so it is important to determine why the breaker is tripping. If a circuit breaker continues to trip immediately after being reset, it is a strong indication that there is a wiring problem somewhere along the circuit or in one of the appliances connected to that circuit.

Here is a procedure to follow if you suspect a short circuit:

  1. Locate the tripped circuit breaker: At the main service panel, look for an individual circuit breaker with a handle that has snapped to the OFF position. Some breakers may have a red or orange window indicator to make it easy to spot. This tripped breaker will identify the circuit where the problem exists. Leave the circuit breaker OFF as you inspect along the circuit.
  2. Inspect appliance power cords: Inspect all the power cords plugged into outlets along the circuit that has tripped. If you find any that are damaged or on which the plastic insulation has melted, there is a good chance the short circuit is within the appliance or device itself. Unplug these appliances from the circuit. If you find suspect appliances, switch the circuit breaker back on after unplugging them. If the circuit now remains active without tripping again, it is very certain that your problem existed in the appliance. However, if the circuit breaker trips again immediately, proceed to the next step.
  3. Turn off all light and appliance switches along the circuit. Then, turn the circuit breaker back to the ON position.
  4. Turn on each light switch or appliance switch, one at a time. If you reach a switch that causes the circuit breaker to trip again, you have identified the section of circuit wiring where a loose connection or wiring problem exists.
  5. Repair the circuit wiring problem. This is a step that may require the help of a professional electrician. Do not attempt this unless you are very confident of your knowledge and skill level. This repair will involve shutting off the circuit, then opening up outlet and switch boxes to inspect the wires and wire connections and making any repairs that are necessary.

If you cannot find any obvious problem in one of the plug-in appliances or fixture wiring connections, the problem is likely hidden somewhere in the wall wiring. Solving this problem will require you to call a licensed electrician to deal with it. Do not reactivate the circuit until the problem has been identified and repaired—doing so poses a risk of fire and shock to you and your family. Any smell of smoke or signs of charring or melted plastic is a sign you have a serious problem.