What Is a Short Circuit, and What Causes One?

A short circuit happens suddenly and the results can be devastating: sparks, fire, circuits tripped. It may seem like an insurmountable task to find and fix a short circuit. But with enough patient detective work, most homeowners can identify the cause of the short circuit and possibly even fix the short circuit.

What a Short Circuit Is

Electricity wants to flow along the path of least resistance, which is usually the shortest path.

The long path for the energy to flow back to ground is on the intended circuit. But when a shorter path is provided, electricity naturally seeks this route—the path of least resistance. The electricity immediately changes its course to head to ground on this shorter, easier path.

Thus, the term short circuit. The electricity has created a shorter circuit than the intended circuit.

What Causes Short Circuits

Short circuits can be caused by:

• Vermin or pests chewing through wires
• Water or other fluids coming into contact with electrical wiring
• Loose connections in an electrical box
• Old or damaged outlets, switches, lights, appliances, or other electrical devices
• Nails or screws piercing through walls and coming into contact with wires
• Deterioration of electrical cable sheathing
• Build-up or surges of electricity

2 Types of Short Circuits

• Normal short circuit: In a normal short circuit, a powered or hot wire touches a neutral wire. Immediately, resistance drops and the current begins to move in another path.
• Ground fault short circuit: In a ground fault short circuit, a powered or hot wire touches a grounded section of a box, device, appliance, outlet, bare ground wire, or anything else supplied by the electrical circuit.

4 Signs of a Short Circuit

Of the common signs of a short circuit, two are most common and familiar to most users: the devices shut off and the breaker trips.

Devices Turn Off

All devices on a shorted circuit will turn off. Lights, TV, computer, outlets, and anything else on that circuit will instantly turn off due.

Breaker Trips or Fuse Blows

A short circuit usually will usually cause the circuit breaker that controls it to flip off. Older style fuses will blow out. The tripped breaker or blown fuse is why the devices turned off.

Evidence of Previous Short Circuits

Short circuits often do not announce themselves until the moment they happen. In some cases, though, there may be a warning sign of a previous short circuit.

This may be in the form of a charred wire or light switch. If the short circuit was recent, you may sense a metallic smell. Or you may smell burned plastic or rubber.

Ongoing Short Circuits

A clear sign you know a circuit has shorted is when the circuit breaker usually shuts off. Sometimes, there are sparks and a bright light. A loud zapping sound or a boom can accompany a short circuit.

The device powered by the electrical current stops working. GFCI outlets will trip off.

If you are touching the device or if your body happens to be the short within this short circuit, you may receive an electrical shock and often a burn from the intense heat.

Why Short Circuits Are Dangerous

When the human body is introduced as the path of least resistance, the current travels through the body. A short circuit is harmful because it can cause injury or death through electrical shock, electrocution, or fires.

More power is demanded during a short circuit, causing electrical arcs and extremely high temperatures that can melt plastics or set fire to flammable materials such as wood or fabrics.

How to Find and Fix Short Circuits

1. Isolate Circuit

   Identify the circuit. Make sure that you're dealing with only the circuit in question.
   

2. Make Circuit Safe to Work on

   Turn off the circuit breaker if it is not already off. A circuit breaker that is off will have its handle positioned toward the nearest side of the service panel, not the center.
   

   Tip

   It may be necessary to remove the breaker. To do this, remove the front protective panel with a screwdriver. Rock the breaker back toward the side of the service panel. The breaker should lift off. Then unscrew the wires from the terminals.

3. Identify Devices on the Circuit

   Identify all devices on the circuit, including outlets, switches, appliances, lights, A/Cs, and more—even junction boxes.
   

4. Examine Devices

   Check the outside of each device on the circuit. Look for blown fuses on individual devices. Look for signs of a short circuit: sharp smells, melted plastic, or burn marks.
   

5. Look Inside Devices

   Where practical, check wires in each device. Open up electrical boxes and check connections. Look at junction boxes that aren't attached to devices. Remove light fixtures and look inside the fixtures and inside their electrical boxes.
   

6. Check Cables in Walls and Attics

   Where you can, examine wires between devices. Since wires are generally closed up in walls, this may be difficult. But often, wires run along the joists in attics and can be examined with a flashlight.
   

When to Call a Professional

For many homeowners, short circuits can be difficult to find and fix. Call a licensed, qualified electrician for help if you encounter any difficulties.