What is a short circuit?

In electricity, the notion of short-circuit comes up very often. Usually associated with danger, shorting is something you want to avoid. But by the way, do you know what a short circuit is? What happens from an electrical point of view during a short circuit? Find out the answers to these questions in our short circuit guide.

short circuits
short circuits

The definition of a short circuit

Principle of short circuit
In electricity, a basic circuit is represented by a generator (battery, battery, etc.) which supplies current to devices (dipoles) such as a lamp, a capacitor etc. When the circuit is closed, the current leaves from its source and passes through the equipment before returning to the generator.
In the event of a short circuit, the current takes a shortcut. Instead of passing through the circuit equipment, it returns directly to its source. In other words, the current becomes trapped in a loop that causes the intensity to increase and heat up. A short circuit can be accidentally caused when a connecting wire strikes its insulating plastic sheath. It can also be caused by inserting a jumper wire between the terminals of the generator or dipole.
An example of an electrical short circuit
Take the example of an electrical circuit comprising a generator and three bulbs connected in series. In normal times, the current leaves the generator and successively feeds the three bulbs. If one of the bulbs is short-circuited, the current will no longer flow through this dipole. As a result, the current will only feed two lamps. Concretely, this means that the short-circuited lamp will not light up. On the other hand, because of the short circuit, the other two lamps will shine brighter and run the risk of burning out. To learn more details about short circuits click here

The different types of short circuits

There are different types of short circuits. They can be classified according to different parameters. For example, considering the duration, we will speak of a self-extinguishing, fugitive, semi-permanent or permanent short-circuit.
The cause of the short circuit can also vary. In the event of a broken conductor, we will speak of a mechanical short-circuit. This is the case, for example, if you forget a tool that connects the terminals of your car battery. The short circuit can also come from an electrical surge after a lightning strike, for example. Finally, humidity, ageing or degradation of the insulation can also be responsible for a short circuit.
Finally, the nature of the connection makes it possible to qualify a short-circuit. For example, a short-circuit is said to be “free” during direct contact between the two terminals, unlike a so-called “impedance” short-circuit. For a three-phase network, short circuits can be:
Single-phase in the case of one phase with neutral;
Biphase for two phases;
Three-phase for three phases;
Two-phase-earth when two phases are connected with the earth.

The short circuit from the point of view of theory

Another way to understand what a short circuit is is to go back to the fundamentals of electricity from a formula point of view.
Ohm’s law in direct current dictates that the voltage (U) is equal to the product of the resistance (R) and the current (I): U = R * I. The resistance reflects the property of a component to oppose the passage of a current. For example, a bulb has a certain resistance. During a short circuit, the component is bypassed by the current. Nothing (or almost) then opposes the flow of current and the resistance of the circuit tends to zero. Therefore, the intensity increases towards infinity.
The increased intensity causes heat dissipation via the Joule effect. This conforms to the following formula: P = R * I², where P represents the power dissipated by the Joule effect.
In short, a short circuit causes the current to rise sharply, causing the circuit to heat up. This is why short circuit phenomena must be able to be detected early and corrected. Otherwise, you risk damaging your circuit. Adding heat can cause the insulation covering your equipment to melt and burn out. On a larger scale, the short circuit can be responsible for household fires.

How to protect your electrical installation against short circuits?

Considering the dangers of a short circuit, there are different pieces of equipment to protect the electrical installation of a home.

The electric circuit breaker

The circuit breaker is an electromagnetic device that automatically cuts off the current when a short circuit is identified. The circuit breaker can be thermal or differential. Once the problem has been eliminated, the circuit breaker can be reset.
Circuit breakers are widely used in a home. This is how you have a branch circuit breaker installed next to the GFCI on a row in your electrical panel. The role of this circuit breaker is to protect a given circuit against short circuits or overloads. A subscriber circuit breaker is also installed upstream of your electrical panel in the technical shaft of the accommodation. Its role is to provide the interface between the general electrical network and your personal installation. Only the electricity supplier is authorized to operate this circuit breaker.

The basics of electricity

Installation, replacement of electrical circuit breaker
The electric fuse
The electric fuse works on the same principle as a circuit breaker. In the event of a short circuit or an electrical overload, it automatically cuts off the power. The main difference with a circuit breaker is that the fuse cannot be reset. It must therefore be replaced after each use. A fuse is in fact made up of a piece of metal (lead for old fuses) that melts during a short circuit or overload. Once the filament is melted, electricity cannot pass through. Compared to circuit breakers, fuses are cheaper and allow circuit overloads to be detected more quickly.


Grounding outlets, equipment and the circuit is also a means of protection against short circuits. Earthing concretely consists in creating a physical connection with the ground. In the event of a short circuit, the current is therefore automatically diverted to the ground.