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Short circuit

When everything stops... or worse

Electronics Info

What is a short circuit?

short circuit
A short circuit happens in an electric circuit when current flows through a path of very low resistance, often an unexpected one, which results in an extremely large current – much larger than the circuit was designed to withstand. It happens when, because of a fault or damaged insulation, two conductors at different potentials (e.g. plus and minus, live and neutral) are connected directly or through a very small resistance.

Types of short circuit

Short circuits differ by the number of conductors involved and by how they happen:

  • Single-phase (single-pole): the most common in homes and electronics; an accidental connection of one conductor to ground or to another conductor.
  • Two-phase (two-pole): two phases connected to each other, or a phase connected to earth.
  • Three-phase (three-pole): all three phases connected; the rarest, but it produces the largest current.
  • Partial/intermittent: short circuits caused by occasional contact (loose wires, water, moisture).

Effects and problems of a short circuit

When a short circuit happens:

  • The current rises suddenly, typically to many times the value allowed for the wires and devices.
  • Equipment and wires overheat, so the insulation can burn and cause a fire.
  • Fuses or circuit breakers cut off the current to prevent greater damage (if there is any protection).
  • In the worst case, the device is completely destroyed, or there is an explosion or a fire.

Symptoms to look out for:

  • A sudden loss of power (a tripped fuse)
  • Burnt-out parts of the circuit
  • A smell of burnt materials
  • Overheated conductors or parts

Historical examples of catastrophic short circuits:

• Power station fire in New York (1930): a short circuit caused a huge fire and material damage, and the city was left without power for hours.
• Notre-Dame fire (2019): one of the suspected causes of the fire was a fault in the electrical system, where a short circuit played a part in spreading the fire.

Protection against short circuits

Electronic circuits are most often protected against short circuits with fuses, circuit breakers, protective relays and electronic fault-detection circuits. The main job of these components is to monitor the current and, if it suddenly rises (which is typical of a short circuit), to cut off the power automatically to prevent damage to the circuits or a fire. The type of protection depends on what the circuit is for: simpler devices use fuses, while complex systems include sensors, microcontrollers and complex relays that analyse current, voltage and temperature.

Protecting an Arduino from a short circuit on an output pin

Arduino microprocessors (e.g. ATmega328/328P) don’t have physical fuses built into their pins, but there are some internal protections that limit the damage:

  • Each digital pin has a recommended maximum output current of about 20 mA, and the absolute maximum is 40 mA (ATmega328P on the Arduino Uno R3). The newer Arduino Uno R4 allows only about 8 mA per pin. If a pin is shorted directly to ground or to the supply without a resistor, the port is overloaded immediately, which can permanently damage the microcontroller.
  • The pins have no overload protection at all, neither in hardware nor in software: a program cannot stop too much current from flowing. That is why the real protection is a series resistor (for LEDs 220–330 Ω on the Uno R3 and at least 470 Ω on the Uno R4; larger for other devices).
  • The USB port on a computer or an adapter often has its own internal protection that can prevent larger currents and switch off the power in a short circuit, but that is not protection for the Arduino board itself.

Rules for working with Arduino:

• Always use resistors on output pins (especially with LEDs).
• Don't connect motors/solenoids that draw a high current directly; use transistors or relays with their own power supply.
• Follow the microcontroller's datasheet for the maximum allowed loads.
A combination of correct wiring and extra external protection (fuses, resistors) keeps an Arduino safe from permanent damage caused by a short circuit or by drawing too much current from its output pins.

What did we learn?

  • A short circuit happens when electric current unexpectedly finds a path of very low resistance, usually because of a fault, damaged insulation or incorrect wiring, and it can then cause a high current, sparks, fires or failures of electronic devices.
  • Electronic circuits and Arduino are protected from short circuits by using resistors, fuses, switches, good-quality wiring and correct circuit design.
  • Arduino output pins are not protected against overload by themselves, so to drive more power-hungry components (motors, LED strips) we need to use transistors, relays or dedicated drivers.
  • In everyday practice it is important to check connections regularly, use suitable protection, not overload circuits and know the basic rules for working safely with electronic components.
  • With a smart approach and an understanding of what a short circuit is, you can develop more complex projects with Arduino and enjoy the creativity of electronics – while also taking care of your own safety and the safety of those around you.