What Are Short Circuits and Open Circuits?

A short circuit and an open circuit are two extreme conditions in a circuit. In a short, current flows through an unintended, very low-resistance path; in an open, the path is broken and no current flows.

What Is a Short Circuit?

A short circuit is when current bypasses the intended load and flows through a near-zero-resistance path. By Ohm's law, as resistance approaches zero the current rises sharply (I = V / R); this leads to excessive heating and damage.

What Is an Open Circuit?

An open circuit is a broken current path (a broken wire, a blown fuse, a loose connection). The resistance is taken as infinite and the current is zero; the circuit doesn't work.

Risks and Protection

A short circuit is the most dangerous condition; that's why circuits use a fuse or a circuit breaker. These cut the path when the current exceeds a safe limit, protecting the components and wiring.

Detection with a Multimeter

You can find a short/open with the continuity range of a multimeter: no beep where there should be one means an open; a beep where there shouldn't be one means a short.

Common Mistakes

• Bridging a fuse: bridging a blown fuse with wire provides no protection and creates a fire risk.
• Measuring under power: a continuity/resistance test is always done with the circuit de-energized.
• Ignoring the symptom: a heating wire or a repeatedly blowing fuse is a sign of a short.