An open circuit is a path in which the current is zero because the circuit is incomplete, for example a broken conductor, an open switch or a missing connection. The resistance of the break is ideally infinite, so the full source voltage appears across it.
Learning objectives
- Define an open circuit and explain why current is zero
- Explain why full voltage can appear across the break
- Locate an open circuit by voltage or continuity measurement
- Distinguish an open circuit from a dead circuit
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
This is why an open circuit is not the same as a dead circuit. A switch that is off still has live voltage on its supply side. Treat an open circuit as live until you have proved otherwise.
Intermediate How it behaves in a circuit
An open circuit can be caused by a broken wire, a failed solder joint, a blown fuse, a faulty switch, a disconnected connector or a cracked PCB track. Finding it means measuring voltage along the circuit, because a voltmeter draws almost no current and shows where the voltage stops.
Advanced Limits, assumptions, engineering detail
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage | V | volt (V) |
| Current | I | ampere (A) |
| Resistance | R | ohm (Ω) |
Worked examples
Open switch
Problem. A 12 V battery feeds a lamp through a switch. The switch is open. What current flows and what voltage appears across the switch?
Solution. The path is incomplete, so I = 0. With no current there is no drop in the lamp, so the whole source voltage appears across the open switch.
Open load after a series resistor
Problem. A 12 V source is connected through a 1 kΩ resistor to a load whose connection is broken. What is the voltage at the open terminals?
Solution. No current flows, so the drop across the 1 kΩ resistor is I × R = 0 × 1000 = 0 V. The source voltage appears at the open terminals.
High-resistance leak
Problem. A damaged insulator of 10 kΩ bridges a 230 V circuit to a metal part. What leakage current flows?
Solution. Ileak = V / Rleak = 230 / 10 000.
Practical applications
- Switch and fuse diagnosis
- Connector and harness faults
- Printed circuit board track breaks
- Sensor and control wiring
- Safety and interlock loops
- Intermittent fault finding
Automotive applications
A broken ground, a corroded connector or a blown fuse creates an open circuit. Voltage-drop testing is more reliable than resistance testing on a live circuit, and wiggle tests help find intermittent opens in harnesses.
Industrial applications
Emergency-stop loops and interlock circuits are designed so that an open circuit stops the machine safely. Monitoring circuits use a small bias current to detect a break before it becomes dangerous, which is a deliberate application of open-circuit behaviour.
Measurement procedure
- Measure voltage across the suspected break while the circuit is energised; the open side will show the source voltage.
- De-energise and verify dead before measuring continuity or resistance.
- Use the voltage-drop method along the circuit to localise the break.
- Be careful with high-impedance circuits: the meter itself can change the reading.
Common mistakes
- Assuming no current means no voltage; an open circuit can still be live.
- Measuring resistance on an energised circuit.
- Using a low-impedance test lamp that loads the circuit and hides the fault.
- Confusing an open circuit with a short circuit.
Safety considerations
- Treat an open circuit as live until it has been isolated and proved dead.
- Discharge capacitors before touching conductors that may be connected to stored energy.
- Use meters and probes rated for the voltage present.