Diagram of an Open Circuit: What It Is and How to Find the Break
This is a free printable diagram of open circuit: download the diagram as SVG or open it and print to paper or PDF.
A diagram of an open circuit shows a broken conductive path where current cannot flow, the circuit voltage appears across the break, and resistance measures as infinite.
An open circuit is a fundamental concept in electrical theory: it is any circuit where the conductive path between the power source and the load has been interrupted. Because electrons have no complete path to follow, current flow is zero — regardless of how much voltage the source applies.
In a circuit diagram, an open circuit is represented by a gap in the conductor symbol, an open switch symbol (two lines not touching), or a broken wire symbol. A clearly labelled open-circuit diagram shows the break point, the voltage source, and the load, and annotates the key measurable effects: zero current (I = 0), infinite resistance across the break (R = ∞), and the full source voltage appearing across the open gap.
This last effect — voltage appearing across the break — is the most important and counterintuitive aspect of open circuits. Ohm's Law applied to a series circuit tells us that voltage drops are proportional to resistance. Because the open gap has effectively infinite resistance compared to every other part of the circuit, essentially all of the source voltage appears across it. This is why a multimeter probed across a break in a live circuit reads nearly full supply voltage — a useful diagnostic clue.
Open circuits occur due to many causes: a blown fuse (a deliberate protective open circuit), a broken wire strand, a corroded or loose connector, a failed solder joint, a tripped circuit breaker, a burnt-out lamp filament, or a failed resistor. In automotive circuits, vibration-induced connector looseness is an extremely common cause. In household wiring, open circuits most often occur at terminal screws that were not tightened adequately during installation.
Contrast an open circuit with a short circuit (resistance approaches zero, current approaches infinity) and a closed circuit (complete path, normal current flow). Understanding all three conditions is essential for fault diagnosis in any electrical or electronic system.
How to wire diagram of open circuit
- Identify the circuit's expected behaviour Before fault-finding, understand what the circuit should do: which loads should energise, what current and voltage levels are normal, and which control devices (switches, relays, fuses) are in the circuit path.
- Perform a visual inspection Look for obvious breaks: burnt fuse elements, visibly broken wires, corroded terminals, connectors that have pulled apart, or components with visible physical damage. In many cases, an open circuit fault is found by eye before any instrument is needed.
- Use the voltage test (half-split method) With the circuit live, probe midpoint nodes with a multimeter set to voltage. If you measure full supply voltage at a node, the path from source to that point is intact, and the break is further along the circuit toward the load. If you measure zero, the break is between the source and that node. This halving approach finds the fault efficiently.
- Verify with a continuity test With the circuit isolated from power, use a multimeter on continuity or resistance mode. Probe along the circuit path systematically. A reading of OL (infinite resistance) across a section of wire or a component indicates the open fault is within that element.
- Inspect and repair the break Once located, repair the break: replace a blown fuse (always investigate why it blew first), re-terminate a loose connector, repair or splice a broken wire using a properly rated connector or solder joint, or replace a failed component.
- Test the circuit before returning to service After repair, restore power and verify the circuit operates correctly under load. Measure current with a clamp meter if possible to confirm it is within the expected range — abnormally high current suggests a secondary fault or an incorrectly rated replacement component.
Specifications
| Current in an open circuit | 0 A (by definition) |
|---|---|
| Resistance across an open circuit break | ∞ (infinite) ohms — displayed as OL on most digital multimeters |
| Voltage across an open circuit break (ideal source) | Equal to the full source EMF |
| Voltage across the load in an open circuit | 0 V (no current, no voltage drop across resistance) |
| Power dissipated in an open circuit | 0 W (P = I × V; I = 0) |
| Multimeter display (resistance mode across open) | OL, 1, or ∞ — instrument-dependent |
Safety warnings
- When tracing an open circuit fault in a live mains circuit, use a properly rated CAT III or CAT IV multimeter with insulated probes. Contact with mains voltage (120 V AC or 230 V AC) is potentially fatal.
- Do not assume a circuit is safe to touch simply because no current is flowing. An open circuit may still have full supply voltage present at the break point and at upstream conductors.
- Always isolate and verify circuits are dead before performing continuity or resistance tests. Applying a multimeter's continuity function to a live circuit can damage the instrument and create a shock hazard.
- When replacing a blown fuse, identify and repair the cause of the overcurrent before reinstating the circuit. Replacing a fuse without finding the root cause can result in fire if the fault is a partial short or overloaded conductor.
Tools needed
- Digital multimeter (DC/AC voltage, resistance, continuity modes)
- Non-contact voltage tester (for initial safety verification)
- Test light (12 V DC for automotive circuits)
- Clamp meter (for current measurement after repair)
- Wire stripper and crimping tool (for wire repairs)
- Inspection torch / flashlight
Common mistakes
- Performing a continuity test on a live circuit — the multimeter's internal battery and measurement circuitry are not designed to compete with an external supply, giving erroneous readings and risking instrument damage.
- Replacing a fuse with a higher rating when the original blows repeatedly, masking the underlying fault and putting the wiring at risk of thermal damage.
- Assuming that because voltage is present at a connector, the circuit is complete. A high-resistance corrosion layer on a pin can allow a voltmeter to read nearly correct voltage while the circuit cannot supply meaningful current to the load.
- Not checking the return (ground/neutral) path for an open fault — techs often focus on the supply side and overlook that a broken ground wire produces identical symptoms to a broken supply wire.
- Confusing the 'open circuit voltage' reading on a battery test with a fault — a battery measured without a load will show a higher terminal voltage than under load; this is normal, not an open circuit.
Troubleshooting
- Load does not operate; multimeter reads supply voltage across the load
- Cause: Open circuit in the supply path upstream of the load — blown fuse, open switch, broken wire, or open relay contact Fix: Probe from the supply toward the load using the half-split voltage method to locate the open. Check fuse, switch contacts, and relay outputs in sequence.
- Load does not operate; multimeter reads zero volts at the load terminals
- Cause: Open circuit in the return (ground/neutral) path, or the voltage source itself is disconnected or exhausted Fix: Verify the voltage source output. Then trace the ground/return path from the load back to the negative terminal, checking for open connections, corroded ring terminals, or broken ground wires.
- Continuity test shows open across a wire that appears physically intact
- Cause: Internal conductor strand break within undamaged insulation (common in flexed cables), or high-resistance corrosion at a connector mid-run Fix: Flex the cable through its full range of motion while monitoring the continuity reading — an intermittent reading confirms an internal strand break. Replace the conductor or reconnect via a new solder joint or crimp connector at the break location.
Frequently asked questions
What is an open circuit?
An open circuit is a circuit in which the conductive path has been broken, preventing current from flowing. The break can occur at a switch, a blown fuse, a broken wire, a failed component, or a corroded connection. Current is zero; the full source voltage appears across the point of the break.
What does a multimeter read on an open circuit?
In resistance mode (ohms), a multimeter reads OL (overload), infinity, or a very large number across an open circuit — indicating no conductive path. In voltage mode, probing across the break in a live circuit will read close to the full supply voltage, because the open gap has effectively infinite resistance relative to the rest of the circuit.
How is an open circuit different from a short circuit?
An open circuit has a broken path — current is zero, resistance is infinite, voltage appears across the break. A short circuit is the opposite: an unintended low-resistance path — current is very high (limited only by source impedance and wiring resistance), voltage across the fault is near zero, and a fuse or breaker should operate to protect the circuit.
Why does full voltage appear across an open circuit break?
In a series circuit, voltage drops are proportional to resistance. An open gap has essentially infinite resistance compared to any other component in the circuit. By Ohm's Law (V = I × R), with I = 0, the load has zero voltage across it, and by Kirchhoff's Voltage Law, all of the source voltage must appear across the element with infinite resistance — the open break.
What are common causes of open circuits?
Blown fuses (by design), broken wire strands (especially in flexed cables), corroded or loose connector terminals, failed solder joints, burnt-out lamp filaments and resistors, tripped circuit breakers, and thermal breaks in fuse links. In automotive systems, vibration-loosened connector pins are particularly common.
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