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Revision 5.00 4th Sept. 2013


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Open Circuit Faults

Open Circuits

Current will only flow IN A CIRCUIT. That is, around a continuous path (or multiple paths) from and back to the source of EMF. Any interruption in the circuit, such as an open switch, a break in the wiring, or a component such as a resistor that has changed its resistance to an extremely high value will cause current to cease. The EMF will still be present, but voltages and currents around the circuit wil have changed or ceased altogether. The open switch or the fault has caused what is commonly called an OPEN CIRCUIT.

Remember that wherever an open circuit exists, although voltage may be present there will be no current flow through the open circuit section of the circuit. Also, as Power(P) is V x I and the current (I) = 0, no power will be dissipated.

Looking further at the simple circuit used in Labelling Voltages and Currents let´s put some actual voltages and currents in and see what happens under "Open Circuit" conditions.

Use the drop down box below the following diagram to select a number of open circuit conditions that might occur in different parts of the circuit. Notice how the voltages and currents around the circuit change depending on where the break in the circuit (the open circuit) occurs. Checking the voltages around a circuit with a voltmeter, and noticing where they differ from what would be expected in a correctly working circuit, is one of the main techniques used for tracing a fault in any circuit.

Open Circuit Examples.

Open Circuit Examples

Choose a fault to see the effects of an open circuit.

The opposite extreme fault condition to having an open circuit is having a component or components go "Short Circuit", which is dealt with here.


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