45 Using Meters for Troubleshooting


A is designed to be used on a live circuit. When working on live circuits, all safety precautions and PPE (personal protective equipment) requirements should be adhered to.

A voltmeter is essentially a very high value of . When the voltmeter leads are connected to two points in a circuit, that high value of resistance is connected in with that point and, due to , experiences the same value of . By measuring how much current flows across the meter’s internal resistance, it can calculate the voltage value.

Voltmeters typically read zero volts when measuring between points of equal potential. Closed and connected wires are examples of components that, when energized, are of equal potential.

Voltmeters typically read line voltage when measuring between points of different potential. In a this is line voltage. In the , this is a phase-to-phase voltage, which is different from their phase-to-ground voltage.

Open switches and are examples of components that a voltmeter would measure line voltage across.

When current flows through a circuit, it drops off voltage proportional to the resistance of the device it flows through. Since closed switches and contacts offer nearly zero resistance, there will be no voltage drop measured across them. In every branch of a control schematic, there must be only one load to limit the current, and this device will have line voltage dropped across it when the circuit is operating. The coils of , , and timer relays are examples of loads that when energized would have full line voltage dropped across them. are another example of resistive loads that would experience full line voltage.


Disconnect circuit from power supply first! Before using an in a circuit, use a voltmeter to confirm that power is off and that there is zero potential difference between the two points you wish to measure.

An ohmmeter works by using an internal voltage source to push a small DC current through its leads. By measuring the value of current, it can display a calculated value of ohmic resistance. Because it has an internal voltage source, ohmmeters cannot ever be connected in live circuits as they could cause damage to equipment or injury to the operator.

When using an ohmmeter in a control circuit, there are three typical readings you can get:

  1. Close-to-zero ohms: This means that the leads of the ohmmeter are connected across two points that are . The two terminals of a closed switch or contact would give an ohmmeter reading close to zero .
  2. Very-high-to-infinite ohms: An open in the circuit would allow zero current to flow and so would read as infinite ohms. The terminals of open switches and contacts would give very high ohmic readings.
  3. Some ohms: An ohmmeter measuring across a load such as a pilot light would read a very high (approximately megaohms), but not infinite, value of ohms. This is one way of confirming that a pilot light is in good working condition. The terminals of a coil should have continuity and a low (approximately tens to hundreds of ohms), but not zero value of resistance. If a coil is shorted and reads zero ohms, it needs to be replaced.

When using an ohmmeter to test , first confirm they are removed from the circuit. If a fuse is in good condition, it should give a close-to-zero reading of resistance. If the fuse has blown due to a fault, then it should behave as an open and give an infinite resistance reading.


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Basic Motor Control by Aaron Lee and Chad Flinn is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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