[Updated] Troubleshooting Thursdays: How to test for an open circuit using a voltmeter (Tip 11)

Posted on: September 27, 2015 by in RSSMasher
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Welcome back, Troubleshooters! In the past weeks on Troubleshooting Thursdays, we looked at the differences between an open circuit and a short circuit, as well as how to test open and short circuits safely. We also went over the importance of determining whether you should test the circuit while it is live or dead—before you test for a fault.

Following up on testing, today on Troubleshooting Thursdays (TsT), we will be diving into testing an open circuit safely, with the help of a voltmeter.  

Note: We recommend you read the information below before you begin testing an open circuit:

  • Be familiar with the voltmeter’s features—read the instruction manual before using.
  • Ensure it’s free from obvious damage and safe to use.
  • Test the voltmeter on a known voltage source before using. Your meter should read the correct voltage.
  • Keep your fingers behind the finger guards on the test probes when taking measurements.

We recently covered the Do’s and Don’ts for using a meter.

How to test for an open circuit using a voltmeter

A voltmeter allows you to test for the difference in voltage between two points. It is important to understand what the voltmeter reading tells you about the components and the circuit.

Above is an example of a typical circuit used in the demonstration video below.  

Testing for Opens with a Voltmeter

Voltmeters are the best tool to use for finding open circuits—if you can safely turn the power on. The following video discusses techniques for finding open circuits using a voltmeter.

This video shows a few possible faulty circuit scenarios. Try your hand at Simutech’s Troubleshooting Electrical Circuits and test your troubleshooting skills. Get your Free TEC Licence today!

In this scenario, we have tried to operate the circuit but the light will not go on. Our observations show no obvious defects or causes.

Upon analysis, we have determined the problem area to be the whole circuit, since an open in any part or wire of this series circuit could prevent the light from lighting. The most probable cause, however, is a burnt out light bulb.

Now let’s begin our testing. First, let’s check the meter on a known source. Connect the black probe to a reference, the neutral in this case, and then connect the red probe to the line terminal. The meter reads 115 volts.

Next, let’s test the light bulb. Leave the black probe on the common and move the red probe to the line side of the light. The meter reads zero volts, meaning it is not getting the correct voltage and that there must be an open before the light.

For the next test, let’s choose a location midway in the circuit. This allows us to divide the circuit into two and determine which part is defective. The meter reads one hundred and fifteen volts, indicating the fault is between the test point and the light.

Dividing the circuit again, the next meter reading of zero volts indicates the fault is before the test point.

The next test also reads zero volts. This indicates that the S2 switch is defective since the meter reads one hundred and fifteen volts on one side and zero volts on the other. There should be no difference in voltage across a closed switch.

Since our tests are now complete, we can turn the meter off. To hide the meter, select OFF.

To replace the switch we must first lockout the circuit and verify it is dead. Then, using the wrench we can replace the defective switch. Notice that the time and cost of the replacement are shown. After removing the lockout and closing the breaker, we can test-operate the circuit to make sure the repairs have fixed the problem.

It is important to understand what the voltmeter reading is telling you about the component and the circuit.

This next video features the same circuit, but in this example, it is the presence of voltage that reveals the problem, rather than its absence.

Testing for open neutrals with a voltmeter

This video shows a few possible faulty circuit scenarios. Try your hand at Simutech’s Troubleshooting Electrical Circuits and test your troubleshooting skills. Get your Free TEC Licence today!

In this scenario, the light does not go on when the switches are closed, and like the previous example, the most probable cause is the light bulb.

To begin our testing, let’s check the meter on a known source.

Now we can test the light bulb. Leaving the black probe on the common, move the red probe to the line side of the light. The meter reading of one hundred and fifteen volts means the circuit feeding the light is okay and the open must be in the light bulb or the neutral wire.

The next test is the other side of the light bulb. If the light bulb is bad, it should read zero volts. However, the meter reads one hundred and fifteen volts, indicating that the open is not in the light bulb but in the neutral wire.

To replace the wire, we must first lockout the circuit, verify it is dead, and disconnect one end of the wire with the screwdriver.

Then, using the wrench we can replace the wire by clicking on the disconnected wire end. Notice that the time and cost of the replacement are shown. After removing the lockout and closing the breaker, we can test-operate the circuit to make sure the repairs have fixed the problem.

These videos showed a few possible faulty circuit scenarios. Try your hand at Simutech’s Troubleshooting Electrical Circuits and test your troubleshooting skills. Get your Free TEC Licence today!

Coming up next week:

Finding opens using an ohmmeter on a dead circuit
Sometimes you can’t energize a circuit for testing even though the fault is open. In these cases, you must use an ohmmeter.

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