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Welcome back, Troubleshooters! In the past week on Troubleshooting Thursdays, we looked at the importance of a voltmeter, testing for open circuits and open neutrals with a voltmeter. To continue with our series on testing using a meter, today on Troubleshooting Thursdays (TsT), we will be diving into testing for opens, with the help of an ohmmeter.
Note: We recommend you read our information listed on our “do’s and don’ts for using a meter” post.
What is an ohmmeter?
An ohmmeter measures resistance in a circuit and is the best tool for finding short circuits, but can also be used to find opens in circuits. In this instalment, we will look at best practices for using an ohmmeter to find open circuits.
Things to keep in mind before you begin to test for opens using an ohmmeter:
- Power to the circuit must be shut off and locked out before taking an ohmmeter reading.
- Remember to put the leads together and take a reading to prove the meter is operating correctly before starting.
- It is always good practice to remove one wire from a component before taking a reading to ensure there are no parallel paths in your reading.
When is an ohmmeter used?
Sometimes you can’t energize a circuit for testing even though the fault is an open. In these cases, an ohmmeter must be used.
Above is an example of a simple circuit that we will be using in our demonstration video below, in the “Finding Opens with an ohmmeter section.” This is the same circuit diagram used in our previous Troubleshooting Thursdays post that covered testing for an open circuit using a voltmeter.
Finding opens with an ohmmeter
The symptom in the following scenario is that the light will not go on when all switches are closed. Our analysis is that the problem area is the whole circuit and the most probable cause is the light bulb.
In this scenario, the light does not go on when the switches are closed and like the previous examples, the most probable cause is the light bulb.
First, we need to “Lockout” the circuit and verify it is dead. Next, let’s disconnect and remove a wire at the component we have identified as the most probable cause. Then, connect one probe of the ohmmeter to a reference point and the other probe to the suspect component. Note that the meter reads “26 ohms,” which is the resistance of a light bulb. This indicates that the open is before the test point.
Reconnecting the wire and disconnecting another in the direction of the fault, we can divide the remaining circuit into equal portions. Leaving the black probe connected to the neutral, we can measure the resistance to the disconnected wire. The meter again reads the resistance of the light bulb, indicating that the open is still before the test point.
Again, the circuit is sectionalized and the resistance measured. The meter now reads “Infinity”—indicating that the open is after the test point. With one more test we can determine the switch ‘S1’ is defective since a closed switch should have a very low resistance. Let’s now replace the defective switch.
After removing the “Lockout” and closing the breaker, the circuit can be test operated to ensure it functions correctly.
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