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Welcome back, Troubleshooters! We’re continuing on with our series on opens and shorts. Last week on Troubleshooting Thursdays we learned one method for finding shorts in circuits using an ohmmeter. Today we’ll show you a second method. The best tool for locating shorts is an ohmmeter. In TsT Tip 12, we discussed what an ohmmeter is, and when to use it when testing for opens. In TsT Tip 13, we learned what a short circuit is.
Finding Shorts Using an Ohmmeter: Method 2
Note: We recommend you read our “do’s and don’ts for using a meter” post.
Recall from last week that, for the purposes of this example, we are assuming that it is possible to have a short to ground anywhere on a wire or component, and that there are no visual clues as to the location of the short.
WARNING: Working with electrical equipment can be hazardous. The electrical energy contained in many circuits can be enough to injure or kill. Make sure you follow all of your company’s safety precautions, rules, and procedures while troubleshooting.
The schematic below shows a circuit that consists of two series circuits in parallel. All of the switches in series with the light must be closed in order for the light to go on.
Here’s the same circuit as it appears in the demonstration video in our Troubleshooting Electrical Circuits (TEC) module:
In this example, the fuse has already been found to be blown, and the circuit locked out and verified dead. This method is different from Method 1 in that the ohmmeter leads are left in one location while testing. First, let’s remove the blown fuse. With the ohmmeter leads on the ground terminal and the load side of the fuse, the meter reads a low resistance, indicating a short to ground:
Let’s remove the wire at switch S1. Note that the meter reading did not change:
This means that the short is in the highlighted portion of the circuit, since the meter still reads a low resistance:
Now, we’ll reconnect the wire and remove the wire at switch S4:
Note that the meter now reads 26 ohms. Since the meter no longer sees the short, it must be between the open point and the light bulb:
Next, let’s reconnect the wire at switch S4, and open the connection at switch S5:
Note that the meter still does not see the short, and therefore, it must be after the open point:
With the next test, we determine that the wire from switches S5 to S6 is shorted to ground:
With the other end of the wire disconnected, a final test confirms that the wire is the problem. We change the wire, replace the fuse, remove the lock-out, and close the breaker. When tested, the circuit now operates properly. Be sure to claim your free TEC module, where you’ll this and other practical exercises to test your skill. Check TsT next week, when we’ll discuss best practices for testing. Follow Troubleshooting Thursdays: Tune in next Troubleshooting Thursdays’ for reliable tips, general troubleshooting process, and industry insights. Stay up-to-date with Simutech Multimedia:
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