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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 what short circuits are, and next, we’re going to look at how to find them. Today we’ll teach you one method for finding shorts in circuits using an ohmmeter, and next week in Part 2, we’ll show you a second method.

Finding Shorts Using an Ohmmeter: Method 1

The best tool for locating shorts is an ohmmeter. Two weeks ago (see TsT Tip 12), we discussed what an ohmmeter is, and when to use it when testing for opens.

Note: We recommend you read our “do’s and don’ts for using a meter” post.

Now, before we get started, for the purposes of the following example, let’s assume 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.

Okay, here we go.

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 is a series circuit in which all switches must be closed in order for the light to go on.

Shorts-Ohmmeter

The most probable cause of the short is the fuse. To check it, use the voltmeter and test the load side and source side of the fuse. The readings (zero on the load side and 115 on the source side) indicate that the fuse is blown.

Before replacing the fuse, however, we need to determine if there is something obvious causing it to blow.

First, lock out the circuit, verify it is dead, and then remove the fuse. We won’t replace it just yet.

With an ohmmeter, test from the load side of the fuse to ground:

finding shorts

Our reading of a low resistance indicates that there is a short to ground. Since there is no obvious location for this short, let’s choose a location that will divide the circuit roughly in half:

finding shorts 3

Remove the wire at this location, and test at the component terminal:

finding Shorts

Since the meter reading is low, the fault must be below the test point:

finding shorts

It’s important to have only one wire disconnected at a time, so we must reconnect the wire and then open a new test point:

Finding shorts

The low resistance reading of this test indicates the short is still below the test point:

Finding Shorts

Next, let’s reconnect the wire and open the next test point. This reading indicates that the short is in the wire between S3 and the light bulb:

finding shorts

With the other end of the wire disconnected, a final test shows that this wire is in fact shorted to ground:

finding shorts

First, replace the wire, and then replace the fuse, remove the lock-out, and close the breaker.

When tested, the circuit now operates properly.

Our TEC module contains a practice exercise for this technique. 

Be sure to check TsT next week, when we’ll discuss a second method for finding shorts with an ohmmeter. In the coming weeks, we’ll also discuss best practices for testing.

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Tune in next Troubleshooting Thursdays’ for reliable tips, general troubleshooting process, and industry insights. Stay up-to-date with Simutech Multimedia:

Have a subject you would like Troubleshooting Thursday to cover? Send Simutech Multimedia an email at [email protected]

Looking to give simulation learning a try? Get our award-winning first module (Troubleshooting Electric Circuits) for free, get started here: Get Free TEC.

However, in our scenario for Method 1, the light will not turn on when the switches are closed.

Looking to test for shorts with the help of an ohmmeter? Get our award-winning first module (Troubleshooting Electric Circuits) for free, get started here: Get Free TEC.

Here’s the same circuit as it appears in the demonstration video in our Troubleshooting Electrical Circuits (TEC) module:

The most probable cause of the short is the fuse. To check it, use the voltmeter and test the load side and source side of the fuse. The readings (zero on the load side and 115 on the source side) indicate that the fuse is blown.

Before replacing the fuse, however, we need to determine if there is something obvious causing it to blow.

First, lock out the circuit, verify it is dead, and then remove the fuse. We won’t replace it just yet.

With an ohmmeter, test from the load side of the fuse to ground:

finding shorts

Our reading of a low resistance indicates that there is a short to ground. Since there is no obvious location for this short, let’s choose a location that will divide the circuit roughly in half:

finding shorts 3

Remove the wire at this location, and test at the component terminal:

finding Shorts

Since the meter reading is low, the fault must be below the test point:

finding shorts

It’s important to have only one wire disconnected at a time, so we must reconnect the wire and then open a new test point:

Finding shorts

The low resistance reading of this test indicates the short is still below the test point:

Finding Shorts

Next, let’s reconnect the wire and open the next test point. This reading indicates that the short is in the wire between S3 and the light bulb:

finding shorts

With the other end of the wire disconnected, a final test shows that this wire is in fact shorted to ground:

finding shorts

First, replace the wire, and then replace the fuse, remove the lock-out, and close the breaker.

When tested, the circuit now operates properly.

Our TEC module contains a practice exercise for this technique. 

Be sure to check TsT next week, when we’ll discuss a second method for finding shorts with an ohmmeter. In the coming weeks, we’ll also discuss best practices for testing.

Follow Troubleshooting Thursdays

Tune in to Troubleshooting Thursdays for reliable tips, general troubleshooting process, and industry insights. Stay up to date with Simutech Multimedia:

Have a subject you would like Troubleshooting Thursdays to cover? Send Simutech Multimedia an email at [email protected].

Looking to give simulation learning a try? Get started with our award-winning first solution here: Get Demo.

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