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2.2: Troubleshooting principles

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    Troubleshooting principles

    There are really only two rules for troubleshooting using a voltmeter. They are simple and always true:

    1. If you measure a voltage across a switch, the switch is open.
    2. If you measure a correct voltage across a load and the load doesn’t work, the load has failed.

    With digital meters, voltage readings that are considered as zero will often indicate very small voltage readings. For example, when reading across a closed switch, a very small reading could indicate a very slight resistance across the switch contacts or even a meter inaccuracy.

    Notice that the first rule does not say that if you read zero volts across a switch, the switch is closed. There are many situations in which you might read zero volts across an open switch.

    The second rule indicates that the load has failed. This only means that the problem is with the load and you don’t have to look anywhere else for the problem. The actual remedy still has to be determined. This may require a replacement of the load, but there may be other possibilities. For example, there may be an overload that needs resetting.

    Always look for the easy fix first. Check components that are easily accessible first that might explain the symptom that you have observed. For example, one of the first checks is to verify the power supply.

    Voltage tests

    You can troubleshoot a problem using either volt or ohms tests. It is most practical to choose voltage testing. With a resistance test, you have to first disconnect the component being tested from the circuit, and while you are removing the wiring you could jostle things and possibly change the circuit, which may temporarily remedy the problem. In other words, you may not really find the problem.

    When you use your voltmeter to troubleshoot, you will find either a switch that is open or a load that has failed. You can do this without moving any wires and without changing the circuit in any way. You may then remove the device and double check it with your ohmmeter.

    Voltage drops in series circuits

    In series circuits, the total voltage is the sum of the individual voltage drops in the circuit, and the equation E = IR is used to calculate the voltage drop across each resistor. Since the current is the same through each resistor, the voltage drop across each resistor is directly proportional to the value of resistance. In other words, the greater the value of a resistor in a series circuit, the higher the voltage drop. Consider the simple series circuit in Figure 1.

    Figure \(\PageIndex{1}\): (CC BY-NC-SA; BC Industry Training Authority)
    1. Series circuit
      Figure \(\PageIndex{1}\): (CC BY-NC-SA; BC Industry Training Authority)
    2. Voltage across an open
      shock.png Caution! Since the source voltage still exists across the open in a series circuit, this represents a shock hazard. Be careful not to touch the live parts of the circuit!

      Similarly, if a switch is opened, the full-source voltage will appear across the switch contacts. Even though the voltage across the load devices may be zero, if any of those loads are ahead of the switch they will be energized with full voltage to ground.

      Troubleshooting series components

      Sometimes you will be required to troubleshoot a piece of equipment that has stopped working. The first thing you would check for is power. Is the breaker off? Is the switch off? Is there a general power outage?

      Once you have determined that power is still available you can begin using the multimeter to locate the problem. Starting with the first component or the one easiest to check, work your way through the circuit until you reach the component that shows no voltage reading. This is known as hopscotch voltage readings. Figure 3 illustrates this process. The dashed line indicates where the probe has already been placed and removed.


      Figure \(\PageIndex{1}\): (CC BY-NC-SA; BC Industry Training Authority)

    3. Hopscotch troubleshooting
      Figure \(\PageIndex{1}\): (CC BY-NC-SA; BC Industry Training Authority)
    4. Ohm test of a load
      importance.png There may be other circuits that are energized even though the circuit you are working on is not energized. DO NOT TOUCH THE METER PROBES TO ANY ENERGIZED COMPONENTS WHEN TESTING FOR CONTINUITY. YOU MAY DAMAGE THE METER.

      Continuity test

      This is a quick audible alarm test using a digital multimeter to determine whether an electrical circuit or wire is complete or broken.

      This test can be applied to a circuit as a whole or in sections—on individual components or sections of wiring. A break in continuity can be caused by mechanical damage, corrosion of components, or simply a switch being left open.

      Follow these steps to complete the continuity test procedure with an autorange digital meter:

      • Make sure all power is off in the circuit you are testing.
      • Set the selector dial to Ω (audible alarm symbol).
      • Connect the test lead and probes on the load terminals as shown (Figure 5). The audible alarm will indicate continuity without a need for taking your eyes off the work.
      • Touch the probes together to check the leads, connections, and battery life. The audible alarm should sound. With the leads apart the meter should display OL or I, depending on the manufacturer.
      • If this is the last test you are doing, turn the meter to “off” and store it in a safe place.


      Figure \(\PageIndex{1}\): (CC BY-NC-SA; BC Industry Training Authority)

    5. Wiring for a continuity test

      importance.png Note: There may be other circuits that are energized even though the circuit you are working on is not energized. DO NOT TOUCH THE METER PROBES TO ANY ENERGIZED COMPONENTS WHEN TESTING FOR Ω (RESISTANCE). YOU MAY DAMAGE THE METER.