An example of voltage drops in a control circuit will illustrate the use of voltmeters for troubleshooting.
Referring to the diagram above and assuming a line voltage of 120V, if the circuit is operating normally we can determine what all the meters would be reading.
If someone were to press the start button, then only voltmeter A would read line voltage, as it is in parallel with the two loads.
Voltmeter B will measure zero volts across the closed contacts of the stop button.
If an overload occurs and the OLR contacts open, voltmeter C will read line voltage while the start button is being pressed, and nothing when it is released.
Assuming the motor is not running, only voltmeter D would give a reading of line voltage. If the contactor is not engaged, then the normally closed contact will remain closed and the red pilot light will be on.