Although it is not recommended, it is sometimes necessary to connect the LCV to a voltage source while the LCV is connected to a PC, for instance, if real time monitoring is required. If both the computer and the voltage source you are connecting to are connected to wall power (as is true in most cases), an isolation transformer must be used between the voltage source and the LCV voltage inputs. An isolation transformer does not have electrical connections between the primary and secondary windings. The recommended connection is shown in the figure below.
If an isolation transformer is not used, the voltage read by the LCV will not be accurate. The actual reading may be close to Actual Voltage + Line Voltage, or Actual Voltage – Line Voltage. For instance, if the LCV is connected directly to 120V line, without the Isolation Transformer, the reading may be 240V or 0V, depending on how the computer power supply is wired.