The process signal is represented by an electric current and is transferred through an instrumentation current circuit. To measure the loop current, the milliammeter process parameter can be used. The circuit must be “broken” at some point to connect the meter in series with the current, and this means that the current will drop to 0 mA until the meter is connected.
Here in the picture describes a control valve I / P transmitter with two wire connection. If the signal to be interrupted comes from a process transmitter to a controller, the controller must be placed in manual mode so that it does not cause a disturbance in the process (moving the final control element in response to the sudden loss of the signal PV))
In addition, process alarms must be temporarily disabled so they do not cause panic. If this current signal also activates the process shutdown alarms, they should be temporarily disabled so that nothing goes off when the signal is interrupted.
If the current signal to be interrupted is a command signal from a controller to a final control element, the final control element must be replaced manually to maintain a fixed setting while the signal varies, or must be completely avoided. some other device (s). If the final control element is a control valve, this typically takes the form of the opening of a bypass valve and the closing of at least one blocking valve.