Operation of Interlock Systems in Distributed Control System (DCS)

Manual vs. Automatic Operation of Interlock Systems

Operation of an interlock system may take place either through manual or automatic trip.

In a manual trip, the interlock system is manually actuated from a switch or pushbutton, which may be located on a local panel in the field or in the control room. A manual trip allows an operator to trip the system independently of the interlock system in the event of a hazardous situation developing.

An automatic trip, as the name implies, is automatically activated when a hazardous situation is detected. A common example is the de-energizing of a solenoid valve that fails a control valve to its safe position.

Manual vs. Automatic Reset of Interlocks

A tripped component (e.g. a solenoid valve) needs to be reset after a trip had been initiated. The reset may be done either automatically or manually.

Self-Canceling Interlock has automatic reset that returns the interlock system to normal operation when the usual process conditions had been re-established or when the offending situation had been effectively dealt with.

Manual Reset Interlock requires the operator to re-initiate the process before continuing the operation of the equipment involved. This method is generally preferable over automatic reset because it requires an investigation of the possible causes for the trip. Positive action by the operating personnel to return the operating conditions to normal is required before the interlock can be cancelled.

Manual reset on a solenoid valve is most commonly carried out by the use of a latching lever that locks the valve when a trip occurs. Unlatch of the lever is required to return the solenoid valve to its normal operation. Alternatively, a solenoid valve can simply be reset by the use of a pushbutton that energizes the valve.