Interlocks
- Last UpdatedFeb 06, 2024
- 5 minute read
In a production environment, an interlock is a physical connection between two pieces of equipment that links their operation, so that an event in one piece of equipment will trigger an action in the other. Typically enabled using PLC logic, interlocks are built into a production system to maintain safety, protect equipment and minimize production costs.
Plant SCADA allows you to represent the interlocks that exist in your system within a Situational Awareness project. The activity associated with an interlock can then be presented to an operator at runtime in the Information Zone.
For example, the "Mixing" page in the ExampleSA project includes a mixer that receives ingredients from four supply tanks on the "Ingredients" page.
The mixing tank has an outlet valve (OV07). To ensure that this valve is not open while ingredients are being delivered to the mixer, interlocks exist between OV07 and the valves and pumps that deliver the four ingredients for mixing.
This example will be used to demonstrate the role each of following components play when configuring interlocks in a Situational Awareness project.
Equipment References are used to represent interlocks in a Situational Awareness project, as they allow you to build relationships in an equipment model that reflect those created by the interlocks that exist in the field.
To support the example above, the following equipment references are configured in the ExampleSA project.
The Referenced Item property specifies the equipment item that will trigger the interlocked state. For example, DRV01 will trigger an interlock when the digital variable associated with its "Running" item equals 1 (TopMilk_DRV01_Running = 1).
The Comment property can be used to provide a description of the event that caused an interlock to trigger, for example "Sugar DRV01 is Running".
When triggering occurs, it causes the "state item" in the interlocked piece of equipment to change. By default, the state item for a piece of equipment is "Running". If this is not the item that should respond to an interlock, you can use the runtime parameter "Interlock_StateItem" to specify a different state item for a piece of equipment.
For example, OV07 does not have a "Running" item as it is a valve. Interlock_StateItem is used to specify "Open" as the correct item.
This enables the interlock behavior demonstrated in the following diagram.
Note: The IsTag field for a Runtime Parameter defaults to TRUE if no value is specified. If you do not enter FALSE, interlocks will not function correctly.
Alarms are used to generate the events required to track the activity associated with interlocks. They determine how long an interlock has been active, or when it was last active.
In the ExampleSA project, the following digital alarms have been configured for the two pieces of equipment that supply sugar to the mixing tank (DRV01 and VLV09).
These alarms monitor the items that are configured to trigger an interlock, providing a record of the start and end times.
Similarly, alarms are configured for the six other pieces of referenced equipment to monitor the items that trigger an interlock.
The following alarm has also been configured to monitor the "Open" item for the outlet valve OV07.
Note: These alarms are intended to be hidden from the operator at runtime. This can be achieved using an alarm category that has ShowOnActive and ShowOnSummary set to FALSE.
Each equipment reference can also be assigned to a Category. In the case of interlocks, the category is used to identify the nature of an interlock. The Situational Awareness Starter Project provides the following default categories.
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Safety — conditions that cause equipment to stop running.
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Process — conditions that cause equipment within a process to stop running.
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Permissive — conditions that stop equipment from starting.
Safety and Process conditions are similar in that both types stop an action when the interlock is triggered. Permissive interlocks are slightly different as they stop an action or process from starting when an interlock is triggered. Process and Permissive interlock conditions may overlap.
In the ExampleSA project, the Category specified for each of the equipment references associated with the OV07 output valve is "Interlock.Process".
When you view the interlocks at runtime, these categories are represented as a set of tabs in the Information Zone. If an interlock is currently active for a selected piece of equipment, a lock symbol is displayed next to the corresponding category tab.
Based on the category specified, all the interlocks associated with OV07 will appear on the "Process" tab.
The interlocks are listed on each tab from oldest (triggered first) to most recent.
Interlocks can be configured to support a bypass. This means an operator can manually override the interlock, forcing the locked process to be available to start (see Configure a Bypass for an Interlock). When enabled, an interlock can be bypassed by an operator in the Information Zone.