Instrumentation and SCADA · Signals and Basic Control
What Are Interlocks and Permissives?
Interlocks and permissives help prevent unsafe or damaging operation.
A permissive is a condition that must be true before equipment can start. An interlock can stop or block equipment when a harmful condition exists. Bypassing either can create serious risk and requires strict authorization.
A pump may need an open discharge valve and enough liquid level before it can run. If a condition is missing, the operator finds the cause.
When equipment refuses to start, the control system may be doing exactly what it was designed to do. Look for the missing permissive instead of repeatedly pressing Start. A low wet-well level, closed valve, tripped overload, or unavailable downstream process may block operation. The screen may show the cause, but field checks can still be needed. Never defeat a protective condition just to make the equipment run. Escalate the problem under the plant procedure.
A blocked start often means a protective condition is working correctly.
Key point: A permissive must be true before equipment can start; an interlock can stop or block equipment when a harmful condition exists.
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Wastewater Instrumentation, SCADA, and Process Control Fundamentals
This page comes from the course. It covers plant instruments, signals and control, flow, level, pressure and water-quality measurement, SCADA and alarms, and instrument maintenance, one step at a time, with short knowledge checks and a certificate of completion.
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