Interlocks and Permissives
Interlocks block dangerous actions and permissives allow safe ones; together they enforce that a system operates only within its safe envelope.
Enforcing Safe Sequences
An interlock is logic that prevents an action unless conditions make it safe, or that forces an action when a condition becomes unsafe. A permissive is the complementary idea: a set of conditions that must all be satisfied before an operation is allowed to proceed. Together they ensure equipment cannot be operated in a hazardous sequence or outside its safe envelope.
Examples of the Logic
- A high-voltage supply cannot energize unless access doors are closed and grounded
- A valve cannot open unless downstream pressure is within limits
- A magnet cannot ramp unless cooling and quench protection are confirmed active
- Loss of any required permissive trips the operation to a safe state
Hardwired Versus Software Interlocks
Interlocks exist at more than one layer. The most critical are often hardwired: implemented in relays or simple logic that acts independently of any computer, so a software fault or crash cannot defeat them. Software interlocks add flexibility and richer conditions but are considered a lower-integrity layer. Defense in depth uses both, with the hardwired layer as the last line that no software error can override.
Permissive Chains
A permissive chain gathers many conditions into a single go or no-go signal. Every condition must be true for the operation to be enabled, and the loss of any one removes the permissive. Designing the chain to be fail-safe, so that a lost signal or lost power removes the permissive rather than granting it, ensures that faults in the interlock system itself do not enable an unsafe action.
Role in the Safety Architecture
Interlocks and permissives are the enforcement mechanism of the broader safety-instrumented system. They translate the safety analysis, which conditions are dangerous and what must be prevented, into logic that acts automatically and independently of operator judgment and normal control. In complex facilities they are documented as an explicit matrix of conditions and actions so that every protective response can be traced, tested, and verified.