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Interlocks in Ladder Logic: Protecting Your Equipment and People

 Interlocks in Ladder Logic: Protecting Your Equipment and People

Of everything you'll ever touch in a ladder program, interlock logic deserves the most respect. Everything else in this series — timers, counters, seal-in circuits — is about making equipment do what you want. Interlocks are about stopping equipment from doing what could hurt someone or destroy a machine. Get this part wrong, and the consequences aren't "the conveyor didn't start." They're injuries, fires, and six-figure equipment damage.

What an Interlock Actually Is

At the logic level, an interlock is nothing exotic — it's a permissive contact, often several of them in series, that has to be true before an output is allowed to energize. The complexity isn't in the instruction set; it's in getting the conditions right, and making sure nothing can bypass them.

Real example: 

A raw mill can't start unless the mill fan is already running, the separator is up to speed, and no E-stop is active anywhere in the chain. Each of those is a contact in series ahead of the mill start coil. Miss one, and you risk starting a mill into a system that can't handle it — or worse, into a maintenance condition where someone is still inside a guarded area.

Two Categories You Need to Keep Straight

Permissive interlocks 

 conditions that must be true before something is allowed to start. Fan proven running before damper opens. Lube oil pressure established before a gearbox drive engages.




Protective (trip) interlocks 

 conditions that stop something that's already running. High bearing temperature trips a motor. Low lube pressure trips a compressor. These often use latching logic (from our seal-in post) so the trip stays flagged until someone investigates and clears it — you don't want equipment to silently restart the moment a transient fault condition clears itself.





Hardwired vs. Soft (PLC) Interlocks

This is a distinction every technician needs to internalize early: not every interlock should live in the PLC program. Safety-critical interlocks — emergency stops, certain guard interlocks, some over speed protections — are often (and in many cases, by design code, must be) hardwired independently of the PLC, so a PLC fault, a program bug, or a communication failure can't defeat the safety function. The PLC handles process interlocks — the ones that protect equipment and production, where a brief delay or a PLC restart isn't a life-safety issue. Knowing which category a given interlock belongs to is a design decision, not something to improvise in the field.

The Mistake That Should Never Happen (But Does)

Bypassing an interlock in the program to "get through a shift" during a fault-finding session, and forgetting to remove the bypass afterward. I've seen this happen — a jumper rung added around a permissive contact to prove a motor could actually run mechanically, left in the program because the shift ended and nobody documented it. Weeks later, a completely unrelated technician is troubleshooting something else, has no idea the interlock is defeated, and the protection simply isn't there when it's needed.

If you ever have a legitimate reason to bypass an interlock temporarily, the rule should be: document it, tag it, time-box it, and make it visible to the next shift — never a silent, permanent change buried in the logic.

Reading Interlock Chains When You're New

When you open an unfamiliar program and see a rung with eight contacts in series feeding one coil, don't panic — trace them one at a time, in order, and ask what each one is actually protecting against. Nine times out of ten, each contact maps to a specific, nameable hazard or equipment condition, and once you've identified all eight, the rung tells its own story.

Next in the series: 

The PLC scan cycle — the invisible clock that determines exactly when your rungs actually execute, and why rung order in your program matters more than most people realize.


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