Interlocking

Some things must never be on at once. Interlocking is the wiring that makes "both on" impossible — and it is the safety foundation under every reversing circuit.

Why interlocking exists

Some pairs of devices must never be on at the same time. The classic case is a reversing starter: the FORWARD contactor and the REVERSE contactor swap two of the motor's power legs. If both ever closed together, you'd connect two power phases straight to each other — a dead short. Big bang, blown contactor, tripped breaker, possible injury. Interlocking is the wiring that makes "both on" physically impossible.

Electrical interlocking

The most common method uses each device's own contacts against the other. You run coil F's circuit through a normally-closed contact controlled by coil R, and coil R's circuit through a normally-closed contact controlled by coil F. Now whichever coil energizes first opens its NC contact in the other's rung — locking the other one out until the first drops.

Mechanical interlocking

Beyond the wiring, reversing contactors are often bolted together with a mechanical interlock — a physical lever or seesaw so that if one armature pulls in, the other physically can't. Good designs use both electrical and mechanical interlocking: belt and suspenders, because the failure here is dangerous.

What to take away

Interlocking guarantees two devices can't energize together. Electrically, you wire each coil through the other's NC contact so the first one in vetoes the second. Mechanically, a physical link prevents both from pulling in. You'll use this in every forward/reverse circuit — which is exactly where we go next.