Contactors, ladder logic and fail-safe design

A contactor (KM) is a switch worked by an electromagnet, its coil. A small control current through the coil closes large power contacts that carry the motor current. Its contacts come in two kinds:

NO, normally open (IEC terminals 13-14): closed only while the coil is energised.
NC, normally closed (for example overload terminals 95-96): open only while actuated.

Control circuits are drawn as ladder diagrams: two vertical rails (supply and neutral) with horizontal rungs between them. A rung's coil is energised when a path of closed contacts connects the rails.

The seal-in circuit

Press START and the coil KM1 energises. Its own auxiliary contact KM1 13-14, wired in parallel with START, closes. When you let go of START, current still reaches the coil through that contact: the circuit holds itself in. STOP (NC) or the overload relay F2 (NC) breaks the rung, the coil drops out, and the seal-in opens.

Try it: ladder logic starter

Overload relay tripped (F2)

Motor stopped

Hold START briefly, release it, and watch the KM1 seal-in contact keep the coil energised.

FoundationStart here if this is new to you

The seal-in contact is like propping a door open with your foot after pushing it. The push (START) opens it; your foot (the KM1 contact) keeps it open after you let go. STOP is someone pulling your foot away. A useful side effect: after a power cut the motor does not restart on its own, which is safer for anyone working on the machine.

Star-delta and interlocks

Switch the simulator to star-delta. The main contactor KM1 and the star contactor KM2 start the motor in star. After the timer KT1, KM2 drops out and the delta contactor KM3 closes. An NC contact of KM3 sits in KM2's rung and an NC contact of KM2 in KM3's rung. This interlock makes it impossible for star and delta to be closed together, which would short-circuit the supply.

Turn on "slow-breaking star contactor" and then remove the interlocks. The short circuit appears. Put the interlocks back and it cannot happen, even with the slow contactor.

Safety chains

A safety chain is many NC contacts in series (door locks, overspeed governor, limit switches, emergency stops) feeding one safety relay. Current reaches the relay only if every contact is closed: a logical AND.

Predict first

Why are safety contacts wired as normally-closed switches in series, instead of normally-open switches in parallel?

It uses less wireA broken wire then stops the machine instead of disabling the stopSeries circuits carry more current

The brake follows the same idea. It is spring-applied and electrically released: power holds it open, so losing power clamps it shut. This is fail-safe design: every failure pushes the system into its safe state.

Finding the fault. Measure the voltage to neutral at each node along the chain. The faulty element is the one with voltage on its input side and none on its output side. Halving the chain each time finds it in $\log_2 n$ measurements (Tutorial 3.3).

ExplorerGo deeper: derivations and open questions

Scan order matters. A PLC evaluates rungs top to bottom and a coil written in one rung is seen by the rungs below it in the same scan. In the simulator the star rung comes before the delta rung, so on the changeover scan KM2 drops before KM3 is evaluated. Swap the order in your head: would the interlock still protect you?

Reliability of a series chain. If each of $n$ contacts works with probability $R$ , the chain works with probability $R^n$ . More contacts means more nuisance trips, but each added contact is another condition that must be safe before the machine can move.