Self‑Locking, Interlocking, and Safety Interlock Circuit Diagrams

First, let’s examine the IEC (International Electrotechnical Commission) standard terminal numbering system for 230 V ±10% AC contactors.

Front

Top:A1 A2 1,3,5 (13 NO)

Bottom:A2-2 2,4,6 (14NO)

When terminals A1 and A2 of the contactor are connected to the neutral (N) and live (L) wires, the contactor coil energizes and pulls in.
The normally open (NO) contacts close (become conductive). At this moment, terminals 1-3-5 and 13-14 allow current to pass, achieving the ON state of the circuit.

You can set the multimeter to the continuity (buzzer) mode.
The multimeter will emit a beeping sound, indicating that the circuit is connected (has continuity).

circuit diagram

Self-Locking

A self-locking circuit uses the contactor’s normally open auxiliary contact to maintain the coil energized, ensuring the motor continues running.
It is also called a start-stop holding circuit.

Mutual Locking

The normally closed auxiliary contacts of two contactors are inserted into each other’s coil control circuit to prevent the equipment from operating at the same time.
It is commonly used in motor forward-reverse control circuits.

Interlocking

Sequential (Interlock) Circuit

Through logical relationships, the circuit ensures equipment starts or stops in a specific order, preventing incorrect operation and protecting the equipment from damage.