Mechanical Interlock in Contactors

In electrical control systems, especially where two contactors are used together, it is important to prevent them from being energised at the same time. If both contactors were to close simultaneously, it could cause a short circuit, equipment damage, or even safety hazards. This is where the mechanical interlock comes in.

What is a Mechanical Interlock?

A mechanical interlock is a physical device built between two contactors that prevents both of them from being closed at the same time. It works like a locking bar or sliding mechanism that allows only one contactor to operate. If one contactor is energised, the interlock physically blocks the other contactor from moving to its closed position.

This type of interlocking provides a fail-safe method of preventing dangerous conditions, even if the electrical interlocking fails due to wiring errors or contact malfunction.

Where is it used?

Mechanical interlocks are commonly used in:

• Forward and reverse motor control circuits (to prevent shorting of supply lines).

• Star-Delta starters (to ensure Star and Delta contactors are not ON together).

• Changeover systems (to switch safely between the main supply and backup supply).

How It Works

1. Two contactors are mounted side by side.

2. A mechanical interlock bar connects them.

3. When Contactor A is energised → the bar shifts → Contactor B is physically blocked.

4. When Contactor B is energised → the bar shifts the other way → Contactor A is blocked.

5. Only when one contactor is OFF can the other operate.

This ensures mutual exclusivity—only one contactor can remain ON at any given time.

Difference Between Electrical and Mechanical Interlock

• Electrical Interlock: Uses auxiliary contacts to prevent both contactors from being energised simultaneously through wiring.

• Mechanical Interlock: Uses a physical locking mechanism to prevent contactor movement.

In practice, both methods are often combined for extra safety.

Advantages of Mechanical Interlock

• Prevents dangerous short circuits.

• Provides an additional layer of safety beyond electrical wiring.

• Reliable even if the control wiring fails.

• Simple and durable mechanism.

Example: Forward-Reverse Motor Control

In a forward-reverse motor control circuit, two contactors are used:

• Forward Contactor (F) connects the motor to rotate clockwise.

• Reverse Contactor (R) connects the motor to rotate counterclockwise.

If both were ON together, it would short the power supply. The mechanical interlock ensures that when F is ON, R is locked, and vice versa.

The mechanical interlock in contactors is a crucial safety device in motor control and changeover systems. By physically preventing two contactors from closing at the same time, it safeguards equipment, reduces the risk of short circuits, and ensures reliable operation.