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Mechanical interlocks are widely used in:

• Reversing motor starters
• Contactor-based transfer switches
• Dual power supply systems
• Industrial control panels

Unlike electrical interlocking, which depends on auxiliary contacts and control wiring, a mechanical interlock provides a physical safety barrier independent of the control circuit. For contactor assemblies, mechanical interlocking solutions are commonly designed according to the requirements of IEC 60947-4-1, the international standard covering contactors and motor-starters.

Why Electrical Interlocking Alone Is Not Enough

A standard reversing starter normally uses two contactors with an electrical interlock. Each contactor coil circuit includes a normally closed (NC) auxiliary contact from the opposite contactor.

When one contactor energizes:

• Its auxiliary contact opens
• The opposite contactor coil circuit is interrupted
• Both contactors are prevented from closing under normal conditions

However, electrical interlocking relies on electrical components and wiring integrity. Certain faults can bypass this protection.

Common failure conditions include:

• A welded auxiliary contact that fails to open the opposing coil circuit
• Incorrect control wiring during installation or maintenance
• A damaged auxiliary contact block
• External control faults that energize both coils simultaneously

A mechanical interlock adds an independent physical protection layer. Because it operates through the contactor mechanism itself, it does not rely on coil voltage, auxiliary contacts, or control wiring.

Mechanical Interlock vs Electrical Interlock

Although both systems prevent simultaneous contactor operation, they work in different ways.

In industrial motor control applications, the highest level of protection is achieved by using both electrical and mechanical interlocking together. The electrical interlock prevents the blocked contactor coil from remaining energized, while the mechanical interlock prevents physical simultaneous closing.

How a Mechanical Interlock Works

A mechanical interlock is a mechanical linkage installed between two compatible contactors. The interlock connects with the operating mechanism of each contactor. When one contactor closes, the interlock moves into a blocking position and prevents the opposite contactor from moving into the closed position.

The mechanism may use different designs depending on the manufacturer, including:

• Rocker mechanisms
• Sliding blocks
• Mechanical locking levers

The working principle remains the same:

• Only one contactor can reach the closed position at any time.

The KRIPAL UKC1 series uses a modular interlock accessory designed for compatible contactors of the same frame size. The interlock fits between two contactors and provides mechanical blocking protection against simultaneous operation.

Mechanical Interlock Wiring Diagram

In a typical reversing contactor application, two types of interlocking are used together:

1. Electrical Interlock Wiring

The normally closed auxiliary contact of each contactor is connected in series with the opposite contactor coil.

Example:

• Forward contactor NC auxiliary contact → Reverse coil circuit
• Reverse contactor NC auxiliary contact → Forward coil circuit

When the forward contactor energizes, its NC auxiliary contact opens and prevents the reverse contactor coil from energizing.

2. Mechanical Interlock Installation

The mechanical interlock is installed between the two contactors.

Its function is independent of the control circuit:

• Forward contactor closes → reverse contactor physically blocked
• Reverse contactor closes → forward contactor physically blocked

This creates two independent protection layers.

Reversing Contactor Application

A reversing starter uses two contactors to change the rotation direction of a three-phase motor by swapping two phases.

Typical connection:

Forward contactor:

• L1 → U
• L2 → V
• L3 → W

Reverse contactor:

• L1 → W
• L2 → V
• L3 → U

If both contactors close simultaneously, the phase arrangement creates a phase-to-phase short circuit. A mechanical interlock ensures that even if both contactor coils receive power due to an electrical fault, only one contactor can physically close. The blocked contactor may produce a humming sound because its coil is energized but the mechanism cannot complete the closing movement. This indicates a control fault that should be investigated.

For reversing motor starters, KRIPAL recommends using both:

• Electrical interlocking
• Mechanical interlocking

The mechanical interlock prevents simultaneous closure, while the electrical interlock prevents unnecessary coil heating.

Transfer Switch Application

Mechanical interlocks are also used in contactor-based transfer switching systems where two power sources supply a common load.

Examples include:

• Utility power and generator power
• Two independent AC supplies
• Emergency backup systems

The mechanical interlock ensures that only one source contactor can close at a time, preventing accidental connection between two power supplies. However, large automatic transfer switch (ATS) systems may use dedicated transfer mechanisms rather than standard contactor interlocks. The correct solution depends on the system design and switching requirements.

Mechanical interlocks are especially useful in applications where equipment may remain inactive for long periods because they rely on simple mechanical operation without electronic components.

Installation and Testing Requirements

A mechanical interlock should be installed according to the manufacturer’s instructions.

General installation steps include:

Recommended testing:

• Manually operate each contactor and confirm the opposite contactor is blocked.
• Energize each coil separately with the load disconnected.
• Verify that electrical interlocking prevents both coils from remaining energized.

Common Mechanical Interlock Problems

Frequently Asked Questions