In Australian critical power systems, uninterrupted supply is essential. Data centres, hospitals, transport hubs, and industrial plants rely on Automatic Transfer Switches (ATS) to shift loads between mains and generator supply during grid failure.

For this transition to occur safely, the control system must receive precise positional feedback from the main switching devices. The Auxiliary Changeover Switch provides this essential interface, converting mechanical movement into low-voltage control signals.

Designed in accordance with AS/NZS 60947-6-1 and integrated under AS/NZS 3000, it forms the backbone of ATS interlocking and monitoring logic.

Mechanical Linkage and Snap-Action Operation

An auxiliary changeover switch is mechanically linked to the primary contactor or circuit breaker. It does not carry load current. Instead, it mirrors the position of the main device.

When the main breaker moves to ON or OFF, a cam or plunger actuates the auxiliary mechanism.

High-quality designs incorporate snap-action switching to ensure:

• Instant contact transition • Minimal contact bounce • Stable signal output • Reliable digital state reporting

Contact bounce can cause false signals to PLCs or generator controllers. Snap-action mechanisms eliminate delayed switching and maintain precise synchronisation with the main device.

Contact Configuration and Form C Logic

Auxiliary changeover switches commonly provide Form C contacts:

• Normally Open (NO) • Normally Closed (NC) • Common (COM)

This configuration enables changeover logic, where one circuit opens as another closes.

Common variants include:

• 1NO + 1NC • 2NO + 2NC • Delayed make or delayed break types

Correct configuration is essential for ATS interlocking sequences.

Utilisation Categories and Inductive Loads

Auxiliary contacts frequently switch relay coils or control circuit solenoids. These are inductive loads.

When an inductive circuit opens, back electromotive force (Back-EMF) creates a voltage spike. If the auxiliary contact is not rated for AC-15 or DC-13 utilisation categories, arcing can occur, leading to:

• Contact pitting • Surface erosion • Welding • Signal failure

Professional-grade auxiliary changeover switches use silver alloy contacts with arc-resistant geometry to withstand inductive switching cycles reliably.

Low Voltage Signal Integrity

Modern ATS and BMS systems operate on 24V DC or lower control voltages.

At low voltage, oxidation on contact surfaces can prevent reliable conduction due to insufficient wetting current.

Advanced auxiliary switches incorporate wiping contact action, where contact surfaces slide slightly during closure. This scrubbing action removes oxide layers and maintains low contact resistance.

For ultra-low voltage applications, gold-plated contact options may be specified to further enhance signal reliability.

Electrical Interlocking in Dual-Supply Systems

In generator-backed installations, electrical interlocking prevents simultaneous closure of mains and generator contactors.

A typical arrangement uses:

• NC auxiliary contact of mains device • Series wiring to generator contactor coil

This ensures the generator contactor cannot energise unless the mains breaker is proven open.

This mechanical-electrical interlock enhances system safety and prevents dangerous backfeed conditions.

Galvanic Isolation and Dielectric Strength

Auxiliary contacts provide galvanic isolation between high-voltage power circuits and low-voltage control systems.

Their dielectric housing must withstand voltage surges and impulse conditions without flashover into control wiring.

This separation protects PLC inputs, fire systems, and generator controllers from transient faults originating in the main distribution circuit.

Integration with SCHNAP Electric Products

SCHNAP Electric Products supports reliable auxiliary installations with compatible mounting and wiring accessories.

High-retention clip mechanisms ensure stable mechanical coupling between auxiliary blocks and main devices.

Bootlace ferrules provide secure termination of fine-stranded control wires, preventing strand bridging between NO and NC terminals.

Cable markers and identification systems ensure every control wire is clearly labelled, simplifying commissioning and future maintenance.

Proper DIN rail support and cable management accessories maintain separation between control and power conductors within the enclosure.

Commissioning and Testing

After installation, auxiliary contacts should be verified by:

• Manual breaker operation confirmation • PLC input status verification • Generator controller handshake testing • Interlock functionality simulation

Routine testing ensures reliable feedback during actual power failure events.

Conclusion

The Auxiliary Changeover Switch is the feedback mechanism that allows Automatic Transfer Switch systems to operate safely and intelligently.

By accurately translating mechanical breaker position into control logic signals, it ensures correct sequencing, interlocking, and generator engagement during mains failure.

In critical power systems, reliable feedback is the foundation of continuity and safety.