Why Overload Relays Always Use 95–96 and 97–98 contacts

Why Overload Relays Always Use 95–96 and 97–98 contacts

Introduction

If you’ve ever opened a motor control panel, you’ve likely seen overload relays marked with 95–96 and 97–98. These numbers appear across Siemens, Schneider, ABB, Allen-Bradley, and countless other brands. At first glance, it might seem arbitrary. But in reality, this numbering is part of a global language of automation, defined by IEC 60947 and NEMA standards.

This article explores why these numbers are standardized, how they function, and why they matter for technicians, engineers, and students.

What is an Overload Relay?

An overload relay is a protective device used in motor circuits. Its job is to protect motors from excessive current caused by overloads or phase failures. Unlike fuses or circuit breakers, overload relays are thermal or electronic devices that trip when current exceeds a preset value for a certain time.

Key functions:

•        Detect overload conditions

•        Interrupt the control circuit to stop the motor

•        Provide auxiliary signaling for alarms or indicators

Why 95–96?

•        95 = Common terminal of the NC (Normally Closed) overload relay contact

•        96 = NC terminal

Together, 95–96 form the trip contact. When an overload occurs, this contact opens, breaking the control circuit and stopping the motor.

This is critical because:

•        It ensures the motor is de-energized during overload.

•        It provides a predictable wiring point for technicians.

•        It aligns with IEC/NEMA conventions, making diagrams universal.

Why 97–98?

•        97 = Common terminal of the NO (Normally Open) auxiliary contact

•        98 = NO terminal

This contact closes during an overload, typically used for:

•        Fault indication lamps

•        Alarms or buzzers

•        Sending signals to SCADA/HMI systems

Other Standard Numbers

IEC and NEMA standards also define numbering for contactors:

•        13–14 → NO contacts in contactors

•        21–22 → NC contacts in contactors

This consistency ensures that whether you’re wiring a Schneider relay in India or an Allen-Bradley relay in the US, the diagrams look familiar.

 

Benefits of Standard Numbering

Transparency

Any technician can read the diagram without brand-specific training.

Safety

Reduces wiring mistakes and hazards. Miswiring in motor control circuits can cause costly downtime or dangerous accidents.

Global Practice

Aligns with IEC 60947 and NEMA standards, ensuring interoperability across industries and countries.

Real-World Example

Imagine a multinational plant with equipment from Siemens (Germany), Schneider (France), and Rockwell (USA). Without standardized numbering, each relay would have different terminal assignments. Troubleshooting would be chaotic. Thanks to 95–96 and 97–98, technicians can quickly identify trip and auxiliary contacts regardless of brand.

Educational Perspective

For students learning PLCs and motor control, understanding these numbers is essential. It’s not just about memorization — it’s about recognizing that automation systems rely on global conventions.

Teaching tip:

•        Draw a motor starter circuit with overload relay.

•        Label 95–96 in series with the coil (trip circuit).

•        Label 97–98 connected to a pilot lamp (fault indication).

This simple exercise reinforces the concept of standardization in practice.

Safety & Compliance

IEC 60947 and NEMA standards mandate these numbering conventions. Compliance ensures:

•        Reduced risk of misinterpretation

•        Easier audits and inspections

•        Safer installations across industries

Future Outlook

As industries move toward smart relays and IoT-enabled devices, the numbering convention remains unchanged. Even with advanced diagnostics and communication protocols, 95–96 and 97–98 continue to serve as the backbone of relay wiring.

Conclusion

Overload relays don’t use 95–96 and 97–98 by chance. These numbers are part of a global language of automation, defined by IEC and NEMA standards. They ensure transparency, safety, and interoperability across brands and borders.

So next time you wire an overload relay, remember: 95–96 isn’t random — it’s a universal code that keeps the world of automation connected.

Key Takeaways

95–96 = NC trip contact (opens on overload to stop motor)

97–98 = NO auxiliary contact (closes on overload for fault indication)

Standardization prevents chaos in wiring diagrams

IEC 60947 & NEMA define these conventions globally

Benefits: transparency, safety, global practice