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