Overload Relay (Thermal or Electronic)

Product Specifications

Technical details and manufacturing context for Overload Relay (Thermal or Electronic)

Definition

An overload relay is a critical safety component within a motor starter assembly. It continuously monitors the electrical current supplied to the motor. When the current exceeds a preset safe threshold for a sustained period (indicating an overload condition), the relay activates a trip mechanism. This action opens the control circuit, which in turn de-energizes the motor starter's contactor, safely disconnecting power to the motor. This protects the motor windings from overheating and insulation failure caused by excessive current due to mechanical overloads, phase loss, or locked rotor conditions.

Overload relays are available in thermal and electronic versions. Thermal types use a bimetallic strip or solder pot that heats proportionally to the motor current. Electronic types use current transformers or sensors and a microprocessor to compare measured current against programmed trip curves. Both types are designed to meet the requirements of IEC 60947-4-1, which defines parameters such as rated operational current, setting range, trip class, rated insulation voltage, rated operational voltage, frequency, trip current accuracy, ambient temperature, degree of protection, reset type, electrical life, and mechanical life.

Typical parameters include a rated operational current range of 0.1–630 A, an adjustable setting range of 0.1–630 A, trip classes 10A, 10, 20, and 30, rated insulation voltage of 690 V, rated operational voltage of 690 V AC, frequency of 50/60 Hz, trip current accuracy of ±5%, ambient temperature range of -25 to 55 °C, degree of protection IP20, manual or auto reset, electrical life of 100,000 operations, mechanical life of 1,000,000 operations, and weight from 0.1 to 2.5 kg. These values are reference ranges; the actual model must be selected based on motor full-load current and application requirements.

Always verify model-specific values and standards with the legal manufacturer or supplier before purchase and installation.

Working Principle

Thermal overload relays use a bimetallic strip or solder pot that heats proportionally to the motor current. Excessive current causes the strip to bend or the solder to melt, mechanically releasing a latch to trip the relay. Electronic overload relays use current transformers or sensors to measure current, with a microprocessor comparing the measured value against programmed trip curves (e.g., Class 10, 20, 30) and time delays before sending a signal to trip the output contact.

Common Materials

Thermoplastic Housing, Copper Alloy Contacts, Bimetallic Strip (Thermal) / Electronic Circuit Board (Electronic)

Technical Parameters

Parameter	Typical range	Notes & selection driver
Rated Operational Current	0.1–630 A	Select according to motor full-load current.IEC 60947-4-1
Setting Range	0.1–630 A	Adjustable to match motor FLA.IEC 60947-4-1
Trip Class	10A, 10, 20, 30	Class 10 for normal motors, 20/30 for high-inertia loads.IEC 60947-4-1
Rated Insulation Voltage	690 V	Maximum voltage for insulation design.IEC 60947-4-1
Rated Operational Voltage	690 V AC	Maximum operating voltage.IEC 60947-4-1
Frequency	50/60 Hz	Compatible with standard power supplies.
Trip Current Accuracy	±5 %	Accuracy of trip current at ambient temperature.IEC 60947-4-1
Ambient Temperature	-25–55 °C	Operating range; outside this may affect accuracy.IEC 60947-4-1
Degree of Protection	IP20	Finger protection; higher IP available with enclosure.IEC 60529
Reset Type	Manual/Auto	Manual for safety, auto for remote operation.
Electrical Life	100000 operations	Number of operations under rated conditions.IEC 60947-4-1
Mechanical Life	1000000 operations	Number of mechanical operations without maintenance.IEC 60947-4-1
Weight	0.1–2.5 kg	Depends on current rating and features.

Ranges are indicative industry figures for RFQ preparation, not a supplier commitment. Confirm every value and standard with the legal manufacturer before ordering.

Components / BOM

Current Sensing Element Part
Measures the motor line current. In thermal relays, this is a heater coil or bimetallic strip. In electronic relays, this is a current transformer or Hall-effect sensor.

Material: Nickel-Chromium Alloy (Heater) / Ferrite Core & Copper Windings (CT)
Trip Mechanism
Mechanically or electronically actuates to open the NC (normally closed) auxiliary contact when an overload is detected.

Material: Spring Steel / Plastic Actuator
Auxiliary Contacts Part
NO (normally open) and NC contacts used to signal the trip status to the control circuit or PLC.

Material: Silver Cadmium Oxide / Silver Tin Oxide
Microprocessor Optional
Compares the measured current against the trip curves in electronic relays.
Trip Curves Optional
The programmed Class 10/20/30 curves the measured current is compared against.

Industry Taxonomies & Aliases

Commonly used trade names and technical identifiers for Overload Relay (Thermal or Electronic).

Applied To / Applications

This component is essential for the following industrial systems and equipment:

Motor Starter

View system integration details

Manufacturers of Overload Relay (Thermal or Electronic)

Manufacturer profiles associated with Overload Relay (Thermal or Electronic).

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Frequently Asked Questions

What is the difference between thermal and electronic overload relays?

Thermal overload relays use a bimetallic strip or solder pot that heats with current and mechanically trips. Electronic overload relays use sensors and a microprocessor to measure current and trip according to programmed curves. Both protect motors from overload, but electronic types offer more precise and adjustable protection.

How do I select the correct overload relay for my motor?

Select based on the motor's full-load current (FLA). The relay's setting range must cover the FLA. Also consider the trip class (e.g., 10, 20, 30) based on the starting characteristics of the load. Verify all parameters with the manufacturer's data and the motor's specifications.

What does trip class mean?

Trip class indicates the maximum time in seconds the relay will allow an overload current (e.g., 7.2 times the setting) before tripping. Class 10 trips in 10 seconds, Class 20 in 20 seconds, and Class 30 in 30 seconds. Choose a class that allows the motor to start without nuisance tripping but still protects it.

What standards apply to overload relays?

The primary standard is IEC 60947-4-1, which covers low-voltage switchgear and controlgear for motor starters. It defines parameters like rated operational current, trip class, and accuracy. Also relevant is IEC 60529 for degree of protection. Always verify compliance with the manufacturer.

Data Basis

Editorial classification, named public sources where available, and source-reviewed manufacturer records.

Preliminary Technical Classification

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pressure:	N/A (electrical device, not pressure-rated)
other spec:	Current range: 0.1A to 630A typical, voltage: up to 690V AC, frequency: 50/60Hz, IP rating: IP20 to IP55 depending on enclosure
temperature:	-20°C to +60°C (operating ambient), -40°C to +85°C (storage)

Overload Relay (Thermal or Electronic)

Product Specifications

Industry Taxonomies & Aliases

Applied To / Applications

Industrial Ecosystem & Supply Chain Structure

Application Fit & Sizing Matrix

Reliability & Engineering Risk Analysis

Compliance & Manufacturing Standards