Structured Manufacturing Data (2026)

Overload Relay (Thermal or Electronic)

Based on aggregated insights from structured factory profiles within the CNFX directory, the standard Overload Relay (Thermal or Electronic) used in the Machinery and Equipment Manufacturing sector typically supports operational capacities ranging from standard industrial configurations to heavy-duty production requirements.

Technical Definition & Core Assembly

A canonical Overload Relay (Thermal or Electronic) is characterized by the integration of Current Sensing Element and Trip Mechanism. In industrial production environments, manufacturers listed on CNFX commonly emphasize Thermoplastic Housing construction to support stable, high-cycle operation across diverse manufacturing scenarios.

A protective device in motor starters that monitors current flow and disconnects the motor circuit when excessive current is detected, preventing motor damage from overloads.

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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.
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
  • Full Load Current (FLC) setting range, the adjustable current threshold at which the relay is calibrated to trip. (A) Standard Spec
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

Industry Taxonomies & Aliases

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

Motor Overload Relay OL Relay thermal overload relay for motor protection electronic overload relay current sensing motor starter overload protection device bimetallic strip overload relay industrial adjustable trip setting overload relay

Applied To / Applications

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

Motor Starter
View system integration details

Industrial Ecosystem & Supply Chain Structure

Complementary Systems
Downstream Applications
Specialized Tooling

Application Fit & Sizing Matrix

Operational Limits
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)
Media Compatibility
✓ Indoor industrial environments ✓ Motor control centers (MCCs) ✓ Clean, dry electrical panels
Unsuitable: Corrosive or explosive atmospheres (unless specifically rated for hazardous locations)
Sizing Data Required
  • Motor full-load current (FLC) in amps
  • Motor service factor and duty cycle
  • Ambient temperature at installation location

Reliability & Engineering Risk Analysis

Failure Mode & Root Cause
Thermal Element Degradation
Cause: Repeated overload cycles causing cumulative heat damage to bimetallic strips or electronic sensors, leading to inaccurate trip timing or failure to trip.
Contact Welding or Pitting
Cause: High inrush currents or frequent switching operations causing arcing and material transfer on relay contacts, resulting in stuck contacts or increased resistance.
Maintenance Indicators
  • Audible buzzing or chattering from the relay enclosure during normal operation
  • Visible discoloration, scorching, or melting on the relay housing or wiring terminals
Engineering Tips
  • Calibrate trip settings annually using manufacturer-specified test equipment to ensure accurate response to overload conditions
  • Implement infrared thermography inspections quarterly to detect abnormal heating patterns in relay components and connections

Compliance & Manufacturing Standards

Reference Standards
IEC 60947-4-1: Low-voltage switchgear and controlgear - Part 4-1: Contactors and motor-starters - Electromechanical contactors and motor-starters UL 508: Standard for Industrial Control Equipment EN 60947-4-1: Low-voltage switchgear and controlgear - Part 4-1: Contactors and motor-starters - Electromechanical contactors and motor-starters (CE marking basis)
Manufacturing Precision
  • Trip time accuracy: +/-10% of rated current setting
  • Contact resistance: <50 milliohms for new contacts
Quality Inspection
  • Calibration verification test: Trip time measurement at multiple current levels
  • Dielectric strength test: 2.5kV AC for 1 minute between live parts and enclosure

Factories Producing Overload Relay (Thermal or Electronic)

Manufacturer profiles with relevant production capability in China

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Manufacturer listings support early research and capability understanding. They are not certification, ranking, or transaction guarantees.

Technical documentation
4/5
Manufacturing capability
4/5
Inspection readiness
5/5
Supplier transparency
3/5

These scores are example evaluation dimensions, not real customer ratings, country-specific buyer feedback, or live inquiry activity.

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Frequently Asked Questions

What's the difference between thermal and electronic overload relays?

Thermal overload relays use a bimetallic strip that bends with heat from excess current, while electronic relays use circuit boards to monitor current electronically, offering more precise trip settings and diagnostics.

How do overload relays prevent motor damage?

They continuously monitor current flow in the motor circuit and automatically disconnect power when detecting sustained excessive current, preventing overheating and winding damage from overload conditions.

Can overload relays be reset after tripping?

Yes, most overload relays feature manual or automatic reset functions after the trip mechanism cools down (thermal) or the fault clears, allowing motor operation to resume safely.

Can I contact factories directly on CNFX?

CNFX is an open directory, not a transaction platform. Each factory profile provides direct contact information and production details to help you initiate direct inquiries with Chinese suppliers.

Data basis: CNFX manufacturer profiles, technical classification, publicly available product information, and ongoing plausibility checks for Machinery and Equipment Manufacturing.

Data Basis

CNFX manufacturer profiles, technical classification, publicly available product information, and ongoing plausibility checks.

Preliminary Technical Classification
This page supports structured research, RFQ preparation, and supplier evaluation. It does not replace buyer-led supplier qualification, standards review, or technical approval.

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