INDUSTRY COMPONENT

Thermal Trip Unit

A thermal trip unit is a protective device in circuit breakers that detects overload currents using a bimetallic strip, triggering disconnection to prevent equipment damage and fire hazards.

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Component Specifications

Definition
The thermal trip unit is an essential component within circuit breakers and main disconnect switches, designed to provide overload protection by responding to sustained excessive currents. It operates based on the thermal effect of current flow, where a calibrated bimetallic strip heats and bends when current exceeds the rated threshold for a predetermined time. This mechanical action releases the trip mechanism, opening the contacts to interrupt the circuit. Unlike magnetic trip units that respond to short circuits, thermal units protect against gradual overloads that could cause insulation degradation, overheating, or fire. They are characterized by an inverse time-current curve, meaning trip time decreases as overload severity increases. Modern versions may integrate with electronic trip units for enhanced coordination and monitoring.
Working Principle
The thermal trip unit operates on the principle of thermal expansion. A bimetallic strip, composed of two metals with different coefficients of thermal expansion, is heated by the load current passing through it. Under normal conditions, the strip remains straight. During an overload, the increased current generates excess heat, causing the strip to bend due to unequal expansion of the metals. This bending actuates a latch or lever mechanism, which releases the spring-loaded contacts to open the circuit. The trip time depends on the magnitude of the overload, following an inverse-time characteristic: higher currents cause faster tripping. The unit resets automatically after cooling or via manual intervention, depending on the design.
Materials
Bimetallic strip (typically nickel-iron or copper-steel alloys), insulating housing (thermoset plastics like phenolic or polyester), contact materials (silver alloy or copper-tungsten), springs (stainless steel or phosphor bronze), and calibration screws (brass or steel).
Technical Parameters
  • Trip Class 10, 20, 30 (e.g., Class 10 for motors)
  • Mounting Type Fixed or plug-in
  • Rated Current 10A to 6300A
  • Mechanical Endurance >10,000 operations
  • Calibration Tolerance ±10% to ±20%
  • Operating Temperature Range -20°C to +70°C
Standards
ISO 8820, DIN EN 60947-2, IEC 60947-2

Industry Taxonomies & Aliases

Commonly used trade names and technical identifiers for Thermal Trip Unit.

thermal trip unit circuit breaker overload protection bimetallic strip electrical protection device thermal-magnetic breaker IEC 60947-2 trip class Thermal Trip Unit in Electrical Equipment Manufacturing

Parent Products

This component is used in the following industrial products

Circuit Breaker/Main Disconnect

A safety device that automatically interrupts electrical current flow in a circuit during overload or short circuit conditions, and provides manual isolation capability.

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Engineering Analysis

Risks & Mitigation
  • Failure to trip during overload leading to fire
  • Nuisance tripping from ambient temperature changes
  • Calibration drift over time
  • Material fatigue in bimetallic strip after repeated cycles
FMEA Triads
Trigger: Bimetallic strip contamination or corrosion
Failure: Reduced sensitivity, delayed or no trip
Mitigation: Use sealed housings, regular inspection, and replace per maintenance schedule
Trigger: Mechanical wear in trip mechanism
Failure: Inconsistent tripping or failure to reset
Mitigation: Design with durable materials, lubricate moving parts, and conduct operational tests
Trigger: Overheating from high ambient temperatures
Failure: Premature tripping below rated current
Mitigation: Select units with temperature compensation, ensure proper ventilation, and derate as needed

Industrial Ecosystem

Compatible With

Interchangeable Parts

Compliance & Inspection

Tolerance
Trip current tolerance typically ±10% at reference temperature (e.g., 30°C), with time delays per trip class (e.g., Class 10 trips within 10 seconds at 600% current).
Test Method
Testing per IEC 60947-2: apply specified overload currents (e.g., 1.05 to 1.3 times rated current) and measure trip times; verify inverse-time characteristics and calibration stability across temperature ranges.

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Frequently Asked Questions

What is the difference between thermal and magnetic trip units?

Thermal trip units protect against overloads by responding to heat from sustained excessive currents, with an inverse time delay. Magnetic trip units protect against short circuits by responding to sudden high currents, tripping instantaneously. Many circuit breakers combine both in thermal-magnetic designs.

How do you calibrate a thermal trip unit?

Calibration involves adjusting the bimetallic strip tension or position using screws, based on testing at specific current levels and durations per standards like IEC 60947-2. It ensures the unit trips within the specified time-current curve. Factory calibration is typical; field adjustments require specialized tools.

Can thermal trip units be used in DC circuits?

Yes, but performance may vary. Thermal units rely on current-induced heating, which works for both AC and DC. However, calibration and trip characteristics might differ due to DC's constant current flow, so use models rated for DC applications.

Can I contact factories directly?

Yes, each factory profile provides direct contact information.

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