Thermal Trip Unit: Rated Current 10A to 6300A

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

Parameter	Typical range	Notes & selection driver
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

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

Standards

ISO 8820, DIN EN 60947-2, IEC 60947-2

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

Typical Suppliers & Equivalents

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.

Procurement Evaluation Criteria

A practical evidence checklist for RFQ preparation and supplier evaluation.

Technical documentation

Request current drawings, revision history, and a signed specification sheet.

Manufacturing capability

Verify equipment lists, process limits, capacity, and representative production evidence.

Inspection readiness

Confirm test methods, calibrated equipment, sampling plans, and traceable reports.

Supplier transparency

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Manufacturers of Thermal Trip Unit

Manufacturer profiles associated with Thermal Trip Unit.

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Compiled and source-reviewed by the CNFX editorial team. Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review

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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.

Data Basis

Editorial classification, named public sources where available, and source-reviewed manufacturer records. See the editorial policy.

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.

Thermal Trip Unit