What is the difference between an electromagnetic release and a thermal release in a circuit breaker?

Differences Between Magnetic Trip Units and Thermomagnetic Trip Units in Circuit Breakers

In circuit breakers, magnetic trip units (Magnetic Trip Unit) and thermomagnetic trip units (Thermomagnetic Trip Unit) are two different protection mechanisms that detect and respond to overcurrent conditions in distinct ways. Below are the main differences between them:

1. Working Principle

Magnetic Trip Unit

Working Principle: A magnetic trip unit detects short circuits or instantaneous high currents through electromagnetic induction. When the current exceeds a preset threshold, the electromagnet generates enough force to actuate the tripping mechanism, rapidly disconnecting the circuit.

• Response Speed: The magnetic trip unit is highly sensitive to instantaneous high currents and can respond within a few milliseconds, making it ideal for short-circuit protection.

• Current Range: It is typically used to detect short-circuit currents, which are significantly higher than the rated current.

• Temperature Influence: The magnetic trip unit is not affected by temperature changes because its operation is based on electromagnetic induction, not temperature.

Thermomagnetic Trip Unit

• Working Principle: A thermomagnetic trip unit combines both thermal and magnetic effects. It uses a bimetallic strip (composed of two metals with different coefficients of thermal expansion) to detect prolonged overload currents. When the current exceeds the rated value, the bimetallic strip deforms due to heat, triggering the tripping mechanism. Additionally, it includes a magnetic trip component to detect instantaneous high currents.

• Response Speed: For overload currents, the thermomagnetic trip unit responds more slowly, as it relies on the thermal expansion of the bimetallic strip. This typically takes several seconds to a few minutes. For short-circuit currents, the magnetic part of the thermomagnetic trip unit can respond quickly.

• Current Range: The thermomagnetic trip unit provides protection against both overload and short-circuit currents, covering a wider range of current levels, especially for prolonged overload conditions.

• Temperature Influence: The thermal trip portion of the thermomagnetic unit is significantly influenced by ambient temperature, as it operates based on the thermal expansion of the bimetallic strip. Therefore, the design of thermomagnetic trip units often accounts for temperature variations to ensure accurate operation under different conditions.

2. Application Scenarios

Magnetic Trip Unit

• Applicable Scenarios: Primarily used for short-circuit protection in applications requiring rapid response to instantaneous high currents. Examples include industrial equipment, power distribution systems, and motors.

• Advantages: Fast response time, effectively cutting off short-circuit currents to prevent equipment damage.

• Disadvantages: Only suitable for short-circuit protection and cannot effectively handle prolonged overload currents.

Thermomagnetic Trip Unit

• Applicable Scenarios: Suitable for both overload and short-circuit protection, especially in situations where both types of overcurrent need to be addressed. Examples include residential circuits, commercial buildings, and small industrial equipment.

• Advantages: Can handle both overload and short-circuit currents, providing a broader range of protection. For overload currents, it offers a delayed response, preventing nuisance trips due to brief current surges.

• Disadvantages: Slower response to short-circuit currents compared to a pure magnetic trip unit.

3. Structure and Design

Magnetic Trip Unit

• Simple Structure: The magnetic trip unit has a relatively simple structure, consisting mainly of an electromagnet and a tripping mechanism. It lacks complex mechanical components, enhancing reliability.

• Independence: The magnetic trip unit typically operates as an independent protection unit, specifically for short-circuit protection.

Thermomagnetic Trip Unit

• Complex Structure: The thermomagnetic trip unit combines a bimetallic strip and an electromagnet, resulting in a more complex structure. It has both a thermal trip section and a magnetic trip section, allowing it to handle both overload and short-circuit conditions.

• Integration: The thermomagnetic trip unit is usually integrated into the circuit breaker as a single protection device, suitable for multiple protection needs.

4. Cost and Maintenance

Magnetic Trip Unit

• Lower Cost: Due to its simple structure, the magnetic trip unit is generally less expensive and requires minimal maintenance.

• Simple Maintenance: Maintenance for the magnetic trip unit is straightforward, primarily involving checking the condition of the electromagnet and tripping mechanism.

Thermomagnetic Trip Unit

• Higher Cost: The more complex structure of the thermomagnetic trip unit makes it relatively more expensive, especially for high-quality units.

• Complex Maintenance: Maintenance for the thermomagnetic trip unit is more involved, requiring periodic inspection of the bimetallic strip to ensure proper operation under varying temperatures.

Summary

• Magnetic Trip Unit: Best suited for short-circuit protection, offering fast response times, a simple structure, and lower costs. However, it only handles instantaneous high currents.

• Thermomagnetic Trip Unit: Suitable for both overload and short-circuit protection, with a slower response to overload currents but a broader application range. It is more complex and costly but provides comprehensive protection.