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Today, we’ll discuss the distinctions between thermal relays, fuses, and circuit breakers. While many can outline the basics, a deeper understanding of their respective application scenarios often remains unclear. This article will provide both an introductory overview and a more in-depth exploration.

1. Thermal Relay

(1) Principle

A thermal relay operates on the principle of the thermal effect of current. It contains a thermal element, often a resistance wire, which generates heat as current flows through it. Most thermal relays use a bimetal strip composed of two metals with different thermal expansion coefficients. When an overload current occurs, the heat produced causes the bimetal strip to bend, triggering the relay.

(2) Function

Thermal relays are primarily used for overload protection in devices like motors. They operate with a delay, making them suitable for prolonged overloads but not for quickly interrupting short-circuit currents. Their response to short-circuit currents is relatively slow, potentially allowing significant damage before tripping.

Tripping Curve Insights:
From the typical tripping curve, we see that a thermal relay is effective for long-term overloads. For example, at 17 times the rated current, the relay requires about 1.5 seconds to trip. In comparison, fuses and circuit breakers respond in milliseconds under similar conditions.

2. Fuse

(1) Principle

The fuse's core component is the fuse element, typically made of metals like lead-tin alloy or copper. Under normal current, the fuse element withstands the heat generated without melting. However, when a short-circuit or overload current exceeds the fuse's rated value, the element rapidly heats up and melts, breaking the circuit.

(2) Function

Fuses are mainly used for short-circuit protection, although they can provide limited overload protection. Their overload protection, however, is not as precise as that of thermal relays or circuit breakers.

Tripping Curve Insights:
Fuses respond more slowly to overloads and are typically oversized to focus on short-circuit protection. While their overload protection is less precise, their strength lies in handling large short-circuit currents quickly and with high breaking capacity.

3. Circuit Breaker

(1) Principle

Circuit breakers employ more complex mechanisms and offer multiple tripping modes.

· Thermal tripping: Utilizes the thermal expansion of a bimetal strip under overload currents.

· Magnetic tripping: Uses electromagnetic forces to trip instantly during short circuits.

(2) Function

Circuit breakers combine overload and short-circuit protection, along with features like undervoltage protection. They offer comprehensive and precise protection for electrical equipment. Additionally, many modern circuit breakers come with adjustable parameters, display and recording functions, and even intelligent features.

Key Takeaways

Each device has its advantages and specific application scenarios:

1. Thermal Relays: Ideal for prolonged overload protection in motors but unsuitable for short-circuit protection.

2. Fuses: Best suited for short-circuit protection, with limited precision in overload scenarios.

3. Circuit Breakers: Provide a combination of precise overload and short-circuit protection, often used for broader system safety.

In some cases, these devices are used in combination. Always review the tripping curves and application requirements when making a selection.