This is just a circuit breaker. However, there is enough complexity and confusion in specifying circuit protection, and many engineers are designing devices with too little or too much protection. The protected circuit makes the device vulnerable to destructive electrical surges. Over-protected circuits will increase costs and may cause false trips. Like Goldilocks and The Three Bears, the goal is to specify "just right" circuit protection.

As a global circuit breaker manufacturer with more than 50 years of history, ETA has selected the 12 most common pitfalls for this article.

The first mistake made was to specify the wrong circuit breaker technology for the application. There are four options for circuit breaker technology: thermal, magnetic, thermomagnetic and high performance. Each has different trip curves related to time and current, and each has different mechanical characteristics.

The electromagnetic circuit breaker is operated by a solenoid, and once the threshold current is reached, it trips almost immediately. This type is suitable for pulse disconnection in printed circuit board applications and control applications.

The thermal circuit breaker contains a thermally responsive bimetallic strip or disk. This type has a slower characteristic curve and can distinguish between safe temporary surges and long-term overloads.

Thermal magnetic circuit breakers combine the advantages of thermal and magnetic circuit breakers: delay can avoid false trips caused by normal surge currents, and rapid response under high currents.

When reliable operation under adverse conditions is required, high-performance circuit breakers can provide high breaking capacity and excellent environmental specifications. Generally, these circuit breakers are designed for aerospace, defense, and similar heavy-duty applications where extreme vibration, mechanical shock, and other conditions exist, and circuit breaker performance is absolutely critical.

Most engineers are worried about accidental tripping because they often specify a circuit breaker with a much higher rating than they should. Part of the reason is the confusion between fuses and circuit breakers. Engineers are accustomed to oversize the fuse to prevent accidental tripping. However, it is not necessary to make the circuit breaker too large.

Unlike the fuse rating, the circuit breaker rating tells you the maximum current that the circuit breaker will always maintain at ambient room temperature. Therefore, the 10A circuit breaker will maintain 10A current without accidental tripping. In fact, a typical 4A circuit breaker with a slow tripping curve can withstand a temporary 10A current surge without false tripping.

It is important to maintain the recommended minimum spacing requirements between non-temperature-compensated thermal circuit breakers. Only 1 mm spacing between circuit breakers is required. Without this tiny thermal gap, the circuit breaker would heat up and increase the sensitivity of the bimetallic tripping mechanism. If the circuit breakers must touch each other, derate them to 80% of their normal rated current.

Terms such as drip protection, fire protection, splash protection, and dust protection are commonly used, but unless standard definitions are applied, they can be misleading. When specifying, use established standards as the measurement standard, such as EN 60529/IEC 529, which defines the protection level of electrical equipment. Use these criteria to decide which protection is suitable for the application.

The circuit breaker is manually reset by the actuator. There are many types of circuit breaker actuators, including push-to-reset, push-pull, push-push, rocker, toggle, baton, and push-to-reset with manual release. The type of actuator is not only a consideration of appearance. For example, critical applications often require push-pull actuators because they are the most resistant to accidental actuation. The type of actuator you choose will depend on the location of the circuit breaker, lighting needs, operator safety or convenience needs, and the consequences of accidental engagement.

Many circuit breakers are designed to be both circuit breakers and on/off switches. The advantages of combined equipment are reduced components, reduced panel space consumption, reduced wiring and increased protection for ordinary switches.

Circuit breakers with plug-in quick connect terminals simplify installation and replacement (they must also be welded). Screw terminal connection is safer, suitable for high current and high vibration environment. Quick connect terminals can be used for circuit breakers with a rated current up to 25A.

Although fuses provide inexpensive circuit protection, the cost savings should be weighed against the lower total cost of circuit breaker ownership. The most important thing is that the circuit breaker can be reset quickly so that the circuit can be restored in the shortest downtime. In addition, there is no guarantee that the replaced fuse has an appropriate rating. If the fuse is replaced with a higher rated fuse, overheating and catastrophic equipment failure may occur.

Over time, the performance of circuit breakers is relatively stable, but as the fuse ages, their tripping characteristics will change. This may cause false trips and increase downtime.

Generally, the trigger of the magnetic circuit breaker is a hinged metal armature, which closes in response to the movement of the magnetic coil. This design makes magnetic circuit breakers (and magnetic hydraulic circuit breakers) particularly susceptible to vibration, which can cause the armature to close prematurely.

According to experience, the rated load of the circuit breaker should be 100%. However, some applications require continuous operation of the circuit breaker at high or low temperatures. In these cases, please follow the manufacturer's derating guidelines. For example, an application requiring 10A protection requires a 12A rated thermal circuit breaker when operating at 50°C.

The performance of thermal circuit breakers is very sensitive to fluctuations in ambient temperature. In a cold environment it will trip at a higher amperage, and in a hot environment it will trip at a lower amperage.

A common mistake is to assume that the thermal circuit breaker needs to be derated in an environment where the ambient temperature rises. In fact, the performance of the thermal circuit breaker tracks the performance requirements of the system, assuming it is exposed to the same heat source. For example, a motor winding at 90°C requires more overheat protection than the same winding at 20°C.

The breaking capacity is the maximum amperage that the circuit breaker can safely break. The circuit breaker manufacturer publishes this specification and the number of times the circuit breaker performs this function. For example, ETA has published two types of interrupt capacity specifications. One is called Icn, or normal interrupt capacity. Icn is the maximum current that the circuit breaker can interrupt (minimum three times, according to IEC934/EN60934 PC2). Icn gives a rough idea of ​​circuit breaker quality. Another specification is UL 1077 breaking capacity. UL1077 breaking capacity is the maximum current that a circuit breaker can safely break at least once without causing a fire hazard.

In order to comply with various standards, engineers must specify a circuit breaker with sufficient breaking capacity. Unfortunately, applying appropriate standards can be confusing.

If you keep these tips in mind, it is easy to specify the correct circuit protection measures at the lowest cost. Start the selection process by trying to really understand your load. Then decide which type of circuit breaker is suitable for your application. Avoid common mistakes and you will get a reliable design.

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