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Selecting the right circuit breaker for your application

01 November 2009

Short circuits and overloads put different demands on circuit breakers, and it is important that engineers know how to protect their designs against both dangers. The correct choice of circuit protection will enhance equipment reliability, reduce cost of ownership, and play an important role in differentiating the serious manufacturer from the ordinary

Circuit breakers are used in a variety of ways. They are mounted in panel boards to protect branch circuit wiring, and they are built into equipment to protect it. For this range of applications a circuit breaker must provide both short circuit and overload protection.

Interrupting a short circuit current that is limited only by the resistance of the wiring is a very severe test of a circuit breaker, and if the interrupting capacity of the breaker is not adequate, it can be catastrophic. Overload currents that reach 2 to 5 times the normal rating of the breaker are handled differently, and very often the circuit breaker must carry the current for an appreciable time without tripping. Protection against short circuits and overloads are the greatest concern when choosing a circuit breaker.

A Circuit Breaker for Equipment (CBE) is defined in EN60934 as: ‘A mechanical switching device, specifically designed for the protection of equipment, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and automatically breaking currents under specified abnormal conditions.’ To ensure that the protection of a diverse range of equipment and circuits can be correctly satisfied, CBEs are available in a wide range of basic types and configurations, all affording a choice of performance characteristics and features.

Protection against short circuits
All circuit breakers are tested for short circuits but the severity of a short circuit depends on where the device is used in the circuit. Not all devices will continue working after opening a short circuit. Under EN60934, there are two short circuit ratings for CBEs – PC1 and PC2.

CBEs compliant with the PC1 performance category are permitted to fail under overload conditions provided they do so in a safe condition without risk of fire. This reflects established US practice and is usually quoted with reference to UL standards. By contrast, category PC2 CBEs must clear a fault current and remain fit for further use. This tends to be the norm for European manufactured devices.

This means that maximum specified current levels quoted for PC1 compliant CBEs are likely to be significantly higher than for PC2 ratings, which is rather misleading, so users should be careful if they are using PC1 rated devices in Europe.

Protection against overloads
Overloads can be short-term or long-term. The chosen protective device must not trip on momentary or short-term over-current events that are normal for the piece of equipment it is protecting. Electronic devices, for example, may create inrush currents as their internal power supply and filter circuits start up. These inrush currents typically last only a fraction of a second, and seldom cause a problem.

Another class of short-term over-current is a motor start-up surge. Most motors, especially those that start under load, draw several times their normal current when starting. Other over-currents may last even longer, and will still be part of normal operation.

A piece of motor-driven equipment, for example, may draw 50% more than normal current for several minutes at a time and the breaker should not trip under these conditions. If the overload lasts longer than normal, the breaker should open to prevent overheating and damage. What gives the breaker this ability to discriminate between normal and damaging over-currents is the delay curve.

Delay curves
There are five choices of delay curves in circuit breakers: thermal, thermal-magnetic, hydraulic-magnetic, magnetic and electronic. Each has a different trip profile in relation to time and current, and each has distinct mechanical characteristics.

Thermal breakers incorporate a heat-responsive bimetal strip or disk. This type of technology has a slower characteristic curve that discriminates between safe temporary surges and prolonged overloads (see Figure 1). It is appropriate for machinery or vehicles where high inrush currents accompany the start of electric motors, transformers, and solenoids. There are some thermal circuit breakers with hot-wire elements, which provide faster switching. They provide a low cost solution for appliances and printed circuit board protection, among other applications.

Thermal-magnetic breakers combine the benefits of a thermal and a magnetic circuit breaker: they have a thermal delay that avoids nuisance tripping caused by normal inrush current, and a magnetic solenoid for fast response at higher currents. The size of overload protection can be selected - examples are shown in Figures 2 and 3. Typical applications for this type of CBE include communications and control equipment.

Both standard thermal and thermal-magnetic circuit breakers are sensitive to ambient temperature. However, they can be selected to operate correctly within a wide temperature range.

A magnetic circuit breaker can be combined with a hydraulic delay to make it tolerant of current surges. These hydraulic-magnetic breakers are similar to the thermal-magnetic in that they have a two step response curve - they provide a delay on normal over-currents, but trip quickly on short circuits. Many hydraulic-magnetic circuit breakers are available in a selection of delay curves to fit particular applications. Hydraulic-magnetic circuit breakers are not affected by ambient temperature, but they tend to be sensitive to position.

Pure magnetic circuit breakers operate via a solenoid and trip nearly instantly as soon as the threshold current has been reached. This type of delay curve is appropriate for sensitive equipment such as telecommunication equipment, printed circuit boards, and impulse disconnection in control appliances.

Electronic circuit breakers provide over-current protection with active limitation of in-rush and short circuit currents for the selective protection of components in industrial plants. On tripping they provide safe physical isolation of the load.

EN60934 covers circuit breakers for use in a wide variety of equipment types; choosing a circuit breaker carefully can save considerable cost and will provide protection tailored to that application. The drawback to this is that the designer must do more homework to select the appropriate device, but the better manufacturers will provide advice. Considering the money that can be saved, that is probably time well spent.

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