Machine Design

Trends in circuit protection

Circuit breakers now sport advanced features that include remote control and operational status reporting.

Ken Cybart
Applications engineer
E-T-A Circuit Breakers
Mt. Prospect, Ill.

Electronic circuit breakers like this ESS20 model from E-T-A Circuit Breakers feature selectable current settings and internal diagnostics for fault and status indications.

The E-1048 solidstate remote power controller from E-T-A is typical of smart power relays that combine an electronic power relay, an overcurrentprotection-device, and a status indicator into a single unit.

Engineers have long considered circuit protection a stable if somewhat unglamorous area. However, that stability is diminishing rapidly. New sophisticated devices reshape the way engineers apply circuit protection and open new possibilities for the savvy designer.

Electronic systems are shrinking and circuit-protection devices are no exception. To take up less space, circuit breakers may double as power-control relays.

Programmability has also come to circuit breakers. Smart protectors sport programmable trip values and overload time delays. More-sophisticated protectors measure and report voltage and current values, alert control systems about tripped conditions, and can be reset remotely.

Breakers can be classified into magnetic and thermal types. Magnetic breakers operate via a solenoid that trips a mechanism at almost the instant it sees a threshold current. The near instantaneous response is appropriate for printed-circuit boards and sudden power surges as from short circuits or emergency shutdowns from crowbartype overvoltage monitors.

Magnetic breakers often get paired with hydraulic delays to tolerate current surges as generated during motor startup. Mounting the breaker horizontally keeps gravity from influencing solenoid movement. Breakers mounted vertically may need derating.

Magnetic breakers have a reputation for low voltage losses. The solenoid coil they use has little resistance resulting in low I R drop. Thermal or thermalmagnetic breakers generate heat that is applied to a bimetallic strip or disk. This heating mechanism generally produces a higher voltage drop though not as high as engineers tend to assume.

Thermal breakers rated under 5 A generally add more resistance to the power circuit than equivalently rated magnetic breakers. But many thermal breakers rated 5 A or higher have the same or lower resistance than magnetic breakers. So nothing precludes the use of these higher-rated thermal breakers if the application would benefit.

The bimetallic strip in a thermal breaker consists of two metals with different coefficients of expansion. As the strip heats one metal expands more than the other to warp the strip. The warped strip either opens a set of electrical contacts directly or triggers a mechanism to trip out the breaker.

The thermal lag from heating the bimetallic strip gives thermal breakers a slower trip. The slow-trip response helps discriminate between safe temporary surges and prolonged overloads. These breakers work best for machinery or vehicles where high inrush current accompanies the start of electric motors, transformers, and solenoids.

As global markets become more important, designers must consider how circuit-protection devices can meet both domestic and international standards. Traditional UL and CSA product approvals may not suffice. Engineers and designers may need to consider VDE, the German Association for Electrical, Electronic, and Information Technology, or the broader European CE mark for products sold in European Union countries.

On the other side of the world the CCC mark, or China Compulsory Certification, is mandatory for products exported to or sold in China. CCC approval covers low-voltage electrical products including circuit breakers, electric tools, household appliances, and telecom equipment.

Embedded microprocessors now get built into a variety of products, and circuit-protection products are no exception with the arrival of intelligent devices. Many smart breakers include sensing circuits. These sensors feedback information to PLCs or other control units on such factors as circuit status, current flows, and other relevant data. Some solid-state circuit breakers provide an analog output signal proportional to current.

Programmable technology using solid-state power control makes it possible to monitor and limit maximum current flow during short circuits. Embedded microcontrollers let engineers program both breaker trip points and speed profiles on the fly. Factory information systems can retrieve high current values, cycle times, and other information from circuit breakers with internal memory storage.

Such features were unheard of with traditional circuit breakers. A small amount of processing power and the ability to communicate via standard industrial networks makes remote programmability a reality today.

One final trend to mention is the steady spread and acceptance of ISO-14001 standards — manufacturing products with environmentally safe materials. Many countries now demand that products be certified as lead-free. Manufacturers are starting to conform to the Restriction of Hazardous Substances directive of the European Union. This directive takes effect in July 2006 and restricts the use of specific hazardous materials such as lead, cadmium, and mercury in products manufactured or sold in EU countries.

E-T-A Circuit Breakers
(847) 827-7600

How circuit breakers work

This cutaway view of a thermalmagnetic circuit breaker illustrates the typical internal workings of these circuit-protection devices.

Thermal-magnetic circuit breakers combine the instantaneous trip ability of a magnetic breaker for shortcircuit currents with the overload tolerance of thermal breakers for normal motor-starting demands. When reset, the movable contact completes the electrical circuit between the stationary contact on the line terminal and the bimetallic strip. The pawl on the strip keeps the contacts engaged. Current flows from the movable contact through the bimetallic strip to the release arm and into the electromagnetic coil. The coil connects to the terminal supplying power to the load.

There are three ways to trip this circuit breaker. First, a prolonged high current flowing through the bimetallic strip creates heat. The strip begins bending from the heat because of the different coefficients of expansion between the two metals. When it bends enough the pawl releases the movable contact and spring tension snaps the contacts open breaking the circuit.

The second trip mechanism uses the electromagnetic coil as a solenoid. When current through the coil is high enough the solenoid core is pulled into the coil, pushing downward on the metal plate where the bimetallic strip is fastened.

As the metal plate is pushed down, the bimetallic strip is again pulled away from the movable contact popping it open.

The final trip mechanism places force on the solenoid core by pressing the manual release button.

TAGS: Technologies
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