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Circuit breakers

Circuit breakers are absolutely essential devices in the modern world, and one of the most important safety mechanisms. Whenever electrical wiring in a building has too much current flowing through it, these simple machines cut the power until somebody can fix the problem. Without circuit breakers (, electricity would be impractical because of the potential for fires and other mayhem resulting from simple wiring problems and equipment failures.

Circuit breakers monitor electrical current and cut off the power when current levels get too high. The circuit breaker is an incredibly simple solution to a potentially deadly problem.

Electricity Basics

Electricity is defined by three major attributes:

Voltage is the "pressure" that makes electric charge move. Current is the charge's "flow" - the rate at which the charge moves through the conductor, measured at any point. The conductor offers a certain amount of resistance to this flow, which varies depending on the conductor's composition and size.

Voltage, current and resistance are all interrelated - you can't change one without changing another. Current is equal to voltage divided by resistance (commonly written as I = v / r). This makes intuitive sense: If you increase the pressure working on electric charge or decrease the resistance, more charge will flow. If you decrease pressure or increase resistance, less charge will flow.

Why You Need a Circuit Breaker

The power distribution grid delivers electricity from a power plant. Inside the electric charge moves in a large circuit, which is composed of many smaller circuits. One end of the circuit, the hot wire, leads to the power plant. The other end, called the neutral wire, leads to ground. Because the hot wire connects to a high energy source, and the neutral wire connects to an electrically neutral source (the earth), there is a voltage across the circuit - charge moves whenever the circuit is closed. The current is said to be alternating current, because it rapidly changes direction.

The power distribution grid delivers electricity at a consistent voltage (120 and 240 volts in the United States), but resistance (and therefore current) varies in your location. All of the different electrical devices offer a certain amount of resistance, also described as the load. This resistance is what makes the device work. A light bulb, for example, has a filament inside that is very resistant to flowing charge. The charge has to work hard to move along, which heats up the filament, causing it to glow.

In building wiring, the hot wire and the neutral wire never touch directly. The charge running through the circuit always passes through a device, which acts as a resistor. In this way, the electrical resistance in devices limits how much charge can flow through a circuit (with a constant voltage and a constant resistance, the current must also be constant). Devices are designed to keep current at a relatively low level for safety purposes. Too much charge flowing through a circuit at a particular time would heat the device's wires and the building's wiring to unsafe levels, possibly causing a fire.

This keeps the electrical system running smoothly most of the time. But occasionally, something will connect the hot wire directly to the neutral wire or something else leading to ground. For example, a motor might overheat and melt, fusing the hot and neutral wires together. Or someone might drive a nail into the wall, accidentally puncturing one of the power lines. When the hot wire is connected directly to ground, there is minimal resistance in the circuit, so the voltage pushes a huge amount of charge through the wire. If this continues, the wires can overheat and start a fire. The circuit breaker's job is to cut off the circuit whenever the current jumps above a safe level.

Basic Circuit Breaker Design

The basic circuit breaker consists of a simple switch, connected to either a bimetallic strip or an electromagnet.

The hot wire in the circuit connects to the two ends of the switch. When the switch is flipped to the on position, electricity can flow from the bottom terminal, through the electromagnet, up to the moving contact, across to the stationary contact and out to the upper terminal.

The electricity magnetizes the electromagnet. Increasing current boosts the electromagnet's magnetic force, and decreasing current lowers the magnetism. When the current jumps to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. The entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit. The electricity shuts off.

A bimetallic strip design works on the same principle, except that instead of energizing an electromagnet, the high current bends a thin strip to move the linkage. Some circuit breakers use an explosive charge to throw the switch. When current rises above a certain level, it ignites explosive material, which drives a piston to open the switch.

Advanced Circuit Breaker Design

More advanced circuit breakers use electronic components (semiconductor devices) to monitor current levels rather than simple electrical devices. These elements are a lot more precise, and they shut down the circuit more quickly, but they are also more expensive.

Another circuit breaker device is the ground fault circuit interrupter, or GFCI. These sophisticated breakers are designed to protect people from electrical shock, rather than prevent damage to a building's wiring. The GFCI constantly monitors the current in a circuit's neutral wire and hot wire. When everything is working correctly, the current in both wires should be exactly the same. As soon as the hot wire connects directly to ground (if somebody accidentally touches the hot wire, for example), the current level surges in the hot wire, but not in the neutral wire. The GFCI breaks the circuit as soon as this happens, preventing electrocution. Since it doesn't have to wait for current to climb to unsafe levels, the GFCI reacts much more quickly than a conventional breaker.

Amps, Volts, Ohm & Watts

The most basic units in electricity are voltage (V), current (I) and resistance (r). Voltage is measured in volts, current is measured in amps, resistance is measured in ohms.

A neat analogy to help understand these terms is a system of plumbing pipes. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size. There is a basic equation in electrical engineering that states how the three terms relate. It says that the current is equal to the voltage divided by the resistance.

I = V/r

Let's see how this relation applies to the plumbing system. Let's say you have a tank of pressurized water connected to a hose that you are using to water the garden. What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of the hose. The same is true of an electrical system: Increasing the voltage will make more current flow.

Let's say you increase the diameter of the hose and all of the fittings to the tank. You probably guessed that this also makes more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases the current flow.

Electrical power is measured in watts. In an electrical system power (P) is equal to the voltage multiplied by the current.

P = VI

The water analogy still applies. Take a hose and point it at a waterwheel like the ones that were used to turn grinding stones in watermills. You can increase the power generated by the waterwheel in two ways. If you increase the pressure of the water coming out of the hose, it hits the waterwheel with a lot more force and the wheel turns faster, generating more power. If you increase the flow rate, the waterwheel turns faster because of the weight of the extra water hitting it.

In an electrical system, increasing either the current or the voltage will result in higher power. Let's say you have a system with a 6-volt light bulb hooked up to a 6-volt battery. The power output of the light bulb is 100 watts. Using the equation above, we can calculate how much current in amps would be required to get 100 watts out of this 6-volt bulb.

You know that P = 100 W, and V = 6 V. So you can rearrange the equation to solve for I and substitute in the numbers.

I = P/V = 100 W / 6 V = 16.66 amps

What would happen if you use a 12-volt battery and a 12-volt light bulb to get 100 watts of power?

100 W / 12 V = 8.33 amps

So this system produces the same power, but with half the current. There is an advantage that comes from using less current to make the same amount of power. The resistance in electrical wires consumes power, and the power consumed increases as the current going through the wires increases. You can see how this happens by doing a little rearranging of the two equations. What you need is an equation for power in terms of resistance and current. Let's rearrange the first equation:

I = V / R can be restated as V = I R

Now you can substitute the equation for V into the other equation:

P = V I substituting for V we get P = IR I, or P = I2R

What this equation tells you is that the power consumed by the wires increases if the resistance of the wires increases (for instance, if the wires get smaller or are made of a less conductive material). But it increases dramatically if the current going through the wires increases. So using a higher voltage to reduce the current can make electrical systems more efficient. The efficiency of electric motors also improves at higher voltages.

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Trends in Circuit Protection

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.

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Circuit Breaker Maintenance

The most common type of re-settable over current protective device is the molded case circuit breaker. The case functions as both an outer wrapper and to retain in proper position the breaker?s internal components. These cases are made from various types of electrical insulating and fire retardant plastic. Cases are typically not hermetically sealed; this allows them to be subject to corrosion from environmental factors. They are limited to 600 volts and less. They are typically available in either single, two, or three pole models. This type of circuit breaker is now available as AFCI, GFCI, and magnetic, hydraulic-magnetic, and thermal-magnetic types.

MCCB?s have many years of life built into them, requiring little maintenance. This should not be understood to mean that periodic maintenance is not required. NETA (InterNational Electrical Testing Association Inc.) has developed and published a book titled "Maintenance Testing Specifications" (NETA-MTS-01) that provides some guidance as to how various types of electrical equipment including MCCB?s should be tested.

The following is a short overview of some MCCB maintenance tasks. It is recommended that at least once a year a properly trained and equipped qualified electrician perform the following maintenance task:
  • Visually inspect the case to determine if any portion indicates overheating; replace the breaker if overheating indications are found.

  • Check connections for indications of overheating.

  • Cycle the breaker five times manually.

  • Check and record the voltage drop across the breaker using a calibrated digital voltmeter (capable of reading three places to the right of the decimal point).

  • The load should be operated at full load for three hours, or until the breaker reaches normal load temperature; scan the breaker with an IR type non-contact thermometer and record the readings.

  • Record voltages and note any voltage imbalance from phase to phase.

  • Current readings should be taken with a true RMS type meter due to the increasing harmonic content in many electrical systems in commercial/industrial facilities today.

  • Current readings on equipment grounding conductors (where required) for specific machines should be noted. Clamp on type ground-rod circuit resistance reading meters should be used for this task as they can detect both the impedance and the level of current on the conductor if any is present, as other clamp on type amp-meters will not indicate Ma levels.
Breaker test sets are commercially available from several sources testing of circuit breakers is a very specialized area requiring special training and test equipment and should be conducted only by competent personnel. NEMA has published a valuable guideline (AB-4-1991).

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