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Copyright 2009 Robert J. Scoff, PE 1

Fuse and Circuit BreakerOperation and Coordination

Robert J. Scoff, PE

Copyright 2009, Robert J. Scoff, PE

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Table of Contents Page

1. Introduction 42. Fuses 43. Voltage Ratings of Alternating Current Fuses 64. Current Ratings of Alternating Current Fuses 8

5. I2t Ratings of Fuses 116. Amps Interrupting Capacity (AIC) 117. Fusing for DC Circuits 128. Selective Coordination for Fused Circuits 139. Circuit Breakers 1510. Arc Dissipation Techniques 1611. Amps Interrupting Capacity 1712. Ground Fault Circuit Interrupters (GFCIs) 1813. Arc Flash Protection Circuit Breakers 1914. Single Phase 120/240 Volt Breakers for Residential Construction 1915. Time Current Characteristic Curves 20

16. Circuit Breaker Selective Coordination for Home Wiring 2217. Industrial 480 Volt Three Phase Systems 2218. Selective Coordination for Three Phase Circuit Breakers 2619. Conclusions 28

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List of Illustrations

Figure Title PageNumber

2.1 Basic Idea of a Fusible Link 4

2.2 Time Current Characteristic Curve for Buss LPS-RK Fuses 52.3 Approximate List Prices for Motor Protection Fuses 63.1 Model Showing What Must Happen When a Fuse Blows 63.2 Model of How a Higher Voltage Fuse Is Constructed 74.1 Possible Shape for a Fusible Link 84.2 Diagram of 1 Circular Mill (CM) 84.3 Resistivities of Materials Used in Fusible Links in English and Metric Units 94.4 Melting points of Various Materials Used In Fusible Links 94.5 English System of Determining Resistance of a Fusible Link 94.6 Metric System of Determining Resistance of a Fusible Link 104.7 Chart Expanded to Show Temperature Coefficient of Resistance 10

6.1 Diagram Showing How Current Is Limited in an Electric Circuit 117.1 Diagram Showing How a Reverse Biased Diode Helps Limit Load Turn

Off Arc 128.1 Single Line Diagram of a Typical Fuse Protected System 138.2 Chart Taken from the Bussman Website to Show Selective Coordination

Ratios of Various Types of Fuses.(Given for reference. Only as an example) 14

9.1 Sketch Showing How a Magnetic Trip Circuit Breaker Operates 159.2 Sketch Showing How a Thermal Trip Circuit Breaker Operates 169.3 Sketch Showing a Breaker with both Thermal and Magnetic Breaks 1610.1 Using Arc Dissipation Plates to Help Extinguish an Arc 16

10.2 Sketch Showing How a Blowout Coil Works 1710.3 Sketch Showing Spring Assisted Mechanism to Separate Contact Points 1711.1 Sketch Showing Why a Circuit Breaker Needs to Have an AIC Rating 1812.1 Sketch Showing How a GFCI Works 1813.1 Sketch Showing How an Arc Could Form In a Circuit Element Such as

A Switch 1914.1 Drawing Showing Typical Single Phase 120/240 Volt Circuit Breaker

Protection 2015.1 Time Current Characteristic for a Siemens EQ Frame Two Pole Circuit

Breaker 2117.1 One Line Diagram for Typical Industrial Plant 23

17.2 Pictures of Typical Siemens Three Phase Breakers 2417.3 Square D NW Series Circuit Breaker Showing Available Options 2517.4 Chart Showing How the Cost of Circuit Breakers Increases With Current

Rating 2618.1 Example of Typical Time Current Trip Curves for 10 and 100 Amp

Breakers 2718.2 One Line Drawing of a 100 Amp Breaker Feeding a 10 Amp Breaker 28

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1. Introduction

Circuit protection methods have been with us as long as there has been the useful utilization of electricalpower. Thomas Edison suggested the use of fuses as safety elements in 1879. He also suggested parallelcircuits, as used in modern electrical distribution systems, at that time. The same principles of modernfuses were used in the early designs. However, as the needs of circuit protection have changed over the

years, modern fuses have become more and more sophisticated. Fuses, by their very nature, are one timedevices that need to be replaced after they have done their job of protecting a circuit. When a fuse doesits job of protecting a circuit, it is said to be blown.

An early form of circuit breaker was described in an 1879 patent of Thomas Edison. However, inEdisons first power systems, fuses were used for circuit protection. This is probably because, at the time,fuses were considered more reliable. What could be more reliable than a piece of wire that burns outbecause too much current flows through it? The circuit breaker not only has to detect the current, but alsocause the circuit to be interrupted. Therefore it needs contact points and a way of separating them.Circuit breakers have the real advantage of being resettable. When a circuit breaker does its job ofprotecting a circuit is said to be tripped.

2. Fuses

Fuses are really small pieces of wire that get hot and melt if too much current flows through them. Figure2.1 shows the basic theory that applies to all fuses.

Figure 2.1 Basic Idea of a Fusible Link

The fusible link is a piece of wire with a certain length and cross sectional area. Determining that lengthand cross sectional area is far beyond the scope of this course. However, we can know that every wire hasa certain resistance, and if current flows through that resistance, it will get hot. To add to the complexity,as the wire gets hotter, its resistance increases. This causes it to dissipate more power, and get hotter.When the fusible link finally melts, the current flow is interrupted, and an arc will form. Then, when thearc is extinguished, the circuit is interrupted and current flow is stopped. As a mater of interest, an arc hasa voltage drop of 30 to 40 volts across it.

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For most fuses, the fusible link is enclosed in an insulating tube of some sort. This keeps the heatgenerated by the current flowing in the fuse from dissipating into the surrounding air. This affects thetime-current curves of fuses. The time-current curve of a fuse shows how long it takes for a fuse to blowa certain current levels. Figure 2.2 shows a typical characteristic curve for a Buss LPS-RK fuse.

Figure 2.2 Time Current Characteristic Curve for Buss LPS-RK Fuses

Using the 60 amp dual element time delay fuse as an example, note that in 300 seconds, 100 amps willcause the fuse to blow. X marks that place on the curves. This is actually a dual element, time delay,motor protection fuse. Other applications include transformers, branch circuits, and solenoids. Many ofthese circuits have a large inrush current on turn on that decreases rapidly afterwards. Motors, forexample, have about a 6 times running current on turn on. Looking again at the 60 amp fuse curve, noticethat it takes 0.1 seconds to blow the fuse at 600 amps. Y marks that place on the curves. Since that is10 times the rated current, there should be plenty of time to start a motor and have the current drop to less

than 60 amps. Also, as a matter of interest, The National Electric Code allows motors to be fused at 125% of the full load rated motor current. That means that if a motors FLA (Full Load Amps) is 50 amps, itcan be fused at 62.5 amps, or the next larger standard size. For most fuse types, that would be 70 amps.For cost considerations, a 60 amp fuse would probably be selected. The reason is that 60 amps is the highend of a smaller (and less expensive) fuse body. The 70 amp fuse would be the low end of the 100 ampsize fuse body. The standard breaks in fuse sizes for LPS-RK are as follows: 0 to 30 amps, 31 to 60amps, 61 to 100 amps, 101 to 200 amps, 201 to 400 amps, and 401 to 600 amps. The higher currentrating sizes are physically bigger and more expensive. The enclosures for the larger fuses are also more

X

Y

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expensive. List prices for FRS motor protection fuses were obtained from an electrical distributor. Theyare shown below in Figure 2.3.

Fuse Type Approximate ListPrice

FRS-R-30 $8.45

FRS-R-60 $14.48

FRS-R-100 $29.92

FRS-R-200 $59.86

FRS-R-400 $119.69

FRS-R-600 $173.70

Figure 2.3 Approximate List Prices for Motor Protection Fuses

Notice that each larger fuse body size is approximately double the price of the next smaller fuse body.FRS and LPS fuses are both motor protection, time delay fuses. To qualify as a time delay fuse, the fusemust not open in less than 10 seconds at 500 % current rating. The LPS fuses are also current limitingfuses that will blow before a certain current limit is reached. One bit of information that can be gotten

from the fuse pricing chart is that bigger fuses are very expensive. This would encourage maintenancepeople to determine the cause of a fuse failure before replacing a blown fuse with another good one. If afault is not removed, the only result of replacing a fuse with a new one would be that the new one willalso blow. A 100 Horsepower motor would draw 124 amps at full load and be protected by 150 ampfuses. These 150 amp fuses would cost about $60 each. Blowing 2 of these fuses would quickly cost$120.

3. Voltage Ratings of Alternating Current Fuses

When a fuse blows, an arc forms where the fuse link melts. This arc must be extinguished in some way.If the circuit being protected is a low voltage, there is no problem. An arc needs 30 to 40 volts to be

sustained. Extinguishing the arc is of very little concern for low voltage fuses. However, as circuitvoltage is increased, some means must be established for extinguishing the arc that can form when acircuit is interrupted. Obviously, the higher the voltage, the more difficult this becomes. If a short circuitexists, the problem is compounded because the possibility of a very large current exists. Figure 3.1 showsa model of a fuse protected circuit with a short circuit.

VAC Load

Fuse

Short

Circuit

Figure 3.1 Model Showing What Must Happen When a Fuse Blows

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4. Current Ratings of Alternating Current Fuses

Fuses come in current ratings of a fraction of an amp to at least 6000 amps. When a fractional amperefuse blows, there is very little energy released in the fuse. As the fuse size increases, more and moreenergy is released in the blowing process. The fusible link is physically larger for larger fuses, and thelink must not melt for the rated current.

The material chosen for the fusible link is also very important. It must have a low enough melting pointto actually melt on over current conditions. Some materials that are used are various alloys of tin andlead. Large power fusible links may also be made of silver or copper. The fusible link can be a wire or astrip of metal. Some fusible links are in the form of a coil or are made longer by forming the link into anS shape as shown in Figure 4.1.

Figure 4.1 Possible Shapes for a Fusible Link

The design of the fusible link depends upon a number of factors. The right material must be chosen sothat it melts at a certain temperature. It must have a high enough resistance so that it will get hot at thedesired current. The arc must self extinguish. Some fuses, called current limiting fuses, must blow at acertain current to prevent high currents from flowing on short circuit conditions. Lets cover these issuesone at a time.

All materials have a property called resistivity. In English units, resistivity is given in Ohm CircularMills per foot. A Circular Mill is the area of a circle 0.001 inches in diameter. Figure 4.2 shows this.

Figure 4.2 Diagram of 1 Circular Mill (CM)

Resistivities of various materials at 20 degrees C used in fusible link construction are as shown in Figure4.3. Note that the most common materials used in fusible links, tin and lead, also have higher resistivities.Alloys of tin and lead are also commonly used. This helps give the fusible links a higher resistance for acertain length and cross sectional area. Tin and lead and their alloys also have lower meltingtemperatures. Figure 4.4 shows the melting points of some materials commonly used for fusible links.The eutectic alloy 63 % tin, 37% lead is shown because it has a definite melting point, and it is the lowestmelting point for any tin lead alloy. This makes it a good material to use as the fusible link.

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Material Resistivity inOhm-Meter

Resistivityin OhmCM per ft

Aluminum 2.65 * 10 exp -8 15.93

Copper 1.725 * 10 exp -8 10.37

Lead 20.6 * 10 exp -8 123.84

Silver 1.59 * 10 exp -8 9.558

Tin 11.0 * 10 exp -8 66.128

Figure 4.3 Resistivities of Materials Used in Fusible Links in English and Metric Units

Melting Points of Various MaterialsUsed In Fusible Links

Metal Degrees F Degrees C

Tin 450 232Lead 621 327

Aluminum 1220 660

Silver 1761 961

Copper 1981 1085

63/37 Tin Lead Alloy 361 183

Figure 4.4 Melting points of Various Materials Used In Fusible Links

Both English and Metric units are shown for convenience. Figure 4.5 shows an example for Englishunits.

Length in

feet

Areain

CM

Fusible Link

Figure 4.5 English System of Determining Resistance of a Fusible Link

If Metric units were used, the drawing would look like Figure 4.6.

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Figure 4.6 Metric System of Determining Resistance of a Fusible Link

The reason that this is important is that when current flows through a fusible link, heat is dissipated andcauses the link to get hotter. Remember that the link is enclosed in a tube that holds the heat in. The heatenergy will slowly move to the area outside of the fuse, but this is a slow process. The power beingdissipated is equal to:

P = I2 * R = E2 / R

Another effect that takes place is that as the fusible link heats up, its resistance increases. This is becausein most materials that are used as fusible links, the resistance increases as the temperature of the materialincreases. This too follows a very predictable pattern. The effect is called The Temperature Coefficientof Resistance. The real effect is to cause the fusible link to get hotter faster as the temperature of the linkincreases. This poses a significant design problem for the fuse manufacturers. The actual ambienttemperature also needs to be taken into account. Because of these effects, a fuse will take differentlengths of time to blow for different over current conditions. Fuse blow/time curves will be addressedlater. Figure 2.2 also shows how the time current fuse blow characteristics are affected. Figure 4.7 shows

the Temperature Coefficient of Resistance for the most commonly used fuse materials.

Material Resistivity inOhm-Meter

Resistivity inOhm CMper foot

TemperatureCoefficient ofResistance1/Degree C

Aluminum 2.65 * 10 exp -8 15.93 0.00429

Copper 1.725 * 10 exp -8 10.37 0.00429

Lead 20.6 * 10 exp -8 123.84 0.0039

Silver 1.59 * 10 exp -8 9.558 0.0061

Tin 11.0 * 10 exp -8 66.128 0.0045

Figure 4.7 Chart Expanded to Show Temperature Coefficient of Resistance

Using an (Temperature Coefficient of Resistance) of 0.004, a 20 degree C rise in temperature causesabout an 8% increase in resistance. This affects the fuse characteristic. Of course, most circuits do notrun the current to the limits. But the fuse designer must take these effects into account. Thermal

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expansion will also have to be taken into account. The mounting of the fusible link has to be designed totake care of the expansion and contraction of the metals used in the fusible link. There is a lot more tothese simple circuit elements than is seen at first glance.

5. I2t Ratings of Fuses

A very important characteristic of fuses is their I

2

t rating. This is a measure of the amount of energyneeded to cause a fuse to blow. If we think about the power equation, P = I2*R, and the energy equation,W = P * t, we can see how this idea of I2t as a measure of energy came about. In these two equations, Pis power in watts, I is current in amps, R is resistance in ohms, and W is energy in joules. Resistance isjust assumed to be a constant. And, as was shown earlier, this is only approximately true. Resistancedoes change with temperature, but this was not taken into account. When fuses are tested for their I2tratings, the resistance change does change the real energy released, but the time to blow for certaincurrents is found by testing.

The reason that this is an important concept is that some fuses need to blow slowly, as for motor startingapplications, and some fuses need to blow quickly, as in semi-conductor protection fuses. For motor

starting applications, the fuse needs to have a high I

2

t rating so that the short time high currents thatmotors have on starting does not blow the fuse. This can be 6 to 10 times the FLA (full load amps) of themotor. To help in this area, the National Electric Code allows fuses used in motor applications to be 125% of the full load current of the motor. Semi conductor protection fuses, on the other hand, need to havea small I2t rating. Semi conductor protection fuses are quite often called I2t or current limiting fuses.When failures occur on solid state circuits it is desirable to have the fuse blow quickly to protect the semiconductors in the circuit. These fuses are quite often used to protect solid state AC and DC motor speedcontrollers

6. Amps Interrupting Capacity (AIC)

Another important parameter of circuit protection devices is Amps Interrupting Capacity or AIC. Whatthis means is that the circuit protection device is able to operate properly or blow when the power supplyis able to supply a certain amount of short circuit current. For example, if a power source is able tosupply 10,000 short circuit amps, the fuse must have an AIC rating of at least 10,000 amps. All powersupplies are inherently capable of supplying a certain current limited by the series impedance of thesource. This can be seen by looking at Figure 6.1.

LoadGenerated

Voltage

Total Series Impedance Fuse

ShortCircuit

Figure 6.1 Diagram Showing How Current Is Limited in an Electric Circuit

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If the fuse is not capable of stopping the current that the source is capable of supplying through the totalseries impedance, it could fail and become an arc or even a short circuit itself. The short circuit availablecurrent can be determined by an analysis of the electrical supply. That is beyond the scope of this course.

7. Fusing for DC Circuits

Everything that is true for fusing alternating current circuits is true for direct current circuits. There is onemore thing that needs to be taken into account. DC voltages do not go through zero volts twice a cyclelike AC voltages do. That means that they do not break a DC circuit as easily as they would an ACcircuit. The current wants to keep flowing and sustain the arc that exists when a fuse blows. Often, a fusewill have two voltage ratings, one for alternating current, and one for direct current. The direct currentvoltage rating is typically lower for these dual rated fuses. Not only does the voltage not go through zerovolts, but an inductive load will want to keep the current flowing whenever the circuit is opened. Thisprocess can often be addressed by placing a reversed biased diode in parallel with the load. An exampleof how this is done is shown in Figure 7.1.

Figure 7.1 Diagram Showing How a Reverse Biased Diode Helps Limit Load Turn Off Arc

I` shown in Red will flow whenever the disconnect switch opens, or the fuse blows. In each case, theenergy of the inductive load will be dissipated by the current flowing through the reverse biased diode.This happens because the current flowing in the inductive load cannot instantly change magnitude ordirection. The basic reason that this is true is that it is impossible to instantly change the energy level ofan inductor. And that energy level depends on the current flowing through the inductor. The energyequation is:

W = * L * I2

Where W is energy in Joules, L is the inductance in Henries, and I is the current in Amperes.

If the reverse biased diode is not present, whatever energy is stored in the inductor will cause an arc toform at the place where the circuit is opened. This could be the switch or the fuse. The arc will besustained until the energy in the inductor is dissipated. DC fuse design has to take into account thepossibility that the reverse biased diode is not present in the circuit. If the load is a motor, it is definitelyan inductor. Real DC circuits are becoming less and less common in the workplace, as AC circuits are

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taking their place wherever possible. This is especially true in the motor speed control world whereVariable Frequency Drives are taking the place of DC Drives.

8. Selective Coordination for Fused Circuits

In most circuits there is more than one fuse in the path of current flow. There are upstream fuses which

are larger and protect branch circuits. Then there are downstream fuses which are smaller and protectindividual loads. The definition from Article 100 of the 2008 National Electric Code is as follows:

Localization of an overcurrent condition to restrict outages to the circuit or equipment affected,accomplished by the choice of overcurrent protective devices and their ratings or settings. Another wayto think of this is to say that when an overcurrent or short circuit occurs, only the nearest upstream fusewill blow, and no other upstream fuse will blow.

An example of a single line diagram of a fused system is shown in Figure 8.1

The whole idea is to select fuses that will not only protect the loads and wiring, but will not cause any

other upstream fuses to blow. In Figure 8.1, if load J fails and causes fuse # 14 to blow, fuses 11, 4 and 1are selectively coordinated (Chosen) to not blow. There are charts given for different types of fuses thatgive ratios for the upstream fuse to the downstream fuse. These ratios vary from 1.5 to 1 to 8 to 1,depending upon the fuse types. A fuse coordination calculator is available on-line at:

http://www.edreference.com/images/SelectiveCoord.jpg

The various fuse manufacturers also publish charts for determining the ratios needed to insure selectivecoordination. A chart was taken from the Bussman website to show a typical chart of this type. It isshown in Figure 8.2. If it is ever necessary to selectively coordinate a fuse protected circuit, you will needto look up your specific fuse types and do your design from there. One thing to notice is that FRS motorprotection fuses have a very high (8 to 1) ratio. This affects the total design, as much bigger switchgearand wiring will need to be used where the upstream fuse is required to be 8 times the size of thedownstream fuse.

Figure 8.1 Single Line Diagram of a Typical Fuse Protected System

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Figure

8.2

CharttakenfromtheBu

ssmanWebsitetoShowSelectiveCoordinationRatiosof

Variou

sTypesofFuses.ThisisgivenforReverenceonly,asanEx

ample.

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To show another example, if a short circuit occurred at Load J, and Fuse 4 blew, this would cause LoadsE, F, G, H, I, and J to be turned off. Not only does this make trouble shooting more difficult, but a lot ofunnecessary outages occur. As an extreme example, Fuse 1 could even blow if the selective coordinationwere done particularly badly. Then the whole system would shut down. Troubleshooting this failurewould then be a real nightmare, and a whole system would be shut down, while the maintenance crewsearched for and corrected the cause of the problem.

9. Circuit Breakers

Circuit breakers are resettable circuit protection devices. This means that they co not have to be replacedwhen they open because of an over current condition. Generally, they co not operate as quickly as fusesused in similar applications. And, like fuses, they are made in a wide range of voltage and current ranges.This paper will only cover a small number of the available types, but will give a general overview of whatis available.

Most circuit breakers operate on one of two principles. The first is Magnetic Trip circuit breakers. Inthese devices, a coil carries all of the current going to the load, and when the current is too high for too

long a period of time, a rod will be caused to move, and make a switch open. How this works isillustrated in Figure 9.1.

Load

Figure 9.1 Sketch Showing How a Magnetic Trip Circuit Breaker Operates

Notice that there is some sort of locking mechanism to prevent the switch from reclosing after the breakertrips. If a breaker trips, or if a fuse blows for that matter, the reason for the tripping should be found.Circuit breakers can be reset without additional cost, but new fuses must be purchased if they blow.However, even though it costs nothing extra to reset a circuit breaker if it trips, the cause of the failureshould be found and corrected.

Another way for a circuit breaker to operate is by means of a Thermal Trip device. A bi-metallic strip is

one way for this to work. Two metals with different thermal coefficients of expansion are welded orriveted together. Current can either flow through the bi-metallic strip or through a resistor that heats thebi-metallic strip. In either case, the strip bends and causes a switch to open. This is illustrated in Figure9.2.

Some breakers have both a magnetic trip and a thermal trip built into them. These breakers are commonlyused on motor protection circuits. The thermal trip then protects against long term motor overloads, whilethe magnetic trip protects against short circuits and too high of a starting current. This could happen if the

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motor rotor is blocked in some manner. A sketch of how a breaker with both thermal and magnetic tripsis shown in Figure 9.3. Either the thermal device or the magnetic device will cause the breaker to trip.

LoadLatching

Mechanism

Voltage

Source

Heater

Bi-Metalic Strip That Bends

When Heated to Activate Switch

Figure 9.2 Sketch Showing How a Thermal Trip Circuit Breaker Operates

Load

Figure 9.3 Sketch Showing a Breaker with both Thermal and Magnetic Breaks

10.Arc Dissipation Techniques

Circuit breakers, as well as fuses, have a problem with extinguishing the arc that forms when the breakeroperates. Fuses can be designed so that sand or a gas evolving substance, such as boric acid, flows intothe space where the fusible elements melts. Either substance helps extinguish the arc. This technique willnot work for circuit breakers, unless some way is found to add more sand or boric acid each time thebreaker trips. Therefore other methods to extinguish arcs in circuit breakers were developed. One way isto use arc dissipation plates as shown in Figure 10.1.

Figure 10.1 Using Arc Dissipation Plates to Help Extinguish an Arc

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Another thing that is done is to put a small coil in series with the breaking contacts. This will generate amagnetic field whenever current flows through it. The separating contacts are placed in a location so thatthe magnetic field that is generated causes the arc to distort and blow out. This is shown in Figure 10.2.Quite often, a circuit breaker will contain both arc dissipation plates and a blowout coil.

LoadVoltage

Source

Arc BlowoutCoil

Magnetic

Field

Arc in Process of

Being Blown Out

Tripping

Mechanism

Figure 10.2 Sketch Showing How a Blowout Coil Works

Another thing that is done on circuit breakers is that the contact points are held in position with a latchingmechanism, and when the breaker trips, the contacts are pulled apart with a spring loaded device. Figure10.3 shows a sketch of how this is accomplished. Of course, in actual practice, these are threedimensional devices and are best seen by actually taking different breakers apart and looking at the actualconstruction. Some very ingenious engineering has gone into the design of these devices.

Figure 10.3 Sketch Showing Spring Assisted Mechanism to Separate Contact Points

11.Amps Interrupting Capacity

Along with fuses, circuit breakers also have an Amps Interrupting Capacity (AIC). This means that thecircuit breaker must be able to trip and disconnect the load (or short circuit) from the source that iscapable of supplying a certain current. One way to think of this is to consider a circuit that has a certainline impedance. If a short circuit is placed across the load (or if the load becomes a direct short circuit),the source will be able to supply a certain current limited only by the source voltage and the total lineimpedance.

Figure 11.1 shows a circuit whose maximum current is only limited by the total of the line and sourceimpedance. If the breaker can not disconnect the source from the short circuit, it could fail in the shorted

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condition and cause a serious arc flash to occur. Two failure modes are the points could actually weldtogether or an arc could be established between the points and not be extinguished. Either conditionwould cause the breaker to fail. On high power circuits, serious explosions could occur.

Load

Total Impedance

Source and Lines

Voltage

Source ShortCircuit

CircuitBreaker

Figure 11.1 Sketch Showing Why a Circuit Breaker Needs to Have an AIC Rating

12.Ground Fault Circuit Interrupters (GFCIs)

A special type of breaker has been developed to help prevent death by electrocution. These breakers arecalled Ground Fault Circuit Interrupters or GFCIs. They work by measuring the current in both the hotwire and the neutral wire and comparing the two currents. If the currents are different by even a smallamount, the trip mechanism in the breaker will cause the breaker to open. As an example of how small ofa difference in currents will cause the GFCI to trip, a 20 amp breaker will trip with as little as .05 ampsdifference between the hot and neutral currents. Figure 12.1 shows how a GFCI circuit breaker works.

GFCI Trip

Circuit

o ov

(

c c

Black Wire

White Wire

Green Wire

Load

Ground Fault

Current

Trip

Linkage

Voltage

Source

Earth

Ground

Figure 12.1 Sketch Showing How a GFCI Works

When the load is operating properly, the black wire current is equal to the white wire current. If, for somereason, such as a person touching the hot wire while grounded, the white wire current becomes not equalto the black wire current, the GFI trip circuit will cause the breaker to open. This will prevent the person,whose hand was where it shouldnt have been anyway, from being seriously shocked or even killed. Thelow current limit of about .05 amps (50 milliamps) was chosen because it is thought that this low a currentcan cause the heart to go into ventricular fibula ion. When that happens the heart starts beating erratically,the blood pressure drops to zero, and shortly thereafter, the patient dies. GFCIs are required by the

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National Electric Code in many applications where a person could be easily shocked. These areas includewet areas, areas near swimming pools, kitchens, bathrooms, crawl spaces, unfinished basements, andoutdoor areas.

13.Arc Flash Protection Circuit Breakers

Arc flash protection circuit breakers, often called Arc Flash Circuit Interrupters (AFCI), are now being

required in many locations. They work by detecting a fast rise and fall of electrical current, as an arcingconnection could do. They are now required by The National Electric Code for bedrooms in newconstruction. This has been true since the 2002 issue of The NEC. These devices do not protect againstground faults or shorts, but to arcs that could occur at a connection point in a circuit. This could occur ina switch or circuit device. An arc could also form at a connection point in the circuit, such as a pointwhere wires are connected together, or where wires are connected to a circuit device. Figure 13.1 showshow an arc could occur without tripping either a regular circuit breaker or a ground fault interruptercircuit breaker.

Voltage

Source

ACFI Circuit

Breaker

Switch

Small Gap or

Dirty Connection

Where an ArcCould Form

Figure 13.1 Sketch Showing How an Arc Could Form In a Circuit Element Such as a Switch

The arcs energy would be limited by the load, but the temperature could be up to 6000 degrees

Centigrade. Enough energy might be available to cause a fire. It is important to note the difference ofenergy levels between an arcing connection and an arc flash. An arc flash, which is caused by a shortcircuit, releases a great amount of energy in a short time (pdhengineer has a course on ARC Flash), whilean arcing connection would release a small amount of energy over a much longer period of time. Anarcing connection would only occur when the circuit is completed, such as when a switch to a load isclosed. An arcing connection could be happening for weeks or months before the energy got high enoughto cause a fire to start. Loose connections such as could occur at wire nuts, switch contacts, or screwconnections could cause arcing conditions to occur. Checking for arcing conditions would require eitherlooking for intermittent noise on the voltage supply or fast turn ons and turn offs in the current supply.Most AFCIs work by detecting fast changes of current. It is possible to build circuit breakers thatincorporate normal over current trip, GFCI trip and AFCI trip.

14.Single Phase 120/240 Volt Breakers for Residential Construction

The most common power supply that people are exposed to is the house wiring 120/240 single phasesystem. The power supply, often called the source, consists of two 120 volt supplies connected in series.The center tap of the two 120 volt sources is connected to ground. Thus, there is never more than 120volts to ground in a system of this type. Figure 14.1 shows how this system is designed. Note that thereis a double pole breaker, called the Main Breaker, feeding two buss bars. The grounded conductor, whichis connected solidly to earth ground, is not protected by a breaker pole. Since it is at zero volts, or

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ground, it is not necessary to protect it. In Figure 14.1, a double pole breaker and a single pole breakerare shown. Note that the transformer primary also has a protection fuse. It will be of a voltage rating ofthe power companys high voltage and a current rating to match the volt ampere rating of the transformer.

Primary FuseProtection

High Voltage fromPower Company

To 120/240 VoltSingle Phase Load

Such As an ElectricDryer

Note: SafetyGround Not Shown

120 VAC120 VAC

240 VAC

Double Pole

Main Breaker

Double PoleCircuitBreaker

Main Buss

Bars

To 120 VoltSingle Phase

LoadNote: SafetyGround Not

Shown

Grounding andNeutral Bar

Single PoleCircuit Breaker

CustomerEarth

Ground

Power CompanyEarth Ground

Power CompanyEarth Ground

Figure 14.1 Drawing Showing Typical Single Phase 120/240 Volt Circuit Breaker Protection

Note that the breakers in a panel such as shown in Figure 14.1 can be standard over current breakers,

GFCIs, or AFCIs. The breaker type will be determined by The National Electric Code and any localcodes that might be applicable. In any case, the AHJ (Authority Having Jurisdiction) has the final say inwhat is approved or not. In a typical application, the main breaker is commonly 100 to 200 amps, and theindividual circuit breakers are from 15 to 30 amps for one pole (120 Volt) applications and 15 to 60 ampsfor two pole (240 Volt) applications.

15.Time Current Characteristic Curves

Time Current Characteristic Curves are a very important consideration when applying circuit breakers.

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Figure 15.1 Time Current Characteristic for a Siemens EQ Frame Two Pole Circuit Breaker

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Figure 15.1 shows a Time Current Characteristic Curve for a Siemens 200 amp EQ Frame CircuitBreaker. It is a log curve with time on the vertical axis and current on the horizontal axis. One of thethings to notice is that there is a range of trip currents at a given time. For instance, from 0.02 seconds to1 second, the trip current could be anywhere from about 1200 to 2000 amps. The maximum clearing timefor this breaker is 1 cycle or about 0.016 seconds. This will happen at currents above 2000 amps. TheAIC (Amps Interrupting Capacity) is 22,000 amps. This is shown as the sharp vertical line on the bottom

right hand side of the characteristic curve. For a better view of the characteristic curve, it can bedownloaded at:

http://www.murrayconnect.com/NR/rdonlyres/3819A875-0CD2-48C5-98AB-07EF9E3A9165/0/M2150A225A.pdf

Circuit breakers do not trip instantaneously at their rated current. For this breaker it takes at least 10,000seconds (about 3 hours) of full load current to cause a trip to occur. At 300% of rated current (600 amps),the maximum trip time is 63 seconds. Since these are thermal breakers, ambient temperature is alsoimportant. The NEMA (National Electrical Manufacturers Association) says that the time current tripdata is to be taken at 25 degrees Centigrade.

16.Circuit Breaker Selective Coordination for Home Wiring

When a main circuit breaker is feeding a buss bar where a number of other circuit breakers are used, as ina typical home panel, it is important that the tripping of one of the auxiliary circuit breakers does notcause the main breaker, or any other upstream circuit protective device to trip or open. In other words, thedownstream (and usually smaller) device needs to open before any upstream (usually larger) device. Thisis to keep the circuits affected by a failure or short circuit as the only ones to be turned off. This helps inmaintenance and troubleshooting also. If the main breaker trips before one of 15 or 20 downstreamdevices, finding the real problem could be difficult. In Figure 15.1, the single and double pole circuitbreakers need to always trip before the double pole main breaker trips. When selective coordination is

applied, the curves of the individual breakers need to be drawn on the same graph, and they should notintersect anywhere on the time current graph. For home wiring 120/240 volt circuit breaker panels this isnot usually a problem. However, if a branch circuit of 70 amps were used on a panel with a 100 ampmain, both breakers might trip on a short circuit on the 70 amp circuit.

17.Industrial 480 Volt Three Phase Systems

Industrial power supply systems are a lot more complicated than home wiring systems. They also handlea lot more power. The voltages and currents are higher. In addition, there can be several layers ofprotection equipment before the power gets to the place where it is actually used. To make this easier tosee, single line diagrams will be used to show a typical distribution system. Note that only one 480 volt

line and breaker symbol will be shown. Figure 17.1 is a one line diagram of a typical factory powersupply. The current to the 20 amp loads has to go through a 100 amp breaker, then through a 400 ampbreaker, then through a 1000 amp breaker and all this is supplied by a transformer protected by the powercompany fuse. For selective coordination to apply, all of the upstream devices have to carry the shortcircuit current of the downstream devices until the proper device opens. The Article 100 definition fromThe National Electrical Code defines Selective Coordination as: Localization of an over currentcondition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protection devices and their ratings or settings.

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To MCC # 1

To MCC # 2

High VoltageDisconnect

Supply from Power Company

High Voltage Fuse

High Voltage to

480 VoltTransformer

Distribution Panel

Main Circuit Breaker1000 amps Three

Phase

Spare

5 - 400 Amp ThreePhase Distribution

Breakers

Panel With 4 - 100Amp Three Phase

Breakers

Panel A Panel B

Panels A and B AreAll 20 Amp Three

Phase Breakers

ToVariousLoads

ToVariousLoads

To Building B

To OfficeCircuits

Figure 17.1 One Line Diagram for Typical Industrial Plant

There are many types of three phase circuit breakers available. They are easily available in ratings from10 Amps to 5000 Amps. The smaller ones are physically smaller than the bigger ones. Frame size is anindication of the physical size of breakers. Obviously, higher current rated breakers must be physicallybigger. This is not only to handle the larger currents, but also the forces that exist between the currentcarrying parts of the devices. And when short currents of as much as 200,000 amps are interrupted, an arcwill be established which must be extinguished. These devices must be able to do this, and then reliablyreclose when the short circuit condition is removed. And there are times when a circuit breaker isreclosed on a short circuit. It the breaker is not strongly constructed, an electrical explosion could result,with possible injury or death. All of these factors must be taken into account when designing and

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building these devices. Even the small breakers that are used in home wiring single phase systems haveto be designed with all of these factors in mind. Figure 17.2 shows some typical three phase circuitbreakers.

Figure 17.2 Pictures of Typical Siemens Three Phase Breakers

The only way that I know of to get a real feel of circuit breakers, or almost anything else for that matter, isto actually work with, and even touch these devices. The pictures above do not, and can not, do circuitbreakers justice. In the past, bigger low voltage Thermal magnetic trip circuit breakers just had anextension handle and would trip on long term over current or short term short circuits. It took quite a bitof force to turn one of these breakers on. The extension handle made it possible. Turning them off wassomewhat easier, but still required some strength. The Sentron and VL lines shown in Figure 18.2 areexamples of this type of breaker.

Modern circuit breakers with current ratings of up to 6000 amps can do so many more things than justinterrupt a circuit under overload or short circuit conditions. Many have a display to show operatingconditions. This includes the ability to display present voltages and currents. They can have built inGround Fault Detectors. It is possible to order them with the capability to meter electrical usage,including demand. Some can come with built in power quality measurement meters similar to whatpower analyzers do. This includes the measurement and recording of voltage, current, watts, volt-

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amperes, power factor, and even harmonics. Since they have a built in micro-processor with memory,they can even record and save trip and alarm history, and even do waveform capture. And, now all thisinformation can be sent to a central station by means a communications capability. Trip and reset is nowdone manually by pushing a button. The breakers have some sort of energy storing mechanism, such as aspring, that takes the place of an extension handle to trip and reset the breaker. To make sure that thespring can be recharged, an external power supply can be utilized. Some breakers have a way to manually

recharge the spring if power is not available. With all of this computing ability, the time current tripcharacteristics of the breakers can be changed to meet individual needs. This helps with selectivecoordination. The technology has come a long way since the breakers that needed an extension handlewas needed to turn a large breaker on. And I can tell you from personal experience, that if the extensionhandle was misplaced, it was quite an adventure to operate a 1200 amp circuit breaker. Now, we justpush the red or green pushbutton to operate one of these breakers up to 6000 amps.

Figure 17.3 Square D NW Series Circuit Breaker Showing Available Options

Figure 17.3 shows the available options for a Square D NW series breaker. These devices obviously costa lot more than circuit breakers whose only function was to open a circuit on overload conditions. Pricingcan be obtained from an electrical supply house for these more advanced circuit breakers with prices forthe various options and add-ons.

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For information purposes, Figure 17.4 shows the approximate list prices for Siemens molded case circuitbreakers. This list is given for information purposes only, to show the relative cost of circuit breakers asthe current rating increases. Discounts are usually offered on these items.

TypeCurrentRating

ListPrice

ED125Amps

$1,594.00

FD250Amps

$2,456.00

JD400Amps

$7,841.00

LD600Amps

$6,290.00

MD800Amps

$8,491.00

ND 1200Amps $17,120.00

PD1600Amps

$24,776.00

RD2000Amps

$24,808.00

STD3200Amps

$35,466.00

Figure 17.4 Chart Showing How the Cost of Circuit Breakers Increases With Current Rating

There is a big jump when going from 800 amps to 1200 amps because GFI (Ground Fault Interrupter)protection is required at ratings of 1000 amps and above. On these large breakers there is a currenttransformer to measure current flowing in the grounding wire. Care must be taken to run the groundingwire through this current transformer. Otherwise GFI protection will be inoperable. Engineers whose jobis to inspect and approve installations should be careful to note this possible code violation. It is veryeasy to bypass the Ground Fault Current Detector and thus make the GFI circuit useless. If the GFIcurrent detector is by passed, a ground fault condition could occur and injure someone or cause equipmentdamage. Also, a properly operating GFI trip will open a circuit on a low level ground fault current beforea fault can escalate into a high level phase to phase fault. A high level phase to phase fault can easily turninto an arc flash incident with a possibility of death or injury and a great deal of property damage.Pdhengineer has a course on NFPA 70E concerning Arc Flashes that shows how much energy can be

released in an Arc Flash incident.

18.Selective Coordination for Three Phase Circuit Breakers

The manufacturers of circuit breakers make available these curves so that engineers and designers ofpower systems can design systems that continue to operate as effectively as possible when one part or theother of the system has a fault condition. This is done by an overlay process whereby the time current tripcurves of the breakers in series in the system are drawn on the same graph. If the curves do not overlap,the system is said to be selectively coordinated.

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Figure 18.1 Example of Typical Time Current Trip Curves for 10 and 100 Amp Breakers

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Figure 18.1 shows typical time current trip curves for 10 and 100 amp breakers. They were purposelydrawn so that they would be selectively coordinated. Figure 18.2 shows a one line drawing of how thesetwo breakers might be used in a power system.

InputPower

100

Amps10 Amps

CB 2

CB3

Load A

Load B

Load C

Figure 18.2 One Line Drawing of a 100 Amp Breaker Feeding a 10 Amp Breaker

To be selectively coordinated, the curves for the 10 amp breaker must not overlap the curves for the 100amp breaker. Notice that for very short times and high currents, such as would exist for a short circuit,the 10 amp curves overlap the 100 amp curves. Since the 100 amp curves show that the 100 amp breakeris slower than the 10 amp breaker, the 10 amp breaker should trip before the 100 amp breaker for allcases. The coordination study becomes a lot more complicated for large systems, where 3 or 4 or morebreakers could be in series. This simple example was given so that the concept could be shown.

Modern microprocessor based circuit breakers can have the time current curves modified to suit specialapplications. Coordination could then be achieved by changing the shape of the time current trip curves.

The 2008 revision of the National Electric Code (NFPA 70) introduced article 708, Critical OperationPower Systems (COPS). It is concerned with the need to isolate shutdowns to only the part of a systemthat fails. This is important in places like hospitals and nursing homes, and emergency power systems. Inthese cases the most important thing to do is to keep the power on for critical loads. As a matter ofinterest, selective coordination requirements first appeared in the 1993 NEC and concerned circuitssupplying power to elevators.

19.Conclusions

There are many circuit protection devices on the market today. Each is designed to fill a certain need.This paper is written to give an idea of how these devices operate, and the factors that need to be looked atwhen selecting and applying fuses and/or circuit breakers to any particular circuit protection scheme.

This paper is not designed to be a comprehensive study of circuit protection coordination, but only to

introduce the general concept.

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References and Bibliography

http://encarta.msn.com/encyclopedia_761563582_2/Thomas_Alva_Edison.html

http://en.wikipedia.org/wiki/Circuit_breaker

http://www.bussman.com/2/Products.html

http://www.bussmann.com/pdf/1064.pdf

Fuse and circuit breaker pricing from Emerson Electric in Farrell, PA

http://host1.publiquik.com/bussmann_web/inv1.cgi?sectionId=4

http://www.engineeringtoolbox.com/melting-points-mixtures-metals-d_1269.html

http://www.engineeringtoolbox.com/resistivity-conductivity-d_418.html

http://en.wikipedia.org/wiki/Resistivity#Table_of_resistivities

http://www.cooperbussmann.com/pdf/246a75ec-0c87-4ef6-b0c6-41306a99360b.pdf

http://www.handymanwire.com/articles/AFCI.html

http://www.cpsc.gov/CPSCPUB/PUBS/afcifac8.pdf

http://www2.sea.siemens.com/Products/Residential-Electrical/Product/Circuit-Breakers/Thermal_Magnetic_Product_Page.htm

http://www.murrayconnect.com/NR/rdonlyres/3819A875-0CD2-48C5-98AB-07EF9E3A9165/0/M2150A225A.pdf

http://www2.sea.siemens.com/Products/PowerDistribution/Product/Circuit-Breakers/Circuit-Breaker-Overview.htm

http://www.us.schneider-electric.com/us/products/circuit_breakers.nsf/unid/00B2016F9430468B85256A68004F68E9/$file/lvpowercbsFrameset.htm

http://en.wikipedia.org/wiki/List_of_elements_by_melting_point

http://www.saltlakemetals.com/MeltingPoints.htm

http://en.wikipedia.org/wiki/Solder

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