The Weakest Link- Understanding Fuse Design and Application.pdf

7/27/2019 The Weakest Link- Understanding Fuse Design and Application.pdf

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Spring 2002 1

In any system, the weakest link is frequently the least valuable and

the most expendable component. Yet the system’s weakest link can

also be the most important component. Whether the weakest link is

in a game show, where it is a player who is humiliated and voted off the

show, saving the others, or whether the weakest link is a sheer pin that

breaks before a far more expensive component is damaged, the weakest

link can not only save money, it can save lives — and there is nothing in

any system that is more valuable than the safety of a human life.

That said, I don’t think any of the components in an electrical system

are in danger of being voted out — at least not in the near future. Maybein years to come, but that’s another story. This article is about the properuse, and the abuse, of fuses in electrical systems.

Purpose of a FuseAs the weakest link in an electrical system, the main purpose of the

fuse is to prevent or limit overloads and short-circuit damage to morevaluable and/or larger components within the system. Since the mostvaluable component involved in an electrical system is you, or the user,the main purpose of a fuse is to ensure the safety of anyone associatedwith the system. The fuse not only prevents or limits overloads and shortcircuits from damaging very expensive equipment but prevents or lim-

its that very expensive equipment from damaging you.

How a Fuse WorksWhen a short circuit or an overload occurs, the nearest series-connected

fuse will produce enough heat to melt the element in the fuse and createan opening in the circuit, which limits the fault damage to other compo-nents. The production of this heat is generated by the high currents pass-ing through the resistance of the metal fuse element. If the fuse is ratedcorrectly, this protective sequence will occur safely with little or no ex-ternal damage to the fuse. Internally, however, the fuse goes through anorchestrated self-destruction.

The Weakest Link

Understanding Fuse Design and Application

by Paul Hartman Advanced Electrical Testing

Internal Workings of Fast-

Acting FusesWithin the current-carrying,

metal element, or “link,” there is aseries of “bridges,” or narrowpoints, designed to melt first withthe help of amalgamation, the pro-cess that reduces the melting pointof copper or silver during over-load. When these bridges melt, anair gap is created within the ele-ment at multiple points where the bridges are located. If these gaps

are not sufficient to break the cur-rent path’s short-circuit overload,an arc will develop over the gapand melt more of the metal. Thus,the gap will widen until it is suffi-cient enough to break the fuse’scurrent path and prevent furtherarcing, as is illustrated in Figure 1.



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Figure 1 — a. Fuse element with normal load,b. Fuse element arcing during a fault, c. Arc extinguished

with gaps in the fuse element

Also residing in the internal portion of the fuse isthe “filler” which surrounds the element and is usu-ally composed of quartz, silica sand, or gypsum. Thepurpose of this filler is to absorb the heat and energycaused by the overload/short circuit and to helpquench any developing arcs. A byproduct of the filler’senergy/heat absorption is nonconductive glass whichgenerally forms between the air gaps and helps to in-sulate the gaps.

Internal Workings of Time-Delay FusesThe metal element, or “link,” residing in the inter-

nal portion of a time-delay fuse has no “bridges” andis generally thicker than the element within a fast-acting fuse. This metal is comprised of a eutecticalloy that is thermally sensitive and provides at leasta ten-second delay at 500 percent overload of the fuseampere rating. When an overload occurs, the tempera-ture of this element rises until it reaches its meltingstate. At this point, the eutectic metal element imme-diately changes from a solid to a liquid state. Gravitywill then push the liquid metal away from the contactpoints and create an air gap.

Selecting the Right FuseTo select the right fuse for the right application, theuser must look at all the characteristics of that fuse to be sure of the correct type. Listed below are the char-acteristics to look for in a fuse and explanations of these characteristics:

Voltage RatingThe voltage rating of a fuse does not indicate the

ampere-carrying capacity of that fuse at its rated volt-age. The rating is the capability of the fuse to prevent

arcing across its completely melted element after fault has occurred.

The correct voltage selection must be equal to ogreater than the circuit voltage at which the fuse is t be used.

A note concerning ac fuses verses dc fuses: Theyare not necessarily interchangeable. Even though somfuses may work in both applications, a fuse must bclearly identified as being designed for the type o

current it is intended to interrupt.Continuous Current Rating

The key to selecting the correct continuous currenis to select a rating that does not exceed the thermacapacities of the conductors it protects. The ratinshould be slightly greater or equal to the load currenof the circuit to which the fuse is applied. A coordination study is one of the best tools for determining thcorrect rating of a fuse for any given circuit.

Interrupting Capacity Rating (IR rms)The interrupting capacity of a fuse is the maximum

amount of short-circuit current a particular fuse caninterrupt safely without external damage and possiblfuse detonation. Listed below are the maximum IRratings for the different classes of fuses.

Class H 10,000 amperesClass G 100,000 amperesClass K 200,000 amperesClass J, L, R, T, CC 200,000 amperes

The available short-circuit current varies throughout an electrical system. The location, or the point oapplication, of the rated IR fuse must not be less thanthe short-circuit current at that location.

Figure 2 — Sign wave analysis of a current limitingfuse clearing a high current fault



Spring 2002 3

Current-Limiting RatingA current-limiting fuse is de-

signed to open at one-half cycle orless of its rated “peak let-throughcurrent.” In terms of a sine wave,this fuse will interrupt a short cir-cuit before it reaches the sinewave’s first peak, as illustrated inFigure 2.

Conversely, a noncurrent-limit-ing fuse does not interrupt a shortcircuit until approximately twocomplete sine waves have passedthrough it. In other words, roughlyfour short-circuit current peakswill pass through the noncurrent-limiting fuse and onto the circuit before the fuse will open.

Because the cost of these cur-rent-limiting fuses is much greaterthan noncurrent-limiting fuses, theuse of these fuses is often limitedto locations in electrical systemsthat are capable of producing veryhigh short-circuit fault currents. Ashort-circuit study will identify

UL Sample of Classes of Fuses:

Current-Limiting

MAXIMUMCLASS VOLTAGE AMPERES IR AMPERES APPLICATIONS

J 600 1-600 200,000 Feeder Circuits, MotorOvercurrent Protection

L 600 601-6000 200,000 Service Entrance EquipmentHigh-Current Applications

RK1 250/600 1/10-600 200,000 Feeder Circuits, CircuitBreaker Backup Protection

RK5 250/600 1/10-600 200,000 Transformers, Motors,High-Inductive Loads

CC 600 1-15 200,000 Branch Service Protectionfor Motor Circuits

Noncurrent-Limiting

MAXIMUMCLASS VOLTAGE AMPERES IR AMPERES APPLICATIONS

K5 600 1-600 100,000 Switches, Panelboards,Electric Heat

H 600 1-600 10,000 Switches and FusibleEquipment

Figure 3 — Fuses with keyed slots that fit

areas where current-limiting fuses will be required. Two common loca-tions for these type of fuses is electrical equipment near utility substa-tions and on the load side of power transformers.

Physically, if a fuse is labeled “current limiting,” one of the fuse ends

will be physically different and will not be interchangeable with non-current-limiting fuse holders. The prevention of this interchangeabilityis accomplished by using a fuse block that is physically designed to ac-cept only the design characteristics of current-limiting fuses, as illus-trated in Figure 3.

Frequency RatingGenerally, most fuse ratings are

unaffected by circuits with fre-quencies up to 100 hertz. The fuseratings are, however, affected byfrequencies above 100 hertz. The

most affected rating is the “peaklet-through current” on current-limiting fuses. During a short cir-cuit, these fuses are designed toreact and open at one-half cycle,or less, of their rated peak let-through current. Any frequencyover 100 hertz is too quick for thefuse to clear in less than one-half cycle. Typically, one or more com-plete cycles will go by before the fuse reacts. By then, thecurrent-limiting rating of the fuse

is defeated.

Fast-Acting vs. Time-DelayRatings

As the nomenclature implies,fast-acting fuses are designed toquickly interrupt overloads in acircuit. Namely, these fuses are de-signed to interrupt high-currentshort circuits. As stated earlier,



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time-delay fuses delay the interruption of an overloadafter a designed time delay. This delay is typically setat a minimum of ten seconds at 500 percent overloadof the fuse ampere rating.

SummaryUnderestimating the importance of the weak links

in any system can lead to less- than-desirable resultsand even personal endangerment. To return to the

game-show analogy, we can easily imagine a contes-tant underestimating the importance of the weakestlink among the other players and by doing so actu-ally becoming the weakest link. If this player was onthe game show The Weakest Link, he or she wouldlikely hear the infamous, yet humiliating, phrase ut-tered by the host, “Goodbye!” This phrase also im-plies, “Goodbye, money!”

Both of these phrases directly apply to electricasystems. If you choose to underestimate the importance of fuses and carelessly replace or install thesfuses into the wrong type of applications, you mayvery well hear the words “Goodbye, money!” whenyour expensive equipment goes up in smoke. Worsyet, you may end up saying “Goodbye!” to your lifas well.

Paul Hartman has over 16 years experience in start-up, commissioning, maintenance, and training in power generation, including international projects in Pakistan, Indonesia, ThailandBrazil, and Korea. He has been an instructor for state certifiecontinuing education programs as well as an associate instructowith the San Francisco State University program. Paul is currently Vice President of Advanced Electrical Testing. He is a regular contributor to NETA World and a frequent speaker at NETA’Annual Technical Conference.

Documents

The Weakest Link- Understanding Fuse Design and Application.pdf