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430 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 44, NO. 2, MARCH/APRIL 2008

Determination of the Cause of Arcing Faultsin Low-Voltage Switchboards

H. Bruce Land, III

AbstractIt is widely recognized that arcing faults in switch-boards contain large amounts of power and can create significantdamage, including melting switchboards, destroying substations,disabling ships, and causing human fatalities. While arcing faultsoccur with a fairly high frequency, electricity is so ubiquitous inour lives that most engineers will not personally be associated witha major arcing fault. The Navy has invested 25 years investigatingthe causes, behavior, and prevention of arcing failures in low- andmedium-voltage switchboards. Laboratory testing used to help un-derstand the behavior of arcs in switchboards is presented. Thosedata are then used to analyze actual switchboard arcing eventsand, thus, to determine the root causes of the events. Additionaltesting used to confirm the cause of each event is discussed.

Index TermsArc, arc fault, arc forensics, electric break-down, fires, power-distribution faults, power-system protection,switchgear, switchgear fires, voltage breakdown.

I. INTRODUCTION

B REAKERS are designed to protect against bolted faults.Arcing faults are high-impedance faults, and their cur-rents frequently fall within the range of normal working loads.

Thus, breakers are frequently ineffective against arcing faults.Some arcing events exist for tens of seconds until manually

interrupted, while others proceed until entire switchboards are

reduced to slag. While the first concern is the safety of thepersonnel and the facility, at some point, ascertaining the cause

of the event will be desired due to possible liability issues andto use the lessons learned to prevent other possible events from

occurring.

Unfortunately, it is difficult to learn from most of the in-vestigations of arcing events in the commercial world, as the

reports remain hidden behind legal settlements, and thus, the

industry looses the lessons learned. The National Fire Protec-tion Association (NFPA) publication [1] contains a very useful

section on interpreting damage to electrical systems, but it

focuses on wiring rather than bus bars and switchboards. Thechapters on recording the scene and on identifying, collecting,

and examining the evidence are enlightening and of value toany fault investigator. Magee and Hittel [2] and Alonzo [3] have

published articles that discuss procedures beyond the NFPA in

investigating electrical fires. McClung and Vallejo [4] go into

Paper PID-07-05, presented at the 2005 IEEE Electric Ship TechnologiesSymposium, Philadelphia, PA, July 2527, and approved for publication inthe IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Petroleumand Chemical Industry Committee of the IEEE Industry Applications Society.Manuscript submitted for review August 31, 2005 and released for publicationSeptember 5, 2007. This work was supported by D. E. Strawser of NAVSEA05Z43 under Contract N00024-97-C-8119.

The author is with The Johns Hopkins University Applied Physics Labora-tory, Laurel, MD 20723 USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/TIA.2008.916595

more details of the process of conducting a forensic analysisof electrical fires. However, these articles focus on procedure

rather than the details of the fault analysis.

Three electrical fires will be discussed to illustrate how anarc behaves in low-voltage switchboards. Two of these sets of

switchboards were constructed to stringent Navy switchboardspecifications and one set to commercial specifications, but the

results of the arcing failure are independent of the specifications

used to construct the switchboard. The events were investigatedusing the experience based upon over 2000 laboratory arcing

tests, conducted by The Johns Hopkins University Applied

Physics Laboratory (JHU/APL), ranging from a few hundredamperes to 30 000 A. In each of these cases, the presumed cause

was recreated in the laboratory to confirm the conclusions, thusthis information may prove useful in the investigation of other

arcing events.

II. INITIAL INVESTIGATIONS OF AN ARCING EVENT

When one is initially requested to investigate a switchboardfire, a request should be made that the scene be secured and left

as undisturbed as possible. Generally, there is a conflict between

the needs of the investigation to retain an uncompromisedscene and the safety and economic issues, so completion of the

investigation in a short amount of time is imperative.If available, examine the switchboard drawings before exam-

ining the switchboards. Ascertain all possible current sources

to the affected switchboards, as this will be needed to identifythe direction of arc movement.

The black coating left on all surfaces from the fire will

soak up light and make obtaining the proper flash exposure achallenge. Photographs of the scene from many different angles

will help to compensate. Photograph, tag, and secure important

items so that they will be available for later detailed evaluations.For sketches of the switchboard, it will be useful to tie the

damage photographs and the power flow together, helping one

to determine the origin of the fault.Interview witnesses and operators and attempt to construct

a time line of every electrical and mechanical event whichmight have had any effect on the switchboards immediately

before the event. Be wary of accepting operators opinion of

the duration of the arc, as during catastrophic events such asthese, a humans view of time becomes very subjective. Obtain

drawings of the switchboards, if possible, as they may behelpful in determining the possible origin of loose components.

III. ARC MARKS

The results of arc testing and analysis agree that when a high-

powered arc strikes between two parallel bus bars, it will be

0093-9994/$25.00 2008 IEEE


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LAND: DETERMINATION OF THE CAUSE OF ARCING FAULTS IN LOW-VOLTAGE SWITCHBOARDS 431

Fig. 1. Evolution of barbs on bus bars. The top bar is new and shows no barbs.Thesecond barsustained a fast moving arc andshows small barbs.The next twobars experienced successively slower moving arcs, creating larger barbs.

Fig. 2. Close-up of small barbs from rapidly moving arc.

forced away from the source of current due to magnetic forces

[5]. The velocity at which the arc moves is proportional to the

current and will exceed 1000 ft/s at a current of 7000 A. The arcdoes not appear to move smoothly along the bus but seems to

skip a few tenths of an inch between attachment points. Giventhat the temperature at the point the arc contacts the metal can

exceed 20 000 C [6], the arc will cause some surface damage to

the copper bus in spite of its short resonance time. The amount

of damage will be related to the amount of time the arc is incontact with any given position on the bus bar.

As the arc travels along the bus bar, a small amount of copperis melted at the point the arc contacts the copper. Since the arc is

rapidly moving, the bulk of the molten copper does not vaporize

or run down the bus but simply solidifies in place. The surfacetension of the molten copper will cause the formation of small

almost-spherical balls attached to the edge of the bus bar asbarbs. With high-current rapidly moving arcs, these barbs may

be sharp enough to cut your skin but may be small enough to

require magnification to be seen.It has been previously reported that insulation on the bus

will slow down the arc motion [5]. Tests were conducted withincreasing amounts of insulation to slow down the speed atwhich the arc moves up the bus and to examine the barb

formation. Current and bus spacing were held constant for this

set of tests. Fig. 1 shows an edge view of four pieces of standard0.25-in-thick copper bus bar. The top bar is new. Its corners,

where the sides meet, are not sharp, as they have a slight radius,and they are quite smooth.

The second bar had a single linear strip of fiberglass in-

sulation applied to the edge of the bar, and the barbs are0.0200.040 in in diameter. The third bar was tested with

a spiral wrap of 0.5-in-wide insulation, and the barbs were

0.0500.100 in. The fourth bar had an overlapping spiral wrap

of 1-in-wide insulation and produced globules from 0.1 to0.2 in. Fig. 2 shows a close up of the 0.0200.040-in barbs.

Fig. 3. Top of vertical bus bars showing the erosion of the bus bars and thekeyhole cut by the arc through the side of the switchboard.

IV. END OF THE BUS BAR

Given a fast moving arc and no obstructions, the arc willgenerally blow out once the magnetic field drives it to the end

of the bus bars. However, if there is some obstruction near thefront, back, or end of the bus-bar gap, then the arc may persist

for tens of seconds. If the bus is mounted adjacent to a side

of the switchboard, the plasma from the arc cannot expandin the direction of the side panel. This causes the plasma to

concentrate and increases the temperature of the plasma. The

increase ion concentration makes the arc more stable.Once reaching the end of the bus, the arc will first attach to

the sharp corners of the bus. Due to the increased concentration

of ions on the panel side of the bus, the arc will billow out in

that direction. If the side panel is conductive, the arc will splitinto two pieces. One piece of the arc will pass current from thebus bar to the side panel, and the second piece of the arc will

pass current from the side panel back to the second bus bar.

Metal will melt, vaporize, and oxidize from the tip of the busbars and from the panel. Magnetic forces will cause the arc to

extend outward from the current source until the gap becomes

too long for the available voltage. At this point, the arc will goout and then restrike at the lowest position down the bus that is

still within the plasma cloud.Once restruck, the arc will again move to the end of the bus

and repeat the process of blowing out and restriking within the

plasma cloud. Each time, the plasma cloud becomes a bit larger,

and the arc can restrike further down the bus. Since the arcspends more time at the top of the bus than at the strike point,

more material vaporizes from the bus bar and the adjacent panelat the top than down in the restrike zone. This will produce a

classic keyhole-shaped hole in the panel and a relatively smooth

wearing back of the bus bars (Fig. 3). While this may be the areaof maximum damage, it is the terminus of the arc and rarely

the origin of the arc. Therefore, knowledge of the directions ofcurrent flow and checking for arc barbs can assist in tracing the

arc back to its initiation point.

V. CAS E 1

In 1987, a ship was cruising off of the West Coast. Thecrew had just completed a training exercise, which involved


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the remote opening and closing of a number of breakers. They

were awaiting an order from the Captain of the ship to beginanother exercise when there were several loud noises and bright

flashes from both the front and back of a satellite switchboardlocated approximately 8 ft outside of the electrical plant control

room. Flames issued from the switchboard and smoke filled the

adjacent areas reducing visibility to less than 6 ft.Since the crew was already poised to react for a training

exercise, they quickly determined the affected circuits, opened

the appropriate generator breaker, and limited the damage.

It should be noted that a trained full crew was present at

the electrical control panel. While it may have taken a few

seconds for them to identify the nature of the problem and

open the proper breakers, manual intervention could not have

occurred any more quickly than in this instant, thus this case

eloquently makes the argument for the need for automatic arc-

fault protective equipment.

After a brief investigation to identify affected components,

power was routed through alternative switchboards and the ship

proceeded toward port. A casualty-analysis team was assem-

bled and transferred to the affected ship at sea so that they could

complete their work before the ship reached port. The arc-fault

scene was left undisturbed, which greatly assisted the analysis.

The switchboard sustaining the arcing fault was a typical

shallow switchboard containing two columns of push in

motor-controller modules. Power entered the bottom of the

switchboard via cables that are bolted to three bus bars that ran

vertically up the back of the switchboard. Three short horizontal

bus bars in the bottom of the switchboard fed power to the

set of vertical bus bars behind the second column of motor

controllers. The bus bars ended approximately 2/3 of the way

up the back of the switchboard. Each of the motor-controllermodules contained a breaker of 100 A or less. The cables

supplying power to this satellite board were bolted directly

to the bus bars in the generator switchboard, and there was

no intervening breaker between the generator breaker and the

satellite board.

This switchboard was a heavy-duty spray-tight switchboard.

Examination of the switchboard showed that the pressure pulse

from the arc had bent the heavy-duty quarter-turn fasteners and

blown the covers off. Centered above the end of the center bus

was a 3-in-diameter hole in the rear of the switchboard. Smaller

holes existed over the ends of the other two bus bars. The square

ends of the bus bars were rounded. This three-hole pattern isthe result of the chassis being used as a current path for the

arc current traveling from phase to phase. Numerous JHU/APL

laboratory tests have duplicated this behavior [5]. An irregular

horizontal hole 1.52 in high and 13.5 in wide existed 6 in

below the ends of the bus, and the bus bars for all three phases

had 1.75-in gaps at this location (Fig. 4). These bus bars are

2.5 in wide by 0.25 in thick and are space 1.75 in apart.

The speed of the arc is proportional to the current [5] and, in

this case, was estimated to have exceeded 15 in/ms. Due to therapid motion of the arc, it was expected that arc barbs would

be found, but none were present with the unaided eye. Lightlybrushing a hand along the edges of the bus bars revealed barbs

on back facing edges of the AB and BC bus bars below thegap in the rear of the switchboard. Lower in the switchboard,

Fig. 4. Arc-damaged motor-controller switchboard. Note the three holesabove the damaged tips of the bus bars. The horizontal white space is a gash cutthrough the bus bars and the back of the switchboard by the arc.

Fig. 5. Lower part of switchboard showing the faulty connection where thearc originated.

the barbs only existed between phases BC. Since the source of

current was in the bottom of the switchboard, the arc could notinitiate in the top of the switchboard and then travel down over

30 in. Therefore, the arc must have initiated near where the arcbarbs began in the bottom of the switchboard and then traveled

to the top of the bus bars.

In the lower portion of the switchboard, it was found thatcrimp lugs on three conductors had been attached to the bus

with a single bolt. While all three conductors were severed fromtheir lugs, only one conductor showed evidence of arcing. One

connection fed a spare breaker and another was very lightly

loaded. Only one wire was carrying a full load at the time of thefault. The remains of the three lugs were still firmly attached to

the screw (Fig. 5).The most likely sequence of events is that a high-resistancepoint was formed at the interface between the crimp lug and the

conductor due to improper crimping or work hardening during

installation. With cycling and vibration, the resistance increaseduntil enough heat was generated by the current passing through

the load to melt the conductor. Next, an in-line or series arc wasformed, but the arc current was limited to the current flowing

through the load. The addition of pyrolytic products from the

wire insulation helped to sustain the arc and allowed the plasmacloud to grow until it spanned the distance between phases

B and C.

Laboratory testing has shown that if the bolt had been loose,

the heat would have been generated at the bolt. The copper busbar has a lot of mass and conducts heat well, thus it is difficult


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to overheat the bus bar. The bolt and the copper lugs have much

less mass and thus heat up more rapidly and reach a highertemperature. Since the copper lug has a typical melting point

of 1084 C, it will melt before the steel bolt which typically

melts at 1370 C. Additionally, since the cross-sectional area ofthe lug is smallest where it transitions from flat to round, the lug

will melt here if the heat source is from the bolt. In this case,the cable melted at the end of the lug-to-cable interface, which

eliminates the bolt as the heat source. This complete sequence

was subsequently duplicated in the laboratory.Once struck between phases BC in the bottom of the switch-

board, the arc rapidly rose due to magnetic forces. As it rose,the plasma cloud expanded, and the arc struck between phasesBA and the two arcs rose until they reached the end of the bus.Here, the arc lodged and began to consume the corners fromthe ends of the bus. As the plasma cloud expanded, it removedthe paint from the rear of the switchboard, and the current path

went from bus to switchboard and back to bus due to the trappedplasma. This caused the typical three holes behind the bus bars.

Due to rapid heating of the air and the expansion of thevaporized materials, there was a large pressure rise which blewthe covers off and forced large quantities of smoke, sparks,molten metal, and debris out the holes in the switchboard andout the front. Metal splatter was found on cabinets that are10 ft from the switchboard. At some point, the plasma cloudexpanded enough to become lodged under a bus support 2 in

below the end of the bus. The plasma trapped below the bussupport caused the arc to restrike at those points and to continueat the lower location until the large gash was cut through the busbars and through the back of the switchboard (Fig. 4). At thispoint, power was manually removed.

While the resistance of the cables tying this satellite switch-board to the main bus helped to limit the current, they expe-rienced a large unexpected magnetic force due to the current.Inspection in the feeder switchboard showed that all threephases of the cable had been secured together to a support with

0.1875-in nylon cable ties with a rated working load of 50 lband a burst strength of 75 lb. The magnetic forces due to thefault current were large enough to cause the cables to snap thecable ties. One cable moved with such great force against a boltprotruding from a bus bar that the insulation was pierced andarcing began between the cable and the bolt. Fortunately, thebreakers were opened before arcing began on the main bus orthe damage to the ship would have been much higher. Damage

was limited to the replacement of the satellite switchboard,replacement of the feeder cable, and some unexpected timein port.

This incident points out that the point of origin of the arcis frequently not the location of most damage. Generally, thepoint of origin is the damage point closest to the source ofthe current. It also emphasizes that hidden damage due to thearcing event may be present in switchboards other than the onewhich actually incurred the arc. Thus, subsequent to an arcingevent, all switchboards upstream of the arc should be carefullyinspected for collateral damage.

If thermal imaging could have been performed on the con-nection in question, the fault could have been predicted. The

Navy has a program of regular thermal imaging, but the con-nection in question was in an inaccessible location and could

not be viewed during inspection. While thermal imaging iseffective and useful, inspections must be performed after theswitchboards are brought up to operating temperature, and it is

estimated that only half of the connections are visible duringthe inspection process. While additional inspections duringinstallation might have caught the potential problem, no known

routine-inspection technique performed after commissioningthe switchboard could have prevented the fault. No event couldmake a clearer case for automatic arc-fault protection, as even

the best trained crew, in place and poised for action, cannotreact quickly enough to an arcing event to limit the damage.

After this event, the Navy data from over 20 years of

shipboard fires were collected and analyzed. Analysis of the

data by the Johns Hopkins, Science Applications InternationalCorporation (SAIC), and the Navy concluded that the majority

of all shipboard arcing failures occur at a faulty connection.

Those faults may be caused by improper torque or crimpingupon installation. Others are caused by the constant shipboard

vibration or the corrosive salt-laden atmosphere. While this

failure initiated at the connection, it is difficult to determine if itwas due to the crimp connection or due to work hardening of the

conductor at the crimp connection. Work hardening of conduc-tors can occur any time the conductor is bent past its acceptable

bend radius or bent repetitively. Work hardening can change

the local properties of the material, enough to increase itsresistance and lead to a fault. Inadequate crimp force raises the

local resistance, allows corrosion to occur, and leads to faults.

VI. CAS E 2

In 2000, a ship at sea was moving under steady normal

power. No loads were being changed or shifted. Two of the

three generators were online. A junior engineer and anotherperson were in the switchboard room when there was a sudden

bright flash from the switchboard followed by a loud noisedescribed as a roar. Almost immediately, fire came out of the

switchboard and rolled along the overhead of the compartment

(room). The two men quickly exited the switchboard room andsaw that the generators were hopping on their mounts. After

about 60 s, both generators were manually shut down. Thelights flickered several times during the event, as the automatic

bus transfer switched back and forth from main to emergency

power. The ships crew discharged several CO2 extinguishersinto the switchboards and contained the fire.

By the time the ship had been towed to port, the crew hadremoved most of the covers and breakers. Most of the debrisfrom the bottom of the switchboards and from the compartment

had been swept up and placed in buckets. It was a great

disappointment to find so much of the evidence disturbed,but, fortunately, the Coast Guard had taken many photographs

before the cleanup, which help to correlate the remains withtheir origin. While immediate cleanup is frequently done in an

effort to bring the system back online, it disturbs the scene and

makes the determination of the cause much more difficult, andyet, due to long lead time for parts for older systems, it may not

shorten the time to restore the system.

The switchboard was a single lineup of 12 sections of un-

equal width. The three generator breakers were in sections threethrough five at the left end, and the very hefty horizontal bus


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Fig. 6. Section 12 of the switchboard. The front cover and the breakers havebeen removed, revealing the extensive damage inside of the switchboard.

bars traversed the bottoms of all of the sections from left toright. Sections 8, 10, and 12 contain distribution breakers and

sections 9 and 11 contain only ventilation louvers. Extensivedamage was found in sections 10 and 12 (Fig. 6), and less dam-

age was found in section 8. The compartment was thoroughly

covered in thick black soot deposits, and the plastic overheadlighting covers in the compartment were melted.

Since the generators were reported to have been jumpingon their mounts during the arcing fault, some personnel pos-tulated that they were the cause of the fault. Their reasoning

was that the generators created a voltage spike, which jumped

the bus-bar gaps and initiated the arc. The closest bus-barseparation was found to be 0.5 in.

Paschens law (1) states that the breakdown voltage (V) of agap is roughly given as a function of the distance in centimeters

(d) and the pressure in atmospheres (p) [7], [8]

V= 30 pd + 1.35 kV. (1)

While this equation is affected by many factors, such as

surface irregularities, dust, gas, etc., the breakdown voltagein normal atmospheres is generally accepted as approximately13 kV/cm. Voltage spikes of 12 kV, caused by switching

transients, have previously been recorded in shipboard switch-

boards, but these voltages will not jump the gaps. No switchingoccurred immediately prior to this incident. Since the voltage-

regulation circuits of the generators were found in good order,it is unrealistic to expect that the generators produced voltage

spikes in excess of 15 kV needed to initiate the arc on this bus-

bar spacing.While first-hand information can be useful, it must be rec-

onciled with the evidence. One observer reported that, during

or immediately after the fire, he felt a recently installed T200

cable leaving the switchboard on the opposite side of thebulkhead and found it much warmer than the adjacent cable,

and he concluded that a fault in this load or cable must have

caused the fire. An investigation showed that if there had beena high-resistance point in this cable that there was a second

parallel T200 cable which would have carried the load. Noproblems were found with the load, the load connections, or the

cables. The cooler cable was found to be armored, while the hot

cable was not. The larger diameter of the armored cable resultsin a larger surface over which to dissipate heat, and thus, it will

operate cooler. Thus, the arc was not created due to a problem

related to this cable.It was postulated that some type of reverse current between

the two generators might have caused the fault to occur.Records showed that the reverse-current relays had passedtesting three weeks prior to the event. Visual inspection of therelays showed no problems, and the mechanical trip flags werenot set. Thus, a reverse-current event did not cause the fire.

The internal failure of a breaker can expel enough hot gasand plasma from the breaker chutes to create an arc on the

adjacent bus. Each breaker was disassembled and inspected and

no problems or abnormal discoloration were found with theinternals of any of the breakers in the area of the arc.

There were no side panels isolating the sections so that thearc was free to move between sections. Using the understandingof arc motion gathered from laboratory testing, it was possibleto isolate the origin of the arc to the right most switchboardsection 12. It was also concluded that the arc began on thefinger bus supplying current to the 400-A breaker. While the bus

bars on the source-side of the breaker were severely damaged,portions of all of the connections were present, and there wasno evidence of a faulty connection. It appeared that someconductive object had created the arc by shorting out the busat that point.

Several pieces of stainless-steel cable ties were found in thebottom of switchboard section 10. It was postulated that oneof these pieces might have fallen down, shorted out the bus,and created the arc. It was found that these metal cable ties areTeflon coated and have an insulation value greater than 2 M.Pieces of these cable ties were placed across bare copper busbars in the laboratory and did not create an arc. The switchboardwhere the pieces were found was not where the arc originated;the location where the pieces were found was not over busshowing arcing damage; and testing was unable to create arcsby shorting out bus with the ties. Thus, the burnt pieces of cableties were collateral damage and not the basic cause of the arc.

A loose bolt (Fig. 7) with unique arcing damage was foundon an angle brace inside the front of the switchboard approxi-

mately 1/3 of the way up from the deck and just to one side ofthe origin of the arc. Reconstruction of the switchboard showedthat all other bolts were accounted for, thus this bolt did notvibrate loose from a connection within the switchboard. Thebolt in question must have been left inside of the switchboarddue to some previous maintenance action.

The loose bolt found had a washer welded to it. Arc damagewas found at one point on the outer edge of the bolt and on the

end of the bolt. It was postulated that the loose bolt had fallendown across the bus behind the breaker and initiated the arc.Some observers stated that such action would have resulted in

the bolt welding to the bus or completely vaporizing, and thus,the bolt could not cause the arc.


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Fig. 7. Loose bolt found inside of switchboard. Note the arcing damage to the

tip of the threads and how the washers are welded together and to the bolt.

Fig. 8. Sketch of bolt showing how it contacted the bus bar and created an arc.Note that the locations of damage shown in the sketch match the actual damageshown in Fig. 7.

Identical bolt and washer combinations were placed across

similar bus-bar configurations in the laboratory and power was

applied. In each case, an arc was initiated on the bus by the bolt,and the bolt was blown free of the bus. Observe that when the

bolt lies on its side, the washer will cock over at an angle as

shown. As current flows from one bus to the other through thebolt, the current is forced to flow through the sharp corner of the

washer inside diameter (ID). This concentration of current willproduce localized overheating that welds the washer ID to the

bolt (Fig. 8). With the light weight of the bolt, the gas pressure

from the arc generally kicks the bolt up and off of the bus. Thebolt tends to rotate around its heaviest point, the head, and flips

off of the bus in the direction the head is pointing.Note that the observed damage to the bolt in question and

to the test bolts agrees with the postulated damage. Thus,

while the immediate cause of the arcing failure was the loosebolt left in the switchboard, the root cause was inadequatemaintenance procedures, which allowed a loose bolt to remain

in the switchboard and an inspection that did not find the loosebolt. Testing has shown that automatic arc-detection equipment,in low-voltage switchboards, can detect this type of arcing eventquickly enough to limit the damage to the loose bolt and the

need to clean the bus where it came into contact with the bolt.

VII. CAS E 3

In 1994, a ship was responding to a flank-speed bell

(maximum speed) by shifting some electrical loads. Suddenly,

a loud bang was heard, and power was lost to a port electricalbus. It was determined that an arc-fault-detector (AFD) system

Fig. 9. Top of the switchboard. The arc damaged the end of two bolts whereindicated and left smoke on the top of the switchboard.

Fig. 10. Middle of the switchboard where the arc initiated. A drill shavingwas found on a ledge behind the bus bar below the arrow. The arc rose up thevertical bus.

had removed power from the bus. Based upon the indicators ofthe AFD system, it was found that an arc had occurred briefly

within one section of a switchboard. While the damage was

limited to surface smoke damage and the ends of two bolts,which could have been corrected at sea, the ship returned to

port for analysis of the cause of the fault.Fig. 9 was taken looking in from the rear of the switchboardand shows the smoke damage to the top of the switchboard. A

small amount of arc damage can be seen on the ends of the

indicated two bolts. There was also a small amount of damageto the corners of the bus near the damaged bolts. The flow of

current was up the bus bars on the left side of the photograph,across the bus bars at the top, and then forward into the supply

side of two breakers. Damage was not immediately apparent

elsewhere in the switchboard.A search of the switchboard revealed several aluminum

drill shavings lodged approximately 1/3 of the way down the

switchboard on a bus support at the point indicated in Fig. 10.

This photograph was taken looking in from the front of theswitchboard so that the vertical supply bus bars are on the right.


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One of the shavings was shaped like a coil spring with the

coil diameter of 0.5 in and a length of 1.5 in. These shavingswere most likely produced by recent drilling into the aluminum

switchboard structure just above this point.While no arcing marks were visible on the bus bars in this

area, it seemed likely that a shaving lodged on the ledge could

vibrate down and short out the bus. Gentle rubbing of onesfingers along the bus bar showed that small arc barbs existed on

the right most side of the vertical bus. These barbs began at a

point just below where a shaving was found and continued upand onto the horizontal bus bars in the previous figure.

The most likely cause of the arc was a drill shaving similar

to the ones found. While inspections should have caughtthe presence of these shavings, sometimes the pressure to

return a switchboard to service reduces the time allocated for

modifications and inspections. There are many ledges andcomponents within a switchboard where objects can hide from

inspection, and yet, the object can vibrate lose at a later timeand create an arc.

VIII. CONCLUSION

Two of these arcing failures resulted in spectacular arcingfailures. All three resulted in the loss of power to large portionsof the ship, and if they had happened at a more inopportunetime, it could have put the ship at risk. With each failure, many

different possible causes were presented by the evidence and byother observers. Each possible cause had to be examined in lightof the total evidence to coalesce upon the true cause. Lessonslearned from many years of laboratory arc testing aided in draw-ing conclusions that were later verified by testing to be correct.

Understanding how the motion of the arc is governed bymagnetic and thermal forces and when each takes precedencewill assist one in recognizing the end point of the arc. Oncethe end point is recognized, this understanding can be used to

trace the arc back to its origin and help to identify the cause. Insome cases, the initiation of the arc may destroy any evidenceof the root cause. In some of these cases, it may be possible toidentify the most likely cause by ruling out all other possiblecauses. Therefore, it behooves one to carefully document allevidence of the arcing event immediately after its occurrence,as it may not be clear at first which pieces of the evidencewill be the most important. Recognize that significant people,who have little background in arcing events and did not see the

event, will read the report and second guess any conclusion.Therefore, it is suggested that the report be written to reachits final conclusion only after presenting and disproving a widerange of other possible causes.

One can theorize that each of these events might have beenprevented by better maintenance and inspection procedures.However, the complexity of the switchboards and the pressureto restore power make it likely that events such as these willcontinue to happen. The use of arc-resistant switchboards fornew construction and the use of arc-fault detection systems on

both old and new construction can significantly reduce the riskto personnel when such events occur. The need for automaticarc-fault protection led to the American Bureau of Shipping

requirement for arc-fault detection systems in the Naval VesselRules [9].

ACKNOWLEDGMENT

The arc research at JHU/APL began in 1978 and manyhave contributed to a large team effort. The author would liketo thank R. T. Cusick who initiated the program, J. Georgewho was program manager during a portion of this paper,L. R. Gauthier who led portions of the laboratory testing, and

H. E. Benden who was the lead test mechanic for the arcingtests. The author has been the Principal Investigator and Pro-gram Manager since 1991.

REFERENCES

[1] Guide for Fire and Explosion Investigations, NFPA, Quincy, MA.NFPA 921.

[2] A. H. Magee and M. J. Hittel, Securing and preserving the scene of anelectrical accident, in Proc. IEEE Ind. Commercial Power Syst. Tech.Conf., May 1516, 2001, pp. 2730.

[3] R. J. Alonzo, Electrical incident investigation procedures, in Proc. IEEEReg. 5, Annu. Tech. Conf., Apr. 11, 2003, pp. 5963.

[4] L. B. McClung and J. M. Vallejo, The process of conducting and com-municating forensic analysis of electrical failures, in Proc. IEEE Ind.

Appl. Soc. 44th Annu. Petroleum Chem. Ind. Conf., Sep. 1517, 1997,pp. 191198.[5] H. B. Land, III, The behavior of arcing faults in low voltage switchgear,

IEEE Trans. Ind. Appl., vol. 44, no. 2, pp. 437444, Mar./Apr. 2008.[6] B. R. Baliga and E. Pfender, Fire safety related testing of electric cable

insulation materials, Univ. Minnesota Inst. Technol., Minneapolis, MN,Res. Rep., Sep. 1975.

[7] J. M. Meek and J. D. Craggs, Electrical Breakdown of Gases. Oxford,U.K.: Clarendon, 1953.

[8] [Online]. Available: http://home.earthlink.net/~jimlux/hv/paschen.htm[9] American Bureau of Shipping (ABS), Guide for Building and Classing

Naval Vessels, Jul. 15, 2005.

H. Bruce Land, III received the B.E.E. degree fromThe Johns Hopkins Whiting School of Engineering,Baltimore, MD, in 1984.

He is currently a member of the Principal Profes-sional Staff with the Milton Eisenhower ResearchCenter, Johns Hopkins University Applied PhysicsLaboratory (JHU/APL), Laurel, MD. He is an In-strumentation Engineer with a broad background insensor development, sensor deployment, programmanagement, and field-testing. He has managedmany sensor programs including the Advanced

Flight Vehicle Instrumentation, Landfill Energy Recovery, Hit Flash Detection,Arc Fault Detection, and others. He has led instrumentation programs thatrecorded data from an exploding missile warhead. He has been conductinghigh-energy arcing tests related to the development of arc-protection equipmentand arc-forensics techniques since 1979. He was a Program Manager for theArc Fault Detection Program, which resulted in the arc-fault detection and

continuous thermal monitoring systems currently protecting switchboards inall U.S. Navy submarines and nuclear aircraft carriers. He led over 1000 arcingtests at power levels from 100 kW to over 10 MW to understand arc physics andto design and verify arc-protective systems. Arc-protection systems are requiredby the 2004 Naval Vessel Rules of the National Shipbuilding Code for all newships. These systems are credited with saving ten ships and with no false alarmsafter over 1000 system years of operation. He is recognized by the U.S. Navyas an expert in electrical-fire forensics and has investigated numerous electricalfires for them. He is the holder of one issued patent in arc fault protectionand 12 patents are pending. He supervised instrumentation and controls forthe JHU/APL Avery Advanced Technology Laboratory for ten years. He iscurrently investigating arc-plasma thrusters for use on satellites and steerablemunitions. He has 24 technical publications in journals and symposiums of theISA, IEEE, Journal of Chemical Education, Joint Army Navy NASA Air Force,International Test & Evaluation Association, etc. He is listed in Marquis WhosWho in Science and Engineering 9th Edition (20062007) and Whos Who in

America 61st Edition (2007).Mr. Land is an ISA Fellow and is a member of the Board of Directors of theAerospace Industries Division, ISA.

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