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INSTALLATION AND APPLICATION CONSIDERATIONS OF ARC

RESISTANT MEDIUM VOLTAGE CONTROL EQUIPMENT

Copyright Material IEEEPaper No. PCIC-2007-5

John A. Kay, CET

Engineering Manager Rockwell Automation Canada

[email protected] 135 Dundas St.,Cambridge, ON

Canada, N1R 5X1

Paul B. Sullivan, P.E.Electrical Consultant

DuPont Engineering, Camden [email protected]

Camden Regional Engineering OfficeP O Box 999, 719A US Hwy. 1 South,

Lugoff , SC 29078

Michael Wactor, P.E.Senior Design Engineer, R&D

Powell Electrical SystemsHouston, TX USA

[email protected] 8550 Mosley Drive

Houston, Texas 77075

Abstract - Improved medium voltage (MV) controlequipment designs, including enhanced structural protectionsystems, have continued to evolve in support of improvedpersonnel protection. Recent changes to some standards,

such as the NFPA-70E; have emphasized the need to look for improved safety compliance to mitigating the risks associatedwith the operation and maintenance of electrical equipment.The requirements for employee safe work practices have alltargeted reducing the risks of electrical arc hazards.However, arcs accompanied by explosions continue to occur in electrical systems. Factors such as inappropriate humaninteraction with the equipment, equipment malfunctionsbecause of misuse or lack of regular maintenance or unforeseen events continue to contribute to the unexpectedrelease of explosive electrical energy in the workplace. Newarc resistant MV control equipment designs provide anadditional level of protection if properly installed and applied.

This paper will outline the added benefits of arc resistant

equipment along with the details surrounding the appropriateinstallation and site application considerations when arcresistant MV control products are being considered. Alsoincluded is a case history where arc resistant medium voltagemotor control equipment was installed at a production facilityof a large North American chemical company.

Index Terms - medium voltage, control equipment, arcresistant, NFPA-70E

I. INTRODUCTION

Failure within a piece of MV control equipment, whether from a defect, an unusual service condition, lack of maintenance, or mal-operation, may initiate an internal arc.

Standards and guides have been developed over a period of many years, through the cooperative efforts of users, thosewho specify equipment, manufacturers, and other interestedparties to evaluate the ability of a given design to withstandfault conditions. These test conditions were traditionallyrepresentative of "down-stream" events, referred to as boltedfaults, that simulate a short-circuit condition outside theequipment under test.

In the 1970’s, principally in Europe, interest in evaluatingelectrical equipment under conditions of internal arcing arose

As a result, a draft Annex AA to IEC 298 [1] “A.C. MetalEnclosed Switchgear and Controlgear for Rated Voltages Above

1kV and Up to and Including 52kV” was prepared in 1976 andapproved by the IEC in 1981. The last edition of IEC 298 wasapproved in 1990. The document was renumbered as 62271200 [2] during its most recent revision cycle and was published in2003.

Knowledge of the arc resistance testing guide in IEC 298spread to North America, and was used as the basis for EEMACG14-1, 1987, “Procedure for Testing the Resistance oMetal-Clad Switchgear Under Conditions of Arcing Due to anInternal Fault” [3]. EEMAC G14-1 incorporated improvements inknowledge and understanding in over a decade of use of Annex

AA of IEC 298 in Europe.

The development of IEEE Guide C37.20.7 "Guide for Testing

Medium-Voltage Metal-Enclosed Switchgear for Internal ArcingFaults" [4] rests heavily on Annex AA of IEC 298 - 1981 and

Amendment 1 - 1994, and incorporates many of the refinementsoriginated in EEMAC G14-1. Since its original publication in2001, IEEE C37.20.7 has undergone numerous revisions toimprove the consistency of the testing, add additional voltageratings, and to incorporate test procedures for new technologiesin arc fault protection. The latest revision of this guide, whichshould be published in 2007, has a new title "Guide for TestingMetal-Enclosed Switchgear Rated up to 38kV for Internal ArcingFaults". It is expected to specifically include informationregarding testing of low voltage equipment. It should also benoted that the Canadian Standards Association (CSA) isadopting the words of IEEE C37.20.7 into their C22.2 series ostandards [5].

II. EQUIPMENT CONSIDERATIONS

A. Protection Levels of Arc Resistant Equipment

Each test guide or standard defines the safe approach areasand arc flash protection boundaries depending on the style ortype of arc resistant protection level.

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B. IEC Classifications

The IEC standard 62271-200 defines this as the Internal ArcClassification, (IAC). The IAC makes allowance for internaloverpressure acting on covers, doors, inspection windows,ventilation openings, etc. It also takes into consideration thethermal effects of the arc on the enclosure and of ejected hot

gases and glowing particles. It does not take into account anydamage to the internal aspects of the equipment. The testprocedure and ratings are provided in a normative annex.

In standard 62271-200 a distinction is made between threetypes of accessibility to the metal-enclosed switchgear andcontrolgear that are possible in the site of installation:

a) Accessibility Type A,Restricted to only authorized personnel.

b) Accessibility Type B,Unrestricted accessibility, including that of thegeneral public. [2]

The metal-enclosed switchgear and motor control centersmay have different types of accessibility on the various sidesof the enclosure.

The following code is used:- F for Front side*- L for Lateral side (left or right)- R for Rear side

* The Front side shall be clearly stated by themanufacturer

A third accessibility type is provided for pole mountedequipment.

c) Accessibility Type C,Restricted by installation out of reach. The minimumadmissible height of installation shall be stated by themanufacturer.

C. IEEE Classifications

The IEEE C37.20.7 guide describes accessibility types in aslightly different manner.

There are also two primary levels of protection described byIEEE C37.20.7. These levels correspond directly to the testindicator placement as described below.

a) Accessibility Type 1,

Equipment with arc resistant designs or features atthe freely accessible front of the equipment only.

b) Accessibility Type 2 ,Equipment with arc resistant designs or features atthe freely accessible exterior (front, back, and bothsides) of the equipment only.

These basic levels can be modified by additionalrequirements indicated by a letter suffix after the Typenumber. The application of suffix "B" to Accessibility Type 1

or Type 2 indicates the instrument compartment is accessiblewithout compromising the arc resistant protection.

The application of suffix ‘‘C’’ to Accessibility Type 1 or Type 2indicates that the equipment meets the additional requirementsto reduce the collateral damage to adjacent compartments andequipment, and should not be interpreted to indicate any

additional degree of protection for personnel.

The application of suffix "D" to Accessibility Type 1 indicates adesign tested in a manner similar to IEC 62271-200 where eachprotected surface is individually identified and rated.

D. EEMAC Classifications

The EEMAC G14-1 standard uses a similar accessibilitydefinition system to the IEC and IEEE standards.

a) Accessibility Type A. Equipment with arc resistant designs or features at thefront only.

b) Accessibility Type B. Equipment with arc resistant designs or features at thefront, back and sides.

c) Accessibility Type C. Equipment with arc resistant designs or features at thefront, back and sides and between compartments withinthe same or adjacent cells. The only exception is that afault in a bus bar compartment of a feeder cell isallowed to break into the bus compartment of anadjacent feeder cell.

III. SITE CONSIDERATIONS

A. Installation Considerations

The accessibility type rating of the equipment’s arc resistancapability will dictate some of the aspects or the installationConsideration has to be made regarding the physical location othe equipment in relationship to the electrical room configurationand location of adjacent equipment. The following informationaddresses equipment qualified to IEEE C37.20.7. Similaevaluations can be made for any of the Accessibility Typesdefined in any of the testing guides previously mentioned.

B. General Considerations

Equipment designed to mitigate internal arcing faults does soby providing a mechanical barrier between the operator and the

fault and by exhausting the arc fault by-products from theequipment to a location that is controlled and safe. This exhausis typically from the top. To successfully pass the requirementsof the testing guides, the mechanical integrity of the equipment isa must and generally remains unaffected by the site installationThe exhaust function, however, is greatly influenced by theinstallation. If the arc gases exhaust from the top, there must beclear space above the arc resistant equipment. This means atotally clear space: devoid of cables, conduits, cable trays, pipingand such. The rationale being that the hot arc plasma releasedfrom the top of the structure needs adequate space to exhaust

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and cool prior to reaching a location where contact with anoperator or any other flammable item could occur.

The determination of accessibility type is made based onthe accessibility of surfaces of the equipment. Where allsurfaces are potentially accessible, type 2 equipment shouldbe specified. Accessibility type 1 equipment is typically only

applied when the sides and rear are inaccessible. Where type2 equipment is not available at the required fault ratings,accessibility type 1 equipment may be installed. However,special considerations must be made regarding cablemanagement and the appropriate labelling and identificationof the flash boundary around the exposed sides and rear,since the sides and rear of the type 1 style of arc resistantenclosure provide limited or no arc resistant protection.

The testing guides and standards specify that adequate clear space is located above the arc resistant control equipment topermit arc gas dissipation. This means total clear spacedevoid of cables, conduits, cable trays, piping and such. Therationale here is that the hot arc plasma released from the topof the structure needs adequate space to be displaced into.

If items encroach into the clear space above the equipmentthey could cause two specific problems:

1. Equipment in the path of the approximately 16,000 to35,000 degrees F arc gases [6] may instantly bevaporized or become ignited unless the arc gaseshave cooled sufficiently before contact is made.

2. The hot arc gases could be re-directed and ‘splashdown’ on anyone in and around the controlequipment.

It is common in arc resistant designs to address this issueby utilizing a plenum above the equipment to collect the gases

and direct them to a designated location within the room or outside the building.

C. Use of Plenums

Many suppliers provide a plenum system on the top of their arc resistant enclosures. Top-mounted plenums allow for theceiling height directly over the switchgear to be reducedslightly. (Refer to installation considerations.)The plenum facilitates channelling of the dangeroussuperheated air and arc contaminates to a safe and controlledlocation which is typically external to the electrical equipmentroom.

The plenum release point could be external into acontrolled, fenced off area. In locations where the externalventing of hot flammable materials is not possible,containment rooms can be used to dump the arc waste into.The size of the containment area has to be determined basedon the recommendations of the equipment supplier.

The use of a plenum requires several additional installationconsiderations:

a) Where will the arc gases be exhausted?

b) Is there a clear path to the exhaust release point?

c) What length does the plenum need to be to reach theexhaust point?

d) How many turns will be required to reach the exhaustpoint?

e) If the release point is external to the control room areais it an area that can be controlled, restricted or isinaccessible?

f) If the release point is internal to the control room, is therelease area large enough and can the area becontrolled and restricted?

g) Do I have attachment locations on the building structureto support the weight of the plenum?

h) Where top entry is necessary, is there adequate spacefor top entry if plenum and exhaust ducts are used?

i) Does the plenum affect the continuous current rating othe equipment?

Fig. 1 Typical arc resistant controller with plenum

Consideration also needs to be made in regards to the entiresub-station and control equipment configuration. Depending onthe site configuration, a close coupling of the switchgear to themedium voltage control equipment may be necessary. In thiscase, consideration has to be made as to whether the individua

functional sections, circuit breaker cabinets and motor controllecabinets have common or separate plenums.

If the plenums are to be connected together, then the size ofthe plenum has to be reviewed to ensure compatibility. In caseswhere different vendors are selected for different aspects of thecontrol, i.e. switchgear is purchased from one supplier - motorcontrol centers from another, validation of a common plenumconnection must be provided by the suppliers.

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D. Cabling Considerations

Since arc resistant equipment is designed to withstand acertain amount of pressure related to the arc, specialconsideration has to be made for cable entry and exit points.

If the equipment is designed to exhaust from the top, then

power cable entry space may be limited. It is preferred thatcable entry be made from the bottom. However, this may notbe always possible. Some equipment configurations mayneed to be constructed with extra deep cable sections to allowboth cable and exhaust openings. Some configurations mayrequire additional width.

Additionally, the cable trays must not compromise the areaabove the equipment in such a way that exhausting gasescould be reflected or redirected into the qualified aisle ways asdefined by the Accessibility Type Rating.

Further, when routing the cable trays outside the perimeter of the equipment, lighting becomes an issue. Care must betaken to adequately illuminate the area so these trays do not

create "dark spots" on the equipment that could lead tomisreading meters or relay settings.

Where control and power cable entry is made intocompartments that are exposed to the arc by-products, thesepoints must be sealed to insure that the pressure wave doesnot propagate into unintentional locations through rigid or flexible conduits. The pressure impact is a function of theconduit fill rate, length and number of bends. Knowledgeableengineering judgement should be used regarding this.

There are different sealing systems available to assist in theprevention of arc gas propagation and pressure wavesthrough conduits. One of the simplest systems is the use of ‘sealing putty’ to fill in the gaps around the cables and the

conduit. The sealing putty should be pushed tightly aroundthe cables and into gaps at the end of the conduit (referenceFig 2). The putty should be placed at each end of the conduitto insure full protection. The putty is not adversely affected bythe high temperature since the putty is exposed to the hotgases for a very short period of time and is primarily used onlyto block the pressure wave.

Fig. 2 Conduit/Cable cross-section with sealing putty

There are also intumescent/elastomeric sealing systemsthat expand under high temperatures to provide a tight sealaround cables or an opening. These systems typically do notperform well under the rapid temperature rise associated with

an arc blast. These types of sealants are not usuallyrecommended for this type of application.

Alternatively there are conduit/cable sealing systems availablethat provide an air tight seal. These systems use compressionstyle sealing rings to seal tightly around each individual powerconductor (reference Fig 3).

Fig. 3 Typical Conduit Conductor Sealing Bushing

CFC-free polyurethane expanding foams, specifically designedfor use with cables and conduits, can also be used to seal theend of the conduits. These typically come as a two part systemThe two parts are mixed and, after a specific time, can expandup to approximately eight times in volume.

All of the installation considerations for power cables alsoapply to a bus duct entry system. An appropriate system isrequired to limit egress of arc gases into a bus duct system.

E. Room layouts

Several decisions must be made concerning the room beforethe equipment can be specified. It should be noted that theseconsiderations need to be addressed for any installation opower equipment, not just arc resistant equipment, as theseconditions affect the safety of the operator in all cases. It iscritical to address these concerns for arc resistant equipment, sothat the expected increase in operator safety is not compromisedby the conditions of the installation.

Review the room/site for the following,1. Will the fault gases be vented into the room or out of the

building?2. If the gas is to be vented into the room:

a) Can the room physically withstand the expectedoverpressure?

b) What collateral damage can be expected?c) Is adequate protection provided to the operator for

potential exposure to gases and smoke?

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d) Is there a designated place to vent the faultgases within the room?

3. If the fault gases are vented external to the room:a) Is the vent area protected from personnel?b) Is the atmosphere in the vicinity of the exhaust

point volatile?

c) Is there equipment nearby that may becompromised?d) Are there any items in the vicinity of the exhaust

point which are flammable?

Once these questions are answered, the basic effects to thesite are identified and the general arrangement of theequipment may be established.

The next step is to determine where the equipment will beplaced in the room.

F. Other Installation Issues

Installation of arc resistant control equipment within an

indoor room or outdoor enclosure requires special review of the room geometry in relationship to the equipment’splacement. This is especially true when the fault gases will bevented into the room through a plenum system and exhaustduct. The room or building which houses arc resistantequipment to which the gases will be exhausted into must bedesigned to withstand overpressures of up to 15-20 poundsper square inch (PSI), on a transient basis

The plenum size, weight and exit point needs to beconsidered in the room layout. The plenum size is critical toproper pressure relief of the equipment. Restrictions in theventing area, numerous turns in the exhaust duct, andextreme lengths of exhaust duct can all contribute to reducingthe effectiveness of the pressure relief. The exhaust duct

arrangements can also create structural issues for the buildingdue to its weight.

The plenum discharge location has to be carefullyconsidered since this point will need to be identified with anarc flash boundary and marked accordingly. Additionally, theplenum and duct concentrate the fault by-products into aspecific opening and the resultant outflow of gases canproduce significant risk of injury from the thermo-acousticwave. Room geometry can increase the effect as the wavereflects and crosses itself, often multiplying the forces. Whenthe plenum discharge is to the outside, suitable accessrestrictions to the exit point area may be necessary.

If the location will not support an exterior exhaust of the arcgases, an interior containment system may be necessary. Thecontainment room or building must be designed to withstandthe overpressures associated with the rapid release of the arcgases.

In the case of installations where no plenum is used, specialcare needs to be made regarding the ceiling materials usedand proximity of other flammable items at or near the exhaustpoints. This should be reviewed even if the manufacturer’srecommended space is provided. Special warning signs need

to be affixed to the building warning of the need to keep the areaabove the control equipment clear of all items.

Finally, the determination of Accessibility Type is made, basedon the accessibility of surfaces of the equipment. Where asurfaces are potentially accessible, Type 2 equipment should bespecified. If Type 2 equipment is not available at the required

fault ratings, then the unprotected areas surrounding theequipment must be quarantined where possible and the areasclearly marked for flash hazard.

G. Checks during commissioning and before energization

Installation practices for this type of equipment are veryimportant. Sloppy or incomplete equipment installation practicescould result in mitigating the overall protection capabilities of thearc resistant equipment installation.

Special care has to be taken to insure that the following itemsare taken care of and even double checked to insure integrity ocompliance:

a) The equipment must be installed per the supplier’s

installation instructions and recommendations.b) All internal and external cover/access plates must be

completely installed and sealed per the manufacturer’sinstructions.

c) All mounting hardware for these plates must be reinstalled and specified torque properly applied to theretaining hardware.

d) All removable cover plates may have a sealing strip oother sealant material attached to them (when present

– it must not be damaged.e) Sealing of seams and gaps during installation, such as

plenums joints or between adjacent cabinets, requiresspecial attention. The sealant materials used should beonly those supplied or recommended by the arcresistant controls manufacturer.

f) Verify all plenum and exhaust ducts are tightened andsealed appropriately.

g) Verify all conduits entering the arc resistant cabinetsare sealed appropriately.

h) Insure the appropriate warning labels been affixed peNFPA-70E [7].

IV. CASE STUDY

A. Background

A large chemical manufacturing company has a manufacturingfacility in Delisle Mississippi. The site experienced significanflooding during hurricane Katrina in August 2005. The flooding

caused significant damage to existing equipment, including MVmotor control equipment. Figure 4 provides you a visuarepresentation of the flood’s size and its impact on the facility.

The following information describes the work involved and theconsiderations made for the replacement of the damagedmedium voltage motor control equipment with new arc resistanmotor control equipment.

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Fig. 4 Flood Level

B. Equipment Damage

The existing 2400 V MV motor controllers were flooded withat least 3 feet of dirty salt water. Many controllers werecompletely submerged. Figure 5 shows results of theflooding.

The facility personnel initially investigated the possibility andexpected success of reconditioning the equipment in placeinstead of replacing it. This option was quickly discarded.The work was then started to specify, purchase, and installnew equipment.

Fig. 5 Flooded Equipment

C. Equipment Selection

Most of the existing MV motor control equipment was notarc resistant and used air magnetic controllers. Equipmentpurchased for facility projects in the last several years usedarc resistant equipment with vacuum contactors. Thedecision to use arc resistant equipment was based on

improved protection for personnel as described in other parts othis paper.

Arc resistant equipment was therefore preferred for thisreplacement work. However, the new equipment had to meeseveral requirements.

D. Delivery

The equipment had to meet the critical project schedule. Thefacility needed to have electrical service restored as soon aspossible with a December 2006 start-up time for some of thesystems.

To facilitate the equipment design, a standard design foprotection, metering, and motor control circuit was developed.

The main bus of each line-up of motor controllers would havethe same phase and ground relay protection. This protectionwas not in the existing equipment as the motor controequipment was supplied power from 13.8kV/2400V, 3750kVAtransformers that had fused switches on the primary of the

transformers. The new 15kV switchgear included circuibreakers. Therefore secondary relaying could be used in thecontrol equipment to trip the primary breaker for a secondary busfault.

The main bus of each line-up of motor controllers would eachhave the same metering system installed. The existingequipment did not have a main bus metering system. Themeters were supplied current from the same currentransformers use for relay protection. Voltage transformesignals came from a set of open delta connected voltagetransformers located in the incoming power cable compartment.

Each controller used the same control circuit design. Thisstandard design helped both the equipment manufacturer and

the installation teams. The new design used a commonmicroprocessor based motor protection relay that included motodifferential protection even though some motors did not requiredifferential protection.

The standardizing on the protection, metering, and controllecircuits enabled the controller equipment manufacturer to easilyduplicate the equipment design. This standardization enabledthe manufacturer to meet the rapid delivery needs of the project.

E. Layout

The equipment had to fit in the footprint of the existingequipment. Some minor adjustments to field wiring could bemade, but those adjustments were minimal. The scope of thereplacement included reusing all of the existing cables.

The cables entered the equipment from the top of theenclosures. Each exposed end of the cable was thoroughlycleaned, tested, and checked for damage. The cable wasreused only if it passed the inspection and testing.

The equipment configurations were either back-to-back or sideto side line ups. The arrangement requirements werecommunicated quickly to the new equipment manufacturer so

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the equipment layout could be developed. Since the newequipment is modular in design, the layout of the equipmentwas not a critical factor at the beginning of the process.

F. Arc Resistant Protection Level

The new equipment is installed such that personnel have

access to all sides of the equipment. Type 2 accessibilitylevel equipment, as defined in C37.20.7, was chosen toprovide protection to personnel on all sides of the equipment.The equipment supplier met this requirement when theequipment plenum is installed.

G. Current Rating

The short circuit current rating of the equipment had to meetthe minimum system requirements of the existing electricalsystem. Equipment rated at 60kA was selected to meet thisneed.

H. Removal of Damaged Equipment

The first work involved the removal of the flooded motor controllers. The work was done to minimize the damage tothe existing wiring. Each conductor was labeled when it wasremoved. This allowed identification of the correct cable andconductor when the new equipment was installed. After eachcable was pulled out of the equipment, the flooded equipmentcould then be removed. The result of this work is shown inFigure 6.

Fig. 6 Field Wiring with Equipment Removed

I. New Equipment Assembly at Shipping Splits

The new equipment was delivered and unpackaged in itspermanent location. Construction crews moved theequipment into position and began the assembly process.

Many of the construction members working on theequipment had not installed arc resistant equipment and werenot familiar with its installation requirements. The first fewmotor control line-ups were used as a learning opportunity bythe crews. The equipment manufacturers were brought onsite after some small installation concerns. No significantinstallation problems were encountered during their assembly.It just took more installation time as the construction crews

learned how the equipment needed to be connected. Theinstallation of the other line-ups went very well. It is highlyrecommended to engage the services of the original equipmenmanufacturers to assist in the initial installation. This would helpinsure that the proper steps are taken which would reduce re-work and insure the overall arc resistant capabilities of theinstallation.

One of the items learned by the construction group was to becareful to not stand on or otherwise damage the equipment arcvent flaps located on the top of the equipment. This was not aproblem once the construction group understood the purpose othe flaps.

J. Conduits and Cables

The existing control circuit cables were routed and installed inthe new motor control equipment. New conduits were run fomany of these circuits as the existing conduits did not meet thenew equipment layout needs. The 2400V motor feeder cableswere installed in the equipment through a cable hub in the top ofthe equipment.

K. Plenums

The arc resistant motor control equipment was provided withplenums to direct the arcing fault gases away from the top of theequipment and personnel operating it. For these motocontrollers, the decision was made not to install the plenumsThis decision was made for the following reasons and was basedon the project needs and with consultation with the equipmenmanufacturer.

First, the existing conduit, cable tray, and cables interfered withthe installation of the plenums. These obstructions preventedaccess to install and route the plenum exhaust duct to outside ofthe equipment room. For a new installation, these obstructions

can be designed out of the system. In the case of thisreplacement project, these items could not be addressed withouadding significant cost and time to the project.

Second, this equipment is installed in rooms with high ceilings(15’ or more). The project team responsible for this work felt thathe high ceilings would give the opportunity for the exhausgases to be vented directly through the top of the equipment intothe room space. Although the gases enter the same room as theperson who is operating the equipment, it was felt the personwould not be exposed directly to the arc flash energyEliminating this direct exposure is a significant benefit of the arcresistant equipment. Personnel could still be exposed to somelevel of arc gases and direct particulate fallout materials resultingfrom an arc event. Additionally, there was no considerationmade for the acoustic sound wave. These were compromisesmade for the installation. Since the plenums could not beinstalled, the equipment no longer officially met its type 2accessibility rating. But it did meet type 1 accessibility ratingBecause of the reduced accessibility type rating and thepossibility for exposure to reflected gases and particulate, sitepersonnel decided to require full arc flash protection whileoperating the equipment.

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L. Arc Flash Energy and Personal Protective Equipment

The arc flash energy for this equipment was calculatedusing a computer software program. The analysis wasperformed for the existing system. This would be a basis tounderstand how the secondary relaying and the arc resistantequipment changed the arc flash energy hazard.

The arc flash energy for the main bus of the existing motor control equipment was calculated to be 7.5 calories/cm

2. The

protective device that operated to clear the fault was a fuse onthe primary of the substation transformer. The calculationused a 36” working distance.

The arc flash energy for the main bus of the new equipment,not considering the benefits of the arc resistant design, wascalculated to be 21 calories/cm

2. This was a surprise to the

project team. The increased incident energy was due to thelimited available settings of the protective relay chosen for thisapplication. In this design, the existing fuse provided faster clearing at the calculated arcing fault current level. If adifferent protective relay with more flexible settings had been

chosen, arc flash energy could easily have been reduced toless than the existing installation. It is important to performarc flash energy calculations as part of the facility designprocess. This way if relaying or equipment choices negativelyaffect arc flash energy, other solutions can be reviewed andchosen before equipment orders are placed. For thisinstallation, there was not time for this review.

The arc flash energy for the main bus of the new equipmentwas then performed considering the benefits of the arcresistant design. The arc resistant design ensured theintegrity of the enclosure for the front of the equipment, wherethe equipment operator could be standing. The workingdistance to the arc could be significantly greater than 36” asthe arc byproducts would not be coming from the front of the

equipment. A conservative distance of 5’ was used and thecalculations were performed again. The arc flash energy wasthen calculated to be 13 calories/cm

2. This was a significant

reduction in arc flash energy from both cases previouslymodeled as compared to when the 36” working distance waschosen. The increases arc distance reduced the arc flashenergy by approximately 38%.

The facility personnel realized the risk that some energycould be reflected from cable trays, cables, and conduits,back towards a person standing near the equipment.Therefore the site chose to require 25 calories/cm

2protective

equipment to be worn when operating this equipment. Thisprotection level was the lowest protection level equipmentavailable at the site. The arc flash protection equipmentincludes a hood, arc flash suit, voltage rated gloves, safetyglasses, and safety shoes.

IV CONCLUSIONS

The installation of arc resistant control equipment providesthe best level of personnel protection from arc flash/blasthazards. The designed level of arc blast protection provided

by the equipment is only maintained if the equipment is properlyinstalled and maintained. Failure to install or maintain theequipment per the supplier’s recommendations will result in afalse sense of protection and could expose those around theequipment to the same hazard level as non-arc resistanequipment.

Special care and considerations have to be made during theinstallation of this type of equipment to insure that the entiresystem is not compromised by its improper installation. It isrecommended that you directly engage the services of theequipment manufacturer in at least the initial phases of theequipment’s installation.

When selecting your installation team, make sure you outlinethese requirements in your specifications and include a completeequipment inspection prior to energization. This will help insurethe integrity of your arc flash protection system.

Sometimes compromises may be required when performingthe complete installation. These compromises should be doneonly with the involvement of the equipment manufacturer and

only after a full safety and system analysis of incident energylevels.

VII. REFERENCES

[1] IEC Standard 298-1994-11, “High-voltage switchgear andcontrolgear–Part 200: AC metal-enclosed switchgear andcontrolgear for rated voltages above 1 kV and up to andincluding 52 kV”, International Electrotechnical CommissionGeneva, Switzerland.

[2] IEC Standard 62271-200-2003-1, “High-voltage switchgeaand controlgear–Part 200: AC metal-enclosed switchgear andcontrolgear for rated voltages above 1 kV and up to andincluding 52 kV”, International Electrotechnical Commission

Geneva, Switzerland.

[3] EEMAC G14-1-1987, “Procedure for testing the resistance oMetalclad Switchgear under conditions of arcing due to aninternal fault”, Electrical and Electronic Manufacturers

Association of Canada. (Electro Federation of Canada

Mississauga, Ontario, Canada).

[4] IEEE Standard C37.20.7-2001, “IEEE Guide for TestingMedium-Voltage Metal-Enclosed Switchgear for Internal ArcingFaults”.

[5] C22. (various), Canadian Electrical Codes Part I and Part IICanadian Standards Association, 2006.

[6] R. H. Lee, “Pressures developed by arcs”, IEEE Transactionson Industrial Applications, vol. IA-23, no. 4, July/August 1987, pp760-764.

[7] NFPA 70E, “Standard for Electrical Safety Requirements foEmployee Workplaces”, 2004 Ed. Quincy, MassachusettsNational Fire Protection Association, 2004.

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