Fluke Arc Flash Electric Shock

7/29/2019 Fluke Arc Flash Electric Shock

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Concerned about arc-flash

and electric shock?

Want to comply withNFPA 70E CSA Z462?

How can IR Windows help?

Infrared thermography has become a

well-established and proven method for

inspecting live electrical equipment. To

carry out tests, the thermographer usu-

ally works with live energized equipment

and requires a clear line of sight to the

target. Thermographers must therefore

be especially aware of the hazards, the

legislation and safety issues, and the

techniques and equipment best suited

to minimizing the risks when working in

these dangerous environments.

Introduction

Electrocution is the obvious danger faced byanyone working on or near live electrical equip-ment and it is clearly important to understand

shock hazards and wear appropriate protection.However, most electrical accidents are not theresult of direct electric shocks. A particularlyhazardous type of shorting faultan arc faultoccurs when the insulation or air separationbetween high voltage conductors is compro-mised. Under these conditions, a plasma arcanarc flashmay form between the conductors,unleashing a potentially explosive release ofthermal energy.

An arc flash can result in considerabledamage to equipment and serious injuries tonearby personnel. A study carried out by the US

Department of Labor found that, during a 7-yearperiod, 2576 US workers died and over 32,000suffered injuries from electrical shock and burninjuries. 77 % of recorded electrical injurieswere due to arc flash incidents. According tostatistics compiled by CapSchell Inc (Chicago),every day, in the US alone, there are 5-10 tenarc flash incidents, some fatal.

NFPA 70E is the leading internationally recog-nized safety standard for electrical safety in theworkplace. The Canadian Standards Associationhas developed its own set of standards basedon NFPA 70E: CSA Z462.

These standards define a set of safe require-

ments for personnel working on electricalequipment. To comply with the standards,employers must carry out a hazard risk assess-ment and ensure that all employees working ina potential arc-flash hazard zone use appropri-ate equipment and wear the right protectiveclothing. Although it is not the responsibil-ity of the thermographer to put in place theappropriate safety procedures, it is importantto recognize and understand their need, and toensure that the correct procedures, equipmentand protective clothing are used.

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2 Fluke Corporation Concerned about arc-flash and electric shock?

The installation of IR windows, panes or portsallows a thermographer to inspect live electri-cal equipment without the removal of protectivecovers and the exposure of equipment. Anarc-resistant window, unlike a port or pane,provides additional protection for the thermog-rapher in the event of an arc flash resultingfrom unexpected component failures or workon other parts of the system. This substantiallyreduces the hazard rating for the inspectionsand, in most cases, may allow the thermogra-pher to work more safely minimizing the needfor excessively bulky and cumbersome protec-tive clothing.

What is an arc-flash?

An electrical system can be subject to two typesof shorting faults: Bolted faults Arc faultsBolted faultsA bolted-fault is everyones idea of a shortcircuit: such as energizing the circuit with aground set in place. A bolted fault results in avery high current; it is a low impedance shortbecause of the solid connection. Bolted faultsbehave predictably and so conductors can berated to withstand the overcurrent for the timerequired for an interrupt device to operate.Bolted faults rarely result in an explosion.

Older switchgear which holds a fault-ratingwill usually be rated for its ability to withstandthis high current for a particular time period. Abolted fault-rated piece of equipment will usu-ally have a BIL (Basic Impulse Level) highlightedon the casing itself in the form of a fault currentfor a set duration. E.g 100 kA for 5 seconds.

Arc faultsThe secondand far more destructivefault isan arc-fault. This occurs when the insulation,

or more specifically the air separation, between

electrical conductors is no longer sufficient towithstand their potential difference. This canoccur for many reasons. A dropped tool or anyother conductive element (even rust), introducedbetween or near energized components maycompromise the insulating clearances. Often,incidents occur when a worker mistakenly failsto ensure that equipment has been properlyde-energized. Incidents can even occur whena worker is simply removing a cover from apiece of equipment. A significant proportionof arc faults occur simply due to some form ofcomponent failure and is not limited to humaninteraction alone.

In contrast to the low impedance required fora bolted fault, an arc-fault is a high impedanceshort because the discharge occurs through air.

Figure 1. Demonstration of the power of an Arc Flash.Photo courtesy of ewbengineering.


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The current flow is therefore comparativelylow but the explosive effects are much moredestructive and potentially lethal. Unlike abolted fault, it is difficult to predict exactly howmuch energy will be released by an arc fault. In

particular, it is difficult to predict the durationof an arc fault as this depends on many factors,feedback mechanisms and the response of theover current protection devices.

When an arc fault occurs

NIOSH (National Institute for Occupational Safetyand Health, Pittsburgh, USA) has publishedthe results of a survey of electrical accidentsreported by MSHA (Mine Safety and HealthAdministration, US Department of Labor) in themining sector over the period: 1990-2001. Inmore than two-thirds of the cases of arc flash

injuries, the victim was performing some formof electrical work such as troubleshooting andrepair. More surprisingly, 19 % of the accidentsarose from the direct failure of equipment duringnormal operation. Overall, 34 % of the accidentsinvolved some form of component failure.

The key components involved in the acci-dents where: circuit breakers (17 %), conductors(16 %), non-powered hand tools (13 %), elec-trical meters and test leads (12 %), connectorsand plugs (11 %). Of the cases that reported thearcing voltage, 84 % occurred with equipmentat less than 600 V and only 10 % with equip-ment at more than 1000 V.

Arc fault make-up

When an arc-fault is triggered, a plasmaarcthe arc flashforms between the shortedcomponents. Once established, the plasma arc

has a virtually unlimited current-carrying capac-ity. The explosive energy release causes: A thermoacoustic (dynamic) pressure wave A high intensity flash A superheated ball of gasThe thermoacoustic wave is a dynamic pressurewave caused by the instantaneous expansion ofgas local to the fault. It causes panels to rupture,flying debris and barometric trauma. The wavefront travels outwards, away from the fault, andas it impacts surfaces it increases in energy: aneffect known as pressure piling.

A common misconception is that an arc flashwill always result in panel rupture. However, byincorporating high-speed interrupt devices andadditional protection systems, an engineer canreduce the arc flash energy to a level where thethermoacoustic wave front does not have suf-ficient energy to rupture the panel.

Although the thermoacoustic wave resultingfrom an arc fault can be very destructive, it isnot the only characteristic of an arc flash. Unlikea chemical explosion, the energy of an arc flashconverts primarily to heat and light energy.Temperatures at the epicenter of an arc flashcan reach 20,000 C (four times hotter than thesurface of the sun) within a millisecond. Such

high temperatures are capable of explosivelyvaporizing metals such as copper. The presenceof vaporized metal can then feed and sustainthe plasma arc and exacerbate its power.

An arc flash essentially lasts until the over-current protective devices open the circuit. Afast-acting fuse may open the circuit as quicklyas several milliseconds.

The consequences of an arc flash

Arc faults are potentially fatal to any person-nel in the vicinity. The intense heat of the arcflash can severely burn human skin and ignite

the clothing of anyone within several feet ofthe incident. Treatment for arc flash burns caninvolve years of skin grafts.

Without proper eye protection, projectilesand molten debris can cause eye damage. Theintense UV radiation associated with the flashcan cause retinal damage. Superheated vaporscan injure lungs and impair breathing. Thethermoacoustic blast can damage hearing withruptured eardrums, cause collapsed lungs anddamage other internal organs. The blast canknock personnel off their feet; falls may result inbroken bones or lead to electrocution or furtherinjuries on other parts of the system.

19 % of the accidents arose from thedirect failure of equipment duringnormal operation


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Inevitably, a serious arc flash will damage oreven destroy the affected equipment. This leadsto extensive downtime and expensive replace-ment and repair.

An incident may also represent a failure on

the part of the employer to comply with indus-try guidelines and regulations. This could resultin a fine, litigation fees, increased insurancecosts, expensive legal actions and accidentinvestigations.

Standards and guidelines

The potential dangers of an arc flash canbe reduced by following the relevant safetyguidelines and using personal protectiveequipment PPE.

In the USA, the following OSHA and NFPAregulations apply to personnel working with

energized electrical equipment: NFPA 70 (National Electric Code) NFPA 70E (Standard for Electrical Safety in

the Workplace) OSHA Standards 29-CFR, Part 1910 (S)

1910 333 OSHA Standards 29-CFR, Part 1926 Subpart K IEEE Standard 1584-2002, (Guide for

Performing Arc Flash Hazard Calculations.)

Many other countries have their own broadlysimilar standards and regulations. For example,Canadas regulations can be found in CSA Z462.In the UK, compliance with EAWR (Electricity at

Work Regulations) 1989, section 5 is required.NFPA 70Ethe safety standard

NFPA 70E defines the safe parameters forpersonnel working on electrical equipment.Although adherence is not a legal require-ment, the standard provides a benchmark formost industries to demonstrate compliancewith OSHAs General Duty clause. An employeradopting the guidelines offered in NFPA 70Edemonstrates a clear commitment to safe work-ing practices and the protection of employeesfrom shock and arc flash hazards.

According to the standard, if personnel will beoperating in the presence of energized equip-ment, then certain safety considerations areapplicable. 70E recognizes that there may bethe potential for arc flash and arc blast evenwhen conductors are not exposed. Qualifiedpersonnel responsible for the work must: Conduct an arc flash hazard analysis Implement qualified and general worker

safety training based on the results Establish shock and flash protection

boundaries

Provide protective clothing and personalprotective equipment to ANSI standards

Put warning labels on equipment Authorize the job with a live work permit

Steps for an arc flash hazard analysisSection 4 of IEEE 1584-2002 outlines a 9-stepprocedure for arc flash hazard analysis. Thepurpose of this analysis is to identify the flashprotection boundary and the incident energy atassigned working distances...

The nine steps are:1. Collate system data. Collect system and

installation information for a detailed shortcircuit assessment. You will need to describethe system and the arrangement of itscomponents in a one-line drawing withnameplate specifications for each device

and the lengths and cross-sectional areas ofinterconnecting cables.

2. Consider all modes of operation. Examinethe different ways that the system operatesand how this may affect the risks and magni-tudes of arc hazards.

3. Calculate bolted fault currents. Using thedata gathered in the first two steps, calculatethe highest bolted fault current expected toflow during any short circuit.

4. Calculate arc fault currents. During an arcfault, the current flow is normally lower thanthat of a bolted fault in the same equipment

because of the added impedance of the arc.For example, for a bolted fault of 40 kA at480 V the corresponding arc fault would beexpected to yield about 20 kA. IEE 1584-2002 provides formula for estimating arc faultcurrents.

5. Determine protective device characteris-tics and arc durations. Estimate how overcurrent protection devices will react duringan arc fault. These may react more slowly,extending the duration and power of the arcflash. Through the analysis, it may be possi-ble to reduce the arc flash hazard and lowerthe PPE requirement by replacing existingcircuit breakers. For example, modern, cur-rent-limiting fuses may considerably reducearc flash energies by reacting more rapidlyand at lower over current values.

6. Document system voltages. Establish busgaps and operating voltages.

7. Estimate working distances. Determine thedistances from arc fault sources to a workersface and chest. Although hands and armsmay be closer to any incident, injuries areunlikely to be life-threatening.


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8. Determine incident energy. Calculate theincident energy resulting from an arc fault atthe working distance.

9. Determine flash protection boundary. Usethe same calculations to estimate a safe

distance from the source of the arc hazardbeyond which PPE is not required. This paperdescribes the Flash Protection Boundary inmore detail later.

Shock hazard analysis

As the name implies, the determination of theshock hazard is an analysis designed to reducethe risk of electrocution. NFPA 70 recommendsthe identification of three boundaries to definethe safe working limits for personnel working inan area with shock hazards. Each area is associ-ated with a level of training and PPE.

NFPA 70E data (contained in Table 130.7(C)(2)allows you to calculate the boundaries using aformula based on the voltage of the equipment.

The limited approach boundaryThis is the minimum permitted distance thatunqualified and unprotected personnel mayapproach a live component. Before crossing thelimited approach boundary and entering thelimited space, a suitably qualified person mustuse the appropriate PPE and be trained to per-form the required work. An unqualified personmay enter the limited approach area if they areunder the supervision of a qualified person.

The restricted boundaryTo cross the restricted boundary and accessthe restricted space, personnel need to havebeen trained in shock protection techniques, bewearing the correct PPE and have a written andapproved plan for any work in the zone. Theplan must make it clear that the worker mustnot enter the prohibited space or cross the pro-hibited boundary either personally or by usingany equipment or tool.

Prohibited boundaryNo worker should cross the prohibited boundaryand enter the prohibited area unless:

The responsible authority has carried out afull risk assessment. The work has been documented and it has

been fully established why it must be carriedit out on live equipment.

The qualified worker has been trained towork on live electrical equipment.

The worker has been equipped with appro-priate PPE. In terms of safety, any workercrossing the boundary must be equipped andprotected as they would be for making directcontact with the exposed live equipment.

Establishing these boundaries is an importantstep in protecting staff from the dangers ofelectrocution. It ensures that personnel use thecorrect equipment and procedures when in theproximity of live electrical equipment.

However, if the live equipment also poses anarc flash hazard, it is important to establish asafe distance for this eventuality: the arc flashprotection boundary.

The flash protection boundary

The arc flash protection boundary (FPB) is theminimum safe distance from energized equip-ment that has a potential for an arc fault. It isdefined as the distance at which, in the eventof an arc flash, a worker would be exposed to athermal event with incident energy of 1.2 cal/cm for 0.1 second. With this exposure, a workermay receive a 2nd degree burn to exposed skin.

If it is necessary for workers to cross the flashprotection boundary, and potentially be exposedto higher incident energies from any arc flash,

they must be wearing appropriate PPE. Forexample, at an incident energy greater than1.2 cal/cm, clothing could ignite and bare skinwould sustain 2nd degree burns.

Figure 2. The shock hazard boundaries


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An important point here is that the flashprotection boundary and the rules govern-ing access within it take precedence over theshock hazard boundaries. So, for example, ifthe flash protection boundary is greater than the

limited approach boundary then no unqualifiedperson can be permitted in the limited approacharea and even qualified workers must wearappropriate arc-resistant PPE here.

You can determine the flash protection bound-ary for an electrical system using the calculatingmethods contained in NFPA 70E and IEEE Std1584. The equations are based on the voltagelevel, fault level and the trip time of the protec-tive device.

The conditional flash protection boundaryis 48 inches for low voltage (


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What is appropriate PPE?

Clearly, the potential injurious effects of anarc flash can be reduced by using a fire flameresistant (FR) suit of a suitable calorie rating

to reduce the indecent energy on the body toan extent that the burns suffered are not lifethreatening.

While safety is the paramount concern, it isimportant to select PPE appropriate for the task.It might seem a good idea to insist on category4 PPE for all live work, perhaps to avoid a time-consuming arc flash hazard analysis, and thismay outwardly appear to be a safe policy forpersonnel. However, the use of restrictive or

excessive PPE can also be hazardous: an over-heating worker struggling with poor visibilityand restricted movement is more likely to havean accident. More accidents, more downtime.

In addition, there are scenarios where HRCmay not be enough arc-flash protection. TheHRC does not account for arc-blast.

NFPA 70E provides several tables listing thePPE appropriate for work within the flash pro-tection boundary: clothing and equipment suchas gloves, hats or hoods.

In addition to the HRC classification, PPE isoften described by the arc thermal performancevalue (APTV). This corresponds to the capability

of the garment to withstand a particular inci-dent energy (in cal/cm).

As we have already discussed, the flashprotection boundary defines the distance wherean arc flash would produce incident energy of1.2cal/cm: a level at which 2nd degree burnscould occur. This corresponds to risk category0. In practice, a worker will need to approachthe electrical system much closer than the flashprotection boundary. It is therefore importantto calculate the likely incident energy for theworking position and select PPE accordingly.

Most garments are tested and rated for incidentradiation at a distance of either 18 inches or24 inches. This roughly corresponds with theposition of the head and chest when work-ing directly on equipment. Of course, during

an arc flash incident, the hands and arms maybe much closer to the arc fault source and mayneed protective equipment with a considerablyhigher rating.

While NFPA 70E and IEEE 1584 cover thePPE requirements for arc flash protection, thereare also other considerations and standards toinclude in any safety appraisal. Workers mayrequire eye protection, insulating gloves, earand hearing protection, head impact protectionand reinforced footwear.

Heeding the warning signs

Equipment such as Switchboards, panel-

boards, industrial control panels, meter socketenclosures, and motor control centers in otherthan dwelling occupancies, which are likelyto require examination, adjustment, servicing,or maintenance while energized, shall be fieldmarked to warn qualified persons of potentialelectric arc flash hazards. The marking shall belocated so as to be clearly visible to qualifiedpersons before examination, adjustment, servic-ing, or maintenance of the equipment.

Although the basic requirement is for a signwarning of the arc flash hazard (see Figure 6) itis more helpful for engineers if the sign includesother useful information such as: Operational voltage Fault current Flash hazard boundary Incident energy at the normal working dis-

tance for the arc fault hazard. If this information is not present on the

warning sign, it should be documented andaccessible to all relevant personnel.

Figure 5. Examples of PPE for arc ash protection.Copyright image courtesy of Salisbury by Honeywell


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It is good working practice to make labelingclear and accurate; too much information is asbad as too little. If a system has several accesspoints and arc flash hazards, label it with theparameters for the greatest hazard.

The live work permit

Finally, before any work can commence, theresponsible manager must generate and sign anenergized electrical work permit. This describesthe task and why it must be performed with liveenergized equipment, long with the shock andflash boundaries plus related PPE.

Arc flash hazards and the electricalthermographer

The goal of any electrical thermographer is toprevent unplanned shutdowns in a manner thatis reliable, repeatable and above all safe. Sincethermographers work with the target equipment

online and on load coupled with the fact thatIR Cameras cannot see through panel covers,thermographers face severe safety hazardswhen attempting to scan electrical distributionequipment.

Taking measurements with a cover removed isnot a safe option.

The ideal solution is the provision of a perma-nent access point in the equipment housing

Leaving the cover closed:-1. Increases safety.2. Reduces the background reflection quotient

and reduces the interference making readingsmore repeatable.

The safer alternative is the provision of apermanent access point in the equipmenthousing.

Ports, panes and windowsFor infrared thermography, there are three typesof access point: Infrared Port Infrared Pane Infrared WindowAn IR port is simply a hole or series of holes; anIR pane is a thin polymer optic. Both limit physi-cal access (human contact) to live equipment.However, in the event of an arc fault neitheroption provides a protective barrier between thethermographer and the exposed conductor andarc flash source. A pane affords little protectionsince it is a thin polymer with a low meltingpoint.

Figure 6. Energy level of 201 cal/cm @7.2K V


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How can infrared windows help?

An infrared window provides a solid barrierbetween the thermographer and the live con-ductors. By careful design, it is possible to not

only to reduce the trigger effects of an arc butalso provide the thermographer with a far saferworking environment.

Thermography windows are relatively newtechnology, so there is no specific standard thatrelates to construction and testing. However,since they are invariably installed close to arcflash hazards, it is important that windows canwithstand not only an arc flash incident but alsothe rigors of their environment and normal day-to-day operation.

In the United States, Canada and Europe,there are differing standards associated withtesting equipment to be deemed arc-resistant,

specifically: ANSI C37.30.7 (North America),EEMACS G14-1 (Canada), IEC62271 (Europe).These standards are manifestly different andcannot be translated from one to another.For example, a product tested to IEC62271 inEurope cannot be claimed to be suitable forNorth American ANSI C37.20.7 nor CanadianEEMACS G14-1.

End-use requirements vary considerably sotesting and certification needs to be generic inorder to provide an all-round product suitablefor use in most if not all applications.

Manufacturers tend to design viewing panes

to withstand accidental impacts from untrainedpersonnel in transit rather than the com-bined pressure piling and sudden temperatureincrease effects of an internal electric arc.

Infrared windows, on the other hand, are con-structed of crystal optic materials designed tobetter protect the infrared thermographer underan arc-flash condition during scheduled, peri-odic inspection of the internal equipment.

Inspection devices may also feature lockingsecurity covers. This ensures only a trained andauthorized person can remove and completean inspection or scan. It also protects the opticmaterial from day-to-day impacts and offers

further arc-flash protection. Properly constructedcover designs are manufactured from materialsthat offer substantially similar properties as theoriginal panel wall knock-outNational ElectricCode 110.12(A).

Selecting the correct material

There are numerous materials that can trans-mit infrared radiation from low cost thin filmplastics used for home intruder alarm systems togermanium optics for military imaging.

Unfortunately, there are not many materialssuitable for the task of permanent installation

into electrical equipment due to the combined

temperatures and pressure experienced duringan arc-flash.

Typically, infrared windows are manufacturedfrom a crystal optic material that allows infra-red and visual inspection via the same product.

This material choice, if designed and imple-mented correctly, can withstand an electricarc and provide a measure of protection to thethermographer.

Conversely, thin film polymers can transmit IRin certain wavelengthsalthough actual trans-mission is poorhowever the polymer itselfcannot withstand the temperature and pressureof an electric arc and hence could become adangerous molten projectile.

The most widely used optic material is that ofcrystal. A properly coated crystal optic can:1. Maintain IR camera flexibility

2. Allow visual inspection3. Enable corona inspection4. Be arc-resistant

Understanding transmission

Infrared window transmission is a commonlymisused term when it comes to obtaining ameasurement. Since no material is 100 %transmissive, other factors come into play whenattempting to correct for the apparent error.

As any good thermographer knows fromtraining:

Reflection + Absorption + Transmission = 1

A thermographer must think of the window andthermal imager as an integrated system.

A little known fact is that the spectral range ofan infrared camera varies from one model to thenext. This is down to the individual Switch-on,Switch-off parameter of the infrared detector.For example, one longwave camera may have aworking spectral response of 8.1 m to 13.9 m;the next unit to leave the line may operate at7.9 m to 13.5 m. When this differing spectralresponse on the detector is mapped against anIR window product, the apparent transmissionchanges as a function of the detector/windowrelationship.

With a crystal window, this relationship isrelatively straightforward to understand as theroute through the optic is consistent. However,when mesh is introduced into the equation,the relationship becomes more complex still.The route through the combined polymer/meshoptic is confused and inconsistent resulting in avignette problem (a vignette effect is known tophotographers as the way a photograph fadestowards its edgesoften used for effect, it canbe an unintentional result of the optical limita-tions of the cameras lens). Even obtaining agood image consistently is a challenge when

attempting to scan through mesh.


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Crystal windows

With a crystal infrared window, the R + A + T= 1 effects are multiplied with respect to thecamera by a factor of two.

These additional signals can be simplifiedand compensated for by making some practicalassumptions based on real life applications.

The first point to understand when investigat-ing transmission is that the apparent error inthe camera will not always be a negative, i.e.the camera may read LOW or it may read HIGHdepending upon contributing factors.

Figure 9 shows an infrared camera scanningthrough a crystal window at a target. Let usassume that the emissivity of the target is 1.The camera shows a positive error: it is readinghigher than the actual target temperature.

Figure 10 shows the same infrared camera

scanning through the crystal window at thesame target. This time however, the optic tem-perature is at the ambient temperature of 30 C

The camera shows a negative error: it is read-ing lower than the actual target temperature.

At first glance, these two examples seem toindicate that the error associated with electricalinspection via a window cannot be compen-sated for: If the optic temperature is equal to or lower

than the target temperature, the camera willread low.

If the optic temperature is higher than the

target temperature, the camera will read high.However, by thinking in more detail about theapplication, we can make sensible assumptionsthat remove the latter.

Since unpowered switchgear is sitting atambient temperature, the whole of the body andinternalsincluding the window opticis alsoat ambient temperature. When the switchgearis energized, current begins to flow through theinternal conductors. This increases the tem-perature of potential targets above that of theoriginal ambient temperature, leaving the casing(and window optic) at ambient.

Since scanning de-energized equipment willnot provide meaningful results, it is safe toassume that the outer casing and hence thewindow optic is at a similar temperature orlower than that of the target.

Combined polymer and mesh windows

With a combination product such as a mesh/polymer optic, the mesh and polymer compo-nents have differing absorption and reflectioncoefficients. In this case, we can never account,in a repeatable manner, for the infrared radia-tion incident on the detector. The reflected

Figure 8. Additional signals resulting from the use of a window.

Figure 9. An infrared camera scanning through a crystal window at atarget.Target Temperature = 50 C (122 F)Ambient Temperature = 30 C (86 F)Window Optic Temperature = 60 C (140 F)Indicated Temperature = 55 C (131 F)

Figure 10. An example of negative error.Target Temperature = 50 C (122 F)Ambient Temperature = 30 C (86 F)

Window Optic Temperature = 30 C (86 F)Indicated Temperature = 45 C (113 F)

Panel

Target

Panel

Target Optic

Optic

quotient is multiplied many times over becauseof the mesh and polymer material reflectivitydifference.

Another problem with scanning through meshis the deflection angle. This term describes theinconsistent effect of the mesh with respectto reflected infrared radiation. Similar to thefaceted faces used on modern stealth aircraft,combined mesh and polymer optics deflectinfrared radiation in different paths that may ormay not be detected by an IR camera.


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These confusing effects are increased furtherwhen mesh is used on both sides of a poly-mer optic. The radiation from the target is nottransmitted in a repeatable manner throughthe IR window; instead, the radiation is in

part deflected back into the panel and in parttransmitted through the first mesh, only to beabsorbed by polymer and the remaining energydeflected again by the outer mesh.

The resulting signal registered by the cameramay allow a thermographer to locate a hotspotBUT the transmission of the optic cannot beaccounted for and modelled repeatably.

Window Standards and codes

An infrared window is first and foremost anindustrial electrical component: electrical designand testing parameters must be applied.

Within North America, an electrical componentmust be recognized by a Nationally RecognizedTesting Laboratory (NRTL) in order for that com-ponent to be acceptable to OSHA. UnderwritersLaboratories (UL) are probably the most widelyknown NRTL although there are others.

UL have standards and codes that apply todiffering electrical components and infraredwindows are no different. Some of the require-ments are material specific whilst others arefunctionality based.

For example, UL746C is a material standardthat must be adhered to if a product destinedfor use as part of electrical equipment containspolymers. If the product has no polymers usedin its construction, then UL746C does not apply.

UL50, by comparison, is a functional testthat is denoted by a type number and is har-monized to the NEMA environmental test. Inorder to obtain a simple UL50 Type 1 (Indoor)certification, a product must show that it ismanufactured from components that are non-corrosive. There are no environmental tests suchas hose-down or dust applied to type 1 compo-nents. If a component is designed for outdoorequipment, it will have a UL50 Type ratinghigher than Type 1. A typical outdoor rating is

Type 3/12. To achieve Type 3/12, a compo-nent is subjected to a series of vigorous teststo ensure that the sealing system and generalconstruction are sufficient to maintain the envi-ronmental integrity of the component when putinto service outdoors.

It is not possible to derive a North American(NEMA) type rating from European (IP) rating.Terms such as equivalent to IP65 or self-certified are often used by other manufacturerswhose product cannot pass the more stringentNorth American environmental testing.

In order to be sure that an IR Window instal-lation does not lower a panels environmentalintegrity, the original NEMA classification of thehost equipment must be matched or exceededby the NEMA/UL50 Type classification of the

component. Reputable manufacturers will pro-vide third party certification of the type rating ofthe component, to prove due diligence.

Arc-resistant windows

The strength of crystal infrared window opticshas been increased to such an extent that theycan withstand the effects of an arc fault.

With the adoption of NFPA 70E and theindustrys focus on arc-flash safety, installing aproduct that can withstand arc-flash should bea primary concern for any end-user.

Finally, some arc flash myths

It is important to separate the truth from themyth: 99.99 % of arc-flash events occur with the

cover removed.

This statement is untrue: as mentioned earlier,19 % of failures occur because of compo-nent failures with equipment during normaloperation.

A simple industry pointer would be the manu-facture of arc-resistant switchgear. If arc-flashevents only occurred with open covers, therewould be no market for arc-resistant switch-gear: it is only arc-resistant with the coversclosed.

It is imperative that an infrared window prod-uct is designed to withstand the pressure andtemperature of an arc-fault.


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2 C C f ?

Conclusions

An electrical thermographer must work closelywith live energized equipment and be aware ofthe dangers of this environment.

In addition to the obvious hazard of electrocu-tion, workers must be particularly aware of thedangers of arc flash and arc blast events. Up to77 % of all electrical injuries are caused by arcflash incidents. It is important to remember thatNFPA 70E does not protect personnel againstthe effects of arc blast.

An arc flash is an explosive discharge result-ing from a compromise of the insulationbetween two conductors or a conductor andground. The event is characterized by its hightemperature plasma and this can cause seri-ous burns and other injuries to an unprotectedworker several feet from the equipment.

NFPA 70E is the leading internationally rec-ognized safety standard for arc flash preventionand protection. The guidelines recommend athorough arc flash hazard analysis to establishthe nature and magnitude of the hazard, calcu-late the shock and flash protection boundaries,and identify the appropriate protective clothingand personal protective equipment required forLive work. Warning labels on the equipmentmust identify the hazard and summarize thisinformation.

The use of windows can limit the exposureof a thermographer to energized equipment,

reduce the hazards of both electrocution and arcflash and significantly reduce the need for bulkyPPE. It is important that inspection equipment isconstructed from suitable arc-resistant materi-als although there is currently no internationallyrecognized standard covering their manufacture.

For electrical inspections via a crystalwindow, the indicated temperature reading onthe camera will almost certainly be lower thanthat of that target. Due to the consistent mannerassociated with the error, quantitative measure-ments are possible as long as the camera andcrystal window are paired. This is generally aone-time requirement as window transmission

is consistent for each model.Conversely, a mesh/polymer optic cannot

be used for quantitative inspection because ofinconsistent transmission. This results from themesh deflection angle and the differing reflec-tivity of the mesh material and polymer opticmaterial. The thermographer may be able tocorrect for transmission in a lab using a one-time set of parameters but the poor performanceof the mesh/polymer and lack of repeatabilitymeans that the this optic solution cannot be cor-rected for in a manner that is acceptable.

When selecting an IR Window, window orport an end-user must consider the electri-cal implications of the installation and insiston third party certification to back up anymanufacturer claims. Potential mistakes such as

installing a Type 1 component into a NEMA 4panel could cost thousands of dollars in repairsdue to leakage and equipment failure.

Finally, an optic material that may melt duringan arc flash and cause contact burns could bemore hazardous to a thermographer than simplyhaving the panel open for the measurements. Atrue arc-tested optic should be used in order todemonstrate due diligence.

For more information please visit www.fluke.com/irwindows or call 1-800-760-4523.

Fluke CorporationPO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa +31 (0) 40 2675 200 orFax +31 (0) 40 2675 222In Canada (800)-36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116Web access: http://www.fluke.com

2010 Fluke Corporation.Specifications subject to change without notice.Printed in U.S.A. 9/2010 3527077C A-EN-N

Modification of this document is not permittedwithout written permission from Fluke Corporation.

Fluke.Not just infrared.Infrared you can use.

Further reading

NFPA 70-2008. National Electrical Code.

NFPA 70E-2009. Standard for Electrical Safety in the Workplace.

IEEE Standard. 1584-2002. IEEE Guide for Performing Arc-FlashHazard Calculations.

CSA Z462. Standard on Workplace Electrical Safety, CanadianStandards Association

Protecting Miners from Electrical Arcing Injury James C. Cawley,P.E., and Gerald T. Homce, P.E.; NIOSHTIC-2 No. 20032718,

National Institute for Occupational Safety and Health.EWG Engineering, LLC (Electrical Engineers), www.ewbengineer-ing.com

Documents

Fluke Arc Flash Electric Shock