ieee 2014 paper - infrared windows applied in switchgear assemblies - taking another look

8/18/2019 IEEE 2014 Paper - Infrared Windows Applied in Switchgear Assemblies - Taking Another Look

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Fig. 1: The thermal image at left shows an elevated temperatureand possible loose connection at the Phase A line terminal of thecircuit breaker

Fig. 2: Thermographer conducting an IR scan of a low-voltagemotor control center unit with the unit door open.

INFRARED WINDOWS APPLIED IN SWITCHGEAR ASSEMBLIES:TAKING ANOTHER LOOK

David B. DurocherSenior Member, IEEE

Eaton Corporation26850 SW Kinsman RoadWilsonville, OR 97070 [email protected]

David LoucksSenior Member, IEEE

Eaton Corporation1000 Cherrington ParkwayCoraopolis, PA 15108

[email protected]

Abstract - Maintenance of electrical power distributionassemblies applied in industry has been critical inassuring facility uptime and reliability. One importantmetric in assuring reliability is electrical terminations ofenergized conductors. During normal energized service,terminations both at conductor bus joints and at cableterminations are subject over time to thermal expansionand contraction, ultimately resulting in loosenedconnections and excessive heat. Deterioratingterminations left unchecked will ultimately fail, resultingin electrical hazards for personnel and also costly loss ofproduction. Infrared (IR) inspection has proved to be anexcellent maintenance method used to identifyingproblems with loose electrical terminations. However, thedesign of Internal Arc Classified (IAC) switchgearassemblies to address arc-flash concerns has changedassembly designs that now limiting line of sight accessnecessary for IR inspection via windows. This paper wil ldiscuss global Standards, how they affect switchgeardesigns and application of IR windows, then presentsome alternative technologies that in some applicationsmay be more suitable.

Index Terms – Thermal Imaging, Infrared Camera, IRWindows, Arc-Resistant Switchgear, Thermal Sensors.

I. INTRODUCTION

Infrared inspection has proved to be an excellentmaintenance method used to identifying problems with looseelectrical terminations. As shown in Fig. 1, an infrared cameracan effectively survey energized systems and provide athermal image that identifies a potential problem, in this casea loose terminal at the line side of the molded case circuitbreaker at Phase A. This method can be used to pinpoint apotential problem and remedy this issue before a hot spotbecomes an equipment failure.

Many industrial manufacturing facilities have effectively

used infrared scanning of both medium-voltage and low-voltage switchgear and motor control assemblies as a meansof predictive maintenance. Thermal imaging for electricalsystems will be performed on a rotational basis for the entirefacility and maintenance outages are scheduled on an as-needed basis only after an electrical problem has beenidentified. Some industry standards including IEEE Standard1458 “Recommended Practice for the Selection, Field Testing,and Life Expectancy of Molded Case Circuit Breakers forIndustrial Applications” [1] actually recommends the use ofinfrared scanning as a means to detect issues such as the

one illustrated above. Fig. 2 shows a plant thermographerconducting a routine thermal scan using a hand-held camera,in this case inspecting a low-voltage motor control center unitwith the door open. This method although effective, canpresent some risks to the person conducting the infrared (IR)scan. The emerging understanding of arc-flash hazards ofenergized electrical systems has changed the waymaintenance personnel perform IR scanning of electrical

systems. New globally accepted and applied standards suchas IEEE Standard 1584 “Guide for Performing Arc-FlashHazard Calculations” [2] now have clearly defined the hazardsassociated with working on or around energized conductors.

Application of this Standard includes calculations thatdetermine the magnitude of incident or potential heat energyshould an arc flash event occur.

PRESENTED AT THE 2014 IEEE IAS ANNUAL MEETING – MINING INDUSTRY COMMITTEE VANCOUVER, BC: © IEEE 2014 - PERSONAL USE OF THIS MATERIAL IS PERMITTED


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Fig. 5: IR window failure occurring during an arc test of a MV motorcontrol center incoming line compartment.

assembly is designed to direct the arc-flash blast energy fromthe compartment where it is initiated up into the arc plenummounted at the top of the enclosure, above the height ofpersons near the assembly. With this arc rated design, thereare really no reasonable places that an IR window could bemounted in order to inspect live conductors. Perhaps an IRwindow could be installed in the lower front of this section to

inspect the outgoing cable terminations, but in this case, theoptional VT drawer restricts the line of sight necessary forinspection with an IR camera leaving no practical way to scanthese load cable terminations. A window could also bemounted at the lower rear for visible access to cableterminations, but this assembly is designed to be mountedagainst the wall. There also is no reasonable place to locateIR windows for visible access to the breaker or main buscompartments. The low-voltage assemblies are similar as arethe IEEE/ANSI assemblies applied across North America andarc containment tested to IEEE/ANSI C37.20.7.

Another concern is the integrity of the IR window mountedin an arc rated switchgear assembly. In order to offerswitchgear assemblies that are certified and tested to arcrated standard, the manufacturer is required to conduct an

arsenal of tests that assure an arc flash event will besuccessfully redirected away from the enclosure front, rearand sides and channeled to the top-mounted plenum. Thesetests include initiating an arc flash event in each compartment,resulting in extreme temperatures and pressures. The teststandard requires that pressure and heat from the arc bechanneled 2 meters above floor level, up and out the top ofthe enclosure, away from persons standing around theperimeter of the assembly. If IR windows are to be applied inthe assembly, it is incumbent on the switchgear manufacturerto conduct arc tests with these mounted on the assemblyduring the tests. Therefore, it is important when specifying IRwindows in switchgear assemblies that the user verifies arctesting has been performed with IR windows installed.Reputable switchgear manufacturers will publish IR window

make and model numbers that have successfully passed typetesting. Fig. 5 shows the results on one arc test where thecrystal optic failed after an arc was initiated in the panel. Inthis case, the protective cover was closed during the test, butthe crystal was shattered due to the extreme pressure andheat.

One additional consideration is concerning the protectivecover for the IR window. The cover is designed to protect thecrystal optics or polymer/mesh element and is closed andsecured during the switchgear assembly arc testing. Ofcourse, thermal inspection with and infrared camera requiresthat the cover be removed or opened. Since the arc testing isonly conducted with the cover installed, there is really no way

to verify that the integrity of the IR window crystal orpolymer/mesh will survive an arc event with the coverremoved. Following completion of an arc flash hazardassessment, what would the calculated incident energy be atthis electrical point in the system with the cover open orremoved? What personal protection equipment would thethermographer wear during inspection with the camera toassure safety from an arc flash event?

While taking a closer look at IR window application, let’salso consider some issues around application of IR windowsin existing panels. Here, the user must also carefully considersome technical areas of concern. IR windows installed as aretrofit in an existing arc classified or arc tested panel haveobviously not been subjected to the testing criteria defined bythe standards for that existing panel. Users can install IR

windows in existing equipment that will function well as apredictive diagnostic tool for thermal imaging; however thiswill likely compromise the arc rating of the assembly. In somecases, IR window manufacturers will offer test data to provethe component has “survived” and arc flash event as definedby the test standard. This must be validated to show testingwas performed in the specific panel and mounting locationwhere it is proposed to be installed. It is often impossible toprove such testing, unless the IR window supplier has workeddirecting with the switchgear manufacturer when the definedbattery of tests was performed. For assemblies that are notarc resistant or internal arc classified, there is generally not anapplication problem with IR windows. Test requirements forthese assemblies focus on interrupting and withstand ratingsthat are typically not associated with the enclosure structural

integrity during the interruption of an arc fault.

IV. ALTERNATIVE TECHNOLOGIES IN THERMAL IMAGING

Most certainly, IR windows offer unique advantages inthermal imaging. In an environment where there is increasedknowledge and awareness of arc flash hazards, IR windowsoffer a true advantage in assuring a thermographer is notexposed to energized conductors while conducting a thermalsurvey. That said, there are some applications where IRwindows have limitations and alternatives should beconsidered.

Most low-voltage motor control center assemblies includemultiple starter modules in each panel. The individualmodules typically include an incoming overcurrent protective

device (fuse or circuit breaker), a motor contactor andprotective relay, a control power transformer and other variousother electrical components. Each module is separated fromthe other by steel barriers and includes a front mountedhinged door. Referring to Fig. 2 as an example, a thermalsurvey of this subassembly requires inspection of line andload power terminations at up to fifteen electrical points (3-phases of the protective device and motor starter line & load,plus 3-phases a the motor load terminals). Although the fieldof view for IR windows would allow for inspection of multiple


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Fig. 6: An infrared non-conductive, non-contact sensor can bemounted inside a switchgear panel offering 24 X 7 thermalmonitoring.

Fig. 7: A sensor with a millivolt output is used for temperaturedetection of cable terminations.

terminations using a single window, several windows wouldbe needed for each starter module. It is easy to see due toboth space and cost that choosing an IR window here wouldnot be practical. Most industrial users conduct this type ofinspection with the unit door open. Depending on theconnected system, maintenance personnel may need to wearappropriate PPE while opening the unit door with the circuit

energized. However, thermal imaging can be safely performedwithout PPE if the thermographer is some distance way,outside of a zone referred to as the arc flash protectionboundary. In this application, the author recommendsadditional protection through the use of an upstream incomingcircuit breaker with an arc flash reduction maintenance setting[8], similar to would be applied when electricians areperforming energized work such as testing or troubleshooting.

Arc rated low-voltage and medium-voltage switchgearassemblies also present a challenge for application of IRwindows as discussed in previous sections. The balance ofthis paper will focus on three alternative thermal measurementtechnologies which may be better suited, in light of theselimitations.

A. Thermal Monitoring via Infrared Sensors

One technology that looks promising which addressessome of the IR window issues for arc rated assembliesdiscussed previously is a miniature infrared thermal sensor asshown in Fig. 6. Note that this device mounts inside theswitchgear assembly and can be positioned within the variouscompartments, eliminating the line of sight requirementnecessary for an infrared camera inspection through an IRwindow. The sensor can be focused on bus joints andterminations that are likely to present the most problems. Acompanion thermal monitoring device shown in Fig. 7 isdesigned specifically for sensing of cable temperature forterminations at terminals, bus bars or circuit breakers. Bothdevices are self-powered and produce a millivolt outputproportional to temperature rise over ambient. The devicetwisted-pair cable millivolt output signal connects directly to adata card which converts the millivolt signal to Modbus RS485where the information can be connected to a network. Real-time temperature data can then be monitored from a localpersonal computer using the manufacturer’s standardsoftware or connected to a host supervisory system such as aSCADA or DCS. With the advent of 24 X 7 X 365 monitoringin lieu of periodic scheduled inspection via an infraredcamera, temperature data can be trended and system alarms

can be set should temperatures exceed pre-set critical levels.These miniature infrared thermal sensors are commercially

available and typically are offered at a slightly higher pricepoint than their IR window counterparts. Because the IRsensor component has a very narrow field of view, one sensoris required for each bus connection that is to be thermallyinterrogated. So, unlike IR windows where one device can

often be mounted on an exterior panel in a location wheremultiple phases are in the field of view, a single IR sensordevice is required for each phase conductor. All the same,there are advantages when applied to interrogate temperatureof bus joints that are not easily accessible using an externalcamera to record a thermal image via and IR window. The IRsensor can be mounted virtually anywhere in the assemblyand unlike IR windows, it does not require mounting on anexternal cover so testing as a part of the assembly arccertification is not an issue (albeit the device would likely bedestroyed during an arc event that occurred in closeproximity).

There are some challenges when applying the IR sensoror cable sensor millivolt devices in existing panels. Aspreviously discussed, assemblies designed and tested to arcclassified standards have compartmentalized sections, soadding sensors to areas such as the main bus compartmentwould be fairly difficult. Installation in new switchgear is abetter application; understand again that due to limitedaccess, maintenance or replacement of a failed sensor mayprove difficult.

B. Thermal Monitoring via Piezoelectric Acoustic Sensor

Similar to the infrared sensor used to measure the bustemperature, another sensor product that makes contact withthe energized bus in switchgear assemblies is also in stagesof early development. This sensor detects loose bus jointsand loose connections via measuring an acoustic “signature”which occurs as microscopic particles of the bus conductor


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melt and then cool. Thermal cycling of molten metal delivers areliable and repeatable acoustic signal that can be measuredand continuously monitored. A relatively small piezo acousticsensor as shown in Fig. 8 can be connected directly to thebus conductor and an Event Time Correlation (ETC) algorithmis used to confirm a bus overheated condition. The non-metallic sensor makes contact with the energized bus at a

single location; however the measured acoustic signal is ableto travel for some distance away from the sensor. Thisdistance is typically from 5 to 10 feet, dependent on theconductor geometry. At the time of installation a calibrationdevice connected to the switchgear bus determines therequired sensor spacing. The on-board electronics in thissensor derives power parasitically from the energized busbased on continuous currents in the range of 100 amperesthrough 5000 amperes.

The sensors operate independent of bus voltage and canbe applied on any type of bus material including epoxyinsulated for both new and existing switchgear installations. Awireless transmitter in each of the sensors communicatesstatus to a central receiver so temperatures across an entireswitchgear assembly line-up can be trended over time via anystandard industrial network, most typically ModbusTransmission Control Protocol/Internet Protocol (ModbusTCP/IP) or Ethernet Internet Protocol (Ethernet IP). Thepiezoelectric acoustic sensor would perhaps be a betterchoice for retrofit in existing switchgear assemblies than theinfrared sensor discussed previously. An industrial facilitycould schedule installation of the sensors during a scheduledrotational outage when the bus covers were removed while

the main us bars are inspected. At this time, the wirelessreceiver would also be installed and communications witheach device established before replacing the bus covers.Similar to the infrared non-conductive, non-contact sensor, theadvantage using this technology versus IR windows is thatreal-time bus temperature can be monitored and trended overtime, offering a continuous predictive method the identifypossible hot spot failures before they occur. The non-metallicsensor is connected or strapped directly to the energizedconductor as shown in Fig. 9

C. Thermal Monitoring via Conductor Resistance

Recently another thermal detection solution [9] wasrevealed that also looks to be an alternative to infraredthermal imaging. This approach continuously measures theresistance of the conductors using normal load current. In

effect this method converts the conductor itself into an RTDsensor. Using voltage, current and phase angle datacollected from a group of metering devices, the per-phaseresistance between each metered point is calculated. When atermination or junction (e.g. shipping split splice) degrades, adetectably larger voltage drop develops across that junction.This appears as an increase in that conductor’s point-to-pointresistance. By trending this calculated per-phase resistanceand normalizing to current (since the balance of the conductoritself changes with temperature as a result of currentchanges), anomalous deviations in phase conductorresistance are detected.

Similar to a differential relay protection scheme wheremultiple protective devices are sensing voltage and current toprotect the distribution system, this method uses metering and

other devices such as solid state motor overload protectivedevices to measure temperature via impedance. Use of atypical 1.0% accuracy class meter offering time stampedvoltage, current and phase angle or power factor allows real-time measurement of conductor temperature. It is important tonote that application of this approach will most typicallyinclude an energy management system and accompanyingsoftware to perform the calculations and deliver reliabletemperature trending. With the advent of lower cost electronicmetering and a drive toward energy management includingsub-metering for downstream loads, many new industrial

Fig. 8: Piezoelectric Acoustic Sensor with mounting strap and acousticcoupler.

Acoustic Coupler

Fig. 9: Acoustic sensor attached to energized bus bar in low-voltagemetal-enclosed switchgear.


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VI. REFEREN

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Pgs 688-694.

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ieee 2014 paper - infrared windows applied in switchgear assemblies - taking another look

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