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In the electrical distribution industry, transformer vaults play key rolesin converting primary line voltage to secondary voltage(s) for deliveryto customers. Typically crammed full of equipment and often belowground level, these bunker-like structures call for a predictiveapproach to maintenance because a failure within a vault can becostly, dangerous and bad for a utilitys business and reputation.

Inspecting electrical

vaults

For more information on Thermal Imagersgo to your local Fluke website.

Application Note

Applications forThermal Imagers

What to check?

Extreme caution is paramount forthermographers and support per-sonnel preparing to enter anelectrical vault. NFPA standard70E provides specific guidanceon procedures and the level ofpersonal protective equipmentrequired when inspecting thistype of equipment. Check forflooding, poor or hazardous air

quality, confining work clear-ances, poor lighting and anyother conditions that couldimpede work or lead to an injury(or worse). Then, before enteringthe vault, perform a preliminaryIR scan of equipment inside fromthe outside. If a potentially haz-ardous situation presents itself,do not enter the vault.

When you are sure that entryis safe and are inside, scantransformers, breakers, contacts,bus and fuse connections, fuse

clips and any other equipmentthat supports the voltage conver-sion to customer requirements orsustains the integrity of the vaultitself.

Transformers like the oil-cooled unit featured here are often found in electrical vaultsand can be quickly scanned for potential failure points.

Predictive maintenance (PdM)involves monitoring equipmentover time for conditions that indi-cate impending failure. The goalis to determine whether correc-tive action is required and, if so,take that action before theequipment fails with potentiallycatastrophic consequences.

One powerful set of toolsfor monitoring equipment inelectrical vaults are handheldthermal imagers, also known asinfrared (IR) imagers. Thermalimagers capture two-dimensionalrepresentations of the apparent

surface temperatures1 of electri-cal components and otherobjects. And since overheatingas well as abnormally cool oper-ating temperatures may signalthe degradation of an electricalcomponent, imagers provide thepredictive capabilities required todetect potential problems beforethey become costly failures.1Apparent temperature is often significantlydifferent from actual temperature; the differ-ence attributable generally to the emissivityof a materials surface. An understanding ofemissivity and the thermal characteristics ofmaterials is crucial in understanding theapparent temperatures displayed by infraredimages.

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What to look for?

For transformers, follow NFPAStandard 70B, RecommendedPractice for Electrical EquipmentMaintenance, Chapter 9: Powerand Distribution Transformers.Monitor high- and low-voltagebushing connections, coolingtubes and cooling fans andpumps. Overheated connections,comparatively cool cooling tubesand hot or cool pumps or fansindicate potential problems.

Thermography is less effectivefor pinpointing internal trans-former problems because a mal-function must generate enoughheat to be detectable on the out-side of the unit. When overheat-

ing is detected, be aware thatinternal components and con-nections are much hotter thansurface temperatures indicate.

Regarding breakers, contacts,fuse clips and buss, fuse, stuband other connections, beforeyou scan them, measure theload, so that you can properlyevaluate your measurementsagainst normal operating condi-tions. In general, look for spotsthat are hotter than other similarconnections. They signal high

resistance possibly due to loose-ness or corrosion. Caution: Inchecking transformer and otherconnections, if removing protec-tive boots or opening panels isrequired, only qualified person-nel using appropriate personalprotective equipment should per-form these tasks.

A good thermographicapproach to electrical vaultmaintenance is to create vaultinspection routes that include allthe vaults owned by your utility.

Remember, each vault is essen-tial to one or more of your cus-tomers. On a computer, save

thermal images of each vaultcomponent and track tempera-ture and associated data overtime. That way, youll havebaseline images with which to

compare later images. Doingthis will help you determine iftemperature levels are unusualand, following corrective action,help you determine if mainte-nance was successful.

What represents ared alert?

Equipment conditions that pose asafety risk should receive thehighest repair priority. Beyondthat, keep in mind that like anelectric motor, a transformer has

a minimum operating tempera-ture that represents the maximumallowable rise in temperatureabove ambient, where the speci-fied ambient is typically 40 C(It is generally acceptedthat a 10 C rise above itsmaximum rated operating tem-perature will reduce a trans-formers life by 50 percent.

Regarding other equipment,NETA (InterNational ElectricalTesting Association) guidelinessay that when the temperature

difference (T) between similarcomponents under similar loadingexceeds 15 C immediaterepairs should be performed.NETA also recommends the sameaction when the T between acomponent and ambient airexceeds 40 C .

Whats the potential costof failure?

The cost of a failed electricalvault to a utility depends uponmany factors including thenumber and types of customersaffected the proximity of the vaultto an area of high traffic or

human occupancy, whether a fireor explosion occurred andwhether there was any resultinginjury or death.

Follow-up actions

Whenever you discover a prob-lem using a thermal imager, use

the associated software to docu-ment your findings in a reportthat includes a digital photographof the equipment and a corre-sponding thermal image. Its thebest way to communicate theproblems you found and any sug-gestions for correcting them. Fol-lowing corrective action, a newthermal image can be used toassess its effectiveness and eval-uate the materials and techniquesused. With this information, youcan continuously improve your

maintenance program for electri-cal vaults.

2 Fluke Corporation Thermal Applications: Inspecting electrical vaults

Fluke CorporationPO Box 9090, Everett, WA USA 98206

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 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800) 36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116

Web access: http://www.fluke.com

2005 Fluke Corporation. All rights reserved.Printed in U.S.A. 10/2005 2542606 Pub-ID 11055-eng

Fluke. Keeping your worldup and running.

An imaging tip:

When entering an electrical vault, always leave someone at theentrance with instructions not to enter the vault under any cir-cumstances. This persons job is to keep unauthorized people asafe distance from the vault entrance and in the event of anemergency to call for help and avoid becoming a victim, too.