This week’s Question of the Week was submitted by one of our forum members. The question is about infrared thermography (IR scanning) and focuses on the famous 40 cal/cm2 value.

Would you allow IR scanning when the calculated incident energy is greater than 40 cal/cm2 ?

• Yes
• No
• Other - comments are always welcome

Thanks Joe!

If you have a question you would like to see used as part of the “Question of the Week� send it to me at jphillips@brainfiller.com

If the incident energy is that high, the IR scanning should be through an IR window.

Depends a little on how the IR scan is being performed. It would seem IR scanning could be performed without interacting with energized and exposed conductors. I know that IR scan personnel object to arc flash safety hoods because the coloring interferes with interpreting the temperature scans, but that is not a reason to exclude from safety.

IR windows are commonly fitted where possible, but that doesn't cover every installation.

IR windows have VARIABLE transmission depending on angle...basically you can completely miss something. I've tried some examples and they just don't really do what is claimed unless they are large enough to give decent access, which none on the market are.

IR scanning by itself poses an additional arc flash hazard only if the person is within a distance where they might somehow fall into the enclosure..e.g. having a seizure or tripping over something while moving around. The significant danger is opening the door in case something is laying up against it...again, not common, or more likely pinching a cable while closing the door...again, not common, except for covers that are not round and bolted on that could fall in while removing them which is common on buswork on the back side of MCC's.

Not doing IR scanning or equivalent activities is not doing proper maintenance and increases the risk of an arcing fault. So there is a strong reason to provide guidance on safely performing this task, not to ignore it because of a perceived risk, however unlikely. We just had this discussion in a follow up meeting where equipment was NOT being properly maintained because of this very excuse.

This is where the existing task table isn't all that useful, and neither is the note where the "40 cal rule" is mentioned. Right now 70E has an entire section of required maintenance but speaks in broad terms and references 70B and NETA MTS. A lot of tasks in those documents don't have anything to do with safe functioning of the equipment. 70E should spell out the bare minimum tasks that must be done for safety reasons (to ensure that the arc flash hazard and/or likelihood values assumed) and not just paint it with broad brush strokes. Then leave it to 70B and/or NETA MTS for specifics on implementation, and those documents can cover additional steps performed for reliability/cost (PM/PdM) reasons outside of safety. I realize that we're quickly approaching the convergence of safety/maintenance here, too, which is an inherent difficulty in electrical equipment...you can't look at them separately in many cases.

Have you looked at IRISS windows? We added some 2" windows to an enclosure. IRISS came and showed us their line of windows. Granted their round windows came in 2, 3, and 4 inch diameter, but some of the custom windows are quite large. They also have some panel covers with the windows built in. It might be worth having a look at what they currently offer.

Yes. All glass has the issue that the transmission of light through it especially in the IR range changes depending on the angle that the light passes through the glass. There are several articles explaining this phenomenom from the manufacturers themselves.

The ones we are using are not crystal or glass, they are polymer which is more durable. The angle of the camera in relation to the polymer surface is not an issue. However the field of view is different dependant on whether you are holding the camera in landscape or portrait orientation. I did some research on the polymer IR windows for the manual I was writing. Of course I incorporated a lot of data and information provided by IRISS into the attached copy of the chapter on the IRISS Windows.

Would it be correct in allowing this as long as there is no incursion within Prohibited Approach boundary? Example would be opening the door on a 480V MCC bucket, performing the survey, then closing the door?

I have had a couple of cases where opening the door was met by a surprise when a bolt or other debris drops down inside when the door is open. (unexpected interaction?)

Yes, I have had such an unexpected incursion as well. 15 or 20 years ago, I was on a field service call. I was walking up to an enclosure where the door was already open and tripped into the panel, right across the 575VAC fusing.

The plant had installed some angle iron on the floor in front of the enclosure to keep fork lifts from running into the panel. The problem was that if the angle iron had ever been painted with the black and yellow safety striping, it was long gone and was the same color as the concrete floor.

I was not seriously injured, but it could have been far worse. In this case there was no arc flash - just a shock, but had i been holding something more conductive than my hands it could have ended a lot worse.

Also, there does not need to be a piece of angle iron the floor for a person to trip into an enclosure. Granted a 4" lip hiding on the floor is a lot harder to aviod tripping over than a flat surface.

My favorite is removing a bolted on cover. Of course when removing the last bolt, your knee is jammed into the cover trying to keep it in place as the last screw comes out.

Great topic and near nd dear to my heart. Larry is correct -- the CRYSTAL windows are spectral transmitters, resulting in variable transmission in the LWIR spectrum (which all the maintenance cameras are shooting in today). Two of the three manufacturers have published papers which point out that it is impossible to achieve temperature accuracy using a traditional crystal window with today's standard LWIR cameras. Furthermore, when shooting at angles through the crystal windows, transmission varies significantly.

BUT Paul is 100% CORRECT when he points out that the POLYMER optics are reliable and can deliver temperature accuracy. The Polymer used by IRISS and Exiscan (full disclosure -- Exiscan is my company) are superior transmitters in the LWIR. They also are durable, impact resistant, non-reactive with the industrial environment, stable (mechanically and transmission) over time -- in all cases superior for industrial use to crystals. The polymer windows are also larger (IRISS by 27%; Exiscan by 66%) than similar size crystal windows for greater field of view, and can be used when shooting at angles without significant transmission attenuation (unlike crystals).

Finally, the points made about the safety of the windows are unquestioned. They allow the thermographer to eliminate high-risk tasks, and permit access to enclosures that are being marked "Dangerous" and therefore are inaccessible with the new 70E stickers.

I would be happy to answer any questions on the topic.