There is a lot of misinformation out there regarding infrared (IR) windows. End-users are being fed lots of marketing spin about Arc Resistance, IP ratings, environmental stability, etc. all of which on the face of it sound plausible but it’s more smoke and mirrors than fact.
We need to stop all of this misinformation and deal with the facts and use our own common sense to make decisions regarding IR windows. Below is a narrative from a meeting I recently had with an electrical systems engineer regarding these exact same issues. Unfortunately, this is a conversation I have 10 times a week.
I recently met with an engineer who was convinced that a crystal IR window was “Arc Resistant”. Why? It’s simple, because the manufacturer said it was! I asked this very experienced engineer if he thought that the steel plate that we manufacture the electrical enclosures from was “Arc Resistant”.
He scoffed at me and said, “Of course not, that’s why we have different designs of switchgear for different tasks.”
So we now had a common agreement on the fact that steel was not arc resistant, my next question was, “Do you think that the crystal IR window that I have in my hand is stronger than steel?”
He looked at me and the lights went on. “Well of course not,” he said!
My response was simple and very straight forward. “Then how can this crystal be “Arc Resistant” if steel isn’t?”
He relented and said that it couldn’t be and the claims being made by the manufacturer were misleading.
Once we agreed that there was no such thing as “Arc Resistant” IR Windows we started to discuss what standards were actually applicable to IR Windows. We agreed that completing an “Arc Containment” test on switchgear is required to confirm that the design meets the IEEE and IEC standards and if IR windows were installed in the piece of equipment that was being tested then, if successful, an IR window manufacturer could make claim that the IR window met the IEEE or IEC “Arc Containment” requirements for that particular piece of equipment for that particular test. They couldn’t make claims of all encompassing “Arc Resistance” for the IR Window itself, as this would be incorrect and misleading.
Our discussion continued on the subject of relevant IR window certifications and I pointed out that whilst there were regulations from UL and CSA on IR Windows fitted in Electrical enclosures up to 600 Volts, there are no specific regulations from IEEE or IEC on IR Windows. There are, however, IEEE regulations on “Visual Viewing Panes” that have been in place since the inception of these standards. Like all standards, these standards have evolved with industries ability to provide superior, less expensive materials and manufacturing options. From the initial use of steel wire reinforced glass to the common polymeric solutions such as Lexan, Plexiglas and Makralon you now see installed in all switchgear.
A common theme for all “Visual Viewing Pane” testing is Impact and Load Testing. This requires the viewing pane to meet a minimum impact and load test without “cracking, shattering or dislodging” from its housing. Sounds simple doesn’t it? So what’s the problem? Why don’t we just insist that all IR and Visual Windows meet this minimum impact test requirement? Well firstly the IEEE test is required for visual viewing panes mounted in medium and high voltage equipment (>600 volts to 38kv metal clad and 72kv station type gear).
The specific regulation requires visual viewing panes to withstand impact and load per IEEE C37.20.2 Section a.3.6. Unlike UL this IEEE standard does not differentiate between the types of material or give exemptions to crystals infrared windows. Instead it clearly specifies that any transparent material covering an observation opening and forming a part of the enclosure should be reliably secured in such a manner that it cannot be readily displaced in service and not shatter, crack, or become dislodged when both sides of the viewing panes in turn are subjected to impact and load.
This testing method has been in place for many years and is the accepted testing method for visual viewing panes so why not use the same test for IR windows? Well, the fact is that the fluoride based crystal IR windows cannot pass any form of impact, so the crystal IR window manufacturers tend to ignore this regulation and instead lean more towards the UL regulation UL1558 for Impact and Load Testing.
UL 1558 is the impact and load standard for visual viewing and IR window testing. This test is identical to the IEEE C37.20.2 Section a.3.6 test except they doubled the load and impact test.
On the face of it, this testing sounds perfect, but unfortunately, unlike the IEEE test, UL1558 has two different test criteria: one for covers fitted and closed on the IR Window and one for covers opened or removed from the IR Window. When provided with a cover, results are considered to be acceptable if the assembly prevents insertion of a 13 mm (0.50 in.) diameter rod at the conclusion of the test. When no cover is provided for the viewing pane, the results are considered acceptable if the view pane does not shatter, crack or become dislodged (as with IEEE test).
Testing IR viewing windows with the metal cover in place is a complete mockery of the intent of the standard. The crystal IR window lens will shatter during the test which renders the IR window “Electrically Unsafe” because it fails the IP20 requirement which stipulates the largest hole size you can have in your cabinet is ½ inch (13mm). However, the window passes the test due to the fact that you cannot pass a steel rod through the metal or plastic cover, which is ridiculous! If this standard is to be used to certify your IR Window, then It is important to demand that the UL1558 testing be completed with the “covers open” and that it meets the minimum test requirement by not shattering, cracking or dislodging from during or after the test.
At the end of my meeting, the engineer’s perception of the minimum test requirements had turned 180 degrees from where we started. We agreed that there really is no such thing as an “Arc Resistant” IR Window and that there really is NO place for marketing spin when dealing with electrical safety. When it comes to IR windows we need to be more concerned with the mechanical stability of infrared inspection windows as a component within the switchgear assembly and that it meets the minimum requirements for impact and load testing.
IRISS has developed 7 questions that you need to ask before you buy an IR Window”.