Infrared inspection windows let a thermographer scan energised switchgear without opening the panel, which removes one of the biggest risks in the job. Around them has grown a persistent misconception about what "arc rating" means for the window itself. This article explains why IR windows exist, what the arc-rating question really is, and what you should actually be checking when you specify one for electrical switchgear.
Why infrared windows exist
To take a useful thermal image of a live connection inside switchgear, the infrared camera needs a clear line of sight to the target. Traditionally that meant removing covers to expose energised parts, which puts the person doing the survey directly in front of an open panel: exactly the situation an arc flash event is most dangerous.
An infrared window is a permanently installed port, fitted with an optic that is transparent to infrared, mounted in the enclosure. It lets the camera see the internal components through the closed panel. The survey is carried out with the door shut, so the thermographer is not exposed to live parts and the arc flash risk is dramatically reduced. That closed-panel working is the whole point of the technology.
The misconception about "arc rating"
The common misunderstanding is to treat an IR window like a piece of arc-rated PPE, expecting it to carry a cal/cm² rating that describes how much incident energy it can withstand. That is not the right way to think about it. A window is not clothing; it is part of the enclosure. Applying a PPE-style arc rating to it confuses two different things.
The meaningful question is not "what is the window's arc rating" in the PPE sense, but "does this window preserve the integrity of the switchgear it is installed in". A window is only as useful as its ability to stay in place and maintain the enclosure's performance during a fault. That shifts the focus from a single headline number to how the window behaves as an installed component.
What actually matters: enclosure integrity
When you fit an IR window into a panel, you are putting an opening into an enclosure that was designed to contain what happens inside it. The right concern is whether the window maintains that containment. In switchgear rated to withstand or contain an internal arcing fault, an added window should not become the weak point that fails first.
In practice this means looking at how the window is constructed and secured, how it is installed, and whether it has been assessed for use in the type of equipment you are fitting it to. A window that is robustly mounted and keeps the panel's protective function intact is doing its job; a poorly fitted or fragile port undermines the very enclosure it sits in. The safety benefit comes from keeping the door closed, so the window must not compromise the door's purpose.
Crystal versus polymer optics
Infrared windows use one of two broad approaches to the optic, and the choice is often where the arc-rating debate really sits.
|
Optic type |
Characteristics |
Considerations |
|
Crystal (for example fluoride-based) |
Solid optic transmitting a wide infrared band, mechanically rigid |
Durable optic; performance depends on the frame and mounting |
|
Polymer / reinforced |
Grille-backed or reinforced polymer viewing area |
Robust construction; check transmission and field of view for your camera |
Both approaches can work. What matters is that the complete window assembly, the optic, its frame and its fixings, is fit for the electrical environment and installed correctly. Judge the window as a whole component, not just the material of the lens. If you are unsure how a window will behave in your equipment, seek the manufacturer's guidance for that specific switchgear type rather than relying on a single rating figure.
Choosing and using infrared windows
When specifying IR windows, match the optic's field of view and infrared transmission to your camera and targets, so you can actually measure the components you need to see. Confirm the window suits the enclosure and is installed by a competent person, with the equipment isolated during fitting. And remember that the window only delivers its safety benefit if the survey is genuinely done with the panel closed.
Used well, IR windows make routine thermal inspection safer and quicker, which is why they have become standard on critical switchgear. To get reliable results, pair them with a properly configured camera and a thermographer who understands terms like emissivity and reflected temperature, covered in our guide to infrared thermal imaging terminology.
FAQs
Do infrared windows need an arc rating like PPE?
Not in the cal/cm² sense used for protective clothing. A window is part of the enclosure, so the real question is whether it maintains the switchgear's integrity during a fault. Assess the complete window assembly and its suitability for your specific equipment, not a PPE-style number.
How do IR windows reduce arc flash risk?
They let you carry out the thermal survey with the panel closed. The thermographer never opens the enclosure or stands in front of exposed live parts, which is where arc flash exposure is greatest. Closed-panel inspection is the core safety benefit.
What optic material is best?
Both crystal and reinforced polymer designs are used successfully. The better choice is the complete assembly that suits your enclosure, transmits the infrared band your camera needs, and is installed correctly, rather than any one material in isolation.
If you are planning safer thermal inspections of your switchgear, talk to us about infrared windows, the right camera, and our thermal imaging survey service.