It can be a frustrating question if you're a plant manager worried about electrical hardware. Today's higher equipment use rates and lower staffing levels at many plants call for aggressive preventive maintenance on electrical panels.
Until now, however, standard methods for safely IR scanning electrical panels and cabinets to find hot spots and pinpoint future failures have not been appealing. Removing or opening electrical panels always poses a risk when high energy levels are involved. Human error, equipment failure, and other factors can quickly turn stable conditions in a cabinet into a dangerous environment for the inspector and anyone nearby.
Closed-door inspections are safer and a better use of manpower. They let thermographers work without electricians and with much-less protective clothing. But these inspections traditionally require cabinets with costly and fragile IR-transparent windows (or sight glasses), which let a small portion of the cabinet interior be viewed through a camera. Metal-mesh ports are also used.
IR sight glasses of crystalline materials screen out much of the IR light and are typically made of calcium fluoride, germanium, sodium fluoride, or zinc selenide, which are expensive. Just a few square inches cost hundreds or thousands of dollars. Worse, they're extremely fragile and prone to vandalism by disgruntled employees. Most are also highly susceptible to scratching and damage from UV light and corrosive fumes, and require an overlay or coating to protect against breakage and degradation. Unlike glass or acrylic, IR windows shatter easily under pressure. The most common material, calcium fluoride, is also hygroscopic and has a reputation for dissolving in humid environments, leaving gaping holes in cabinet doors.
IR windows and metal screens add an unpredictable amount of IR attenuation or transmission loss, so temperature measurements taken by a thermographer cannot be considered accurate. And metal screens also break and can fall into the cabinet, creating a hazard of their own.
IR windows, even with wide-angle lenses, restrict thermographers to seeing only a portion of the cabinet interior. It can take four or more sight glasses (at a cost of about $200 to 300 apiece) to scan the entire inside of a panel. Sight glasses also mount in relatively large holes in cabinet doors, a safety hazard if the window breaks.
An alternative has recently been developed for closed-door inspections that requires no IRtransparent or metal-mesh windows. It lets a thermographer scan a panel by looking through a 0.62-in. opening in the door. This new system, SpyGlass, combines a viewport and a mating fisheye lens.
This combination of windowfree port and fisheye lens makes company-wide standardization feasible and affordable because the cost comes down as the number of ports ordered goes up. (Prices start at about $50.) Industry leaders such as GE, Exxon Mobil, Boeing, DuPont, and Cargill have installed thousands of these UL-approved ports.
Spyglass lets thermographers inspect energized electrical switch gear without opening the panel. Therefore panels maintain original safety ratings. The device works on cabinets energized with 480 to 13,200 V, as well as motor junction boxes with 4,160 V.
The viewport uses only a 0.62-in. aperture covered by a screw-on cap to maintain cabinet integrity and safety rating when not in use. An optional lock for the cap prevents unauthorized access to the cabinet. The viewport is unaffected by moisture, dirt, UV, and corrosive environments, and it never needs cleaning or replacement.
The viewport mates with the coneshaped tip of a special lens barrel for an IR camera. The plastic-tipped barrel prevents the lens from touching the port or cabinet, and provides a layer of insulation to eliminate a path-to-ground hazard. By contrast, the metal rim of a camera lens touching an IR window might easily damage the window.
In addition to the lens' wide field of view, its tip fits into the viewport like a ball-and-socket joint, letting the camera swivel to look up, down, and to the sides, as well as straight ahead.
The fisheye lens in the viewport has a relatively wide field of view (53° horizontal by 40° vertical, or 66° diagonal). It provides temperature measurement accuracies of ±3°C. The fisheye lens, with a focus range of 4 in. to infinity. (10 cm to infinity) and large depth of field reduces the need to refocus for electrical cabinets with different depths.
The viewport is UL approved for installation at the OEM level or as a retrofit in the field on NEMA-Type 1, 2, 3, 3R, 4, 5, 12, 12K, and 13 enclosures. It is suitable for indoor and outdoor cabinets, as well as vertically and horizontally mounted cabinets.
ELECTRICAL HIGH ENERGY:
The dragon behind the door
The possibility of an arc flash from high-energy electricity makes inspection of motor-control centers and electrical panels a dangerous task. And according to OSHA, 5 to 10 arc-flash incidents are reported daily in the U.S. So technicians should never, under any circumstances, perform energized inspections unless someone has conducted an arc-flash assessment, inspectors are wearing proper protective equipment, and appropriate precautions have been taken. For example, arc-flash boundaries should be calculated according to NFPA 70E or other industry reference to determine what kind of protective gear should be worn.
New technology, such as the SpyGlass, permit inspection with cabinet doors closed. But if you must open the doors, here are a few things you should know:
- "High energy" does not mean high voltage. Most arc flashes stem from stored energy and can be encountered in 480-V motor-control centers, which are considered low voltage. Typically, an arc flash begins when one of the three phases faults to ground, and then it propagates phase to phase. The amount of energy unleashed depends on whether the line is switched or fused, the number and size of the fuses, length of cable from the substation, and several other factors.
- Arc-flash temperatures can reach 35,000°F, hotter than the surface of the sun, for up to a second and be accompanied by a deafening blast (160 dBA) and pressure wave. It can easily cause third-degree burns and other trauma. And this does not take into account lasting neurological damage and personality changes caused by an intense magnetic field passing through the body.
- Arc-flash intensities are rated in calories per square centimeter. For working purposes, one cal/cm2, is roughly equal to the burn you'd get sticking a finger into a butane lighter flame for 1 sec. Arc flashes today range from 20 to 200 cal/cm2. Thus, safety glasses and a hard hat are hardly adequate.
- The basic level of protective clothing starts with gloves, shirt, and slacks of 6-oz/yd Nomex (a Du-Pont product). Lab coats may not properly protect the lower extremities. No other synthetic-fiber clothing such as polyester, rayon, or nylon, should be worn. It can melt and fuse to the skin in an arc flash. Two layers of 6-oz Nomex provide about 20 cal of protection. A typical green faceshield provides about 8 cal of protection and keeps out harmful UV and infrared radiation. A safer alternative, the Nomex beekeeper's hood and face shield, provides 31 cal of protection. And there are suits that give wearers up to 100 cal of shielding. This type of clothing can be hot and tiresome to work in, so it's critical to have a good inspection plan. And never hesitate to simply back off, cool off, and take a few deep breaths.
- It is strongly recommended that arc-flash assessments be made for all high-energy equipment, per NFPA 70E. Equipment should be labeled with proper protective clothing requirements and other precautions.
- At substation levels with 69,000 to 115,000 incoming volts, no thermographer should be inside a barrier fence unless certified and qualified for the task. All inspections should be done from outside the fence using zoom lenses, where protective clothing is not required. Even after power is stepped down to 11,500 V, the thermographer should never open and inspect switchgear panels if covers are bolted on. But if the covers are hinged and arc-flash assessment has been performed, it may be possible to inspect when wearing the proper clothing and taking all specified precautions.
- When inspecting motor-control centers, avoid opening doors on any cabinets behind you. Also minimize the number of doors open at one time. If working in an area where nonelectrical personnel have access, the arc-flash boundary must be barricaded. And unless you're a qualified electrician, never point at or touch anything in an open electrical cabinet. If anything unexpected comes up, back off and replan. Better yet, update your enclosures and thermography tools so that you rarely have to perform "open-door" inspections.