Our service technicians occasionally must enter live control enclosures to troubleshoot the logic. The control enclosures are primarily 120 VAC control circuits but sometimes they contain a couple of 480 VAC motor starters ranging from 5 to 100 HP. These control panels are installed in plants that have not conducted arc flash studies so the risks are unknown. On several occasions we have been successful locating and identifying the upstream overcurrent protection as current limiting so we are reasonably confident that the arc flash incident energy is quite low as long as the Ibf is at least 10 times the overcurrent device's rating. What concerns me is the source having a high output impedance so the Ibf is significantly less than 10 times the overcurrent device’s rating so the clearing time gets ridiculously long and the incident energy climbs to dangerous.

Is there any way to safely proceed with troubleshooting this equipment without conducting a full SSC analysis or actually measuring the output impedance of the source?

Yes. Follow the table based method in 70E-2015. Your case is exactly what it's for (when you don't have an engineering study to work from). Here are two tasks from the table to consider:

"Work on control circuits with exposed energized electrical conductors and circuit parts, 120 V or below, without any other exposed energized equipment over 120 V including opening of hinged covers to gain access". No PPE required.
"Normal operation of a circuit breaker, fused switch, contactor, or starter" (normal = properly designed, installed, and maintained). No PPE required.

Thank you for the response. My post must not be clear. The 480 volt sections of the control enclosure are inches above the 120 V sections and have exposed live contacts. There is no barrier or guard between the 480 V motor starters and the 120 V relays and timers in the logic circuits - they are all in the same 36 x 48 x 12" box.

Might want to consider de-energizing and adding barriers to isolate the 120V.

Otherwise, I believe you are either going to have to use the tables (if they apply) or do a study.

This sounds like a typical industrial control panel. If it's above then there's no chance of falling tools. That being the case, it may be exposed but you could consider whether or not you are within the restricted approach boundary which is 12", but that doesn't sound like your case either.

The fastest, easiest way to deal with these if you can't "install" equipment is to get a roll of rubber blanket. This stuff is very common in utility work and you can also get nonconductive "clothes pins". It comes in two varieties. One is a roll that does NOT require retesting unlike gloves. The other is a true "blanket" and has a retesting requirement. Buy the roll. Cut off what you need with scissors and cover the exposed equipment. Problem solved.

This is common practice in utilities. They call it "cover up". They use blankets and rubber line hose routinely. It works very easily and solves the problem. If it's no longer exposed, you've solved your problem. On a permanent basis you can install lexan or glastic covers and similar barriers to do the trick but on a temporary basis cover up works the best.

Dear PaulEngr,
Thank you again for your quick response but your response has now left me quite confused. There are posts on this forum and on NFPA Xchange that claim what you proposed is not a solution. Those posts assert that since the aftermarket shields and guards are not tested, approved, or listed they do not meet the requirements for arc flash protection. They further state that the arc blast could come around the shield or through the shield. I have seen the arc blast videos and they are quite impressive so I too am skeptical of an aftermarket shield but I have no personal experience, thankfully.

Is there a reference or standard that you can point me to that would support your proposal? Your help is greatly appreciated.

Several different issues here. First let's identify the different between a rubber insulating blanket and an arc flash blanket. The former is an old product. It has been around for about as long as rubber gloves. The approach is to insulate and protect against creating an arc in the first place. There is something to be said for it's arc flash protective abilities (performs as well as rubber gloves) but that's not really the intended purpose.

In contrast, an arc flash blanket is a relatively new product. Conceptually if you go from the idea that an insulating blanket is similar to rubber gloves and sleeves, the arc flash blanket is similar in design and construction to a multilayer arc flash suit. In this case it does not do anything in terms of preventing an arc flash in the first place, but if it is properly installed it will protect against an arc flash event. I didn't even mention these. Like you said...the critical point with these devices is the installation.

As to being tested, approved, and listed, no arc rated PPE is approved or "listed" either. The clothing at least is not tested either. The textile itself is tested using ASTM 1959 or similar standards. Several pieces of the textile are placed over copper calorimeters that are oriented perpendicular to an arc source and "tested" to a known arc. The data is collected and analyzed on a pass/fail basis on whether the calorimeter registered a value above or below the Stoll curve, and then curve fit to a sigmoidal shaped curve. The 50% crossing point is reported as the ATPV. All testing is done at a single lab (Kinetrics). There is no registration agency such as UL, TuV, ETL, CSA, etc., so there is no "Listing" of a product. And as for approval since there is no AHJ other than the purchaser, there is no "approval" process either.

It is my understanding that the arc flash blanket manufacturers are either working on or have an ASTM-style testing process in place and that they have engineering and design procedures available for customers to use their products. The issue of "blowing by" is a real one and the reason that even if you use these products, the arc rated PPE is still required so this does not provide any real "out". Plus if you are standing behind an opaque blanket how do you do any work in the first place? If you are looking/reaching around the blanket and an arc flash incident occurs in a matter of fractions of a second, how in the world are you going to be protected? These things as far as I know are only used in vaults and manholes where the incident energy is reflected off all the surfaces and is prohibitively high...beyond any available PPE.

Rubber blankets are very common. See IEEE 516 for a standard that references using it as an obvious example since 70E as well as OSHA use this as the reference for their own shock protection standards. They are tested essentially under the same standards as rubber gloves and sleeves. The standards are ASTM F2320, ASTM F1742, ASTM D1048, ASTM D1049, ASTM D178, ASTM F478. Depending on what standard the product falls under (how the product is defined) testing is required either once per year as it is with rubber gloves or when damage is evident.

The "work" standard in this case is the definition of "exposed". If the equipment is insulated, it is not exposed. Thus shock protection for the 480 V equipment vanishes and only shock protection for the 120 V side needs to be considered, which decreases it to a "don't touch" standard instead of a 12" restricted approach boundary. In terms of arc flash it depends on wehther or not the employee is interacting with the equipment in such a way that an electric arc could be produced. Based on your description, you're not interacting at all with the 480 V equipment except that you could brush up against it during the course of troubleshooting the 120 V side. According to NFPA 70E's table based method once you've ensured that none of the 480 V equipment is exposed, the 120 V control circuitry can be worked on without arc flash PPE. Obviously the process of installing the rubber blanket itself would still be working on and around exposed, energized 480 V equipment so up until that point the arc flash PPE would be required. The same problem occurs working on an overhead power line....until cover up is applied energized lines are in the working space and they are a potential shock and arc flash hazard.