These switches are in an electrical room at work. Not sure if they're in service or not. What do the warnings mean? Can't turn switch?
I perform power distribution analysis every day as an EE for an industrial manufacturing company - my work includes estimating Arc Flash Hazard as part of this process.
Simply put, arc flash hazard is the "other" electrical hazard. Traditionally, electrical work has involved shock hazard - touching energized conductors, with resultant current through the body from impedance/resistance of skin/tissue across the system voltage (or from voltage to ground). Arcing faults also will emit incident heat energy that will burn electricians. The heat of an arc is generated by the ionized plasma that conducts the fault current while the fault is arcing. This plasma is about 36,000 degrees F - hotter than any thing on earth; about 4 times the temperature of the sun.
The heat energy can be estimated/calculated in units of calories per centimeter-squared - sort of a "heat flux" value, if you will. This incident heat energy is estimated at a typical working distance - usually 18 inches for 480V equipment.
The magnitude of the incident heat energy depends on the available short-circuit fault current, plus the arcing time before the upstream protective device clears the fault (circuit breaker or fuse). A value of 1. 2 cal/cm^2 is considered the amount that would cause onset of a second-degree burn - just past the level of a "just curable" burn. Levels of hazard can be different for every bus in the system, since fault duty changes at every point and different protective devices are called upon to clear the faults.
Arc flash hazard values are classified into Hazard Risk Categories from 0 - 4. (HRC zero through HRC4) HRC0 is any hazard that's less than 1. 2 cal/cm^2. The upper end of HRC4 is 40 cal/cm^2. Anything more than that is considered "extreme danger" and we recommend that no energized work be performed on equipment with this hazard level.
PPE (personal protective equipment) garments can be used for protection against heat energy while performing energized electrical work. Non-flammable, natural fiber clothing is used to "insulate/isolate" workers from the incident heat energy. PPE clothing is rated for a certain amount of heat energy, so that skin inside the garment would then be protected to a level of energy no more than the 1. 2cal/cm^2 level - thus resulting in maximum of the just curable burn. Layering of garments adds to the level of protection, just like layering of outdoor wear in cold weather.
Since the hazard label in your photo shows an arc flash hazard energy level of 73 cal/cm^2, this equipment is considered in the Extreme Danger category, and should not be worked while energized. The 220-inch distance shown on the label would be the boundary distance from the equipment where energy from the arcing fault would be no more than the 1. 2 cal/cm^2 value.
Arcing faults will also include a pressure/blast wave. This is caused by the rapid expansion of conductors as the heat causes copper to vaporize - copper expands about 67,000 times normal while vaporizing. At this time, we can't estimate this hazard, but it can crush you and will certainly send you flying across the room. (maybe this is a good thing - gets you away from the heat) There's also the deafening sound wave that accompanies the blast wave to worry about too.
Hope this makes some sense; I wrote it hurriedly, and there's a lot more details that go with it. NFPA/IEEE, and other organizations are still hard at work - there are more tests planned in the future to get a better handle on the phenomenon and give us better tools to perform safe electrical work.
Best,
John
Attention: TDR Forum Junkies 
