Can someone explain calories in a way that even I can understand? I am trying to get a handle on the level of hazard I am sometimes exposed to. Calories don't register with me.
I occasionally work with live 460V. Mostly that would involve changing or installing a breaker, or troubleshooting with meters etc. From what I gather, a calorie is the potential energy that can be delivered
to an area by way of a fault? Is that right? Say I was working on a 20amp 460V circuit. Does that equate to some level of calories?
No. The energy in calories depends on the possible fault current, time, and voltage from a fault before any protective devices kick in, not the normal load rating of a circuit.
To maybe give you a better feel for what is being described, a calorie is the energy that is required to heat one gram (one cubic centimeter) of water by one degree C. The PPE levels are expressed in terms of calories per square centimeter.
Based on that, a 10 calorie/cc flash will apply enough energy to each square centimeter of your skin to raise the temperature of that area of your body, 1 centimeter deep, instantly by 10 degrees C. Since the heat will actually be concentrated very close to the skin, the temperature rise and resulting burns will be a heck of a lot worse.
Assuming your skin to be 2mm thick, 10 cal/cc will raise unprotected skin temperature by 50 degrees C, on its way to doing damage below that layer. Note that raising the temperature by 50 C is a lot
worse than coming in contact with 37C + 50C = 188 F water and that in turn is an instant third degree burn.
You may find some other way of visualizing it to work better, but that does well for me.
Another helpful comparison may be that 1 calorie is about four Joules, and one Joule equals one Watt-second. And, again, the PPE calculation is based on calories per cc. So one cal/cc squared is about 26 watt-seconds per square inch of your skin. But that is not a graphic to me.
A third comparison is that one Cal/cc-sq. = 4 Joules per cc squared is the upper limit of medical laser treatment for tattoo removal!
