its the blades on the cord end, not the cord itself, vs the receptacle its plugged into.... unless its a NEMA/SPEC grade receptacle its a fire hazard.
and the insulation type of cords varies from manufacturer to manufacturer.
ambeint temps play a major role in insulation rating.
hysteresis gnerated heat on the cord cannot dissapate if the ambient temp is too high...
an ambeint temp can be greatly affected by a strong
light source.
just because the air in the room is a certain temp, doesnt mean everything in the room is the same temp as the air.
so the cord gets warm, and the heat cant dissapate because your ambeint temp is high, say 78-82deg f. this is compunded by the intense light from the HID lamp. this light strikes the cord where it is visible and contributes greatly to more heat generation. over time, the insulation becomes brittle, untill one day a short circuit condition occurs after the cord insulation has sufficiently deterioted.
next time your in a big building/ warehouse that has HID lamps in it, look up.... if there cord fed its going to be with either armored cable with thwn-2 wire in it, or its going to be SO or SJ cable, which has a very high resistance to heat....
sorry for poor spelling, ive gotten to the point where i dont care about it
edit for clarity-
about cords and heat-
the danger is actually not from overloading the cord.... though overloading is a major, major concern, an overloaded cord becomes apparent pretty rapidly, you can smell it long befor eyou will see it.
the danger is from something that is close to being overloaded, i.e., maxed out... like running 1full rated ampacity of the cord.
ampacity is determined by the ambeint temps vs the voltage and power requirements of the device.
with a maxed out cord, the danger is from thermal expansion and contraction... the insulation on the cord expands as it heats up, then contracts later on when the lights turn off. its this continula expansion and contraction that degrades the insulation, being compounded by both additional heat
and high levels of UV radiation from the HID lamps. UV makes thermoplastic get extremely brittle..... so running an hid lamp off of a cord is really a risky gamble, because its simply a matter if time untill you have a component failure. cords were never intended to run loads like that (an hid lamp)
this is why it takes 5 years of school and over 10,000 hours listed verifiable field exp befor eyou can even call yourself an electrician. theres
alot to know.