Why? Because the standard says so.
Some years ago, I looked at this big and expensive box that we'd just put together for our customer. Couldn't see any glow from the power lamp, so it must be turned off. Foolishly, without checking any further, I put my hand inside and zap - electric shock.
Ouch (and which could have been much worse). My silly mistake of not pulling out the power cable to make sure. But the power lamp was off, so, shouldn't it have been OK?
Actually the power lamp was on, but it looked like it was off.
So what went wrong? The lamp was blue, or to be more precise, a blue LED. And although blue LEDs are everywhere these days, are sometimes used as power lamps, and glow brightly, the work of their inventor Shuji Nakamura (who won the 2014 Nobel Prize for Physics for it) was yet to hit the market. The available blue LEDs were dim things with a very narrow viewable angle. I didn't see that it was on because I was a mere 30° off axis, and their viewing angle was just 20°.
At times like this, after calming down a bit, you go and ask the designer WHY. His answer was that this was what he had to do, because of the standard.
Huh? Well, his explanation came from the venerated MIL-STD-1472 (Human Engineering), which still says (in the 2012 version):
| Red | Malfunction; action stopped; failure; stop action |
| Yellow | Delay; check; recheck |
| Green | Go ahead; in tolerance; acceptable; ready |
| White | Functional or physical position; action in progress |
| Blue | Advisory |
His explanation was that the power lamp couldn't be red, because that would mean that it had failed. Grrr. Blue was OK because it didn't mean anything. And of course, this part of the standard is unchanged since the days when "lamp" meant a tungsten filament light bulb with a colour lens in front.
So after ceasing to hit one's head on the proverbial brick wall, and realising that the rooms where this was going to be used would be full of computer equipment with red LEDs that nobody would ever care about (does anyone really think that the red light that's been on the front of every PC since the 1980's means that its hard disk has failed?), I figured out a solution.
I changed the power LED to GREEN. This was OK because it meant that the power supply is READY (and at the time, green LEDs were already bright, with wide view angles).
At least the designer permitted me to make the change.