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Is there any industry-defined guidelines to select the color of LEDs which is used for power indication in systems? A colleague told me that red color LED is usually used for 12V, green for 5V and orange/amber for 3.3V. I couldn't find any relevant results in the web while googling for that also.

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No, blue is used for "this is a brand-new high-tech must-have gadget". Red means "this gadget belongs to your grandfather and is not worth owning". I'm pretty sure this is an industry standard. Actually this standard may have lapsed in 2012, the current colour for power indicators may be an RGB state-indicator. SO hard to keep up, how much is an IEEE subscription? –  RedGrittyBrick Feb 17 at 10:46
    
I disagree, surely flashing-white is for "this is a brand-new high-tech must-have gadget" - anyway that's what I was told by the priest. –  Andy aka Feb 17 at 10:53
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Well, since I'm a PC user, I'd say orange form +3.3 V, red for +5 V and yellow for +12 V. –  AndrejaKo Feb 17 at 10:58
    
Lol.. lets say if this question is not related to a "hi tech gadget" but to an industrial equipment, then?? –  Avin Feb 17 at 11:01
    
@AndrejaKo Isn't that the color-coding for PC wires? I haven't noticed any orange colored LEDs in the PC motherboard so far. –  Avin Feb 17 at 11:27
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For industrial equipment, IEC 60073 might apply, which reserves red indicators for dangerous conditions or emergency situations. For example on machines with stack lights.

IEC 60073 Basic and safety principles for man-machine interface, marking and identification – Coding principles for indicators and actuators

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Could you possibly summarize this in a bit more detail for those who don't want to spend $220 on a European standard? –  Spehro Pefhany Feb 17 at 16:15
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Red for emergency/danger, yellow for abnormal condition, green for normal, blue for mandatory action - from here –  Martin Feb 17 at 20:30
    
Thank you, that's helpful. –  Spehro Pefhany Feb 17 at 20:32
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There are only the common IEC, ANSI, EN, etc. standards for industrial applications. They usually don't apply to anything outside that field, so if your application doesn't have to comply lets say EN60204-1:98-11 (or any other standard), then you can use whatever colours you want but check any requirements first.

But just for the fun: IF your application would need to comply with EN60204-1, all the LEDs you mentioned would have to be green: (Red = Emergency; dangerous condition / Yellow = Abnormal; check physical dimension for exceeding limits / Green = Normal; physical dimension inside normal range / Blue = Forcing; action required / White = Neutral)

And after all, the standard rule in electronics (as mentioned before) summarized:

  • brand-new high-tech must-have gadget from the last decade: blue
  • brand-new high-tech must-have gadget from this decade: white or way-too-bright blue
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