electrical

An electrical equipment is designed for a certain rating - may be Voltage rating or current rating - based on use at certain standard operating conditions. But, it may not be practicable to achieve all the standard opearting conditions, at the place & time of its use. So, the equipment is given a new rating based on the actual site conditions. This new rating is normally specified as a factor of the actual rating which is called the Rating Factor. While most of the time it is De-rating factor, equipment can also have an uprating factor, based on the site conditions.

De-rating is often done to ensure that the equipment can perform well and will last a long time. For example...

If I needed to regulate 15V down to 12V with 2A of current I would not use a regulator rated for a maximum input of 15V and a maximum load current of 2A. I would typically use a regulator that is rated for 20V input and 3A out. In that instance my regulator input voltage would be 75% of the maximum or 25% derated and my load current would be 66% of the maximum or 33% derated. The same principle applies to all types of equipment and mechanical designs. The brakes on your car, roof loading bridge loading, electrical equipment in cars.

Without derating we would see a lot more feild failures.

Sorry you are wrong and #1 is right.

You are talking about factor of safety, not derating.

Derating is as #1 said- changing the rating as per the actual condition.

Eg a motor is taken up to the top of Mt Everest - there the cooling, and other parameters will burn the motor of say 10HP if the rated load is applied on it.

So the motor is de-rated accordingly (ie it is treated as a lower HP Motor)

De-rating equipment for use at elevations above sea-level. Industrial controls require de-rating when operated at an altitude above 6600 ft. Industrial power semiconductors require de-rating when operated above 3300 ft. The equipment must be derated 1% for every 330 ft above these heights.

Then you may have compensations for RAD (relative air density), Humidity.

The compensations are usually covered under standards (eg NEMA MG1-14.04) and in quite a few cases the manufacturer put added conditions