Ratins of electric equipment under intermittent load

WHEN electric equipment is subjected to intermittent load, as in spot-, flash-, projection-, or gun-welding, it is customary to rate it according to the duty cycle^1,2 under which it is to be used. This is good practice as long as we take duty cycle for what it really is: the proportion of time that the equipment is heated — or current “on” time divided by total time. But we may get into trouble by extending this definition to say that temperature rise is proportional to duty cycle — as is commonly done in rating welding equipment for intermittent load.