There is a difference between "over-current" protection and 'over-load' protection of the motor.
"Over-current" protection is supposed to act FAST. With motors, the amount of over-current protection we can provide is limited by the high amount of current needed to get the motor started.
"Over-load" protection is what we use to keep the motor from working too hard for any period of time. This is what the heaters provide. It will, say, keep you from asking that 1-hp motor from providing 2-hp for any length of time. The breakers or fuses for that same motor, by contrast, won't kick in until you're asking that motor to give you six horses - which will be long after that motor is trash.
