With the introduction of overload relays in the mid-1900's, motor protection reached a new level. When a protective relay was added to a standard contactor, it provided a low-cost level of protection saving time and money on replacing the more expensive pieces of machinery that were being operated down stream of this new "starter".
Overload relays come in many different forms and have evolved over the years. Originally there were bi-metal heaters, which were strips of metal that when heated to a certain temperature, would cause a "trip" and shut down power to the contactor and subsequently the motor being operated. This allowed for automatic protection of a motor in an over-amperage state. One heater was needed for each leg of the motor and these bi-metal heaters were used for many years.
Following the use of bi-metal heaters came the introduction of thermal overload relays. These provided greater flexibility in motor protection with phase loss (single phase sensitivity) tripping, different class trips (from 5 to 30 though 10 is the standard), manual or automatic reset and most importantly a wider AMP range per unit. While bi-metal heaters had a small AMP range, sometimes 1 to 2 AMPS at most, thermal overloads could have AMP ranges ranging 20 to 30 AMP in the higher ratings. This provides for great convenience and portability should an application change from 460 volt down to 230 volt.
The newest innovation in this category is the advent of electronic or solid state overload relays. These relays offer every option imaginable from a 4:1 or even a 5:1 AMP adjustment ratio to class selection all in the same unit. Larger units can have AMP ranges from 20-100 AMP, manual and automatic reset and trip class selections from 5 seconds up to 30 seconds. These units also are now self powered and no longer require a separate power source for operation.