An overcurrent relay has no (or short and constant) delay and acts - as th name implies - on excessive current. Like what you get in a short-circuit situation.
The simplest overcurrent relay is a magnetic coil that trips when current is higher than the trip value. Modern O.C. relays are electronic and usually has several trip levels - each with some small delay associated with them.
The overload relay usually tries to simulate the heating in a connected motor or other load. The time delay is usually an inverse function of current squared with a cooling coefficient that simulates cooling of the actual motor/load.
The simplest O.L. relay is the bimetal strip. It is being heated by load current squared (from RxI^2) and convection cooled by surrounding air. Modern O.L. relays does the simulation numerically and there are several levels of complexity involved. Depending on the application.
For motor loads, an O.C. relay can not be used for thermal (overload) protection. But for several other loads, like resistive loads, an O.C. relay can also serve as an overload relay. Or rather, the overload relay shouldn't be needed in such an application.
Overcurrent protection is a protection against excessive currents or current beyond the acceptable current rating of equipment. Short circuit is a type of overcurrent.
Overload protection is a protection against a running overcurrent that would cause overheating on the protected equipment. Hence, an overload is also type of overcurrent.
An overcurrent relay is used to protect equipment against high abnormal current (short circuit currents. These relays maybe time delayed (IDMT or ANSI 51) or Instantaneous (no time delay or ANSI 50).
An overload relay on the other hand is a type of overcurrent relay which monitors the load current and are set to determine whether the level or overcurrent exceeds the rating of the thermal elements.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
Hello salem
A quick answer is:
Overcurrent protection is protection against short circuits. It generally operates instantly. With standard breakers, between 500% and 1000% of full-load current is the point where the overcurrent protection over-rides the overload protection and opens the circuit instantly.
Overload protection is protection against overheating. It operates slower. Overload protection typically operates on an inverse time curve where the tripping time becomes less as the current increases.
A thermal magnetic circuit breaker is an example of both types of protection in one device.
Given a 100 amps breaker, the overload elements monitor the current and trip the breaker when the current exceeds 100 amps. The overload element has an inverse time element. The greater the current the quicker it trips. This is the thermal element in the thermo-magnetic breaker.
The second, magnetic element opens the breaker instantly when the current exceeds the overcurrent setpoint. Typically 500% to 1000% of full load current.
For example consider a thermo-magnetic breaker rated 100 amps, with the magnetic trips set at 500%.
At a current of 110 amps the breaker will trip eventualy but it will take awhile. As the current increases the tripping time becomes less. at 300 amps the breaker will trip quite quickly but there will still be a delay. When the current is equal to or greater than the setting of the magnetic trip (500 A in this example) the magnetic trip over-rides and trips the breaker instantly.
This is a simple example. The same protection for overloads and overcurrents may be provided by electronic circuits built into a breaker or by external relays controlling a large power breaker.
Can an overcurrent relay be used for overload protection?
skogsgurra has answered this better than I can.
Fuses are sometimes used to provide both types of protection in one fuse. Dual elemant fuses do a better job than standard fuses in that they have both inverse time characteristics in the time delay element and instantaneous characteristics in the other fusible element. The current rating of the instanteneous fuse element is higher than the rating of the time delay fuse element.
respectfully
