Over Current Relay

If you have a short circuit, you obviously want to break the circuit as quickly as possible. If you have a load with a motor starting up occasionally that imposes a moderate overload for a short period of time, you do not want nuisance tripping. You need a magnetic breaker for the first case. A thermal tripping breaker would be more appropriate for the latter case to allow momentary overcurrents but to trip if the overcurrent condition is persistent. Both of these functions are often combined.

"Thermal magnetic circuit breakers[edit]

Shihlin Electric MCCB with SHT

Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions. The thermal portion of the circuit breaker provides a time response feature, that trips the circuit breaker sooner for larger overcurrents but allows smaller overloads to persist for a longer time. This allows short current spikes such as are produced when a motor or other non-resistive load is switched on. With very large over-currents during a short-circuit, the magnetic element trips the circuit breaker with no intentional additional delay.^[6]"

https://en.wikipedia.org/wiki/Circuit_breaker#Thermal_magnetic_circuit_breakers

When you buy a standard MCCB for a panel, it has long-time over current protection by virtue of a rather "sloppy" thermal trip sensor arrangement; a simplistic bi-metal strip. The plus/minus range of tripping action can vary by quite a bit at any given current value, based on the ambient temperature, previous thermal condition prior to an event, how many times it has previously tripped etc. etc. Some people do not want that kind of variability in deciding when current is excessive, so they use an OCR as the deciding element.

Breakers with Electronic Trip Units (ETUs) instead of T-M trips are less variable and more repeatable, so you would rarely see an OCR added to a breaker that has an ETU, that would be a waste of money.