what is the concept of setting the instantaneous and time delayed over current relays?

it is instantaneous over current relays are for short ckt protection and time delayed over current relays for overload protection?

Yes.

Instantaneous Over Current (IOC) protection is usually part of the circuit protection scheme, i.e. circuit breakers or fuses, not usually relays (unless you are speaking of large MV motors). But motor protection relays also have instantaneous protection settings in many cases. This is sometimes used as a backup to your short circuit protective device, i.e. set to trip at a value lower than the fuses to prevent losing the fuse. But in most cases, the IOC in a relay is used AFTER the motor has started fro purposes of preventing mechanical jams from breaking critical components, essentially acting as an "electronic shear pin".

The thermal settings are typically based on an i²t curve and are intended to match the thermal damage curve of the motor, usually the rotor. The curves are defined as "Overload Classes" designated by a number, i.e. 10, 20, 30 etc., which depicts one of the plot points on the curve. The 3 main plot points are, I believe, "Locked Rotor Current", 2 minutes and continuous (over 2 hours) current ratings. The "Class number" denotes the time at that LRC value, which is loosely defined as 600% FLC (but I learned here that it may now be higher), meaning that a Class 10 is 10 seconds at LRC, class 20 is 20 seconds etc., etc. The 2 minute point is called the "pickup point" and is typically 125% FLC and the continuous point is 100% FLC for 2 hours. The curve is then plotted between those 3 points.

More info: http://www.lmpforum.com/inforum/Overload-T...Class-t241.html