What is the difference between fixed-time overcurrent protection and inverse-time overcurrent protection?

Action time characteristics
Fixed-time overcurrent protection: the action time of the protective device is fixed, independent of the size of the fault current. No matter how much the fault current exceeds the setting value, as long as the action conditions are met, a pre-set fixed time will elapse before action is taken to trip or signal is issued. For example, if the set action time is 5 seconds, then when the current exceeds the set value, no matter how much it exceeds, the protection will be triggered after 5 seconds.
Inverse time limit overcurrent protection: the action time is inversely proportional to the size of the fault current. The larger the fault current, the shorter the action time; the smaller the fault current, the longer the action time. That is, the greater the multiple of current over the setting value, the faster the protective device action, can more quickly remove serious faults, more in line with the actual needs of the power system faults.

Principle and realization
Fixed-time overcurrent protection: usually consists of time relays, current relays, etc. The current relay detects the current in the circuit. The current relay detects the current in the circuit, and when the current exceeds the set value, the time relay starts timing, and sends out a trip signal after reaching the set time. Its principle is simple, the realization is more direct, by setting a fixed time to control the protection action.
Inverse time overcurrent protection: generally using special inductive relays or microprocessor-based protection devices to achieve. The inductive relay utilizes the principle of electromagnetic induction to shorten the action time of the relay as the current increases. Microprocessor-based protection devices, on the other hand, use software algorithms to calculate the corresponding action time according to the real-time current detected, realizing the inverse time limit characteristic.