What is the purpose of the delayed third operation of an Overcurrent Relay (OCR)?

The purpose of the delayed third operation of an Overcurrent Relay (OCR) is to provide more time for line fuses on the load side to operate before the relay responds to an overcurrent condition. This feature is critical in protecting the system from unnecessary trips and potential outages due to transient overloads that might not pose a significant risk.

When an overcurrent occurs, there can be a momentary spike in current levels, which could cause fuses to blow if the relay were to trip immediately. By implementing a delay, the relay allows the fuses to clear these transient conditions first. This adjustment is essential for ensuring that the relay trips at the correct moment, contributing to the reliability and stability of the power system while also preventing nuisance tripping that could disrupt service.

In contrast, increasing maximum current or resetting the relay does not pertain to the timed response of the relay in managing overcurrent situations. Moreover, while preventing overheating of the transformer is important in electrical systems, the primary function of the delayed response in this context is specifically related to allowing fuses to operate effectively.