Electromechanical relays (not just earth fault relays) need to be calibrated to insure they are operating in the expected way. A simple over current ground relay will have a tap setting, and a time dial setting. These are chosen by a protection engineer to coordinate with other ground protection devices upstream and downstream. If the relay is not calibrated correctly, the relay will not operate at the expected tap value, or may operate slower / faster due to the time dial calibration being different than expected. This can cause over tripping, removing more equipment from the power system than is necessary to clear a fault.