Core Principle: It calculates the active power internally within the relay based on the measured voltage and current at the generator terminals (or outlet). For thermal overload protection (ANSI Device 49), the pickup is typically set at 115% to 125% of motor full-load amps depending on service factor. For overcurrent. Coordinating overcurrent relays across multiple protection zones is one of the most consequential tasks in power system design — get it wrong and a single downstream fault trips an entire substation. Common calculations. This paper describes the experiences of Energinet. dk is Denmark's transmission system oper-ator. At the beginn ng of the article it is drawn up process to protect power lines.