When the feeding circuit fails, the protective relay detects the fault and the circuit breaker cuts the fault current in the AC electric railway system (with the feeding voltage at 20 kV for narrow-gauge lines and 25 kV for Shinkansen). In recent years, the service load tends to increase with higher frequency of train operation, increased train speeds and power consumption multiplied by new type electric trains. Under the circumstances, the development of protective relays has become very important to ensure the safety of train operation. The significant advancement of digital computer technique has enabled development of protective relay systems with higher reliability. This paper describes phenomenon of the faults in AC feeding circuit, load characteristics of electric cars and recent development of protective system.