Early Detection of Internal Arc Faults in Instrument Transformers Using Voltage Waveform Analysis Failures of combined instrument transformers can lead to catastrophic explosions, posing serious safety risks to substation personnel and causing extensive damage to both primary and secondary equipment. Investigations of recent incidents revealed that, before the explosion, the measured phase voltage of the affected phase exhibits a characteristic distortion caused by internal arcing within the transformer. Based on these findings, a new protection function is being developed and implemented in protective relays. The method continuously monitors phase voltage waveforms and identifies the specific distortion pattern associated with internal arc faults. Early detection enables the affected equipment to be disconnected before catastrophic failure occurs, thereby improving both personnel safety and substation reliability.