Power Line Carrier of the Proven Power has NS40A Audio Tone Frequency Shift Keyed Relay Channel. The carrier channel can be configured in the one-way to transfer trip application protection or in the two ways for the breaker failure protection in single or dual channel operation. 2 or 4 wire audio (600 ohm) or Fiber Optic interface can be used for the termination. The typical replaying system is useful for various applications. It is used in the transfer trip replaying equipment. You can directly transfer-trip it for the protection of equipment. It can perform direct under reaching transfer-trip and permissive over reaching transfer-trip. These are unblocking relays. You can combine them for both unblocking and direct transfer-trip.