VFD protection relay
The EL731 ac/dc Sensitive Earth-Leakage Relay by Littelfuse can detect ground currents.
Littelfuse Inc.’s EL731 ac/dc Sensitive Earth-Leakage Relay can detect ground currents—even in resistance-grounded electrical systems—all the way down to 0 Hz. This makes it ideal for protecting variable-frequency drives (VFDs) that may operate at low speeds, during which ac current approaches dc. The EL731 relay provides complete range of ground-fault protection in VFDs from 0 to 15,000 Hz in one device, saving cost and panel space compared to other relays. The EL731 uses a patented approach that enables the relay to detect the location of ground faults and makes it equally useful for ac, mixed ac/dc, and dc systems, including low-voltage 6-pulse VFDs commonly used on motors up to 250 hp. The relay is ideal for protecting VFDs operating in resistance-grounded electrical systems, which are growing in popularity because of their ability to reduce arc-flash hazards. The on-board ground-fault protection that comes with VFDs is generally not sensitive enough to detect a ground fault in resistance-grounded systems.