Relay protects against ground-fault hazards with variable speed drives
Littelfuse Inc., provider of electrical safety, power distribution, and power control products, offers the Littelfuse EL731 AC/DC Sensitive Earth-Leakage Relay, which can detect ground currents—even in resistance-grounded electrical systems—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 a complete range of ground-fault protection in VFDs from 0 Hz to 15,000 Hz in one device, saving cost and panel space compared to other relays.
The relay 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. It can reliably protect a variety of applications, including conveyor belts, cranes, compressors, pumps, and mixers found in demanding environments, such as mining, oil sands operations, chemical plants, food processing, and other industrial facilities.
At its introduction, Tony Locker, PE, Littelfuse product manager, said, “This is the first relay to adequately address the common challenge of detecting ground faults in VFDs operating at low speed.” (See additional comments and explanation from Locker in the short video clip.)
The new 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.
The EL731 can be installed in place of an existing ground fault relay without replacing the current transformer (CT). Two CTs are required for the entire frequency range, or one CT can be used for only low- or high-frequency detection. The relay has a temperature input, metering capabilities, password protected alarm, and trip settings. Ethernet communications port is optional.
Video clip, above, explains the hazards of VFD ground faults and solution: Because variable frequency drives typically cannot pick up a ground fault, there’s potential for employee shock hazard and equipment damage. There is a solution, according to Locker. VFDs or variable speed drives (VSDs) typically aren’t able to pick up low-level ground faults, which can lead to a safety hazard. Most end-users may be unaware of the risk, Locker said in the video. VFD ground-fault detection is around 20% full-load current, so if you have a 100 A drive or 100 A motor, that means that 20 A is the lowest level at which that drive will detect a ground fault. He said high-resistant ground systems are limited to 5 A. There have been documented cases where a loose connection terminal block on a motor loosened and created a fault to ground, without the drive or motor knowing it. In one case, such a fault burned a 2-in. hole in motor terminal cover, while the motor continued to operate, creating an electrocution and fire hazards, he warned. A ground fault relay could be added to the circuit to avoid such a hazard, and provide filtering, as well, he said.
Learn more about the EL731.
– Text edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer, firstname.lastname@example.org. Video edits by Peter Welander, content manager, CFE Media.