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Quantum leap in transmission line protection performance

The SEL-T400L Time-Domain Line Protection trips securely in as fast as 1 millisecond to protect critical lines.

PULLMAN, Wash. — Schweitzer Engineering Laboratories, Inc. (SEL) announced the world’s first traveling wave relay, the SEL-T400L Time-Domain Line Protection. Using traveling waves and incremental quantities, the SEL-T400L breaks the speed barrier of existing phasor-based protection, reducing operating times from 1–1.5 cycles to 1–4 milliseconds.

“SEL time-domain and traveling wave technologies are delivering dramatic reductions in transmission relay tripping times,” said SEL President Dr. Edmund O. Schweitzer, III. “We are averaging at least 12 milliseconds in time savings, compared to the already fast SEL-411L Advanced Line Differential Protection, Automation, and Control System. Faster tripping means more megawatts transmitted, better system stability, improved safety, less damage to insulators and transformers and reduced fire risk.”

Built for speed and simplicity, the SEL-T400L is easy to set, having only a few protection settings. The SEL-T400L also provides high-resolution (1 MHz) oscillography and single- and double-ended traveling wave fault locating accurate to one tower span.

The SEL-T400L is now available for pre-order for $12,000 USD. For more information on its features, benefits and applications, visit

SEL serves the power industry worldwide through the design, manufacture, supply and support of products and services for power system protection, monitoring, control, automation, communications and metering. For more than 30 years, SEL has provided industry-leading performance in products and services, local technical support, a 10-year worldwide warranty and a commitment to making electric power safer, more reliable and more economical.