STJ typically refers to a “Short-Time Inverse Time Overcurrent Relay” in relay protection systems.

Meaning and Function

In the context of relay protection, STJ is commonly used to denote a short-time overcurrent relay with an inverse time characteristic. This type of relay is designed to trip a circuit breaker after a short delay when the current exceeds a preset threshold, allowing for temporary overcurrent conditions (like motor starting or transformer inrush) without unnecessary tripping. The inverse time characteristic means that the higher the fault current, the faster the relay operates, providing both selectivity and coordination with other protective devices .

Application in Power Systems

• Short-Time Protection: STJ relays are used to protect transformers, busbars, and feeders from short-circuit currents that are higher than normal operating currents but may not require instantaneous tripping.
• Coordination: They are coordinated with instantaneous overcurrent relays (50/50N) and long-time overcurrent relays (51/51N) to ensure selective tripping, minimizing disruption to the rest of the system .
• Inverse Time Feature: The inverse time function allows the relay to delay tripping for lower fault currents while responding quickly to severe faults, balancing protection speed and system stability .

Relation to ANSI/IEEE Numbering

While STJ is not a standard ANSI/IEEE device number, it is often used as a manufacturer-specific designation for a short-time overcurrent relay with inverse time characteristics. In ANSI terms, it corresponds to a device 51 (time overcurrent) with short-time settings, sometimes combined with a letter suffix to indicate the specific function or application .

Summary

STJ relays are essential for short-time overcurrent protection in electrical power systems, providing selective, time-delayed tripping to protect equipment from short-circuit damage while avoiding unnecessary interruptions. They are widely used in transformer, bus, and feeder protection schemes and are coordinated with other overcurrent relays to ensure system reliability and safety .