Electronic Trip Unit
How does an electronic trip unit differ functionally from a thermal-magnetic trip unit?
A thermal-magnetic trip unit uses a bimetal element for overloads and an electromagnet for short circuits, with tripping affected by ambient temperature. An electronic trip unit uses current transformers, a circuit board, and a microprocessor for configurable curves, providing temperature-insensitive, accurate, and repeatable protection with programmable ANSI functions, thermal memory, and communications.
What are 'LS', 'LSI', and 'LSIG' designations on an ETU, and why do they matter for NEC 240.87 and arc-flash?
LS means long-time and short-time (no instantaneous); LSG adds ground-fault; LI is long-time and instantaneous; LSI adds short-time; LSIG adds ground-fault. NEC 240.87 requires energy-reducing measures for breakers without instantaneous trip (LS, LSG) to limit arc-flash energy. Absence or high instantaneous settings increase clearing time and incident energy, affecting arc-flash calculations.
What is 'thermal memory' in an ETU and how does it affect overload tripping?
Thermal memory is an algorithm that models conductor heating and cooling under overloads, so the breaker 'remembers' previous overload exposure. If a circuit was near trip level, the ETU may trip earlier on a subsequent overload, reflecting reduced thermal margin of cables and equipment, preventing cumulative damage.
An electronic trip unit (ETU) is a microprocessor-based overcurrent protective device integrated into a circuit breaker that continuously measures current, digitally processes the signal, and issues a trip command when preset protection thresholds (long-time, short-time, instantaneous, ground-fault) are exceeded. It provides programmable protection, metering, diagnostics, and communications, certified to UL 489 and CSA standards.
On a manufacturing shop floor, ETUs are embedded in main and feeder breakers (400 A and above) to act as the 'brain' for overcurrent protection. They are configured for process-specific loads, such as motor starting currents and transformer limits, using settings like long-time at 0.8× In and short-time at 4× In with 0.2 s delay. ETUs provide temperature-independent overload protection with thermal memory, enabling selective coordination and arc-flash mitigation. They support zone selective interlocking and energy-reducing maintenance switches, critical for NEC 240.87 compliance. Advanced ETUs offer monitoring, diagnostics, and communication via Modbus or Profibus, integrating with plant SCADA for trip analysis and predictive maintenance. From a WorkSafeBC perspective, ETUs are engineered safeguards that limit fault duration and reduce arc-flash risk, essential for documenting overcurrent protection and coordination under the Canadian Electrical Code.
Misconfigured or undocumented settings: Wrong pickup levels or time delays can cause inadequate protection or loss of selective coordination, increasing arc-flash energy. NEC 240.87 requires documentation for breakers without instantaneous functions; ignoring this is a compliance failure.
Failure to maintain/verify ETU functionality: Lack of periodic functional testing (e.g., secondary injection) can leave undetected failures in sensors or electronics, leading to delayed or no tripping under faults, violating OSHA/WorkSafeBC duties to maintain safeguards.
Inadequate integration into arc-flash programs: Energy-reducing features like maintenance switches or zone selective interlocking are often not wired or used, increasing incident energy. NEC 240.87 and CSA Z462 expect these features to be considered; non-use can be criticized during audits.