SIEMENS SB20TLSI

Overview

SIEMENS SB20TLSI is a solid-state LSI trip unit for the Siemens SB frame molded case circuit breaker, rated 2000A, 3-pole, 600V class. LSI stands for Long-time overload, Short-time delay and Instantaneous short-circuit protection; it does not include ground fault protection (G function). This legacy trip release is widely supplied as exchange spare for SB series MCCB switchgear overhaul and feeder protection retrofit.

Features

SIEMENS SB20TLSI provides adjustable long-time overload protection with inverse time curve for thermal overload protection of cables, transformers and motors. The short-time function enables selective coordination with downstream protective devices to reduce unnecessary blackouts, while instantaneous protection trips immediately for severe short-circuit faults. SIEMENS SB20TLSI is self-powered from the breaker built-in current transformers, so core protection functions remain active even when auxiliary control power is lost. It supports front-panel adjustment dials to set pickup current and time delay parameters on site. It records fault events and can be equipped with auxiliary alarm contacts to send fault signals to DCS and SCADA logging systems. It is designed for heavy-duty industrial distribution switchgear and complies with relevant ANSI and UL standards.

Specs

SIEMENS SB20TLSI Platform: Siemens SB frame MCCB. Type: solid-state LSI trip unit. Rated current: 2000 A. Poles: 3P. Voltage rating: 600 V. Protection functions: L (long-time), S (short-time), I (instantaneous). Power source: self-powered via internal CTs. Mounting: installed inside SB frame molded case breaker. This is not a complete circuit breaker; it is the electronic trip release assembly. SIEMENS SB20TLSI is widely supplied as service replacement spare for protection unit failure.

Principle

SIEMENS SB20TLSI continuously samples current signals from the breaker CTs. The internal electronic circuit compares measured current against preset thresholds. For sustained overload, the long-time inverse curve initiates delayed tripping to avoid nuisance trips for normal starting inrush. Short-time delay logic coordinates with downstream breakers, and instantaneous protection triggers fast opening for high-magnitude short circuits. If fault conditions are detected, the trip unit sends a trigger signal to the breaker’s operating mechanism and activates alarm contacts for remote monitoring.

Applications

SIEMENS SB20TLSI is deployed for main feeders, bus tie feeders and large motor feeders in industrial plants, data centers and commercial low-voltage distribution switchrooms. It is selected for spare inventory and panel retrofit projects for existing SB frame MCCB LSI protection loops.