Product Overview

The Allen-Bradley 1756-PLS​ is a specialized Power Line Sensing (also referred to as Phase Loss / Phase Sequence / Undervoltage Sense) module designed for installation in a Rockwell Automation ControlLogix® 1756 chassis. Unlike standard analog or digital I/O, the Allen-Bradley 1756-PLS​ directly monitors a three-phase AC power line (typically the main incoming plant power or a critical motor feeder) to detect phase loss, phase reversal (incorrect phase sequence), and undervoltage conditions in real time. This capability allows a ControlLogix controller to take programmed protective action—such as inhibiting motor starts, initiating a controlled process shutdown, or alerting maintenance—before damage occurs to three-phase motors, drives, or sensitive equipment due to abnormal mains conditions.The Allen-Bradley 1756-PLS​ accepts a three-phase AC voltage input directly (nominally 120 V AC or 240 V AC line-to-line, depending on wiring configuration and the specific catalog variant’s range of 85–264 V AC per phase), measures the RMS voltage on each phase via isolated sensing circuits, and evaluates phase sequence and relative balance against user-configurable thresholds set in Studio 5000 Logix Designer®. Results are reported to the controller through the backplane as Boolean status bits (Phase A/B/C Present, Phase Sequence Correct, Undervoltage, Fault) and integer data words containing measured RMS values. The module features LED indicators for power, module health, phase presence, sequence error, and undervoltage alarm, giving immediate visual feedback at the panel without requiring HMI access.Drawing its operating power from the ControlLogix backplane (typically 300 mA @ 5 V DC, 90 mA @ 24 V DC), the Allen-Bradley 1756-PLS​ occupies a single chassis slot and requires only the three-phase sense wires plus a neutral/reference (as applicable) connected through the supplied or recommended 1756-TBNH / 1756-TBCH terminal block. With cULus Listed, CE, C-Tick, and FM Class I Division 2 approvals (on applicable series), the Allen-Bradley 1756-PLS​ is the purpose-built solution for embedding mains-power quality monitoring into a ControlLogix automation and protection scheme, eliminating the need for separate standalone phase-monitoring relays while leveraging the controller’s logic, alarm, and data-logging capabilities.

Technical Specifications

Main Features and Advantages

Integrated three-phase power quality monitoring inside the control chassis: The Allen-Bradley 1756-PLS​ brings phase-loss, phase-sequence, and undervoltage detection directly onto the ControlLogix backplane, allowing the processor to evaluate mains conditions with the same scan-cycle determinism as any other I/O. This eliminates the need for external phase-monitoring relays with separate contacts to wire back into a digital input—reducing panel components, wiring, and potential failure points.Configurable undervoltage threshold and time delay: Users can set the undervoltage trip point as a percentage of nominal (e.g., 85%, 90%) and program a time delay (0–25 seconds) before the fault is asserted, preventing nuisance trips during brief sags caused by motor starting or utility switching. The Allen-Bradley 1756-PLS​ reports both the instantaneous undervoltage condition and a latched fault bit that can require operator acknowledgment in the control program.Immediate visual diagnostics on the module faceplate: Dedicated LEDs on the Allen-Bradley 1756-PLS​ show per-phase presence (L1/L2/L3), correct phase sequence (green = ABC correct; red/alarm = reversed), and undervoltage alarm status. During commissioning, technicians can confirm correct three-phase wiring at a glance without a multimeter or HMI screen—a significant time-saver when bringing up new MCCs or verifying generator-backed feeds.Controller-readable RMS voltage data: Beyond simple Go/No-Go status, the Allen-Bradley 1756-PLS​ makes per-phase RMS voltage values available as REAL data tags in the ControlLogix program (e.g., Local:x:I.PhaseAVolts, PhaseBVolts, PhaseCVolts). These values can be logged to a trend, compared for phase imbalance calculation, or used to trigger preventive alerts when voltage deviation exceeds a user-defined tolerance—even if it hasn’t yet reached the undervoltage trip point.Programmable response through Logix logic: Because detection results are native controller tags, the Allen-Bradley 1756-PLS​ enables sophisticated responses impossible with a standalone relay: conditional inhibition of specific motor start permissives based on which phase is lost, staggered restart sequences after power restoration, automatic notification to a SCADA/FaultView system with voltage readings attached, or logging of pre-fault voltage levels for root-cause analysis of nuisance trips.Galvanic isolation for safety and noise immunity: The sensing inputs of the Allen-Bradley 1756-PLS​ are isolated from the 5 V DC ControlLogix backplane by 250 V AC working isolation (1500 V AC Hi-Pot tested), protecting the controller and other modules from transients that may be present on the sensed power lines and ensuring measurement accuracy even in electrically noisy MCC environments.