Overview
The Square D B40 thermal overload relay is a melting alloy thermal unit engineered for motor overload protection in industrial electrical systems. This relay provides standard trip Class 20 protection and is compatible with Square D NEMA motor starters, making it a drop-in replacement for existing installations requiring reliable motor circuit protection.
Key Features
- Melting Alloy Sensing Technology: One-piece thermal unit construction that cannot be reset until cooled and repeated tripping does not affect original calibration
- Fast Trip Response: Trips in 20 seconds or less on overload conditions
- Manual Reset: Standard trip, Class 69 classification
- Plug-In Mounting: Easy installation and removal for maintenance without rewiring
- Authentic Square D Product: Engineered by Schneider Electric for quality and reliability
Technical Specifications
- Current Rating: 27.2–29.2 Amps
- Sensing Method: Melting Alloy
- Tripping Class: Class 20 (Standard Trip)
- Reset Type: Manual
- Mounting Style: Plug-In
- Construction: One-piece thermal unit
- Trip Response Time: ≤20 seconds on overload
- Manufacturer: Schneider Electric (Square D Brand)
Typical Applications
- Motor protection in NEMA-sized starters (Sizes 1, 1P, 2, M-1, M-1P)
- Overload protection for AC induction motors 21.5–35.3 Amps (single thermal unit)
- Industrial motor control circuits and machinery
- Equipment requiring automatic disconnection on sustained overload
Compatibility & Replacements
Compatible With
- Square D NEMA Motor Starters (Sizes 1, 1P, 2, M-1, M-1P)
- DPSG-22, DPSG-23, DPSG-32, DPSG-33 starters (25–30 Amps)
- DPSO-42, DPSO-43 starters (40 Amps)
- DPSG-52, DPSG-53, DPSO-52, DPSO-53 starters (50 Amps)
- Standard industrial motor control panels
Replaces / Drop-In Replacement For
- Original equipment Square D B40 thermal units
- Equivalent overload heater elements rated 27.2–29.2 Amps
- Legacy Square D B-series thermal relays in motor starter maintenance
Works With
- Square D Class 8501 and 8502 contactors
- Three-phase and single-phase motor circuits
- Manual and automatic starter configurations
- Industrial control systems requiring Class 69 overload protection
