The C4037A400 is a MicroLogic 7.3 E AL electronic trip unit from the ComPacT NSX400...630 range, rated 400 A at 40 °C for distribution applications. It packs LSI protection (long-time, short-time, instantaneous) with integrated earth-leakage protection Class A, all in a 3-pole (3D protected poles) package that fits fixed-mount ComPacT NSX breakers. The LCD display gives you local readout of metering, alarms, and event logs — no separate power meter needed on the door.
If your panel runs hotter — say 50 °C inside the enclosure — you'll need to derate per the ComPacT curves, or step up to the next frame size. The Ir long-time pick-up is adjustable from 160 to 400 A, so you can dial in the exact cable or busbar rating without changing hardware. Short-time (Isd) and instantaneous (Ii) pick-ups are also adjustable: Isd from 1.5 to 10 x Ir, Ii from 1.5 to 12 x In. That range covers motor start inrush, transformer magnetizing, and selective coordination with downstream breakers. The short-time delay (tsd) can be set from 0 to 0.4 s with I²t on or off — use I²t on for cable protection, off for generator or busway coordination where you need a flat time band. The IΔn rotary switch lets you disable it (OFF position) if the downstream circuit already has its own RCD, but having it built in saves a separate toroid and relay in the panel. If you re-close quickly on a still-hot circuit, the trip curve shifts to account for the residual heat — no nuisance re-trip on a warm restart, and no waiting for the conductor to cool before re-energizing.
This trip unit mounts into ComPacT NSX400, NSX630, and NSX400...630 fixed-mount circuit breakers. The 3-pole (3D protected) form factor matches the standard NSX frame; the LCD display and pushbuttons are accessible through the breaker's front cover once installed. IP40 per IEC 60529 keeps out tools and wires larger than 1 mm — fine for a clean indoor panel, but if the breaker lives in a washdown area or outdoors, you'll need the IP54 or IP55 shrouded version of the NSX frame. The 50/60 Hz network frequency and 440 V AC rated voltage suit standard low-voltage distribution in North America (480Y/277 V) and Europe (400/230 V) alike.