What it is, and what the ratings mean for fit
The Siemens SENTRON 5SU1324-6KX40 is a 2-pole RCBO rated 40 A at 30 °C with Type A fault-current sensitivity. It detects sinusoidal AC and pulsating DC fault currents. The B-curve tripping characteristic suits resistive and general-purpose loads. Breaking capacity is 10 kA per EN 60898 at 110/120 V AC, 50 Hz — enough for most residential and light commercial distribution boards where the prospective short-circuit current stays under that threshold. The 2-pole (2P) form switches both phase and neutral, and the device is rated for overvoltage category III (distribution-level surge immunity) with pollution degree 2.
Derating and thermal reality — what the current numbers actually tell you
The 40 A rating at 30 °C is the headline number, but the real-world current capacity drops steadily as ambient temperature rises inside the enclosure. At 40 °C it's 37.6 A; at 50 °C it's 34.8 A; at 60 °C it's 32.4 A; at 70 °C it's 29.2 A. If your panel runs hot — say 50 °C ambient — you need to treat this as a 34 A device, not 40 A. The mounting position is any, which helps when you're packing DIN rails tight, but the derating curve is what governs the safe continuous load.
DIN-rail footprint and panel integration
Width is 54 mm (3 width units at 18 mm each), depth 77 mm, height 90 mm, with an installation depth of 70 mm into the distribution board. It snaps onto standard 35 mm DIN rail. The IP20 rating applies only when the distribution board is installed with connected conductors — so it's protected against finger contact inside the enclosure, not for wet or dusty locations. Supply can enter from either top or bottom, which saves time when you're retrofitting into an existing board with fixed busbar orientation.
Lifecycle and sourcing posture
Production status is current. This part is in active manufacture by Siemens under the SENTRON brand. No end-of-life notice or last-time-buy window is available. Availability and pricing are confirmed at quote time through independent distribution channels.