What it is and what it does
The Siemens 5SJ6540-7FC is a SENTRON miniature circuit breaker (MCB) with a 1P+N pole design — it switches the live conductor and disconnects the neutral, so you get full isolation on a single-module-width footprint. Rated 40 A with a C tripping characteristic, it handles the inrush from motor starters and lighting ballasts without nuisance trips, while still clearing overloads and short circuits up to 6 kA per EN 60898. That 6 kA rating is the breaking capacity at 230 V AC — enough for most residential and light commercial distribution boards where the prospective fault current stays under that threshold.
Fit and panel integration
This MCB snaps onto a standard DIN rail (mounting position any, so you can orient it vertically or horizontally in the enclosure). It occupies 2 modular width units (36 mm wide), with a depth of 76 mm and an installation depth of 70 mm — that's the dimension from the DIN rail to the front face, so check your enclosure depth if you're retrofitting into a shallow gland plate. IP20 with connected conductors means it's protected against finger contact but not washdown; keep it inside a sealed distribution board. Sealable design lets you lock the toggle position with a padlock for lockout/tagout.