The Siemens 5SP3740-2KK01 is a 1-pole SENTRON miniature circuit breaker rated 40 A with a tripping characteristic E, designed for busbar mounting on a 40-mm busbar system per DIN 43870 Part 2. It's built for residential and infrastructure panels — think meter cabinets, distribution boards, and building supply feeders where the E-curve's delayed magnetic trip handles transformer inrush or motor starting without nuisance trips.
Rated breaking capacity of 25 kA per EN 60898 means this breaker can safely interrupt a fault current up to 25,000 A at 230 V AC — enough for most residential and light commercial service entrances where the prospective short-circuit current stays under that threshold. The 6 kV surge voltage resistance (overvoltage category IV) confirms it's rated for the mains-side of the meter, not just downstream subpanels. Solid and stranded conductors from 2.5 mm² up to 50 mm² fit the box terminal — that's a wide range covering everything from a 2.5 mm² lighting circuit to a 50 mm² main feeder. The infeed is at the box terminal, and the outgoing feeder uses a spring-loaded terminal, so you land the line side on the screw clamp and the load side on the cage clamp. No special tools beyond a screwdriver and a ferrule crimper for stranded tails. Depth of 89.5 mm (76.3 mm enclosure depth) and height of 151.3 mm — check your enclosure depth before committing. The 1.5 width units (roughly 27 mm on a standard DIN rail adapter) mean it takes up one and a half module spaces on the busbar.