Siemens 5SY7540-6 SENTRON MCB — 40 A B-Curve, 2-Pole, 15/20 kA
The Siemens 5SY7540-6 is a SENTRON miniature circuit breaker rated 40 A with a B-curve tripping characteristic, configured as 2-pole (1P+N) in a 2-module-width (36 mm) DIN-rail housing. It breaks fault currents up to 15 kA per EN 60898 and 20 kA per IEC 60947-2, making it suitable for residential and light commercial distribution boards where the prospective short-circuit current stays under those thresholds. The B-curve (3–5× In) means it trips fast on short-circuit but tolerates moderate inrush — a fit for resistive and lighting loads rather than motor circuits.
What the ratings mean for fit
The 40 A rating holds at 35 °C ambient; derate to 36.28 A at 45 °C, 34.96 A at 50 °C, 33.6 A at 55 °C, and 32.16 A at 60 °C (–). If the panel runs hot, size the upstream protection for the derated value, not the nameplate. The 15 kA (EN 60898) and 20 kA (IEC 60947-2) breaking capacities are the maximum fault currents the MCB can safely interrupt — verify the supply-side SCCR at the distribution board. Pollution degree 3 and overvoltage category III mean it's rated for fixed installations inside an enclosure, not for outdoor or wet locations. The 2-pole 1P+N configuration switches the line conductor and provides a switched neutral disconnection — required in some regional wiring rules for final circuits. The combined terminal top and bottom accept both busbar and conductor entry without adapter plates. Mounting position is any, and the quick assembly system clips onto standard 35 mm DIN rail. Operating temperature spans -40 to 75 °C, with humidity limits of 95% up to 55 °C, tapering to 35% at 75 °C.
Construction and compliance
Halogen-free and silicon-free construction reduces corrosive off-gassing in a fire — relevant for clean-room or enclosed panel environments. Sealable with a provision for a lead seal or lockout tag, which satisfies isolation locking requirements in some safety procedures. Touch protection is integral. Energy limitation class 3 means the let-through energy is limited, reducing downstream equipment stress during a fault.