What this MCB is and what the ratings mean for fit
The Siemens 5SY6540-6 is a 2-pole miniature circuit breaker in the SENTRON family, designed as a 1P+N unit — it switches and protects the phase conductor while switching the neutral, but the neutral pole is not protected against overcurrent. The B tripping characteristic means the magnetic trip operates at 3 to 5 times rated current, so it's sized for resistive and general-purpose loads where inrush stays moderate — lighting circuits, small heaters, control transformers. The 40 A rating at 35 °C is the headline figure; at 40 °C ambient it derates to 37.56 A, at 50 °C to 34.96 A, and at 55 °C to 33.6 A, per the manufacturer's published curve. That derating is what governs real panel fit when the breaker sits in a warm enclosure alongside other heat sources. Breaking capacity is rated at 6 kA per EN 60898 and 10 kA per IEC 60947-2. The 6 kA figure is the standard for domestic and light commercial distribution boards; the 10 kA rating under the industrial standard (60947-2) means it can interrupt higher prospective fault currents typical in panel-fed circuits. For a BOM line, the 10 kA at 400 V AC is the number that decides fit in an industrial panel with a transformer or busbar capable of delivering that fault level. The 5SY6540-6 occupies 2 modular width units (36 mm) on a DIN rail, with a depth of 76 mm and an installation depth of 70 mm. It mounts in any position, and the terminals are combined (top and bottom) for feed-through wiring. Pollution degree 3 and overvoltage category III mean it's rated for the industrial environment — conductive pollution and transient overvoltages from the mains distribution side are within its design envelope.
Standards and compliance documentation
The 5SY6540-6 is certified to IEC / EN 60898-1 and IEC / EN 60947-2, and also carries UL1077 recognition. It is halogen-free and silicon-free, which matters for outgassing-sensitive environments like clean rooms or sealed enclosures. The sealable feature (tamper-evident cover) is available for installations that require lockout or anti-tamper compliance. The product designation per EN 61346-2 and IEC 81346-2 is letter code F — the standard symbol for a protective device in functional plant documentation.