Siemens 5SY4540-7CC — 40 A C-Curve MCB, 2-Pole 1P+N
The Siemens 5SY4540-7CC is a SENTRON miniature circuit breaker (MCB) rated for 40 A at AC with a C tripping characteristic. It's a 2-pole design switching both line and neutral (1P+N), occupying 2 modular width units (36 mm) on the DIN rail. Breaking capacity is 10 kA per EN 60898 — sufficient for most industrial and commercial sub-distribution boards where the prospective fault current stays under that threshold. The C-curve means it holds through moderate inrush (5–10× rated current), so it's the usual choice for motor-starter feeders, small transformers, and lighting banks with capacitive drivers.
Temperature derating and panel fit
At 35 °C ambient the full 40 A is available. Derate linearly: 37.6 A at 40 °C, 36.36 A at 45 °C, 35.04 A at 50 °C, and 33.68 A at 55 °C. If the panel runs hot — say a crowded enclosure with drives — the 40 A marking on the handle is not the continuous rating; size the upstream conductor for the derated value at your actual ambient. The 76 mm depth (70 mm installation depth) fits standard distribution boards; the quick-assembly system clips onto the DIN rail without tools. Mounting position is any, which simplifies layout in tight gland-plate zones.