What it is and what it does
The Siemens 3RT5055-6NF36 is a SIRIUS power contactor, size S6, built for switching motor loads and resistive circuits in industrial control panels. Its AC-3e rating of 150 A at 690 V tells you it can handle a 55 kW motor at that voltage without welding the contacts, and at 230 V it's good for 60 kW — that's the sort of headroom a compressor or a big conveyor motor needs. The insulation voltage is rated 1000 V, so it's comfortable in 690 V line-fed panels.
Mounting and panel fit
This contactor is screw-fixed to the panel backplate — no DIN rail snap-on here, so plan for a drill and tap. The mounting position is flexible: you can rotate it ±90° on a vertical surface or tilt it ±22.5° front-to-back, which helps when you're threading it into a tight gland-plate layout. Side-by-side mounting is allowed, so you can gang multiple contactors without a gap, but watch the heat — the 60 °C ambient derating on the 70 mm² cable minimum tells you it breathes better with some air around it.