The Siemens 5TT5051-2 is a SENTRON Insta contactor — a compact, DIN-rail-mount switching device with 3 normally-open and 1 normally-closed contact set, rated for 400 V AC and actuated by a 24 V AC/DC coil. It occupies 3 modular width units (17.8 mm each) on the rail, so it takes about 53.5 mm of panel width. The 24 V coil is universal AC/DC, which simplifies control wiring — no need to segregate coil supply from the DC bus if that's what the panel runs.
The 400 V AC rating covers the standard three-phase line voltage in European and many Asian industrial panels — 400 V phase-to-phase. The 24 V AC/DC coil means it can be driven from a 24 V DC power supply (common in PLC cabinets) or a 24 V AC transformer; the hold-in power is low enough that a typical 24 V DC supply with a few amps headroom can handle several of these in parallel. The manufacturer explicitly notes that spacers are required between units for heat dissipation. In a densely packed panel, you'll need to leave a gap or use a forced-air scheme; stacking these without air movement will derate the contacts.
Supplementary devices (auxiliary contact blocks, surge suppressors) can be installed on the side or top of the contactor, per the product extension note. That's useful if you need to add status feedback or coil suppression without a separate terminal block.
Panel fit and mechanical notes
The 5TT5051-2 is 71 mm deep, 53.5 mm wide, and 90 mm tall. The installation depth (the dimension that matters for enclosure depth clearance) is 64 mm — the extra 7 mm is the front projection of the coil terminals and manual test button. A standard 80 mm deep enclosure gives enough clearance for wiring fingers. At 3 modular width units, it fits the standard 18 mm-per-module DIN-rail grid. That's a common footprint for a 3-pole contactor with an auxiliary NC built in — you won't lose an extra slot for the NC contact.