The Siemens 3RT1016-1HB42-ZW95 is a SIRIUS coupling relay in size S00, rated for AC-3 duty at 4 kW/400 V with a DC 24 V coil. The 1 NC contact is the normally-closed mirror contact per IEC 60947-4-1, used in safety circuits to signal the state of the main poles — it mechanically mirrors the welded or open condition of the power contacts, not just the coil state. That makes it the contact you wire into the safety chain, not the auxiliary that follows the coil. Coil operating range is 0.7 to 1.25 times the rated control supply voltage US, so at 24 VDC nominal it picks at about 16.8 V and holds through 30 V. The 3-pole screw connection matches the standard S00 footprint — 45 mm width on the DIN rail, same as the base contactor in this frame. Multiunit packing (144 units per pack) tells you this was spec'd for a production run, not a one-off swap.
What the AC-3 rating means for fit
The 4 kW at 400 V AC-3 rating governs motor switching — this relay is sized for a 4-pole induction motor up to that power on a 400 V three-phase line. AC-3 covers starting and plugging (inrush up to 8× rated current) but not switching capacitor banks or resistive loads at higher currents. If your load is resistive (AC-1), the relay can handle more current, but the AC-3 figure is the one that decides fit for a motor starter.
Panel integration
Size S00 snaps onto a 35 mm DIN rail per EN 60715. The screw terminals accept 0.5 to 2.5 mm² solid or stranded wire — strip length 8 mm, torque 0.8 to 1.2 Nm. The 45 mm width leaves room for a matching overload relay (Siemens 3RU2 or 3RB3 series) mounted directly below, sharing the same busbar system. No side clearance needed for cooling at this frame size.