The Siemens SIRIUS 3RT6916-1BC00 is a varistor-type surge suppressor designed to clip directly onto S00-size contactors and contactor relays. At 9 mm deep and 28 mm wide, it adds negligible DIN-rail space; it mounts in any position on the contactor's front face.
Clip-on fastening to the contactor's coil terminals — no wiring required beyond the existing coil connections. The suppressor shares the contactor's mounting rail; no extra DIN-rail slot is consumed. The varistor design means it wears with each surge — if the contactor coil fails shorted, check the suppressor first.