The Siemens 3RT2926-1BB00-ZX90 is a SIRIUS varistor surge suppressor sized for S0 contactors, handling AC control voltages of 24-48V at 50/60 Hz and DC control voltages of 24-70V. It clips onto the contactor without extra wiring space — the 12 mm depth and 30 mm width keep the panel density tight. For a mill environment, that means the varistor sits right on the coil terminals, clamping voltage transients before they couple into the 24V DC loop powering your PLC inputs. No unplanned downtime from a spike that took out a DI card.
The varistor is rated for AC 24-48V at 50 Hz and AC 48-24V at 60 Hz — the range reverses because the impedance of the varistor changes with frequency, but the effective clamping voltage is the same. On the DC side it covers 24-70V, which matches the common 24V DC control bus plus headroom for a 48V nominal loop. The S0 size designation means it fits the SIRIUS S0 contactor frame (3RT1023 and similar); it won't physically mate with S00 or S2 frames. Clip-on mounting and any mounting position mean you can snap it onto the contactor in a crowded panel without worrying about orientation. Operating temperature range is -25 to +60 °C, storage -40 to +80 °C — fine for an indoor panel in a paper mill or a heated enclosure, but if the panel sits next to a dryer section or recovery boiler, verify the ambient stays under 60 °C. The varistor itself dissipates negligible heat, so derating is not a concern here.