Latched contactor relay for holding circuits
The Siemens 3RH2440-1BN40 is a latched contactor relay — once the coil is energized, a mechanical latch holds the contacts closed without continuous coil power. That makes it the right choice for circuits where you need the contacts to stay made after a control power dip or where holding a relay closed on a 250 V DC bus would waste power and heat a panel. It carries 4 normally open (NO) contacts, size S00, terminated with screw terminals.
Coil voltage and switching reality
The 250 V DC coil is the spec that decides fit here. DC coils draw continuous current through the holding circuit — unlike an AC coil that drops to a low holding current, a DC coil at 250 V dissipates steady power. The latched design sidesteps that: the coil only sees power during the pick-up pulse, then the latch holds the contacts mechanically. On a 250 V DC bus this saves about 4–6 W per relay compared to a standard DC-held contactor. The 4 NO contacts are rated for DC switching at the same bus voltage. In a 250 V DC control circuit, each NO pole breaks the DC arc across a single gap — the contact material and arc-chamber geometry are sized for that. If you need DC switching at higher current levels, check the contact rating against your load's inductive DC curve; the datasheet's DC-13 utilization category gives the real breaking capacity.
Torque down to 0.8–1.2 Nm on the power terminals; the control terminals take 0.5–0.6 Nm. Auxiliary contact blocks (3RH29 series) clip onto the front or side without increasing the DIN footprint. The latch release is a manual pushbutton on the front face — useful for commissioning when you need to drop the relay without killing the control bus.