3RH1122-1BF40 | Siemens Sirius 3RH1 Contactor Relay (Auxiliary Contactor) Screw Terminal 6 A, 2 NO + 2 NC, 110 V DC, Size S00 – Siemens

Q: Is the 3RH1122-1BF40 rated 6 A for AC-3 motor duty or AC-1 resistive duty?

A: The 6 A rating of this Siemens contactor applies to AC-3 duty (squirrel cage motor starting and running). For AC-1 (resistive/non-inductive) loads, the current can be up to ~1.3× the AC-3 rating. AC-4 (plugging/reversing) requires significant de-rating — consult the Siemens selection table.

Q: What does the 2 NO + 2 NC contact configuration on the 3RH1122-1BF40 mean?

A: This device has both Normally Open (NO) and Normally Closed (NC) contacts. NO contacts close when the device operates (energised/tripped); NC contacts open. This allows both start-interlock and stop-interlock circuits from a single device.

Q: Can multiple 3RH1122-1BF40 contactors be wired for star-delta starting?

A: Yes. Star-delta starters use three contactors: main (KM1), star (KM2), and delta (KM3). The Siemens contactor series supports mechanical and electrical interlocking accessories that prevent simultaneous energisation of star and delta contactors — critical for safety. Ensure all three contactors are rated correctly for the motor FLC and star-delta transition current.

Q: What is the electrical and mechanical life of the 3RH1122-1BF40?

A: Siemens contactors are rated for millions of operations: typically 10–30 million mechanical operations and 1–3 million electrical operations at rated current and voltage (AC-3 duty). High-frequency switching applications (>10 operations/hour) require checking the specific thermal capacity of the contactor for continuous duty without overheating.

Q: Can the 3RH1122-1BF40 be used for reversing duty (forward/reverse motor control)?

A: Contactors used for reversing must be wired with two contactors interlocked so only one can energise at a time. The Siemens series provides mechanical interlock kits that physically prevent both contactors from closing simultaneously — mandatory for reversing and brake applications to prevent phase-to-phase short circuits.