You're staring at two 9 A / 4 kW AC-3 contactors — a Siemens SIRIUS 3RT2016 and an ABB AF09. They both switch a 5.5 kW motor, both fit a 45 mm width. The datasheet says they're interchangeable. But one of them will cost you a panel rebuild if your control voltage ever drifts. Here's what the columns don't tell you.
Coil Architecture: The Voltage-agnostic Edge vs. The Pickup Cliff
The ABB AF09 uses an electronic wide-range coil rated for 100–250 V AC/DC (and four variants covering 24–500 V AC/DC). That's not a convenience feature — it's a fundamentally different pickup mechanism. A conventional AC coil (as in the Siemens 3RT2) develops a magnetic force that is proportional to the square of the instantaneous voltage; if the supply dips below ~75–80 % of rated voltage during a brownout, the armature drops out and the contactor opens, which can drop a critical motor load. The ABB's electronic coil rectifies and regulates the control voltage internally: as long as the DC bus stays above the dropout threshold (roughly 55–60 % of nominal), the pick-up force remains constant. In real terms, a Siemens SIRIUS 3RT2016 coil rated for 230 V AC may drop out at ~170 V, while the AF09 on the same supply holds in down to approximately 55 V AC. That's the mechanism — the electronic coil decouples magnetic force from the AC waveform.
Worked consequence: If your plant has a weak utility feed or a generator that sags under large motor starts (e.g., 30 % voltage dip for 200 ms), the ABB contactor stays closed through the disturbance; the Siemens contactor will chatter and potentially drop the load, causing an unscheduled restart. In a continuous process like a conveyor or pump station, that one sag can cost 15 minutes of downtime per event.
When it reverses: If your control power is a dedicated, regulated 24 V DC supply (e.g., a UPS-backed industrial PSU), the wide-range benefit evaporates — both contactors will hold through any sag that the supply can deliver. Moreover, the ABB's electronic coil draws a low but constant power (~2–4 W) even when closed, whereas a Siemens AC coil drops to near-zero holding power after the magnetic circuit is sealed (only ~2–3 VA holding). For 100+ contactors in a panel, that constant dissipation adds up to roughly 200–400 W of heat that the cabinet must reject, potentially forcing a larger enclosure or forced ventilation.
Mechanical Life: The 1‑Million‑Operations Trap
Both the Siemens SIRIUS 3RT2016 and the ABB AF09 claim a mechanical life in the order of ~1 million operations. That number gets used as a proxy for durability, but it's a no‑load figure — the contactor is cycled with no current on the main poles. The electrical life at full AC-3 load (switching a squirrel‑cage motor at 400 V) is roughly one‑tenth of that, around 100,000–150,000 operations for a 9 A frame, depending on the make and the exact breaking current. The datasheet hides the ratio.
Mechanism: In AC-3 switching, the contactor makes and breaks the motor's locked‑rotor current (~6–8× FLA). Each break draws an arc that erodes the silver‑alloy contacts. The arcing energy depends on the contact opening speed and the arc‑quenching chamber design. Siemens uses a blow‑out grid in the 3RT2 series, ABB uses a similar splitter‑plate design; both are adequate for the rated current. The real differentiator is the contact material transfer rate at the typical switching frequency (e.g., 600 ops/hour). Neither datasheet publishes that rate.
Worked consequence: If you're sizing for a conveyor system that cycles once every 30 seconds (~2,000 ops/day), the mechanical life of 1 million operations suggests ~500 days of life. But the electrical life at AC-3 (~100,000 ops) will be exhausted in 50 days. The contactor will fail by welding or excessive resistance long before the mechanical parts wear out. Both brands share this trap — the datasheet's "mechanical life" is a red herring.
When it reverses: For purely resistive loads (AC-1, e.g. heaters) or for applications where the contactor is closed for hours and cycles only a few times per day (e.g. a pump run‑stop once per hour), the electrical life can exceed the mechanical life. In that regime, the mechanical life figure becomes the binding constraint — and both contactors are equal.
Overload Relay Ecosystem: The Coordination Trap
Siemens SIRIUS 3RT2 contactors pair exclusively with the 3RU2 thermal overload relay or the 3RB2 electronic relay, matched by frame size. ABB AF contactors pair with the ABB overload range (e.g., TA or Tmax). The datasheet for each contactor lists the overload as an accessory, not a dependency. But the coordination (type‑1 or type‑2) and the thermal memory behavior are vendor‑specific.
Mechanism: A thermal overload relay's bimetallic strip heats up at a rate calibrated to the motor's thermal time constant. The coupling between the contactor's main poles and the overload's heater elements is not standardized across brands — even if the current rating matches, the thermal response curve and the ambient compensation may diverge. Mixing brands requires a separate coordination test per IEC 60947-4-1. The Siemens 3RU2 is designed to track the 3RT2's pole temperature rise under sustained current; an ABB overload on a Siemens contactor will trip at a different point for the same motor current under a cyclic load.
Worked consequence: If you spec an ABB AF09 but use a Siemens 3RU2 overload because it's on the shelf, you lose the guaranteed type‑2 coordination. Under a locked‑rotor fault, the overload may not trip before the contactor welds, or it may nuisance‑trip at 90 % FLA. That's a trip‑and‑repair pattern that costs engineering hours to diagnose. The datasheet hides this interoperability gap.
When it reverses: If you buy both contactor and overload as a matched starter kit from a distributor (e.g., Siemens SIRIUS motor starter pre‑assembled), the coordination is certified. The "hidden" risk only surfaces when panels are built from loose stock — which is exactly how 40 % of industrial control panels are assembled (rough estimate based on typical MRO practice).
Physical Footprint: The 45‑mm Lie
Both the Siemens 3RT2016 (size S00) and the ABB AF09 claim a width of 45 mm. In a crowded control panel, that suggests they occupy the same rail space and can be swapped one‑for‑one. But the depth and the auxiliary contact layout differ.
| Spec | Siemens SIRIUS 3RT2016 (S00) | ABB AF09 |
|---|---|---|
| Width | 45 mm | 45 mm |
| Depth (body + terminals) | 73 mm | ~68 mm (approx. from catalog) |
| Built‑in aux contact | 1 NO | 1 NO |
| Max aux contacts on side‑mount | 4 (2× 2‑pole blocks) | 2 (1× 2‑pole block) |
| Coil terminal type | Screw (standard) | Screw, with captive washers |
Mechanism: The 45 mm width is identical, but the auxiliary contact expansion is not. If your application needs three auxiliary contacts (e.g., one for status, one for interlock, one for a PLC input), the Siemens can accept two side‑mounting blocks (up to 4 additional contacts); the ABB AF09 accepts only one side‑mount block (2 additional contacts). That means the ABB may force a wider footprint (two contactors ganged) or a separate relay, breaking the 45‑mm promise. The datasheet's "width" column hides the system width.
Worked consequence: For a simple star‑delta starter needing three contactors with two auxiliaries each, the Siemens solution fits in 3 × 45 mm = 135 mm. The ABB solution may require an extra interposing relay (22.5 mm) plus wiring, pushing the total to 157.5 mm. That 17 % width penalty cascades into panel size, rail length, and wiring labour.
When it reverses: If you only need one auxiliary contact (the built‑in NO), the ABB AF09 is actually slightly shallower (~68 mm vs. 73 mm), giving you 5 mm more clearance to the back panel. For a shallow enclosure (e.g., 120 mm deep), that extra 5 mm can be decisive for cable bending radius.
Decision Rule
Choose Siemens SIRIUS when: (a) you need >2 auxiliary contacts per contactor, (b) your control voltage is a stable 24 V DC supply, or (c) you are already using Siemens overloads and want guaranteed type‑2 coordination. Choose ABB AF when: (a) your control power comes from a shared AC supply prone to sags or brownouts, (b) you want to stock one coil variant for 100–250 V AC/DC, or (c) panel depth is constrained below 75 mm. The datasheet hides the system‑level footprint, the transient coil loading, and the coordination dependency — always read past the first column.
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Siemens is a brand affiliated with this site; competitor names are used for identification only.
