ABB AF vs Siemens SIRIUS 3RT: Which Contactor Fails First in a Tight-Cooling Shelter?

Myth #1: “All Coils Tolerate the Same Voltage Swing – a 24–500 V Coil Just Adds Convenience”

You’ve got a DC bus that sags to 90 V during a generator transfer, then spikes to 135 V when the charger kicks in. A standard Siemens SIRIUS 3RT contactor coil, say the 3RT2016 with a 110–120 V AC/DC coil (e.g., 1.1 W holding power), is designed for a narrow band: typically ±10 % of rated voltage. That means at 90 V DC the coil might not hold in — dropout voltage is around 75 % of rated, so 90 V is borderline. The real failure mode: coil dropout causes the main contacts to open under load, arcing and pitting them, dramatically reducing electrical life. The reheating during the next reclose can weld the contacts.

Worked consequence: In a shelter with a 48 V DC backup, the Siemens 3RT with a dedicated 48 V coil is fine if the battery stays at 52 V. But if you spec the common 110–120 V coil and run it through a DC/DC converter, you’ve added a potential single point of failure. ABB contactor’s AF contactor uses an electronic wide-range coil covering e.g. 100–250 V AC/DC — the same coil handles the sag and the spike without dropout. Per the datasheet, the AF09 coil holds in at 0.7×U_min (70 V for a 100–250 V range). That means the same coil survives a 90 V sag. The reversal: If your shelter has a perfectly regulated 48 V supply and you use the dedicated AF09 24–60 V wide-range coil (also available), the coil’s range covers both normal and brownout conditions. The failure mode only matters when the control voltage varies beyond the narrow coil’s tolerance — which is exactly the real-world case in shelters with generator start/stop cycles. For a fixed, clean 24 V or 120 V bus, the Siemens contactor coil is simpler and cheaper.

Myth #2: “AC-3 Current Rating Is the Only Thermal Constraint”

Both contactors are listed under IEC 60947-4-1. The ABB AF09 is rated 9 A AC-3 at 400 V (4 kW); the Siemens 3RT2016 is also 9 A AC-3 at 400 V. At first glance they’re equivalent. But the thermal reality inside a tight shelter is different: the contactor’s ability to dissipate heat depends on its thermal resistance, which is partly determined by the coil’s power dissipation. The ABB AF electronic coil consumes about 2–3 W holding power, while a standard Siemens AC coil draws 4–7 W holding. That extra 2–4 W inside a 10‑inch wide enclosure, when you have six contactors, adds 12–24 W of heat that the HVAC has to pull out. In a shelter with a 500 W cooling capacity, that’s 2–5 % of the thermal budget. More important: the heat from the coil raises the contactor’s internal ambient above the enclosure’s ambient. If the enclosure ambient is 40 °C, the coil’s internal heat can push the contactor’s local ambient to 55 °C, which is above the maximum operating temperature for some insulation classes. The failure mode: accelerated thermal aging of the contactor’s insulation (reducing dielectric strength) and increased contact resistance from oxide film growth.

Worked consequence: Let’s say you run a 3RT2016 at 7.5 kW (AC-3 16 A) — this is within its size S00 rating. At 55 °C internal ambient, the rated current must be derated per the manufacturer’s table; for a typical contactor, derating can be 0.8 %/°C above 40 °C, so at 55 °C you lose about 12 % of the AC-3 rating. That means the 16 A becomes about 14 A effective. If the motor pulls 15 A, you’re now in overload territory, and the contacts overheat, leading to contact welding. The ABB AF09 with the lower coil dissipation (The reversal: If you use a thermally compensated overload relay (like the Siemens 3RU2) that tracks the contactor’s temperature and trips earlier, the derating is mitigated. But the overload relay itself adds its own heat — the 3RU2 dissipates about 2–4 W, adding another heat source inside the shelter. The key decision threshold: if your shelter’s cooling capacity is less than 1.5× the total heat load from all contactors and relays, ABB’s lower coil dissipation gives you a measurable thermal margin.

Myth #3: “Mechanical Life = Electrical Life – They’re the Same for Both”

The ABB AF09 is rated for ~1 million mechanical operations; the Siemens 3RT2016 is also rated at around 1 million mechanical cycles. But electrical life under AC-3 load is usually a fraction of that — typically 100,000–200,000 operations at full load. The failure mode that kills contactors in a shelter isn’t the number of cycles; it’s contact erosion from repeated inrush currents during fan or pump starts. In a shelter, the HVAC compressor and fans cycle on/off many times per hour. Each start draws 6–8× the rated current for 0.1–0.2 s. The ABB AF09’s silver alloy contacts handle this, but the critical spec is the make/break capacity at 690 V — both are rated for 690 V. The real differentiator: the welding threshold voltage. Under heavy inrush, if the contact material reaches the melting point during the first half-cycle, the contacts can weld shut. The ABB AF contactors use a silver‑tin‑oxide (AgSnO₂) material that has a higher resistance to welding than the conventional silver‑cadmium‑oxide (AgCdO) used in many Siemens 3RT contactors. (Note: this is a typical material differentiation, but verify against specific datasheets — the Siemens 3RT2 may now use AgSnO₂ in some versions.)

Worked consequence: In a shelter with a 7.5 kW HVAC compressor starting 6 times per hour (144 times per day), the inrush current is about 60 A for 0.15 s. Over 5 years, that’s ~260,000 start transitions. With a AgCdO material, the cumulative erosion depth can reach 0.2 mm, reducing contact pressure and increasing resistance — leading to thermal runaway. The ABB AF09’s AgSnO₂ material is roughly 2× more resistant to welding under these conditions (per manufacturer statements). The reversal: If your shelter’s HVAC has a soft starter or VFD that limits inrush to 2× rated current, the material difference becomes negligible. In that case, the Siemens 3RT with a matched 3RU2 overload relay offers a simpler, more cost‑effective solution.

Myth #4: “Auxiliary Contact Count Doesn’t Matter – You Can Always Add a Block”

The ABB AF09 comes with one built‑in NO auxiliary contact; the Siemens 3RT2016 also has one built‑in NO contact. Both allow add‑on auxiliary contact blocks. The failure mode: in a shelter with high vibration (from nearby generators or fans), the mechanical linkage of an add‑on block can loosen, causing intermittent signals to the PLC. The ABB AF09’s electronic coil includes auxiliary contact status feedback that can be used for diagnostics without an extra mechanical block (the coil’s power supply status indicates contactor state). The Siemens 3RT does not offer this electronic feedback; you need a separate auxiliary block. In a shelter where a PLC monitors contactor state for fault detection, the ABB’s built-in electronic status reduces the number of mechanical contacts that can fail. The reversal: If you already have a control relay feeding back the contactor’s state, the extra auxiliary block is redundant, and the Siemens option is fine.