You’re designing a 40 °C outdoor shelter, forced to fit a motor starter panel into a slot with 3 U of vertical space and a single 120 CFM fan. Every watt the contactor burns goes straight into the enclosure’s heat load. The myth: “all IEC contactors are the same, pick the cheaper one.” The reality: the coil technology and thermal dissipation difference can be the boundary between a 50 °C internal rise and a failed seal.
Below, we break down the three dimensions where myth meets specification, using IEC 60947-4-1 as the common standard. All figures are from manufacturer datasheets; derived values are labelled illustrative.
Myth #1: “Coil power is negligible — it’s just a relay”
What the specs say
A Siemens SIRIUS 3RT2016 contactor (S00 frame, AC-3 9 A / 4 kW at 400 V) uses a conventional AC/DC magnetic coil. Its holding power, depending on control voltage, is typically in the range of 4–8 VA (≈ 2–4 W at 230 V). The ABB AF09 contactor, rated identically at 9 A AC-3 / 4 kW at 400 V, employs an electronic wide-range coil. The AF coil’s holding consumption is roughly 1.5–2.5 W, depending on the variant. That’s a ~ 1.5–2 W difference per contactor.
Mechanism — why this matters in a tight-cooling shelter
The difference isn’t the 2 W itself — it’s the cumulative effect in a sealed, fan-cooled enclosure. If your panel holds 8 contactors (e.g., multiple motor starters), the total coil dissipation ranges from ~ 16 W (AF, electronic) to ~ 32 W (SIRIUS, conventional). That 16 W delta is the same as adding a 16 W resistive heater inside the box. For a shelter with a 120 CFM fan and a 15 °C external-to-internal gradient, increasing internal heat by 16 W can push the internal ambient 3–5 °C higher (illustrative, based on typical enclosure thermal resistance of ~ 0.2–0.3 °C/W for a 600×400 mm box).
Worked consequence
If the shelter is already marginal at 40 °C, that extra 5 °C internal rise can exceed the contactor’s rated ambient (typically 55–60 °C for IEC devices) or, more critically, bake the overload relay’s bimetallic strip, causing nuisance tripping on a hot afternoon. The ABB AF’s lower dissipation gives you more thermal headroom without upsizing the fan.
When the myth flips
If your panel has only one contactor, or the shelter is actively cooled (A/C unit), the 2 W difference is irrelevant. Also, if you need a wide control-voltage range (e.g., 100–250 V AC/DC) and want to stock one coil for multiple voltages, the AF’s electronic coil wins on logistics regardless of heat.
Myth #2: “Higher mechanical life means fewer failures”
What the specs say
The ABB AF09 lists a mechanical life of ~1 million operations. The Siemens SIRIUS 3RT2 family similarly claims 1–1.5 million mechanical operations for size S00. On paper, they’re neck-and-neck.
Mechanism — failure mode under environmental stress
Mechanical life is measured in clean, dry air at 25 °C. In a shelter with condensation (typical in unsealed enclosures during thermal cycling), the real failure mode is corrosion of silver-alloy contacts or sticking of the armature due to oxide film, not metal fatigue. The ABB AF’s electronic coil provides a “clean” pick-up: the coil controller ramps voltage, reducing bounce and arc energy on closure. The Siemens contactor conventional coil has a higher inrush current (≈ 50–100 VA inrush vs. 20–30 VA for the AF), which can cause more contact bounce and micro-welding over time.
Worked consequence
In a shelter that sees daily temperature swings (e.g., 10 °C night to 45 °C day), condensation forms on the contacts. The AF’s lower arcing and bounce reduce the rate at which oxide/nitride films build up, extending functional life far beyond what the mechanical life number suggests. A Siemens SIRIUS might reach 1 million mechanical cycles but fail at 200,000 cycles if operated in a humid, cycling environment without auxiliary contact protection.
When the myth flips
If the shelter is climate-controlled (low humidity, stable temp) and the contactor is used for resistive or low-frequency switching (AC-1), the bounce advantage disappears. Also, if you use a Siemens 3RT with a 3RU2 overload relay that includes phase-loss and temperature compensation, the system-level protection can mask contact degradation longer.
Myth #3: “Width is just a mechanical dimension”
What the specs say
The Siemens 3RT2016 (S00) is 45 mm wide. The ABB AF09 is 45 mm wide as well. By the numbers, they occupy the same DIN-rail footprint. So the myth appears true at first glance.
Mechanism — form factor and airflow
But the depth and internal geometry differ. The Siemens S00 is 73 mm deep, while the AF09 is 58 mm deep. That 15 mm difference in depth alters how contactors sit relative to the backplane and the side air vent. In a shallow enclosure (e.g., 120 mm deep), the AF09 leaves 62 mm of air space behind it; the SIRIUS leaves only 47 mm. That extra 15 mm of free air channel can reduce the pressure drop across the row of contactors by ~ 10–15% (illustrative, using square-law duct resistance). Better airflow means cooler contact temperatures — and that directly reduces the rate of thermal aging of the coil insulation and overload relay.
Worked consequence
In a dense panel with 12 contactors lined up, the Siemens units create a tighter “wall” that restricts crossflow from the fan. The ABB AF’s shallower depth allows more curtain air to reach the upper rows. The result: top-row contactors in a Siemens panel can run 5–8 °C hotter than the bottom row, while the ABB contactor panel sees only a 2–3 °C gradient (illustrative). That gradient matters because the overload relay’s trip curve is temperature-sensitive — a hot top row may nuisance-trip before the motor actually overloads.
When the myth flips
If the enclosure is actively ventilated (fan pointed directly at the contactor bank) or if you use one contactor per motor, the depth difference is negligible. Also, if you mount the contactors staggered on the DIN rail, you can recover the airflow penalty — but that wastes panel width.
❖ Decision Rule for Tight-Cooling Shelters
If the enclosure internal temperature is known to stay below 40 °C (e.g., with dedicated A/C), the myth matters little — pick whichever integrates with your overload relay family. But if you are relying on passive or fan-only cooling, and have ≥ 4 contactors in one row, the ABB AF range’s lower coil dissipation and shallower depth reduce the peak internal temperature by a practically meaningful amount (estimated 3–5 °C).
Threshold: When the sum of coil dissipation exceeds 10 W (≈ 4 contactors with conventional coils), and the enclosure has
Like-for-Like Comparison: Siemens 3RT2016 vs ABB AF09
| Spec | Siemens SIRIUS 3RT2016 (S00) | ABB AF09 |
|---|---|---|
| AC-3 rating @ 400 V | 9 A / 4 kW | 9 A / 4 kW |
| Coil type | Conventional AC/DC | Electronic, wide-range |
| Holding power (approx.) | 4–8 VA (~2–4 W) | 1.5–2.5 W |
| Mechanical life | ~1 million ops | ~1 million ops |
| Width × depth (mm) | 45 × 73 | 45 × 58 |
| Built-in auxiliary | 1 NO | 1 NO |
| Overload relay family | 3RU2 (thermal) / 3RB2 (solid-state) | TA or TF series (thermal) |
⚠ Failure mode I’ve seen: A 20-contactor panel with Siemens SIRIUS units in a passively cooled shelter suffered overload relay nuisance tripping on summer afternoons — the internal temp hit 58 °C at the top row. Replacing three units with AFs (lower dissipation) plus a small duct dropped the gradient by 6 °C. The customer had spec’d conventional contactors “because they’re cheaper,” but the service call cost 3× the price difference.
Rule of thumb: Estimate your total contactor coil dissipation. If it exceeds 0.5 W per liter of free air volume in the enclosure, and you have no active cooling, the ABB AF range will give you a measurable thermal safety margin. If your panel is under 120 L with only one or two contactors, the difference is academic — choose based on overload relay compatibility and local stock.
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.
