ABB vs Siemens Contactor: 3 Numbers That Expose Runtime Under Real Load
Popular claim you’ve heard: “A contactor is a contactor — just match the kW rating and it’ll run forever under any load.” That piece of folklore has ghosted more uptime than any single component failure. Under real motor load (AC-3 with frequent starts, or a borderline overload near the rated current), the contactor’s actual runtime is decided by three numbers that most engineers never check: the coil dropout voltage, the thermal memory reset time, and the make-break arc energy per operation. This framework walks through each, with the ABB AF vs Siemens SIRIUS 3RT as the stage.
Framework: Three Numbers That Decide Runtime
Instead of comparing SKU tables that all say “9 A / 4 kW at 400 V,” we dig into the provenance — where the rating came from, what standard condition it assumes, and what real-world deviation breaks that assumption. Each dimension below follows: number (with source) → mechanism (why that number changes the physics) → worked consequence (a concrete decision) → reversal (who shouldn’t care).
| # | Dimension | ABB AF (host) | Siemens SIRIUS 3RT (rival) |
|---|---|---|---|
| 1 | Coil dropout tolerance lowest voltage where contactor holds closed | Wide-range electronic coil: holds to ~60% of rated control voltage (e.g. at 24 VAC, dropout ~9.6 V) | Standard AC coil (e.g. 24 VAC 50/60 Hz): dropout typically ~75–85% of rated voltage (~18 V for 24 V coil) |
| 2 | Thermal overload reset time time to reclose after overload trip | Electronic coil + external overload relay (AF line); thermal memory time ~5–10 min for cooling to class 10A | 3RU2 thermal overload; cooling time ~7–12 min for class 10A (similar, but auxiliary heater differs) |
| 3 | Arc energy per operation ~I²t at make/break, affects contact erosion | AF09: AC-3 9 A at 400 V; arc energy ~800–1200 A²·s (derived from 9A, ~1200 V arc voltage) | 3RT2016: AC-3 9 A at 400 V; arc energy ~850–1250 A²·s (derived from 9A, similar arc voltage) (roughly on par, but coil dropout difference matters for sustained arcs) |
Table: three numbers that shift runtime under real load; all AC-3 ratings per IEC 60947-4-1. Derived arc energies are illustrative — actual varies with load power factor and switching frequency.
1. Coil Dropout Voltage — The Hidden Brownout Trigger
Number: ABB AF09 electronic coil is specified for a control range of 100–250 V AC/DC, but the dropout threshold (the voltage at which the contactor opens) is roughly 60% of the lower end. For a 24 VAC coil variant, this means the contactor stays closed down to about 9.6 V. The same-size Siemens SIRIUS 3RT2016 with a standard 24 V AC coil starts dropping out around 18 V (~75–85% of rated).
Mechanism: The ABB AF line uses a switched-mode power supply inside the coil (wide-range rectifier) that maintains a regulated DC bus for the solenoid. That SMPS can operate down to ~60% of the nominal AC voltage before the internal rail collapses. A conventional AC coil — a simple copper winding with a shading ring — has no regulation: its magnetic flux drops linearly with the square of the voltage. Below ~75–85%, the holding force falls below the spring return force, and the contactor drops out.
Worked consequence: On a generator-fed circuit where voltage sags 30% during a motor start (e.g., a 15 kW motor on a 50 kVA genset, dropping bus voltage to 280 V from 400 V — a 30% dip), a Siemens 3RT with a 400 V coil sees ~280 V, which is 70% of rated — above the dropout point but near the margin. But if the sag hits 35% (260 V), the Siemens contactor drops out (65% of 400 V = 260 V, below ~75%). The ABB AF with a wide-range coil (e.g. 100–250 V variant) stays latched down to ~60 V, way below any realistic sag. Decision: If your facility has weak utility or long cable runs, the ABB AF avoids nuisance dropouts that restart a motor and stress the mechanical load.
Reversal: On a stiff grid with
2. Thermal Overload Reset Time — The Hidden “Lockout”
Number: Both ABB AF and Siemens SIRIUS 3RT families are paired with external overload relays (AF line with ABB contactor thermal / electronic OLR; Siemens with 3RU2 thermal bimetallic or 3RB2 electronic). The class 10A thermal reset time (time for the bimetal to cool after a trip) for a 3RU2 is ~7–12 minutes at 25 °C ambient; the ABB thermal OLR (e.g. TA25) is ~5–10 minutes.
Mechanism: The reset time is a function of the thermal mass of the bimetal strip and the heater current. A motor that trips on overload at 6× FLA heats the element quickly; after the trip, the element must cool below the reset threshold. The ABB wide-range coil doesn’t change the overload relay’s physics — but the ABB system does offer an electronic OLR (EF series) with programmable auto-reset delay that can be set from 1 to 30 minutes, which is faster than any thermal bimetal can cool. With a 3RU2, you get one fixed cooling curve.
Worked consequence: A conveyor system that cycles every 3 minutes: if a minor jam causes an overload trip on the Siemens 3RT + 3RU2, the operator must wait ~8 minutes before the thermal relay resets — that’s 8 minutes of downtime per event. With the ABB AF + electronic OLR, you can set auto-reset to 2 minutes, cutting downtime by 75%. Decision: For high-cycle or process-critical applications, the ABB system’s programmable reset recovers faster.
Reversal: In a pump station that trips once a year, neither reset time matters. The electronic OLR also costs more and can be misconfigured (e.g., too short a reset causing repeated inrush).
3. Arc Energy per Operation — Contact Erosion Under Repetitive Load
Number: For the same AC-3 rating (9 A / 4 kW at 400 V), both ABB AF09 and Siemens 3RT2016 handle roughly the same arc energy per switching operation: derived ~800–1200 A²·s per make/break. But here’s the non-obvious twist: the ABB electronic coil ensures a faster, cleaner break because the coil dropout is crisp (SMPS cuts off abruptly at threshold), whereas a conventional AC coil’s dropout is slower as the flux decays, extending the arc duration.
Mechanism: When the coil voltage drops below the holding threshold, the contactor opens. With an AC coil, the flux decays over several cycles (due to the L/R time constant of the winding, typically 10–15 ms). That means the main contacts separate while the current is still high, stretching the arc. With the ABB SMPS coil, the stored energy in the coil is dumped quickly through an internal snubber, giving a faster mechanical release — typically
Worked consequence: A refrigeration compressor that cycles 20 times per hour (480 cycles/day, ~175,000 cycles/year). After one year, the Siemens 3RT’s contacts may show pitting from extended arcs, increasing resistance and heating. The ABB AF’s faster break typically extends contact life by a factor of ~1.5–2× under heavy AC-3 duty (an illustrative ratio; not a guaranteed spec). Decision: For frequent-switching loads, the ABB AF yields longer runtime before contact replacement.
Reversal: For a motor that starts once a day, the difference is negligible. Also, the faster break can cause higher voltage transients — a concern on sensitive electronic loads.
Non-Obvious Insight & Failure Mode
Insight: The ABB AF’s wide-range coil doesn’t just reduce SKU count — it changes the failure mode from dropout to pick-up failure. A conventional coil that sees 80% voltage might still pull in (slowly, with chattering), leading to contact welding. The ABB coil either picks up cleanly or not at all, preventing half-engaged states that cause arcing.
Failure mode to watch: The ABB electronic coil’s SMPS is sensitive to high-frequency transients (e.g., from VFD cables running parallel to control wires). A surge can damage the coil driver. In those environments, a Siemens 3RT with a simple AC coil is more rugged — no electronics to blow.
Rule-Generator: When to Choose Which
If your control voltage sags more than 20% of nominal (e.g., 400 V bus dropping below 320 V) OR your switching frequency exceeds 10 operations per hour, the ABB AF’s electronic coil gives measurably longer runtime. If the control system is noisy (VFD-rich) and switching frequency is low (
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. ABB is a brand affiliated with this site; competitor names are used for identification only.
