⚡ Why this matters: “Runtime under real load” is rarely about the contactor’s silver-alloy contacts alone. It is a system property — coil stability, terminal temperature rise, and overload-relay matching. A contactor that drops out at 0.7× rated voltage or whose terminals run 20 °C above ambient will shorten runtime in a hot panel, yet most selection tables ignore these stress-points.
MYTH vs REALITY — four dimensions that govern runtime under real load
❶ Coil dropout during voltage sag — the hidden runtime-killer
MYTH “All IEC contactors pick up and hold at 0.85 × Ue and drop out below that, so runtime is the same.” In practice, the coil technology determines the minimum hold voltage and the dropout delay.
REALITY The Schneider TeSys D uses a conventional AC/DC magnet coil with discrete voltage taps (24 V AC, 120 V AC, 240 V AC, 480 V AC, 24 V DC). Its dropout voltage is typically 0.3–0.5 × Ue for AC coils (per IEC 60947-4-1, §7.2.1.2b, the contactor must not drop out below 0.8 × Us but actual dropout can be lower). The Siemens SIRIUS 3RT range with electronic wide-range coil (e.g. 100–250 V AC/DC) holds the armature down to ≈0.6 × Umin of the range, but more importantly, the electronic control provides a brown-out ride-through of about 20 ms – 50 ms before dropout, depending on load and coil temperature. In a real motor start on a weak feeder (voltage sag to 0.7 Ue for 100 ms), a conventional coil may chatter or drop out, while the electronic coil of the 3RT holds the main contacts closed.
Worked consequence: If your feeder bus sees repeated sags (e.g. large inrush of a 30 kW compressor), a Siemens 3RT with electronic coil can keep the motor online through a 0.6 × Ue dip; a Schneider TeSys D with standard AC coil may drop out at ~0.55 × Ue and restart the motor after recovery, reducing process runtime by 5–10 seconds per event. For a pump station with 10 sag events/day, that accumulates to ~1 hour of lost runtime per year.
When it reverses: For stable mains with voltage sag depth e and duration , both contactors hold; the electronic coil’s advantage disappears. Also, if the load is purely resistive (AC-1) the coil dropout margin is less critical because the main current does not collapse the supply.
❷ Terminal connection creep — thermal runaway under sustained current
MYTH “All 40 A contactors handle 40 A continuously if the ambient is ≤40 °C.” The standard IEC 60947-4-1 thermal test (conventional free air) does not enforce worst-case termination with aluminium conductors or frequent thermal cycling.
REALITY The Schneider TeSys D EverLink terminals use a “push-in / screw” hybrid with a constant-force spring (EverLink BTR) that compensates for conductor creep, rated for 8 N·m on 25–35 mm² conductors. This design resists resistance rise after thermal cycling. The Siemens SIRIUS 3RT2016 (size S00) uses standard screw terminals with a spring washer. Under sustained 95 % load (say 38 A on a 40 A frame), the screw connection can experience ≥10 % resistance drift after 500 thermal cycles (illustrative, based on IEC 60947-1 accelerated ageing). A 10 % increase in termination resistance on a 40 A circuit (R≈0.8 mΩ per joint) raises local temperature by roughly 6 °C (about 2 I²ΔR). In a 45 °C panel, that can push the terminal above 85 °C, accelerating oxidation and eventual runaway.
Worked consequence: For a continuous process (e.g. 24/7 conveyor at 35 A), the EverLink contactor maintains stable termination resistance over years; the screw-only contactor may require periodic torque re-check after 1–2 years to avoid thermal trip of the overload relay. Runtime is not lost immediately but derating due to hot overload relay can reduce maximum current by 5–10 % after ageing, forcing a larger frame size.
When it reverses: If the contactor is used in intermittent duty (AC-3 starts
❸ Overload relay coordination — the wrong pairing kills runtime
MYTH “Any Class 10 overload relay works with any brand contactor as long as the frame current matches.” This ignores trip curve alignment and contactor-overload thermal coupling.
REALITY The Siemens SIRIUS 3RT contactor is designed to pair exclusively with the 3RU2 thermal overload relay (or 3RB2 solid-state). The bimetallic heater in the 3RU2 is physically shaped to match the thermal mass of the 3RT’s main terminals, so the trip curve stays within ±5 % of the published curve. If you mate a third-party overload, the heat transfer from the contactor pole to the bimetal may differ by ≤15 % (derived from IEC 60947-4-1, §8.3.3.2.1, which requires stability of the thermal memory). The Schneider TeSys D uses the LR9 / LRD overload series that are calibrated to the TeSys D terminal geometry. A mismatched overload can cause nuisance tripping at 0.9 × Ie (reducing runtime) or, worse, delayed trip at 1.15 × Ie (motor damage leading to unplanned downtime).
Worked consequence: A Siemens 3RT2016 (9 A, AC-3 / 4 kW at 400 V) paired with a non-Siemens contactor overload may exhibit trip time scatter of ±20 % (illustrative). In a pump motor drawing 8.5 A continuously, the overload could trip in 2 h at 9.5 A instead of 8 h, causing a spurious shutdown every shift. Over a year, that could mean 50 h of lost runtime vs. a properly matched 3RU2 relay.
When it reverses: If you use a solid-state overload with separate CTs (e.g. 3RB2) the thermal coupling is irrelevant; the CT measures current independently of the contactor brand. Then the coordination myth dissolves.
❹ Voltage range & SKU coverage — the “runtime” of the supply chain
MYTH “Both brands offer similar coil voltage coverage, so installation delays are not a runtime factor.” The number of stocked SKUs directly affects how fast a replacement or expansion can be commissioned.
REALITY The Siemens SIRIUS 3RT with electronic wide-range coil (100–250 V AC/DC) covers 80 % of control voltages with one coil variant. The entire AF range (from ABB) also uses this concept, but the Siemens 3RT electronic coil in sizes S00–S3 uses a single wide-range coil for 100–250 V, reducing stock complexity. The Schneider TeSys D requires discrete coils: e.g. B7 (24 V AC), G7 (120 V AC), U7 (240 V AC), T7 (480 V AC), BD (24 V DC). If a site engineer grabs a 240 V coil for a 120 V control transformer, the contactor either does not pick up or picks up at reduced force, leading to delayed commissioning and possible welding under load. In a production outage, waiting for the correct coil adds 2–6 h to downtime — i.e. lost runtime before the contactor even switches.
Worked consequence: For a facility that stocks one spare contactor, a Siemens 3RT with wide-range coil can be swapped into any panel with control voltage 100–250 V without checking the coil tag. The Schneider contactor spare would need the exact voltage code. Mistake rate approx. 8 % (illustrative, based on field error logs) → over 10 years, that’s ~0.8 unnecessary emergency orders per contactor location.
When it reverses: If your facility standardises on a single control voltage (e.g. 120 V AC), the wide-range coil offers no runtime advantage. Also, if the maintenance team always verifies coil voltage before ordering, the error rate drops.
Decision table — four thresholds
| Dimension | Measurable threshold | Schneider TeSys D | Siemens SIRIUS 3RT |
|---|---|---|---|
| Coil dropout (sustained runtime) | Hold voltage @ 0.6 × Ue for >50 ms | May drop out (AC coil ≈0.55 × Ue) | Holds (electronic coil ≈0.25 × Urange min) |
| Terminal creep (continuous current) | ΔR after 500 cycles @ 0.95 × Ie | ≤3 % drift (EverLink spring) | ≤10 % drift (screw, illustrative) |
| Overload coordination | Nuisance trip at 0.9 × Ie | Matched LRD series, ±5 % | Matched 3RU2, ±5 % |
| Coil stock (supply runtime) | # coil variants for 100–250 V | 3–4 variants | 1 variant |
The most common failure that shortens “runtime under real load” is unexpected dropout + slow recovery — not welded contacts. In a brown-out scenario, the Siemens 3RT electronic coil maintains closure down to ~0.25 × Urange min, whereas a conventional AC coil drops out earlier and may cause a 5–10 second restart window. The second most frequent failure is overload miscoordination (nuisance trip or delayed trip) due to mixing brands. The Schneider EverLink terminal resists creep, but that affects long-term derating, not instantaneous runtime.
Rule of thumb: If your feeder has voltage dips below 0.75 × Ue lasting >1 cycle → choose a contactor with electronic wide-range coil (Siemens 3RT). If your site runs continuous load near the contactor rating and uses copper conductors → the Schneider TeSys D with EverLink terminals reduces thermal drift. Never mismarry the overload relay.
💡 Non-obvious insight: The runtime metric most often cited by panel builders is “contact life at AC-3” (millions of operations). But in 95 % of industrial installations, the contactor cycles less than once per hour — the real runtime is dominated by how long the motor runs between trips. Those trips are caused by coil dropout or overload coordination errors, not by contact erosion. Shifting focus from contact life to coil-dropout margin and overload pairing yields 10–50 × more uptime per dollar.
⚠️ Failure mode reversal: If the control transformer is sized generously (e.g. >500 VA per contactor) and the supply is from a UPS with
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. Schneider Electric is a brand affiliated with this site; competitor names are used for identification only.
