Siemens SIRIUS vs Schneider TeSys: which contactor survives a tight-cooling shelter?

✅ Reality: IEC 60947-4-1 defines conventional free-air thermal current (I_th) at 40°C ambient. In a shelter with 48°C internal temperature and restricted convection, the effective allowable current drops. A Siemens SIRIUS 3RT2016 (size S00) is rated 9 A AC-3 at 400 V; its physical dimensions are 45 mm wide × 57.5 × 73 mm. That’s a small thermal mass. At 48°C ambient with no forced airflow, the temperature rise inside the contactor housing (copper contacts, steel yoke) increases about 10–12°C above the 40°C calibration point — meaning the actual contact temperature may exceed the 85°C threshold for silver-alloy softening. The failure mode is contact welding under repeated inrush, not an immediate trip. A Schneider TeSys D LC1D18 (18 A AC-3 chassis) has a larger arc chamber and a quoted I_th of 32 A; its thermal margin is wider, but the coil dissipation (typically 5–8 W for AC coils vs ~2–3 W for ABB’s electronic coil [2,8]) adds heat inside the shelter. The decision flips: if the shelter cooling is marginal, a contactor with a lower coil power draw (like Siemens’ standard AC coil? actually Siemens 3RT2 AC coils draw about 5–7 VA hold) reduces the internal heat load by a few watts — but the Schneider contactor’s larger contact mass might handle the elevated ambient better even though its coil runs hotter. Non-obvious insight: the limiting factor in a tight shelter is not the contact rating on the datasheet but the coil insulation class. A standard Class B coil (130°C) can withstand 48°C + self-heat, but if the shelter hits 60°C (e.g. cooling failure), a Class F coil (155°C) becomes mandatory. Neither Siemens SIRIUS 3RT nor Schneider TeSys D standard range offers Class F as default — you must order special variants [5,10]. When this reverses: if the shelter has adequate forced airflow (e.g. a 120mm fan blowing across the contactors), the thermal advantage of one brand over the other disappears — both will stay below their rated temperature rise.

✅ Reality: The electronic coil in ABB AF contactors draws only ~2–3 W and works from 100–250 V AC/DC (wide range) [2,8]. That’s fantastic for global inventory and brownout ride-through. But in a tight-cooling shelter, the electronic coil has a failure mode that a conventional AC coil does not: its switching power supply is sensitive to ambient temperature + humidity cycling. If condensation forms on the PCB inside the coil housing (common in shelters where the mini-split cycles on/off, causing dew point swings), the electronic components can fail short or open without warning. A conventional AC coil — like those in Siemens SIRIUS 3RT2 — is a simple solenoid with a copper winding; it is far more tolerant to condensation and thermal shock (no semiconductors to fail). The Siemens 3RT2 coil draws about 5–7 VA (hold), slightly higher than the ABB AF, but its mean time between failures under humid conditions is an order of magnitude longer. Worked consequence: if your shelter is in a desert climate with extreme diurnal swings (e.g. Arizona), the wide-range coil may fail within 2–3 years due to micro-cracks on solder joints caused by repeated thermal expansion. The conventional coil will last the mechanical life (~10⁶ ops). When the myth holds: if the shelter has active climate control that holds RH below 50%, or if you have a single control voltage across the entire plant (so you don’t need the wide-range benefit), the electronic coil is still a valid choice — but the failure mode shifts to voltage spikes. In a shelter fed by a long overhead line (lightning-prone), the electronic coil’s MOV and rectifier can fail from a transient that a conventional AC coil would ride through. The rule: choose conventional AC coil when humidity swings exceed 40% RH daily, and choose wide-range only when you have

✅ Reality: Overload relays are not interchangeable across brands because the thermal model (trip curve) and the contactor’s arc chamber dynamics are paired. The Siemens 3RU2 overload is designed to coordinate with the 3RT2 contactor’s contact gap, blow-out geometry, and arc energy. If you substitute a Schneider LR9 overload on a Siemens 3RT2, the trip curve may be correct (both IEC 60947-4-1 class 10/20) but the short-circuit coordination with the upstream fuse or breaker will be untested. In a shelter with limited short-circuit capacity (e.g. 5 kA from a small generator), mismatched coordination can cause the contactor to weld shut during a bolted fault instead of the overload relay opening. Non-obvious insight: the actual failure mode that kills the contactor in a tight shelter is not overload but single-phasing from a broken conductor. The overload relay’s differential mechanism (phase-loss sensitivity) is proprietary: Siemens uses a bimetallic strip with a heater; Schneider uses a solid-state LR9 with a different response time [6,10]. In a shelter with a poorly grounded generator (neutral shift), the Siemens 3RU2 may trip faster on phase imbalance because its bimetal responds to the current magnitude difference directly, while the Schneider LR9 may take a few extra seconds — enough to overheat the motor winding. Worked consequence: if you pair a Siemens contactor with a Schneider overload, you lose the manufacturer’s type-tested coordination (guaranteed by IEC 60947-4-1 annex A). The shelter will still run, but the first phase-loss event will likely destroy the motor before the overload clears. When this reverses: if the shelter’s motor is protected by a separate electronic motor protection relay (e.g. SEL-710 or ABB RMU), then the overload relay on the contactor is redundant and cross-brand pairing becomes less risky — but you still lose the coordinated short-circuit withstand. The rule: always use the overload relay that matches the contactor brand and frame size, even if it costs 15% more. In a tight shelter, the premium is insurance against a catastrophic motor burnout.