Siemens Contactor FAQ: Why Your Sirius Contactor Won't 'Reset' and What to Do

I'm a quality and compliance manager at an industrial controls distributor. I review every Siemens contactor that goes into stock—roughly 200 unique line items per quarter—and I've rejected about 6% of first deliveries in 2024, mostly for documentation mismatches rather than hardware faults. These are the questions I still answer most, in plain language.

What's the Difference Between a Siemens Magnetic Contactor and a Relay?

A magnetic contactor is an electrically operated switch built for power circuits. In the Siemens range, the SIRIUS 3RT is the current workhorse, and the older 3TF still shows up in legacy panels. Apply voltage to the coil terminals (A1 and A2), the electromagnet pulls the armature in, and the main contacts close to energize the motor or heater circuit. Remove the voltage, and a spring opens the contacts again.

A relay works on the same electromagnetic principle, but its contacts are designed for control circuits, not power loads. Put a motor on a typical control relay and you'll quickly understand why contactors have heavier contacts, arc chutes, and larger cooling surfaces.

The confusion is understandable. It's the same kind of confusion as the spark plug vs ignition coil question. The ignition coil generates the high voltage; the spark plug fires it across the gap. They work together, but they're not interchangeable. In a starter panel, the relay handles the logic, and the contactor does the heavy switching.

Why Won't My Siemens Sirius Contactor Reset?

Here's the part that catches people: a standard SIRIUS 3RT contactor does not have a reset button. It is electrically held. Energize the coil and it closes; de-energize it and it opens. If it won't close after an interruption, the contactor usually isn't the component that needs resetting. (Exception: if your unit has a mechanical-latch accessory, the latch needs to be re-armed separately.)

Most of the time, what has actually tripped is the overload relay, for example a SIRIUS 3RU2 mounted with the contactor. The overload relay opens the control circuit, the contactor drops out, and the machine refuses to start. It's easy to blame the contactor. In our Q1 2024 audit of customer returns, we found plenty of perfectly healthy contactors—the overload relay had simply been left in its tripped state.

Here's the reset sequence I give customers:

1. Check the overload relay's trip indicator first. If it is tripped, no reset will hold until the cause is addressed.
2. Remove the overload cause: motor overload, jammed equipment, or phase loss.
3. Reset the overload relay. Manual reset is done on the relay front (or via its remote reset accessory, if fitted). In automatic reset mode, the relay re-arms by itself once the trip condition has cleared.
4. Reapply your normal start command. The contactor should pull in immediately.
5. If it drops out again, measure the motor current and verify the overload relay setting.

One old belief I keep meeting is the 'wait an hour' rule from melting-alloy overloads. Modern SIRIUS overload relays have visible trip indicators and their own reset lockout. If the relay isn't re-arming yet, it's telling you it hasn't reset internally—give it the cool-down time its data sheet specifies, then reset again. A healthy contactor doesn't need a time-out.

Can I Test a Siemens Contactor Coil With a 100-Watt Power Inverter?

Short answer: for a bench click-test of a small contactor, maybe. For powering a machine, absolutely not. Don't confuse the two.

A coil draws inrush current while the armature is moving and much less sealed current once it is held in. The inrush lasts only a few milliseconds, but it can be several times the sealed value. A 100-watt power inverter might handle that for a small SIRIUS 3RT, provided its surge rating is real and the output voltage stays within the coil tolerance.

The waveform matters too. AC coils are designed for sine-wave supply. Feed one from a modified-sine inverter and it will usually hum louder and run hotter. If the coil is marked DC—common on 3RT2 contactors with electronic coil control—use the rated DC supply instead of the inverter's AC outlet.

I'm not a power-electronics engineer, so I'll leave the harmonics discussion to people who design inverters. From the returns bench, wrong coil voltage is one of the most common reasons a contactor comes back as 'failed.' Check the rating plate first. A 100-watt power inverter is a testing convenience, not a control transformer.

What About a Portable Solar Generator Like the Patriot Power Solar Generator 2000x?

This question comes from homeowners and small workshops more than from panel builders, and it's a fair question. A one-piece order deserves the same straight answer as a bulk order.

Honest boundary first: I haven't opened the Patriot Power Solar Generator 2000x, so I can't comment on its internal switching components. What I can tell you is the electrical math: a 2,000-watt-class load on a 120 V circuit draws roughly 16 to 17 A. At that current, you want an industrial contactor chosen by utilization category and rated operational current—not a small control relay chosen by guesswork.

The part I won't bend on: connecting any generator or solar generator to building wiring requires listed transfer equipment and, in most jurisdictions, a licensed electrician. A bare contactor is not a transfer switch. I'll help you select the right contactor for a legitimate switching job, but I won't pretend it replaces an interlock or an automatic transfer switch.

That's actually why I wrote this FAQ. Small questions can turn into bigger projects, so the answer should be useful from day one.

Why Does the Same Siemens Contactor Have Different Current Ratings?

Because a contactor rating is meaningless without a load profile. A SIRIUS 3RT can show different maximum operating currents depending on utilization category, operating voltage, ambient temperature, and enclosure conditions.

For AC motor switching, the categories you'll see most are AC-1 (resistive loads) and AC-3 (starting and stopping squirrel-cage motors while running). AC-4 covers plugging and inching, which is far harder on contacts. The same contactor will have a different allowable current in each category—not because the hardware changed, but because the switching stress changed.

That's why I check the rating plate and the data sheet, not just the picture. Siemens contactors are designed and tested to IEC 60947-4-1 and the corresponding UL/CSA standards for North America. The official product page on Siemens Industry Online Support is the right place to confirm coil voltage, utilization category, and accessory compatibility. If the marking on the unit doesn't match the data sheet, treat it as suspicious. Good documentation is part of a quality product.