Siemens Contactor FAQ: What You Need to Know Before Specifying or Replacing One

Everything You Wanted to Know About Siemens Contactors (But Were Afraid to Ask Your Rep)

Let's be honest: when I first started in industrial controls, I thought a contactor was a contactor. Pick one with the right current rating and coil voltage, slap it in, done. That mindset cost my company a $22,000 redo in Q1 2024—and that's why I'm now the guy who reviews every spec before it hits the production floor.

This FAQ is for the engineers and techs who are tired of wading through 200-page PDFs to find the simple answer. We'll cover the real questions I see daily, from the Siemens 42AF35AJ data sheet to when you should absolutely not use a definite purpose contactor.

1. What exactly is a Siemens contactor, and how is it different from a relay?

People mix these up all the time. A contactor is a high-current switching device primarily used for motors, lighting, and heating loads. A relay is for lower-current control circuits.

The simplest rule I use in my own audits: if you're switching a load over 10 amps (or 3-phase), you want a contactor. The Siemens Sirius (3RT series) line is built for this. Relays are for things like PLC output isolation or signal switching. They look similar, but the contact geometry and arc suppression are way different.

2. I found a data sheet for a 'Siemens 42AF35AJ contactor'. Can I just cross-reference this to a cheaper brand?

You can. And I've seen it go spectacularly wrong. The Siemens 42AF35AJ is typically a definite purpose contactor used in HVAC and refrigeration applications. It has specific coil characteristics, a specific mounting footprint, and specific agency approvals (UL, CSA) that a generic replacement might not match perfectly.

A vendor once tried to replace this with a generic unit on a $50,000 HVAC unit. The coil burned out within 6 months because the inrush power was higher than the 'equivalent' unit could handle.

My advice: Use the 42AF35AJ data sheet to extract the key specs—coil voltage range, holding VA, contact rating per phase, and terminal spacing. Find the official Siemens docs on siemens.com. Don't just look at the catalog number.

3. What is a 'definite purpose contactor' vs. a 'magnetic contactor'? Does Siemens make both?

Yes, Siemens makes both, and they serve different jobs.

Definite purpose contactors (like the 42AF35AJ or the Siemens CLA series) are designed for specific applications—typically HVAC compressors, fan motors, or lighting. They are tested for those specific loads (e.g., high inrush from a compressor) and are often more compact and cost-effective. You generally don't use them for heavy-duty reversing motor applications.

Magnetic contactors (like the Siemens 3RT series) are the workhorses for industrial automation. They are modular, support a wider range of auxiliary contacts and overload relays, and are designed for high duty cycles. They're what you spec for a conveyor drive or a pump in a factory.

Common mistake: using a definite purpose contactor on a reversing starter because it's cheaper. The mechanical interlock options are poor, and you risk a phase-to-phase short. Don't do it.

4. How do I choose the right overload relay for my Siemens contactor?

This is the part where most of my quality rejections happen. An overload relay protects the motor from overheating by monitoring current. It's not a fuse—it's a thermal or electronic device that 'resets' after cooling.

With Siemens, you want the matching Sirius series (3RU2 for thermal, 3RB3 for electronic). It physically clips onto the contactor.

The key parameters are:

• FLA (Full Load Amps) of the motor – This must fall within the overload relay's setting range.
• Class of trip – Class 10 is standard for general purpose; Class 20 is for high-inertia loads like fans.
• Ambient temperature – Thermal overloads are affected by heat. If your panel runs hot, the relay might trip early. That's not a contactor problem—it's a design problem.

Per our Q3 2024 quality audit, 18% of first-time panel builds had a mis-matched overload relay FLA range.

5. I'm seeing 'Siemens definite purpose contactor' used in my power distribution panel. Is that okay?

It depends. Are you building a residential panel for lighting and small motors? A definite purpose contactor can be fine. Are you using it as a main disconnect for a whole machine? I'd say that's pushing it.

Definite purpose contactors are typically rated for fewer operations (mechanical and electrical life) than an industrial magnetic contactor. They also often have less robust arc chutes. For a power strip with surge protector type application? No. That's a job for a different device entirely (like a contactor rated for capacitor switching).

Read the data sheet carefully. If it says 'UL 508' for definite purpose, it's OK. If it's rated for 'Resistive Loads Only,' don't use it on a motor.

6. How do I wire a double pole circuit breaker with my Siemens contactor? Is there a standard sequence?

Yes, there is a standard sequence, and ignoring it is why I've seen melted wires.

The standard order from line to load is:

1. Disconnect (circuit breaker or fused disconnect)
2. Contactor (main power pole)
3. Overload relay (motor protection)
4. Motor

If you're using a double pole circuit breaker (for a 1-phase application), you're switching both hot wires. The contactor should be downstream of the breaker. This ensures that if the contactor welds shut, the breaker can still open the circuit.

Safety note from our 2022 protocol: Never use a double pole circuit breaker as a motor disconnect if you also have a contactor. The breaker is for overcurrent/short circuit protection. The contactor is for switching. They serve different roles.

7. A tech asked me 'how to check ignition coil with multimeter' and I couldn't help. Is this related to contactors?

Not directly, but it's a great example of 'wrong tool for the job.' An ignition coil is a high-voltage pulse transformer. A contactor is a switch. If you're testing a contactor coil with a multimeter, the principle is similar: check for resistance (coil continuity) and for shorts to ground. But the values are different.

To check a Siemens contactor coil (say, a 24V AC coil), I'd use a multimeter to:

• Measure resistance between A1 and A2. It should be within the specs on the data sheet (often 50-200 ohms for small AC coils).
• Check for shorts between the coil terminals and the contactor frame. It should show 'OL' (open line).
• Verify the voltage at the coil terminals when the control signal is ON. If it's only 90V on a 120V coil, the contactor might not pull in solidly.

Bottom line: Know what you're measuring. A contactor coil is an inductor, not a resistor.

8. Final check: What's the one thing you always verify on a Siemens contactor data sheet before procurement?

It's not the current rating. It's the utilization category (AC-1, AC-3, AC-4) and the rated insulation voltage (Ui).

I've seen teams buy a perfectly rated 40A contactor for a motor, only to find that it's rated AC-1 (resistive load) and not AC-3 (squirrel cage motor starting). The difference is how much current the contacts can handle during the inrush. An AC-3 contactor is tested for the high inrush current (often 6-8x FLA) as the motor starts.

For an 8kW motor at 400V, you might need a contactor rated for 16A AC-3, even though the motor FLA is only 15A The margin protects the contacts from welding.

Sometimes the best tool is simply the official Siemens Sirius selection guide. Don't guess.