I Ordered the Wrong Siemens SIRIUS Contactor – Then Learned How to Check a Relay With a Multimeter

If you're here because a Siemens contactor won't pull in, buzzes, or just welded its contacts again, I get it. I've been there. For years, I thought I had a quality problem. Then I added up what my mistakes really cost, and the contactor wasn't the problem. I was.

I'm a maintenance planner at a food processing plant. I've been handling electrical component orders for eight years. In that time, I've personally made – and documented – 11 significant mistakes, totaling roughly $18,000 in wasted budget. Now I maintain our team's pre-order checklist. This is the story of why.

What I Thought Was Wrong With the Siemens SIRIUS Contactor

The first time a motor contactor failed in the field, I blamed the component. It was a Siemens SIRIUS contactor, which was supposed to be reliable. The coil didn't close when the PLC command was present. So I reordered the same part number, no questions asked. When the replacement failed exactly the same way, I started to wonder.

The actual cause? I never checked the coil voltage spec against the actual supply. (Yes, I know.) I kept ordering what I thought was “the right Siemens contactor” from the catalogue, without verifying the application.

Let me break down the mistakes that really matter.

Mistake #1: Not Checking the Coil Voltage

In my first year (2017), I made the classic mistake: I ordered a batch of Siemens SIRIUS contactors for a conveyor upgrade and didn't specify the coil voltage. The contactors arrived with 230V AC coils. Our control circuit was 24V DC. Nothing pulled in. The electricians on site spent a day chasing a PLC problem that didn't exist.

That $3,200 order had to be redone. Plus a one-week delay while we waited for the correct parts. The supplier was not happy. My manager was less happy. The embarrassing part (for me) was that a simple resistance check would have caught the issue only if I'd compared it to the data sheet. I didn't.

Honestly, I'm not sure why coil voltage gets ignored so often. My best guess is that contactors look too simple. People treat them like light switches, but a contactor is a device with a coil, main contacts, auxiliary contacts, and an arc-quenching system. Each one matters.

Mistake #2: Using the Wrong Contactor for an Electric Motor Load

The second lesson came when a contactor for a fan motor welded its contacts after about a week. We replaced it, and the new one did the same. “Bad batch,” we said. It wasn't a bad batch.

I had chosen a contactor based on the full-load current from the motor nameplate. That seems logical, but it's incomplete. For an electric motor contactor application, you also need to match the utilization category. According to IEC 60947-4-1, motor starters and contactors are tested in AC-3 (or AC-4 for heavy duty reversing and jogging). Using an AC-1 rated contactor on an inductive motor load is asking for contact erosion and welding.

The Siemens SIRIUS data sheets (available from Siemens Industry Online Support, accessed January 2025) list AC-3 ratings separately from AC-1 ratings. The difference isn't cosmetic; it's the number of operations the contacts can handle while switching motor inrush currents. I don't have hard data on industry-wide failure rates, but based on the corrective maintenance tickets I've reviewed across two plants, my sense is application errors cause more contactor failures than component quality by a wide margin.

Mistake #3: Assuming Everyone Means the Same Thing by 'Safety Contactor'

This one still stings. In September 2022, I ordered what I called “safety contactors” for a guarding circuit. I said 'safety contactor.' The supplier heard 'contactor with a safety cover.' Result: standard SIRIUS contactors with green shrouds. We were using the same words but meaning completely different things. We discovered this when the safety inspection failed and the machine couldn't run.

When I finally looked up 'safety contactor Siemens' in the SIRIUS documentation, I realized I had been treating it as a vague term. A safety contactor for Siemens is not just a contactor with a cover. It's a contactor with positive-guided contacts used in a safety-related circuit, typically with a safety relay that monitors the contacts. The safety contactor isn't just a marketing label. It has to meet specific criteria, and the documentation is explicit about how it can be used safely.

The mistake cost us a 3-day production delay, plus a rushed and expensive expedited shipment. It also taught me to ask the exact question: “What's the SIRIUS part number, and will it work with a safety relay?” If you can't answer that, you haven't finished selecting it.

The Detail That Cost Me $450: Auxiliary Contacts

Somewhere in the middle of all this, I also learned about auxiliary contacts. I ordered 75 contactors for a small material handling upgrade. I specified “one auxiliary contact” because that's what the old contactors had. But some positions needed two auxiliary contacts – one NC for interlock and one NO for feedback. The original contactor had both. My replacement unit had only one snap-on auxiliary contact block. I reused the old one? No. I had already thrown them away.

75 items, $450 wasted, plus the embarrassment of calling the supplier to explain what I meant. The part number for a contactor with factory-fitted auxiliary contacts is different from one without. That's in the catalogue, but I was looking at the main contact rating and nothing else.

How to Check a Relay With a Multimeter (and Why It Works for Contactors)

After the third rejected shipment in Q1 2024, I created our pre-check list. The core of it is simple: verify every contactor before installation, even if it's brand new. Yes, even SIRIUS. You can and should verify a relay or contactor with a multimeter in about five minutes.

You don't need a fancy meter for this. I carry a 20 amp multimeter with me, but that's for measuring motor current. For contactor checks, I set it to resistance (Ω) and continuity mode.

1. Check the coil. Put probes on the coil terminals (usually A1 and A2). A healthy coil should show a resistance value – usually from tens to hundreds of ohms, depending on the voltage and power rating. If it reads open (infinite resistance), the coil is broken. If it reads near zero, the coil is likely shorted. Compare it to the data sheet for your exact SIRIUS part number.
2. Check the main contacts. With the coil de-energized, a normally open (NO) main contact should show no continuity. Press the contactor's armature in by hand. The NO contact should close and show low resistance (near 0 Ω). Release it: the contact should open again. This catches welded or damaged contacts.
3. Check auxiliary contacts. Repeat the same test on the auxiliary contact blocks. For a normally closed (NC) auxiliary contact, the state is reversed: it should be closed when the contactor is resting and open when you press the armature.
4. For safety contactors, check the positive-guided function. Press the armature slowly. The NC contact should open, and the NO contact should close. They should never both be closed at the same time. If they are, the positive-guided safety function isn't working.

This isn't just a theory. It's how I now check a relay with a multimeter before installation. It takes less time than arguing with a supplier about returns, and it's dramatically cheaper.

The Efficiency Lesson I Wish I'd Learned Earlier

I used to think checking every part would slow us down. In reality, the opposite happened. Switching to a documented pre-install check cut our changeover turnaround from 5 days to 2 days on the projects where I actually used it. The automated part of the process isn't complicated; it's a checklist in an Excel sheet with the coil resistance logged for each part number. But it eliminates the “maybe this is the problem” loop that wastes days.

I wish I had tracked our first-pass success rate more carefully from the start. What I can say anecdotally is that our team has caught 47 potential errors with this checklist in the past 18 months. Some were simple wrong voltage. A few were safety components that didn't match the functional safety requirements.

The checklist doesn't replace experience; it makes experience repeatable.

Bottom Line: Don't Blame the Siemens Contactor First

If a Siemens contactor fails, look at the application before you blame the device. Check the coil voltage. Check the AC-3 rating for electric motor loads. Verify the auxiliary contact configuration. And if a safety contactor is specified, confirm it's actually a safety contactor.

Then get out the multimeter and test the coil and contacts. It's boring. It's not glamorous. But it will save you the same $3,200 mistake I made, and probably a lot more.