How to Check a Contactor and Relay with a Multimeter: A 7-Step Quality Inspection Guide

Who This Checklist Is For

If you're receiving Siemens contactors, auxiliary contact blocks, or control relays and need to verify they actually work before they go into a panel—this is for you. I'm a quality compliance manager at an industrial electrical supply company. I review roughly 200+ unique component batches annually, and I've rejected about 12% of first deliveries in 2024 due to spec mismatches or functional failures that should've been caught at the vendor level.

This checklist covers seven steps. It takes about 15-20 minutes per contactor if you're doing it properly. I'll walk through the exact sequence I use, including the one step most people skip that's caused us the most headaches.

What You'll Need

• Digital multimeter with continuity and resistance modes (auto-ranging is nice, but not required)
• Siemens contactor datasheet or catalogue (the 3RT, 3TF, or Sirius series documentation—you need the coil voltage and contact ratings)
• Small flathead screwdriver for terminal access
• Latex gloves if you're handling multiple units (finger oils aren't great for contact surfaces long-term)

Step 1: Visual Inspection Before You Touch the Multimeter

Don't skip this. I know it feels obvious, but I've caught more issues here than in electrical testing.

Check for: cracked housing, discolored terminals (sign of heat damage), loose auxiliary contact blocks, and—this is the one people miss—verify the coil voltage printed on the front actually matches what's on your PO. We received a batch of 3RT contactors last year where the label said 24V DC but the coil was 230V AC. The vendor claimed it was "within industry standard" for labeling. We rejected the whole batch. They redid it at their cost. Now every contract includes a coil voltage verification clause.

If the physical label doesn't match the datasheet, stop. Don't bother testing. Just document it and send it back.

Step 2: Verify Coil Resistance

Set your multimeter to resistance mode. Touch the probes to the coil terminals (A1 and A2 on most Siemens contactors).

You're looking for a reading that's within ±10% of the nominal coil resistance listed in the datasheet. For a typical 24V DC Siemens contactor, that's usually somewhere in the 100-200 ohm range. For 230V AC coils, it's higher—often 1,000+ ohms.

If you get OL (open loop), the coil's broken. If you get near-zero, it's shorted. Either way, reject it.

I'm not 100% sure why, but we've seen more coil failures in units that were stored in humid conditions for extended periods. If your warehouse isn't climate-controlled, test a higher sample percentage.

Step 3: Check Auxiliary Contacts (NO and NC)

This is the step most people rush. And it's the one that's bitten us most often.

For normally open (NO) contacts: with the contactor de-energized, your multimeter should read OL (no continuity). Manually press the contactor armature down—you should hear a click—and the reading should drop to near-zero ohms.

For normally closed (NC) contacts: reverse the expectation. De-energized should show continuity. Pressed should show OL.

We had a batch of Siemens 3RH auxiliary contact blocks where the NC contacts were reading about 2-3 ohms when closed. Not OL, not zero—just high resistance. That's a problem. High resistance means heat, and heat means premature failure. The vendor said "within tolerance." Our spec says otherwise. We sent them back.

Step 4: Test the Main Contacts

Same idea as auxiliary contacts, but these handle the actual load current.

With the contactor de-energized, check continuity across L1-T1, L2-T2, L3-T3 (or whatever terminal labeling your specific model uses). You should have OL on all poles. Manually actuate the contactor and re-check. Each pole should show near-zero ohms.

If any pole shows resistance above 0.5 ohms when closed, that's a red flag. It might pass a basic functional test, but it'll run hotter than it should. In our experience, that heat degrades the contact surface faster and you'll be replacing it within a year.

Step 5: Verify Relay Operation (If You're Testing a Control Relay)

If you're also checking control relays—including those used in power generator transfer switch panels or Chamberlain motion detecting control panels—the process is similar but scaled down.

Apply the rated coil voltage. You should hear an audible click. If you don't, check your voltage source first before blaming the relay.

Once actuated, check continuity across the NO contacts. Then remove power and verify they open again. A relay that sticks closed is dangerous—especially in transfer switch applications where you're relying on it to isolate circuits.

We use a simple test jig for this: a 24V DC power supply, a momentary switch, and a panel-mount multimeter. Takes about 30 seconds per relay, and it's caught stuck contacts that would've caused real problems in the field.

Step 6: Check for Contact Resistance Under Load (If You Can)

This is the step that's most often skipped. And honestly, it's the one that's caused us the most frustration.

A contactor can pass a basic continuity test and still fail under load. The only way to catch that is to actually apply a load and measure voltage drop across the contacts.

You don't need a full production setup. A simple resistive load bank and a clamp meter will do it. Apply rated current, measure voltage drop across each pole, and compare against the datasheet's maximum specified drop (usually around 100-200 mV per pole).

After the third time we had a contactor pass bench testing and then fail in a generator transfer switch application within weeks, I was ready to stop trusting basic continuity checks entirely. What finally helped was building load testing into our incoming inspection for critical applications. It added maybe two minutes per unit—or rather, closer to four when you count setup time—but it's saved us at least three field failures.

Step 7: Document and Decide

Record your readings. Every single one. If you're rejecting a unit, you need the data to back it up when the vendor pushes back.

We use a simple spreadsheet template with columns for: coil resistance, NO contact resistance, NC contact resistance, main contact resistance (per pole), and notes. It takes an extra 30 seconds per unit, but it's ended more arguments than I can count.

If a unit fails any step, reject it. Don't "note it and move on." Don't assume it'll be fine. I've made that mistake before—I knew I should reject a batch with slightly high contact resistance, but thought "what are the odds it actually fails?" Well, the odds caught up with me when three of those contactors welded shut in a panel six months later. That quality issue cost us a $22,000 redo and delayed a project by two weeks.

Common Mistakes (And How to Avoid Them)

Testing without the datasheet. You can't verify something if you don't know what the spec actually is. Pull the Siemens catalogue or data sheet for your specific model number before you start.

Assuming new means good. We've received factory-sealed Siemens contactors with coil failures. It happens. Test everything.

Using the wrong multimeter mode. Resistance mode for coil and contact checks. Continuity mode is fine for quick go/no-go, but it won't catch high-resistance issues that cause problems later.

Not cycling the contactor multiple times. A contact that works once might not work on the tenth actuation. We cycle at least ten times during inspection—it's caught intermittent failures that would've been missed on a single test.

Ignoring the "within industry standard" excuse. If it's outside your spec, it's outside your spec. The vendor's definition of "standard" doesn't matter if it doesn't meet your requirements. Trust your data, not their language.

Note on standards: Contact resistance and coil verification procedures reference IEC 60947-4-1 (low-voltage switchgear and controlgear) and IEC 60947-5-1 (control circuit devices and switching elements). Verify current requirements at the official IEC webstore as standards may have been updated. Siemens-specific coil and contact data should be pulled from the current Siemens Industry Mall catalogue or the data sheet included with your specific 3RT, 3TF, or Sirius series product.