The call came in on a Tuesday morning last fall, which is exactly when you don't want a call like that. One of our long-time customers had a line down. They had six Danfoss VFDs running a packaging operation, and one of them had kicked into fault twice in the same shift. The maintenance supervisor asked the question everyone in my position hates: 'Could this be something you sold us?'
To give you some context, I'm the quality and compliance manager at an industrial automation supply company. I review every order before it ships—roughly 200 line items a month. In 2025, I've rejected about 8% of first deliveries for issues that should have been caught at the manufacturer, not at my loading dock.
When I first started sourcing automation components, I assumed relays and timers were commodity parts. You match the coil voltage, you match the contact rating, and you're done. This call proved that assumption wrong.
The drive was fine. The wiring was fine. The signal wasn't.
The customer sent a photo of the keypad. I did the obvious thing: I opened the Danfoss VFD fault codes PDF download from Danfoss's support portal and looked up the alarm. The code pointed to a loss of the run command while the drive was running. According to the Danfoss manual, that usually means control wiring, a loose terminal, or something in the enable circuit. We checked all three. Everything looked tight.
Our engineer went to site and did the usual checks: incoming voltage, motor connections, grounding. Then he swapped in a spare drive we brought from stock. The same fault came back. That's when he started looking at the control cabinet, not the drive.
The run command came from a PLC output through a small relay on a DIN rail. That relay was part of a bulk relay order we had bought for the project. The same supplier also happened to be our timer supplier for that cabinet, so there were timer relays from the same batch sitting nearby.
Our engineer watched the input LED on the drive while the machine ran. It flickered. Not a long drop, just a split-second blip. The drive saw that as a valid stop command, logged a fault, and coasted to a stop. It wasn't the VFD. It was a relay contact that couldn't hold a clean 24V signal.
What the datasheet didn't say
From the outside, a relay looks identical to a relay. The numbers are the same: 24 VDC coil, 5A contacts, same footprint. The reality is that relay manufacturers make a lot of choices about contact material, coil tolerance, and drop-out voltage before that datasheet gets written.
The bulk relay batch we used had been manufactured to a close tolerance in some areas, but the coil drop-out voltage was uncomfortably close to the expected panel supply voltage. In a warm cabinet, a slight supply dip was enough to let the contact release for a few milliseconds. We also later found that the contact was rated for 5A, but the signal we needed to pass was 24VDC with almost no current. That's a low-level switching application, and we hadn't specified for it.
A senior engineer had warned me about this months earlier. He told me to ask about minimum contact load and coil drop-out voltage. I didn't listen, because the relay spec sheet looked fine. The rework cost us about $11,000, not counting the customer's lost production time or the overnight shipping on replacement parts.
How we evaluate relay manufacturers now
That job changed how we handle bulk relay purchases and how we look for a timer supplier. I don't think every relay manufacturer is the same, and I'm not going to pretend I can judge a factory from a photo. But we now ask a few questions before any large order:
- What's the coil drop-out voltage, not just pick-up? This is the spec that bit us. A relay can pull in at the rated minimum, but drop out too early when the cabinet warms up.
- What's the minimum contact load? If you need to switch a low-current signal, ask for a relay with gold-plated contacts or a stated low-level switching rating. A 10A relay is not automatically safe for a 5mA control signal.
- Can you share lot test reports? We now ask for internal QC data on contact resistance and coil voltage. If a manufacturer hesitates, we mark them as a risk.
- Do they test to IEC 61810? That standard exists for electromechanical relay endurance and contact performance. A manufacturer that actually tests against it usually has a better understanding of real-world failure modes.
- What are the timer relays like? If your timer is electromechanical, the same coil and contact issues apply. We've shifted some applications to solid-state timers, but not all of them, because not every application can use them.
This is the part that sounds contradictory, but it's important. I still buy relays based on price in some cases. For a simple motor contactor circuit with a proper 120VAC coil and a contactor load, a standard bulk relay can be perfectly fine. But for a VFD enable signal, I now specify a relay designed for low-level switching. It might cost a little more and add two days to delivery, but trust me on this one: it is worth it.
The lesson I keep coming back to
The Danfoss VFD fault code wasn't wrong. It was just the first place where the failure became visible. The actual problem lived two feet away in a relay that anyone could have bought online. I still remember how close we came to replacing a whole control panel because we didn't want to question the relay supplier.
If you're dealing with a phantom fault on a VFD, don't assume the drive is the answer. Check what's carrying the run signal. Is it a relay that's undersized for the signal level? Is it from a bulk relay lot that only passed a paper check? Download the Danfoss VFD fault codes PDF, look at the likely causes, and then test the wiring loop from the PLC to the drive input.
There are good relay manufacturers out there. But the honest dividing line is not brand name. It's whether your application matches the part's real-world test limits. Ask the hard questions before you buy, because the fault code won't tell you twice.
Rebecca Sloan
Rebecca Sloan is a power distribution and protection analyst specializing in circuit breakers, switchgear, contactors, fuses, surge protective devices, and coordination. She applies IEC 60947-2 breaker requirements, IEC 60269 fuse characteristics, and IEC 61643-11 tests while examining rated voltage, breaking capacity, time-current curves, selectivity, and prospective short-circuit current. She helps engineers and buyers compare protective devices against documented fault levels, installation conditions, maintenance access, and continuity priorities.