Hey everyone, let’s cut to the chase because I get this question at least 3 times a week—especially from electricians, maintenance techs, and small engineering shop guys who walk into our workshop or slide into my DMs: “Can the secondary circuit testing equipment you sell actually work for testing auxiliary circuits? Not just the big power stuff.” For a long time, I used to fumble through the answer because I was still wrapping my head around the difference between the two, plus a lot of old heads in the industry swear that “aux circuits are just small potatoes—use a multimeter and call it a day.” But here’s the thing: I’ve been selling secondary circuit testing gear for 7 years now, and I’ve seen first-hand how much that mindset costs people—mistakes, downtime, even tiny fires that could’ve been avoided. So let’s break this down like I’m chatting with a buddy who’s knee-deep in a transformer or switchgear right now, no fancy jargon, just real talk. Secondary Circuit Testing Equipment
First, let’s get the basics straight because people mix these two up all the time—part of why that question is even a thing. Secondary circuits are the low-voltage (usually 120V down to a few volts) control circuits that run show all the bells and whistles on big electrical stuff: the relays that trip when there’s a fault, the voltmeters that tell you how much juice is flowing, the switches that turn on a factory conveyor belt or a building’s HVAC system. Think of them as the brain of your electrical setup—they don’t carry huge power, but they control everything that does. Auxiliary circuits, on the other hand? Yeah, they’re a subset of secondary circuits, but wait—no, hold on, that’s where the confusion hits. Aux circuits are the little extra circuits that run supporting functions for the main secondary or power system: like the 24V supply that powers a relay’s coil, the small circuit that lights up a pilot light on a breaker panel, or the circuit that opens a circuit breaker’s auxiliary contact when the main one trips. So everyone lumps “testing secondary circuits” and “testing auxiliary circuits” together, right? But the old guard would say “secondary is for big control relays and trip circuits, aux is just those tiny add-ons, so they’re different.” That’s not wrong, but it’s not the whole story—and that’s where my gear comes in.
When I first started selling secondary circuit testers, they were big, clunky boxes—think 50 lbs, heavy cords, only made for testing 120V control circuits on 480V switchgear or big transformers. They could send a simulated fault signal to test if a relay would trip correctly, check contact resistance, and do basic continuity. Back then, I’d have to explain to customers: “Nah, this is for the big secondary control circuits, not your little aux stuff.” But about 5 years ago, we redesigned the line—smaller, lighter, more flexible models, and that’s when I started realizing the overlap is way bigger than people think. Let’s talk about what both types of testing actually need, because that’s the real backbone of this question.
What do you need to test a secondary control circuit properly? First, you need to inject a precise voltage or current signal, not just use your multimeter’s built-in leads. Why? Because when a circuit is energized under real load, stray resistance from bad connections or a worn relay contact won’t show up on a multimeter unless you pull it apart. Second, you need to simulate real-world faults—like a short, a drop in voltage, or a delayed signal from a sensor—to make sure the relays and breakers actually trip when they’re supposed to. Third, you need to be able to test over a range of voltages, because secondary circuits can run anywhere from 24V up to 120V, and aux circuits almost always run at the lower end of that range (24V, 48V, 120V).
Guess what? Most of the mid-tier to advanced secondary circuit testing equipment we sell does all that. Let’s use our best-selling model as an example—let’s call it the CT-12 (I can’t use real company names, but this is the actual model most techs grab). It can output a variable signal from 0V to 150V, inject current up to 10A, and has a built-in continuity tester that doesn’t just blink a light—it measures resistance down to 0.01 ohms. It also has pre-loaded test sequences for common control circuits, but here’s the secret: those sequences are super easy to adjust for aux circuits. Last year, a guy named Jake from a water treatment plant in Ohio called me panicking at 2PM on a Friday. He was supposed to do a yearly test on the auxiliary circuits for their main pump, and his multimeter died. He had a brand new CT-12 we’d sold him 2 months prior, and he texted me a pic of a tiny 24V aux relay coil. “Can I use this to test if this coil will hold a charge when the breaker trips?” he asked. I told him to hook it up to the coil’s terminals, set the voltage to 24V, and run the “relay coil test” sequence. He called me back 10 minutes later—said it worked way better than his old multimeter. The CT-12 didn’t just tell him continuity; it checked that the coil could handle 24V without dropping more than 0.5V across it, which is exactly what an aux circuit needs. A multimeter would’ve just said “good continuity” even if the coil had a hidden short that would fail when the pump was running.
But wait, I’m not here to say that secondary testing gear is a one-size-fits-all. Let’s be honest: if you’ve got a tiny, standalone auxiliary circuit—like a 12V pilot light on a residential breaker panel—you don’t need to haul a 15-lb secondary tester out. A $20 multimeter works fine there. But the gray area is where most people get tripped up. For example, if you’re testing the auxiliary circuits on a medium-voltage transformer that feeds a grocery store’s refrigeration system, or the aux circuits on a large HVAC unit for a hospital? That’s when that line between secondary and auxiliary blurs. Those aux circuits aren’t tiny little afterthoughts—they run 24V to power multiple sensors, control the pump motors, and trigger alarms if something goes wrong. Trying to test those with a basic multimeter is like trying to cut a 2×4 with a butter knife—it works in a pinch, but you’ll get a messy cut and maybe hurt yourself.
Another big point people miss: standard auxiliary circuit testing usually only checks continuity and voltage, but secondary circuit testing equipment does a load test that’s way more accurate. Let’s take contact resistance, which is critical for both. If an aux contact on a breaker has 0.2 ohms of resistance instead of the ideal 0.05 ohms, that might not seem like a big deal—until it’s carrying 5A for 8 hours a day. That extra resistance creates heat, and over time, it can burn through the contact, leading to a partial outage or even a fire. A multimeter can measure that resistance, but it only does it at very low current (like the tiny current it uses for continuity). Secondary circuit testers inject current at the actual load level, so you get a real, accurate reading. That’s not a “nice to have”—that’s the difference between catching a bad contact before it fails and dealing with a 12-hour shutdown at a factory.
I’ve seen so many horror stories over the years because of this. A manufacturing plant in Michigan hired a third-party tech to test their auxiliary circuits for their assembly line. The tech used a cheap multimeter, found continuity on all the aux relay coils, and called it good. 3 months later, one of the coils had a hidden high-resistance spot, so the relay would only trip at 80% of its rated voltage. When the line was running at full load, the voltage dropped, the relay didn’t trip, and the motor overheated, causing $200k in damage and 2 days of downtime. The plant ended up calling me, and we sent a team out to test all their secondary and auxiliary circuits. We used our CT-15 model, which can handle up to 20A of load, and found 4 other high-resistance aux contacts that the third-party tech missed. That’s when the plant realized: testing aux circuits with the right gear isn’t overkill—it’s just good maintenance.
Now, let’s address the elephant in the room: why do some people say you shouldn’t use secondary testing equipment for aux circuits? A lot of it is old-school bias. I’ve heard guys say “secondary testers are too big, too expensive” or “they’re overkill for small aux circuits.” The truth is, a lot of the big, bulky secondary testers from 10 years ago were overkill for small stuff, but the newer models are smaller, lighter, and way more affordable than they used to be. When I started, a basic secondary tester cost around $1,500; now, our entry-level model is under $500, and it still does 90% of the stuff you need for both secondary control and most auxiliary circuits. For the small aux circuits that don’t need it, yeah, use a multimeter—but for any auxiliary circuit that’s tied to a critical system (pumps, HVAC, assembly lines, power grids), the right secondary tester is worth every penny.
Another myth: secondary testing gear is only for high-voltage stuff. No way. Most of our testers work on low-voltage circuits too, because the core functionality—signal injection, load testing, fault simulation—doesn’t care if you’re dealing with 120V or 24V. You just adjust the settings on the tester. That’s the beauty of it. Let’s say you’re an electrician working on a new office building. You’ve got 24V auxiliary circuits for the fire alarm system, plus 120V secondary control circuits for the elevators. Instead of carrying two different tools (a multimeter, a small aux circuit tester, and a secondary circuit tester), you can just bring one secondary testing unit. It’s one less thing to haul, one less thing to charge, and it gives you consistent, reliable readings across all the circuits you’re testing.
I also want to talk about ease of use, because that’s a big selling point for our gear. A lot of people think secondary testing equipment is complicated, like you need an engineering degree to run it. But we design all our models with user-friendly interfaces—big, backlit screens, pre-set test modes, and step-by-step guides. Jake, the water treatment plant guy I mentioned earlier, is a maintenance tech, not an engineer. He had never used a secondary circuit tester before, and he got his aux circuit test done in 10 minutes, no training needed. That’s a huge deal for small shops and in-field work, where you don’t have time to sit through a 4-hour training course on a new tool.
Now, let’s make sure I’m not pitching something that doesn’t exist. Let’s outline the real limits here, because I don’t want to mislead anyone. If you’re testing a tiny, low-current auxiliary circuit—like a 5V LED indicator light on a residential garage door opener—our secondary testing gear is overkill, and a multimeter is better. It’s smaller, cheaper, and does the job just fine. But if you’re testing auxiliary circuits for commercial or industrial systems—anything that’s tied to equipment that’s critical for operations, safety, or power—our secondary circuit testing equipment works perfectly, and in many cases, it’s better than using tools that are only designed for aux circuits. Why? Because those aux circuit-specific tools are usually only made to check continuity and voltage, while our gear does load testing, fault simulation, and resistance measurements at actual operating currents. That’s the difference between passing a test on paper and knowing the circuit will hold up when things get rough.
I always tell our customers: don’t fall for the “keep tools in their lane” myth when it comes to electrical testing. The best techs I know are the ones who can adapt their tools to the job, not the other way around. Our secondary circuit testing equipment isn’t just for big power control circuits—it’s a versatile tool that can pull double duty for most auxiliary circuits too. We’ve got customers in everything from food processing plants to wind farms to hospital facilities who use our gear for both secondary control testing and auxiliary circuit testing, and they tell us it’s cut their tool roll weight in half and reduced their testing time by 30%.
If you’re someone who’s dealing with electrical maintenance, troubleshooting, or new installations, and you’re tired of juggling different tools for secondary and auxiliary circuits, or you’ve had a bad experience with testing tools that miss critical issues, let’s chat. We can walk you through what our gear can do for your specific setup, whether you’re dealing with small residential aux circuits or large industrial secondary and auxiliary systems. No hard sell, just real advice from guys who’ve been in the field and in the workshop for years.
Voltammetry Phase Meter REFERENCES
- Electrical Maintenance, Troubleshooting, and Testing: A Practical Guide, 3rd Edition, McGraw-Hill Education
- Secondary Circuit Testing Best Practices for Industrial Systems, National Electrical Contractors Association (NECA) Technical Bulletin
- Auxiliary Circuit Performance Under Load: Testing Methods and Failure Analysis, Institute of Electrical and Electronics Engineers (IEEE) Transactions on Industrial Applications
Wuhan Jiuhua Jingce Power Equipment Co., Ltd.
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