When a plant’s main control panel went up in flames, it wasn’t a lightning strike or a voltage surge that caused the damage. The root cause was far more preventable and far more common: an undersized circuit protection device that couldn’t handle the available fault current. The result was catastrophic. The customer lost the entire enclosure, suffered extended downtime, and even named the device’s supplier in a lawsuit, simply because their sticker appeared on the panel.
According to Billy Sonnenthal, Technical Marketer at AutomationDirect, that scenario underscores why getting circuit protection right isn’t just a matter of compliance. “With the correct protection in place, maybe you lose one component and swap it out. Without it, you might lose everything,” he says.
Circuit protection is about more than breakers and fuses—it’s about ensuring personnel safety, preserving equipment, and minimizing downtime when things go wrong. Yet even experienced engineers can make costly assumptions. In this article, we’ll explore five of the most common circuit protection mistakes—and what engineers can do to avoid them.
Mistake #1: Underestimating Available Fault Current
“Many people assume circuit protection devices just stop the current,” Sonnenthal says. “But they must survive the fault condition long enough to open the circuit—and that depends on the available fault current. If it’s too high, and your protection can’t handle it, you might have catastrophic failure.”
This is especially true for industrial environments with high-capacity feeds and/or in close proximity to large transformers. In those cases, fault currents can exceed 20, 30, or even 65 kiloamps. Standard protection devices won’t cut it.
The fix: Always calculate or obtain (via an engineering study, if required) the available fault current at the panel or load location and compare it with the interrupting rating of your protection device.
For high-AFC applications, use current-limiting fuses or fast-acting breakers at the service entrance to reduce the fault current reaching downstream devices. This allows downstream equipment to have lower interrupting ratings and SCCRs while still being protected.
Remember, all protection devices allow some fault current to pass through before they interrupt the circuit. If the let-through current exceeds what downstream components can safely handle, you’ll need a current-limiting fuse or breaker to reduce the energy exposure.
Learn more: What Type of Circuit Protection Do I Need?
Mistake #2: Using Supplementary Protectors Instead of Branch Circuit Protection
One of the most common errors Sonnenthal sees is the misapplication of UL 1077 supplementary protectors in place of UL 489 branch circuit breakers. While they may look similar—and often cost less—these two types of protection serve very different roles.
“Supplementary protectors are meant for internal use, downstream of a proper branch circuit breaker,” says Sonnenthal. “In the control circuit, they’re fine for relays, PLC I/O, or other low-load devices. But they aren’t rated or approved for branch circuit protection and don’t meet NEC requirements when used as the sole protective device. They don’t have the interrupting capacity to handle a serious fault. If someone puts one in where a UL 489 breaker should be, it can fail explosively under a fault condition.”
UL 1077 devices typically offer interrupting ratings from 1 to 10 kA. By contrast, UL 489 breakers are tested and rated for much higher fault currents—commonly 10 kA up to 65 kA or more—and can serve as the primary overcurrent protection in a panel.
The fix: Know which circuits require full branch protection and which are supplementary. If there’s any uncertainty, default to a UL 489 breaker. “When customers ask what they should use,” says Sonnenthal, “I always tell them: ‘If you’re not sure, go with the UL 489. That will protect you either way.’”
Learn more: Branch or Supplementary Circuit Protection?
Mistake #3: Oversizing Fuses or Breakers “Just to Be Safe”
It’s a tempting shortcut: a breaker keeps tripping, so an electrician installs a higher-rated one to “fix” the nuisance. Problem solved—until it isn’t.
“Everything in the system—the wires, the load, the devices—is sized based on a certain current,” says Sonnenthal. “If you increase the breaker size without ensuring the wiring and equipment are rated to safely carry the higher current, you risk exceeding their design limits. That’s a fire risk.”
Oversizing protection devices defeats their primary purpose. In overload situations, excessive current can heat conductors and insulation, degrade components, and in extreme cases, ignite surrounding materials. It also increases the chance that the protective device won’t trip when it should, allowing damage to escalate.
The fix: Investigate the root cause of the nuisance tripping before reaching for a larger breaker. In many cases, inductive loads such as motors, solenoids, and transformers produce temporary inrush currents that momentarily exceed the trip curve of a standard breaker. The solution? Increase the breaker’s time-delay setting if available or use a time-delay fuse or breaker with a slower trip curve, one that’s designed to handle brief inrush events without sacrificing overload protection.
As Sonnenthal puts it, “It’s not about making the breaker stop tripping—it’s about making sure it’s tripping for the right reasons, and at the right time.”
Learn more: 10 Reasons Why Fuses are Essential for Overcurrent Protection
Mistake #4: Inadequate Motor Circuit Protection
Motors are the muscle behind most industrial processes, requiring more than basic overcurrent protection. A common mistake is to size circuit protection devices based solely on the motor’s full-load amps (FLA) without accounting for startup inrush current, thermal overload risk, or phase-related issues. The result? Frequent nuisance trips, motor damage, or premature failure.
“Motor circuits are tricky,” says Sonnenthal. “You need to allow for the high inrush current during startup without oversizing your short-circuit protection. But you also need overload protection that detects prolonged, moderate overcurrents—like those caused by a failing bearing or continuous mechanical strain—which wouldn’t trip short-circuit protection but can still damage the motor over time.”
Effective motor protection typically involves three layers:
- Short-circuit protection, usually provided by time-delay fuses or motor circuit protectors (MCPs) rated to handle inrush current without nuisance tripping, yet capable of clearing short circuits quickly.
- Thermal overload protection, which monitors current over time to detect overheating conditions caused by prolonged overloads, preventing insulation damage.
- Voltage and phase monitoring, to detect phase loss, undervoltage, phase imbalance, or phase reversal—conditions that generally won’t trip overcurrent devices but can cause winding damage or premature motor failure.
Some of this protection may come bundled in a motor starter or built into a VFD, but that doesn’t make external coordination unnecessary. In fact, for critical or expensive motors, Sonnenthal recommends adding current sensors, temperature monitoring, or phase relays to support predictive maintenance and early fault detection.
“A failed motor isn’t just a parts problem,” he says. “It’s a downtime problem. Motors are heavy, hard to replace, and often take days to reinstall. A little extra protection can save a lot of pain.”
Learn more: Group Motor Protection | White Paper
Mistake #5: Ignoring Lockout/Tagout and Multiple Power Paths
Circuit protection isn’t just about components—it’s also about people. One of the most overlooked hazards in control panels is the presence of multiple power sources. A technician may shut off the main disconnect and assume the panel is safe, unaware that backup power, UPS feeds, or auxiliary circuits may still be energized.
“I’ve seen panels where the main disconnect only shuts off the primary circuit,” says Sonnenthal. “Auxiliary circuits such as lighting, ventilation, and PLC power often remain energized from separate sources. Failing to identify and disconnect all power sources or hazards inside an electrical panel can put anyone working on it at serious risk.”
This makes proper lockout/tagout (LOTO) procedures essential—not just for compliance but for life safety. OSHA requires that all hazardous energy sources be isolated and locked out before maintenance is done. That means using physical padlocks and labeled tags, not just flipping a switch and walking away.
In fact, safety codes require all such panels to be properly labelled. Pay close attention to any warning labels during a LOTO session. One final warning is temporary power used during maintenance. Introducing an outside power source for testing after a LOTO session is common and very dangerous if not communicated and done properly.
“In a plant environment, it’s common to see multiple technicians working on the same equipment,” Sonnenthal explains. “Each one should apply their own lock, and the machine shouldn’t be restarted until every lock is removed. That’s how you avoid tragic accidents.”
Beyond electrical hazards, improper LOTO can expose workers to kinematic risks—rotating shafts, gears, or actuators unexpectedly re-engaging during repair or inspection. A simple oversight can turn into a catastrophic injury.
The fix: Always identify and label every external and internal power source. Use lockable disconnects and OSHA-compliant tags with the technician’s name and warning information.
After lockout, always verify that all circuits have been de-energized using an appropriate test instrument. Don’t rely on memory or convenience—build LOTO into your standard operating procedures and maintenance training.
Learn more: Providing Circuit Protection for Safety
Conclusion
When designed and applied correctly, circuit protection devices quietly safeguard people, equipment, and operations. But when misunderstood or misapplied, they can become weak links that expose plants to unnecessary risk, from equipment loss and unplanned downtime to serious safety incidents.
“The key is not just having protection but also having the right protection, properly sized and correctly placed,” says Sonnenthal.
As industrial systems become more automated, compact, and interconnected, the margin for error shrinks. Engineers must account for factors like fault current, trip curves, motor inrush, and human factors like LOTO compliance. And in the face of evolving standards and increased system complexity, taking shortcuts—whether to save time, reduce nuisance trips, or cut upfront cost—can have outsized consequences.
The good news? Most of the mistakes covered here are preventable with careful planning and a basic understanding of protection fundamentals. Take the time to get it right. Because when the unexpected happens—and it always does—solid circuit protection helps you recover quickly, safely, and without burning down your bottom line.
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Want to go deeper?
For a more detailed breakdown of protection types, device ratings, and application guidance—including illustrated charts comparing UL 489 vs. UL 1077 devices—download AutomationDirect’s Circuit Protection Overview. It’s a free, engineer-focused resource that expands on the concepts discussed here. View the guide.