Transformers are built to last decades. With no moving parts and a straightforward design, they're among the most reliable pieces of electrical infrastructure you can spec. But when a transformer fails, the damage goes far beyond the cost of a replacement unit.
Downtime. Lost revenue. Safety risks. Cleanup costs. Project delays that cascade across your entire operation. A single transformer failure can derail schedules, blow budgets, and put people at risk.
Understanding the most common transformer failure modes helps you make smarter decisions before a problem takes your site offline. Here's what causes transformers to fail, and how to avoid it.
Understanding transformer failures: Why equipment breaks down
A properly specified and maintained transformer can run for decades. But still, millions of transformers end up failing each year.
The reasons usually fall into a few categories: design or manufacturing defects, environmental factors, operational stress, and normal aging that degrades insulation over time.
We will explore the most common failure points in more detail below, but if you want to minimize your risk of transformer failure from day one, it comes down to three things:
- Specifying the right equipment for your application
- Following the maintenance guidelines to the letter
- Choosing the right manufacturing partner
Cheap, fast, or poorly engineered units might save money upfront, but they'll cost you later. With this context in mind, let’s explore the most common transformer failures and give you actionable tips for how to avoid them.
1. Insulation breakdown from moisture and contamination
Moisture is one of the biggest threats to oil-filled transformers. When water enters the tank through leaking gaskets, cracked seals, or poor maintenance, it attacks the insulation system from the inside.
The insulation protecting the windings will then begin to degrade, losing its ability to prevent electrical arcing. Over time, this leads to accelerated aging, reduced performance, and, if left unaddressed, catastrophic failure.
Some common triggers for this transformer failure include:
- Cracked transformer tanks or damaged sealants
- Improper installation that compromises seals
- Neglected maintenance and oil contamination
- Poor gasket quality from the factory
You can avoid insulation breakdown by following a few simple steps.
- Specify transformers built with sealing systems and high-quality gaskets.
- Conduct annual oil testing, including dissolved gas analysis and moisture content checks, to catch problems early.
- Work with manufacturers that maintain oversight throughout production so you know every part of your device has undergone rigorous QC.
The difference between a transformer that lasts 40 years and one that fails in 10 often comes down to the quality of seals and gaskets. Don't assume all units are equal when choosing a transformer. Ask questions about manufacturing processes and material specs from day one.
Read more: Electrical transformers: The buyer’s guide to selection, installation, and maintenance
2. Overheating from overloading or poor cooling
When a unit is pushed beyond its capacity or its cooling system can't keep up, the windings and oil start to overheat. The paper insulation that keeps components electrically isolated begins to degrade, and what should last 40 years might fail in 10.
In extreme cases, overheating can lead to catastrophic failure, taking your entire operation offline. Some triggers to keep an eye out for to prevent overheating damage:
- Undersized transformers that can't handle actual load demands
- Blocked radiators or failed cooling fans
- Poor ventilation around the unit
- Misunderstanding cooling class ratings (ONAN/ONAF) and what they actually deliver
You can avoid overheating and protect your equipment by taking a few simple steps.
- Size transformers for your actual load requirements. Remember to leave room for planned growth rather than investing in the exact load requirements you have today.
- Understand ONAN and ONAF cooling classes, and know the actual capacity increase when fans kick in.
- Ensure proper installation with clear airflow paths and functioning fan systems.
- Work with suppliers who provide accurate load calculations and technical support, not generic spec sheets.
Overheating isn't always obvious until it's too late. By the time you smell burning oil or hear strange noises, the damage is often done. You’ll want to be proactive in terms of installation and maintenance to avoid overheating.
3. Design and manufacturing defects
Some transformers are vulnerable to failure before they're ever energized. When the manufacturer doesn’t construct the windings properly, fails to insulate adequately, or uses substandard materials during construction, a unit might fail under normal operating conditions.
The most frustrating part of this type of transformer failure is that they often happen early in service life, right when your project is ramping up.
The trigger for this failure is simple: poorly designed or manufactured transformer units. How can you avoid installing a lemon in your build?
- Source from manufacturers with proven track records.
- Require UL-listed equipment that meets ANSI and IEEE standards.
- Ask about design conformance processes and what factory testing happens before transformers are shipped.
- Choose suppliers with American engineering oversight and transparent manufacturing, not just brokers reselling overseas units.
Not all transformers are built the same. Two seemingly identical units might have completely different build qualities. If you want to protect your project timelines, you need to choose your manufacturing partner carefully.
4. Mechanical damage from external forces
Transformers are pretty sturdy, but that doesn’t mean they’re immune to physical damage. Rough handling during shipping, vibrations from nearby equipment, earthquakes, or poor installation can shift internal components, loosen connections, or deform the core.
When this kind of damage occurs, you might struggle with smaller failures like short circuits or reduced efficiency. Depending on the extent of the damage, you might end up dealing with full catastrophic failure over time.
Avoid mechanical damage and keep your transformers running safely (and at peak performance) using these tips:
- Conduct sweep-frequency response testing upon arrival to detect shipping damage before installation.
- Use vibration-dampening pads in environments with heavy equipment or constant movement.
- Inspect transformers after natural disasters, even if they don’t look damaged from the outside.
- Work with trusted manufacturers who package and ship equipment properly, not the cheapest freight option.
Mechanical damage doesn't always announce itself. A transformer might operate fine for months before a loosened connection or shifted winding causes a failure. Inspect early and often to avoid this type of transformer failure.
5. Specification mismatch with application needs
One of the most preventable (yet common) causes of transformer failure is simply ordering the wrong unit. If the specs of the transformer you order don’t match your facility’s requirements, the transformer will either underperform or fail prematurely.
Some of the common mismatches we’ve seen are:
- Selecting the wrong voltage class for your utility supply.
- Undersizing capacity, meaning your unit cannot handle peak loads.
- Installing the wrong insulation or cooling type for the environment.
- Ignoring harmonic loads in data centers or industrial facilities.
You can avoid these challenges by specifying the right transformer for your use case. Easier said than done, sometimes, we know. We have a whole deep dive on the different transformer types and classifications, but a few key points to keep in mind:
- Provide detailed site requirements to your supplier, including load profiles, ambient conditions, voltage class, and environmental factors.
- Consider K-factor ratings if you're running harmonic-intensive loads.
- Specify the correct cooling class (e.g., ONAN/ONAF for oil-filled; AN/AF for dry-type) and enclosure/NEMA rating. In general, NEMA 1 for general indoor use andNEMA 3R for basic rain-proof outdoor service.
- Work with suppliers who ask the right questions and provide engineering support.
A specification mismatch isn't always obvious until the transformer is installed and struggling. Do the engineering work upfront, and be sure to partner with a supplier that asks detailed questions about your application during the buying process.
6. Lightning strikes and overvoltage events
Lightning and voltage surges can cause catastrophic damage to transformers. If your transformer suffers a direct strike or switching surge, you might end up with damaged windings, insulation, or terminals.
The good news is that this failure is far less common than the other types of transformer failure we’ve discussed in this post. The bad news is that when it happens, it’s incredibly expensive and disruptive.
Of course, it’s impossible to avoid getting struck by lightning. However, you can take preventive measures to minimize damage from lightning and overvoltage events.
- Install proper surge protection and lightning arresters as part of your system design.
- Inspect surge protection systems regularly to ensure they're operating at peak functionality.
- Verify that transformer protection and transformer fusing options are coordinated with your overall electrical system design.
Finally, remember that this type of failure is incredibly rare. If a transformer fails and overvoltage is suspected, you’ll want to double-check. Did nearby electronics also fail? Are there physical signs of high-voltage arcing? Overvoltage is often blamed for failures that actually stem from other issues.
7. Neglected maintenance and aging
Transformers don't require a lot of attention, but they're not maintenance-free. If you aren’t regularly inspecting and maintaining your units, they may degrade prematurely.
Small issues such as oil contamination, loose connections, and minor rust can compound over time, leading to major failures. Substations overgrown with vegetation, transformers with visible leaks ignored for years, and units running hot without intervention are all preventable problems.
Some of the most common causes of transformer failure due to neglected maintenance and aging are:
- Neglected or nonexistent maintenance schedules.
- Overgrown substations that block cooling or hide damage.
- Ignoring warning signs like unusual noise, visible leaks, or high temperatures.
- Running transformers beyond their reasonable service life.
You can avoid maintenance-related failures by regularly monitoring and maintaining your transformers’ health. Let’s cover a few best practices to follow to keep your transformers running efficiently and safely for decades.
- Establish annual maintenance schedules that include oil testing, visual inspections, and thermal imaging.
- Monitor oil quality, operating temperature, noise levels, and pressure relief device status.
- Don't let transformers run beyond 40-50 years without a thorough condition assessment.
- Consider proactive replacement over reactive repairs for very old units.
Maintenance isn't glamorous, but it's the easiest way to extend transformer life and avoid unplanned downtime.
Reducing transformer failures starts with the right partner
Most transformer failures are preventable. As we mentioned at the top of this post, avoiding transformer failures often boils down to specifying the right equipment, maintaining it properly over its life, and working with the right supplier from day one.
At Giga, our transformers are UL-listed and ANSI/IEEE-compliant, with rigorous quality control from drawings through delivery. We deliver transformers in record time without compromising on engineering or materials. Best of all, when you work with us, you have a team of engineers who ask the right questions about your application before you buy, not after something goes wrong.
We've delivered 3.5 GW of equipment and built over 1,000 transformers for customers across the country. We're not brokers. We're builders.
Ready to avoid costly failures and get transformers that deliver? Build a quote or contact our team to discuss your project.
