Why Do Transformers Blow? What Happens, How the Fire Spreads

Updated September 23, 2026. By Jeffrey Demos, Sinisi Solutions (Fire Barrier Experts).

Direct Answer

Transformers blow when an internal fault creates an electric arc. The arc boils the oil into gas until pressure bursts the tank, often starting a fire [1][2]. Overloads, worn insulation, lightning, and animals are the usual triggers. No upkeep stops every failure, so codes require safeguards like spacing or fire barriers to contain it [3][4].

You hear a crack like a gunshot, see a blue flash, and the street goes dark. Sometimes that bang is the transformer itself. Often it’s a fuse on the pole doing its job, cutting power before the damage spreads [5].

TL;DR

A transformer blows when an internal arc turns its cooling oil into gas and the pressure bursts the tank [1][2].
Overloading, worn or wet insulation, short circuits, lightning, and animals cause most failures [6][7].
Most loud pops on a pole are a fuse opening to limit the outage, not the transformer itself [5].
Fewer than 1% of transformers explode, but a substation unit can hold 1,200 gallons of oil or more [6][8].
NEC 450.27, NFPA 850, and FM Global call for spacing, fire barriers, suppression, or containment so one fire doesn’t spread [3][4][9].
The 2025 North Hyde fire knocked two more transformers out of service and shut Heathrow Airport [10].

One-liner: You can lower the odds of a transformer blowing but not remove them, so the real protection is keeping one fire from reaching everything nearby.

Key Numbers

Figure	Value	Source
Share of transformers that explode	Fewer than 1%	[6], 2019
Energy in a severe internal arc	About 150 megajoules	[6], 2019
Gas temperature near an arc in oil (lab test)	Up to about 2,200 K	[1], 2024
Hydrogen share of arc gas (lab test)	70% to 85%	[1], 2024
Typical breaker time to clear an arc (study)	About 80 milliseconds	[2], 2015
Oil in a typical substation transformer	1,200 gallons or more	[8], 2025
Customers who lost power in the North Hyde fire	71,655	[10], 2025
Time before the fire took two more units offline	28 minutes	[10], 2025
Sustained outages caused by squirrels, 2023	7,196	[7], 2024
Minimum insulation life in IEEE standards	About 180,000 hours	[11], 2021
Distance the ground may be energized near a downed line	Up to 35 feet	[12], 2021
Lead time for transmission-scale transformers	Three to six years	[13], 2025

What Happens When a Transformer Blows? How a Transformer Explosion Unfolds

In short: an arc inside the tank turns the cooling oil into gas, the pressure bursts the tank, and the escaping gas and oil can catch fire.

How does a transformer blow? Most large transformers are a steel tank with metal windings sitting in mineral oil. The oil keeps the windings cool and stops electricity from jumping between them. What happens when a transformer explodes is a five-step chain that starts when that insulation fails:

The fault. Insulation between the windings breaks down, and current takes a shortcut it was never meant to take.
The arc. That shortcut becomes an electric arc, a spark that keeps burning. In a 2024 lab test, gas near an arc in transformer oil measured up to about 2,200 K, roughly 1,900°C [1].
Oil turns to gas. The arc breaks the oil down into flammable gases, and hydrogen made up 70 to 85% of the gas in that test [1].
The pressure spike. Gas forms far faster than it can escape. A severe internal arc can release about 150 megajoules, close to the energy in 150 sticks of dynamite [6].
The transformer burst. Once pressure beats the strength of the tank, the tank or its bushings split open [2][9]. Hot gas meets the air and oil sprays out, which is the fireball you see in videos of an exploding transformer.

Picture a sealed soda bottle tossed into a campfire, where the cap gives out first and then the seams.

So how do transformers blow this fast? Protective relays usually spot the fault and trip the breaker quickly, but not instantly. In one study of a large 220 kV transformer, a typical breaker took about 80 milliseconds to clear the arc, and faults cleared in under 60 milliseconds left the tank without lasting damage [2].

Most transformers never get this far. Researchers quoted by IEEE Spectrum put the share that explode at fewer than 1% [6]. When a large oil-filled unit does fail this way, burning oil can spread across the ground toward whatever sits next to it [9].

You Can’t Stop Every Failure: How Codes Keep a Transformer Fire From Spreading

In short: maintenance lowers the odds, but codes assume some oil-filled units will still fail and call for space, barriers, suppression, or containment.

Good maintenance cuts the odds of a blowout, but no code treats it as a guarantee. The rules for oil-filled transformers assume one may still fail. They call for space, a fire-resistant barrier, automatic suppression, or oil containment so the fire stays with that one unit [3][4].

That matters because a power transformer explosion is over in moments. The fire that follows can keep burning long after the power is cut. It can reach the next transformer, a control building, or a plant wall.

The North Hyde substation fire in west London shows how that plays out. On March 20, 2025, moisture in a bushing caused an arc that lit the oil in one large transformer [10]. Flying porcelain and heat damaged a second transformer. Twenty-eight minutes later, protection systems switched that unit and a third out of service [10]. More than 71,000 customers lost power, and Heathrow Airport shut down. The official review found the site had neither the spacing nor the fire barriers that current standards favor. It was built before those standards applied [10].

Which rule applies depends on the site:

Rule	Who it covers	What it asks for
NEC Section 450.27	Outdoor oil-filled transformers on or near buildings, outside utility control	One or more of: space separation, fire-resistant barriers, automatic suppression, oil enclosures [4]
NESC Section 152	Utility-owned stations and lines	Methods such as spacing, barriers, less-flammable fluid, suppression, and containment [14]
NFPA 850	Power plants and high-voltage DC converter stations	A 2-hour firewall or set spacing for units holding 500 gallons of oil or more [3]
IEEE 979-2025	Electric substations	Fire risk design guidance, updated in April 2026 [15]
FM Global Data Sheet 5-4	Insured facilities	Spacing with spill containment, or 2-hour barriers where spacing falls short [9]

Spacing on its own isn’t always enough. FM Global warns that its distances, used alone, “may not protect surrounding property from damage due to overpressure and rupture” [9]. That’s why a 2-hour barrier is common between units that sit close together [3][9]. The local authority having jurisdiction, meaning the official who approves the design, usually has the final say [4].

For distance tables and wall sizing, see when power transformers need a fire barrier and how a fire barrier compares with a blast wall [16][17]. Facility owners who aren’t sure where their site stands can start with a site risk assessment [18].

What Causes a Transformer to Blow?

In short: what makes a transformer blow is usually heat, worn or wet insulation, short circuits, lightning, or animals, and a few of them tend to stack up first.

Why does a transformer blow after years of working fine? Most blowouts don’t come from one bad moment. A transformer wears down for years, and then one event pushes it over the edge. Here’s what causes transformers to blow most often:

Overloading and heat. A transformer carrying more load than it was built for runs hot, and so does one with a failed cooling fan or low oil. IEEE’s transformer standards set a minimum insulation life of about 180,000 hours, roughly 20 years, and extra heat uses that life up faster [11].
Worn or wet insulation. Paper and oil insulation break down with age, heat, and moisture. At North Hyde, a July 2018 oil test flagged moisture in a bushing at a level pointing to an imminent fault. It wasn’t acted on before the 2025 fire [10].
Short circuits. A tree limb, a fallen line, or a fault farther down the line can push a rush of current through the windings. Inside the tank, a short between windings is what starts the arc.
Lightning and switching surges. A strike on or near a line sends a voltage spike into the unit. Surge arresters are there to bleed it off, but a strong spike can still puncture the insulation. Storms are a common trigger for internal arcs [6].
Animals. Squirrels, birds, and snakes bridge live parts and cause a short. Squirrels alone caused 7,196 sustained outages at public power utilities in 2023, third behind trees (8,503) and equipment problems (8,205) [7].
Dirty oil and hidden defects. Water, particles, or sludge in the oil weaken it as an insulator [6]. A poor weld, a loose connection, or a weak spot from the factory can fail years later under stress.

Picture a transformer that ran cool when it went in. Ten years later the same street has new homes, heat pumps, and EV chargers, and the unit spends summer evenings near its limit. Nothing breaks that day, but the insulation ages faster until a storm finds the weak spot.

Most of what can cause a transformer to blow builds slowly, which is why oil sampling and insulation testing catch many problems early. North Hyde shows the other half of the story. A test can spot the fault, but someone still has to act on it. The fire rules above exist for the times that doesn’t happen.

What Does It Sound Like When a Transformer Blows? Transformer or Fuse

In short: transformer popping on a pole is usually a fuse doing its job. A real transformer failure brings a flash, smoke, and often fire.

The bang most people hear during an outage is a fuse, not a transformer. Bristol Tennessee Essential Services says that loud sound is “most likely a fuse, not a transformer malfunctioning” [5]. The fuse cuts the circuit so the problem stays on a short stretch of line and fewer homes lose power [5].

The transformer blowing up sound people describe is usually a sharp pop or a shotgun blast, because the energy is released all at once [5]. A true transformer blast is messier, with a bright flash, smoke, and sometimes flames at the top of the pole or around the pad.

The sound itself tells you a lot about what happened and whether to report it:

What you hear	What it usually means
A low, steady hum	Normal. The metal core expands and contracts as current flows [19]
Crackling, popping, or transformer sparking at the unit	Possible arcing or failing insulation, worth reporting [19]
Buzzing that comes and goes	Possible loose connection [19]
One sharp bang, then the lights go out	Most likely a fuse opening on the line [5]
A boom with a flash, smoke, or fire	A failure at the transformer itself, so stay back and call 911

Does a transformer blowing sound like a gunshot? It can, but the things that come with it give it away. A flash near a pole, flickering lights, or a sudden outage all point to the power lines, not a gun.

To check, look up at the pole, where a blown cutout fuse drops out of its top clip and hangs from a hinge. That hanging fuse is a visible sign it has blown [20]. Then call your utility and tell them what you saw.

Why Transformers Blow During Storms, Heat Waves, and Winter

In short: storms bring water, wind, lightning, and falling trees, while heat and cold both push extra load onto units that may already be worn.

Weather sits behind a large share of outages. In APPA’s 2023 data, trees, storms, and other weather together caused more than 18,000 of the 73,694 sustained outages it tracked [7].

Storms. What causes a transformer to blow during a storm is usually wind, lightning, or water. Wind drops branches on lines and snaps poles, creating shorts that run back to the transformer. Lightning adds voltage surges on top. Water is the quieter threat. It seeps into the paper insulation around the coils and speeds up its breakdown until the coils touch and spark inside the tank [21].

Heat waves. Hot days mean air conditioners running hard, and hot air cools a transformer less. Nearly 35% of the outages in APPA’s 2023 data came in June, July, and August [7].

Winter. Heaters drive the load up, and the risk climbs again in the minutes after an outage ends. Heaters, furnaces, and fridges that normally cycle at random all start at once. Engineers call this cold load pickup, and the first rush of demand can top anything the circuit carried before the outage [22]. That rush lands on equipment the storm may already have weakened.

Neighborhood Transformer Keeps Blowing? Here’s Why

In short: repeat failures on one street usually trace back to the same animals, the same trees, or too much load on one unit.

When the same pole goes pop again and again, something keeps coming back to it:

The same animals. Squirrels often reach lines by jumping from nearby trees, and one Nebraska utility says small outages hitting up to 20 customers are common [23]. Summer tree growth gives them easier access, and squirrel damage tends to peak in October and November [24].
The same trees. Trees were the top cause of outages in APPA’s 2023 data [7]. A limb that brushes a line in the wind can trip the fuse in every storm until someone trims it.
More load than the unit was sized for. New homes, heat pumps, and EV chargers add demand over the years. Heat waves and cold load pickup then stack on top of it [7][22].

So what does a blown transformer look like? An exploded transformer on a pole often shows scorch marks, soot, a split or bulging case, or oil stains running down the pole. A fuse hanging open from its cutout is the easier sign to spot [20].

If it keeps happening, report every outage with the time and what you heard or saw. That record helps the utility find what causes a blown transformer on your street and decide whether the street needs a bigger unit, an animal guard, or tree trimming.

Oil-Filled vs Dry-Type: Which Transformers Burn?

In short: oil-filled units carry the fuel for a big fire, dry-type units carry far less, and the code sets fire rules for both.

The fire risk comes mostly from the oil. A typical substation transformer can hold 1,200 gallons of mineral oil or more [8]. That includes the green pad-mounted boxes in yards and parking lots, which is why an electrical transformer box explosion can end in a fire too. Dry-type units use air and solid insulation instead, so there’s no pool of liquid to spill and burn.

That doesn’t mean the code requires dry-type units indoors. The NEC allows oil-filled transformers inside a building when they sit in a vault, and it allows less-flammable liquid units under set conditions [25]. It also treats larger dry-type units as a fire source, which is why they need a fire-rated room [25].

	Oil-filled	Less-flammable liquid	Dry-type
Cooling and insulation	Mineral oil	High fire point fluid	Air and solid insulation
Burning liquid spill risk	Yes	Lower, since the fluid is harder to ignite	No liquid
NEC rule indoors	Vault, with exceptions (450.26) [25]	Set conditions, such as suppression and liquid confinement (450.23) [25]	12 in. from combustibles up to 112.5 kVA, 1-hour room above that (450.21) [25]
NEC rule outdoors	Space, barrier, suppression, or oil enclosure (450.27) [4]	Safeguards near combustible buildings (450.23) [14]	Fewer restrictions
Where you see it most	Substations, power plants, poles, pads	Buildings and sites where oil is restricted	Inside commercial buildings

For utilities and plants, dry-type often isn’t an option at the sizes and voltages they run. That’s why the fire planning in the earlier section matters most for oil-filled units.

What to Do If a Transformer Blows Near You

In short: stay at least 35 feet back, treat every fallen line as live, call 911 if there’s fire or someone is hurt, and report the outage to your utility.

Whether a transformer blew out down the street or right outside, start here:

Keep your distance. The ground around a downed line can be energized up to 35 feet away, so treat every fallen wire as live [12].
Leave anything touching it alone. Don’t touch a fence, car, or person in contact with the line. Call 911 if someone is hurt or you see fire [12].
Stay in your car if a line lands on it. Honk for help and wave other people away [12].
Report it to your utility. Tell them where it happened and what you saw or heard [26].
Protect your electronics. Turn off and unplug major electronics, since power can come back in surges [26].

If you manage a plant, campus, or substation, the planning starts before anything fails. A site risk assessment shows which transformers sit too close to each other or to a building, and whether spacing, containment, or a fire barrier fits each spot [18][27].

Frequently Asked Questions

What does it mean when a transformer blows?

It means an internal fault made an arc that turned the cooling oil into gas and burst the tank or its bushings, often with fire [1][2]. Many loud pops on a pole are a fuse opening instead of the transformer itself [5].

What is a transformer explosion?

A transformer explosion is the rupture of a transformer tank or bushing after an internal arc turns the insulating oil into gas faster than it can escape [1][2]. The escaping gas and oil often ignite, and that transformer fire is what can spread to nearby equipment [9].

Why do transformers blow up?

Transformers blow up when an internal fault becomes an arc that turns the insulating oil into gas faster than the tank can hold it [1][2]. The usual triggers are overloading, worn or wet insulation, lightning, and animals [6][7].

Why do transformers explode instead of shutting off safely?

Protection has to spot the fault and open a breaker first, and the arc keeps heating the oil until it does. In one study of a 220 kV unit, a typical breaker took about 80 milliseconds, while faults cleared in under 60 milliseconds left the tank undamaged [2].

Why do power transformers blow?

What causes a power transformer to fail is usually insulation that has aged, overheated, or taken on moisture until a fault turns into an arc [6][11]. At North Hyde, moisture flagged in a 2018 oil test led to the 2025 fire [10].

Why would a transformer blow on a clear day?

Because heat and age do most of the damage, not weather. Overloading, a failed cooling fan, low oil, or a squirrel on a live part can all do it with no storm in sight [7][11].

Can a transformer explode?

Yes, but it’s uncommon. Researchers quoted by IEEE Spectrum put the share that explode at fewer than 1% [6]. When a large oil-filled unit does, the fire can reach nearby equipment unless spacing or a barrier stops it [9].

How loud is a transformer explosion?

Loud enough that people often mistake it for a gunshot or a shotgun blast, though most of those bangs are a fuse opening [5]. A true tank rupture on a large unit adds a flash, smoke, or fire.

Does a transformer blowing sound like a gunshot?

It can. Utilities describe the sound as a pop, a bang, or a shotgun blast, and it’s most often a fuse opening [5]. A flash near a pole or a sudden outage points to the power lines, not a gun.

Can a blown transformer damage my appliances?

It can, because power sometimes comes back in surges when it’s restored. Turn off and unplug major electronics during the outage to protect them [26].

Can a blown transformer be fixed?

A blown fuse can be replaced once crews clear the fault. A transformer with a burst tank is generally swapped out, and large grid transformers now take three to six years to build [13].

Why do transformers blow during storms?

Wind drops trees and branches on lines, lightning adds voltage surges, and water seeps into the paper insulation until the coils short inside the tank [7][21].

Definition Bank

Term	Plain-English definition
Electric arc	A spark that keeps burning as current jumps through oil or air instead of along the wire [1].
Insulating oil	Mineral oil that cools a transformer’s windings and keeps current from jumping between them.
Bushing	The insulated fitting that carries a high-voltage line through the transformer tank wall [10].
Fuse cutout	A pole-mounted fuse that opens on a fault and hangs from a hinge as a visible sign it blew [20].
Surge arrester	A device that bleeds voltage spikes from lightning or switching away from equipment.
Protective relay	A device that senses a fault and signals the breaker to cut power [2].
Turn-to-turn fault	A short between neighboring turns of a winding after the insulation fails [21].
Cold load pickup	The rush of demand when power returns and heaters, AC units, and fridges all start at once [22].
Less-flammable liquid	A high fire point transformer fluid the NEC treats under its own rules (450.23) [25].
Dry-type transformer	A transformer cooled by air with solid insulation and no liquid to spill [25].
Fire barrier	A fire-rated wall, often 2-hour rated, that blocks flame and heat between transformers or from a building [3].
Oil containment	A pit or curb that holds spilled oil so burning oil can’t spread across the ground [9].
Authority having jurisdiction	The local official or agency that approves a fire protection design [4].
NFPA 850	The recommended fire protection practice for power plants and high-voltage DC converter stations [3].
IEEE 979	The IEEE guide for substation fire protection, updated as IEEE 979-2025 and published April 10, 2026 [15].

Entity Cards

Oil-filled transformer

Property	Value
Cooling medium	Mineral oil
Oil in a typical substation unit	1,200 gallons or more [8]
Share that explode	Fewer than 1% [6]
NEC rule outdoors	Section 450.27 safeguards [4]
NEC rule indoors	Vault, Section 450.26 [25]

Fuse cutout

Property	Value
Job	Opens the circuit so fewer customers lose power [5]
Visible sign	Fuse holder hangs from a hinge [20]
Typical sound	Pop, bang, or shotgun blast [5]

North Hyde substation fire

Property	Value
Date	March 20, 2025 [10]
Cause	Moisture in a bushing led to an arc that lit the oil [10]
Early warning	Moisture flagged in a July 2018 oil test [10]
Impact	71,655 customers lost power, Heathrow closed [10]
Spacing and barriers	Neither in place, site predated the standards [10]

Sinisi Solutions (Fire Barrier Experts)

Property	Value
Focus	Modular fire and ballistic barriers for transformers and energy infrastructure [28]
Services	Engineering, site assessment and inspection, fabrication and installation [18][28]
Location	75 Main Street, Manasquan, New Jersey 08736 [28]
Phone	732-232-2100 [28]

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Why do Transformers Blow?