A fire in a dry-type transformer is rare, but when it happens it puts your equipment, your building, and anyone nearby at risk. These units are built without flammable oil — that's a big reason they get specified for indoor and occupied spaces. But "no oil" isn't the same as "no fire risk." If you ever face one, your first move is to kill the power and get help on the way, then let trained crews handle the extinguishment. Below we cover what actually causes these fires, what to do in the moment, and how to keep them from starting.

Why a dry-type unit can still burn

Dry-type transformers cool with air and insulate with epoxy resin or varnish instead of oil. That removes the large pool-fire hazard you get with liquid-filled units. But the windings and the insulation around them are still combustible if they get hot enough. A fire starts when heat in one spot climbs past what the materials can take. In normal operation that never happens — the design holds temperatures well inside the rated limits. It's only under a fault that things run away.

Almost every fire-causing fault comes down to one of two things: too much current, or too much resistance.

Cause 1: Overcurrent (too much current)

Two things drive overcurrent:

• A heavy overload, often set off by an external short circuit somewhere downstream.
• Internal insulation failure that shorts the windings together. A severe enough internal fault can be violent — arc flash, pressure, and in the worst case the enclosure can rupture.

The upside: dry-type transformers run behind protection — fuses, breakers, and relays. When overcurrent shows up, that protection usually trips fast enough to save the unit. That's the main reason genuine transformer fires are uncommon in day-to-day service.

Cause 2: High resistance at a loose connection

This one is sneakier. A loose terminal, a poor crimp, or a corroded joint adds resistance. Resistance makes heat, and that joint can run hot while the rest of the unit looks perfectly normal. Protection doesn't catch it, because the current through the joint isn't high — the problem is local. Left alone, that hot spot can ignite the insulation around it. Unlike an overcurrent trip, there's often no alarm until you see smoke.

What to do if a fire breaks out

1. De-energize the transformer. Trip the upstream breaker or open the disconnect. No power means no arc and no live fault feeding the fire.
2. Sound the alarm and clear the area if smoke or flame is spreading.
3. Call the fire department. Tell them it's an electrical fire so they bring dry-chemical or CO₂ extinguishers and treat the gear as energized until it's confirmed dead.
4. Don't fight it yourself unless it's small and you're trained. Use a Class C / dry-chemical or CO₂ extinguisher — never water on live equipment.
5. Stay clear and let it cool. Even after the flames are out, hot windings and debris can reignite.

Keeping the fire from starting

Most of these fires are preventable with basic, routine care:

• Torque and inspect connections on a schedule. Loose terminals are the most common ignition source we see.
• Use thermal imaging during preventive maintenance to catch hot joints before they become a problem.
• Keep ventilation paths clear so the unit sheds heat the way it was designed to.
• Don't load past nameplate, and make sure upstream protection is sized and coordinated.
• Act on winding-temperature alarms instead of silencing them.

When you're installing a new unit, follow the spacing and barrier rules in the NEC — our NEC transformer calculator lays out the clearances for your kVA. Specifying a low-loss dry-type design also helps: cooler-running units give you more margin, and you can model the savings with our transformer loss and TCO calculator. For an extra layer of fault protection, review the isolation transformer types and buying guide. If you're comparing suppliers, our guide to transformer manufacturers and brands covers what to look for. You can also browse our full line of dry-type transformers, or read more across our resource library.

FAQ

Can a dry-type transformer explode? It's unlikely, but a severe internal fault can produce an arc flash and enough pressure to rupture the enclosure. That's why de-energizing fast and keeping people clear matters. The protection system is there to trip before a fault reaches that point.

Do dry-type transformers have oil that can catch fire? No. Dry-type units use air for cooling and solid epoxy or varnish for insulation, with no flammable liquid. That's why they're chosen for indoor and occupied spaces. They can still burn if the windings or insulation overheat, but there's no oil pool fire to deal with.

What extinguisher do you use on a transformer fire? A Class C (or equivalent dry-chemical) or CO₂ extinguisher. Never use water on live electrical equipment — it conducts and can spread the hazard. Once the unit is confirmed de-energized, standard extinguishers are fine.

Why do most transformer fires start at the connections? Loose or corroded joints add resistance, and resistance makes heat right at that spot. Because the current through the joint isn't abnormally high, the overload protection won't trip — so the hot spot can sit there unnoticed until the insulation ignites. Regular torque checks and thermal scans catch it early.

How often should dry-type transformer connections be inspected? At commissioning, then on your preventive-maintenance cycle — typically annually for most sites, and more often in harsh or high-load environments. Add infrared scans during those visits to spot heating before it turns into a fire.