Transformers

A transformer changes electrical voltage from one level to another. They are on utility poles, in green boxes on lawns, in building electrical rooms and inside equipment. Transformers look like quiet metal boxes, but they can hold lethal voltage, hot oil and stored energy, and they can send power backward through a line you thought was dead.

• Electrical
• Utilities
• Facilities

How a transformer works

• A transformer has two or more coils of wire (windings) wrapped around a core. Alternating current in the primary winding creates a magnetic field that induces voltage in the secondary winding.
• The ratio of turns sets the voltage. A step-down transformer lowers voltage, for example from thousands of volts to 120/240 volts for a house. A step-up transformer raises it, such as at a power plant.
• Transformers don’t make power. They change voltage and current. A transformer that lowers voltage raises the current, and the reverse.
• Transformers can be energized from both sides. If the secondary side is fed by a generator or another source, voltage appears on the primary side. This is called backfeed. See Standby Generators & Transfer Switches.

Common types

Main hazards

• Shock and electrocution from high-voltage windings and terminals, from the secondary side, and from touching the case if it’s not properly grounded. See Electrical Injuries.
• Arc flash and arc blast from a fault or a mistake in switching and testing. See Arc Flash & NFPA 70E.
• Backfeed: power from a generator, solar system or battery system feeding the transformer from the low-voltage side and energizing the high-voltage line. Line workers and others may be killed on a line they thought was de-energized.
• Stored energy: some transformers, and the capacitors and equipment around them, hold charge after power is off.
• Fire and explosion: overloaded or failed transformers can overheat, catch fire or rupture. Oil-filled units can spray burning oil.
• Hot oil and surfaces: burns from oil, fins and cases.
• Noise: constant humming, and louder when overloaded. See Noise.
• Environmental: oil spills and, in old units, polychlorinated biphenyls (PCBs).

Current transformers: never open the secondary

• Only trained, qualified workers should work on CT circuits.
• Use shorting blocks and follow written procedures for testing and replacing devices.
• See switchgear and motor control centers and Test Meters.

Working on or near transformers

• Only qualified persons should work on transformers, using proper procedures and PPE. Unqualified workers must stay out of the transformer area. See Qualified vs. Unqualified.
• De-energize and lock out, including all sources, both sides of the transformer, and any backfeed paths such as generators, solar and batteries. See Lockout/Tagout.
• Test and verify that all circuits are dead with a properly rated meter, test it on a known source before and after, and apply grounds where the procedure calls for. See Testing for Absence of Voltage.
• Discharge stored energy and wait for cool-down before touching.
• Wear the right PPE: arc-rated clothing, insulating gloves and eye and face protection as the analysis requires. See Arc Flash PPE Categories and Rubber Insulating Gloves.
• Keep the area clear. Keep a work space in front of equipment, as the code requires. See Panel Clearance. Don’t store materials in transformer rooms.
• Keep a safe distance from live lines and exposed transformers. See Overhead Power Lines.

• Don’t open pad-mounted transformers or cabinets. They’re locked for a reason. Call the utility.
• Don’t climb, lean or put things on transformers, and don’t park or dig near them. See Call 811.
• Don’t assume a quiet transformer is off, or a “dead” line is safe. Treat all as live until verified.
• Don’t bypass protective devices or exceed a transformer’s rating.

Oil, PCBs and spills

• Oil-filled transformers contain insulating oil. Leaks are a fire, slip and environmental hazard. Spill containment, such as a berm or pit, is often required. See Spill Response.
• Transformers made before about 1979 may contain PCBs, toxic chemicals that are regulated by the EPA, generally when concentrations are 50 parts per million or more. Don’t handle suspect oil or units without testing and proper procedures. Leaks, fires and disposal need trained specialists.
• Use the correct PPE for oil handling, and follow environmental rules for disposal.

Heat, overload and maintenance

• Transformers run warm. Keep vents clear, don’t block airflow and watch for signs of overheating: discoloration, a burning smell, leaking oil, unusual noise or tripped protective devices.
• Follow an inspection and maintenance program that includes infrared scanning, oil tests and cleaning. See electrical equipment maintenance and Electrical Safety Programs.
• Don’t overload. Check the nameplate for ratings and impedances.
• Report damage, leaks, bulging or unusual conditions right away.

If something goes wrong

• Fire or explosion: evacuate, call 911 and the utility, and keep everyone back. Don’t fight transformer fires with water. See Fire Extinguishers (PASS).
• Someone is shocked: don’t touch them until power is off. Call 911, then give first aid. See Electric Shock Response.
• Vehicle hits a pad-mount transformer or pole: stay away, assume everything is live, and call 911 and the utility. See Downed Power Lines and Step & Touch Potential.

Transformer safety checklist

• Only qualified, trained workers work on transformers; others stay out
• All sources, including backfeed paths, are locked out, tested and grounded as required
• CT secondaries are shorted before being disconnected
• Arc-rated PPE and insulating gloves match the hazard analysis
• Work space and ventilation are clear; pad-mounts stay closed and locked
• Oil leaks, PCBs and overheating signs are reported and handled by specialists

Last updated: October 4, 2026