Power Transformer Hazard Awareness

Paul Chamberlain, Asplundh Engineering Services, LLCColumns, Safety Corner, Summer 2024 Columns

Performing condition analysis or maintenance on a power transformer and ancillary devices is a potentially hazardous task that requires well-trained test personnel to identify and mitigate risks. Products and equipment that are designed to test and diagnose equipment while in operation, such as infrared cameras, partial-discharge detection equipment, and online oil analysis can be used to determine a power transformer’s condition. The personal protective equipment (PPE) required must be appropriate and adequate for all tasks and their associated hazards.  


A few of the potential hazards of a power transformer and means of safeguarding and mitigating those hazards are described below. We also explore some potentially hazardous situations that can occur while performing this type of work. 

Additional hazards may exist, depending upon the type or condition of the equipment. Take all procedures and instructions seriously, verify that the instruction or equipment operation and maintenance manuals are for the correct equipment, and identify and check for potential hazards before beginning the task by using a pre-job briefing worksheet.

Electrical and Mechanical Hazards

Improperly performed lockout/tagout (LO/TO) is a major contributing factor to injuries suffered while working on a power transformer and associated devices. Controlling all forms of hazardous energy is essential, and many may be involved. Refer to the appropriate OSHA regulation or required procedure, including 29 CFR 1910.147, The Control of Hazardous Energy (Lockout/Tagout) and 29 CFR 1910.333, Selection and Use of Work Practices, and manufacturer instructions to determine the proper LO/TO procedures. 

De-energize the electrical feed to the power transformer and auxiliary devices from its primary energy source and visually verify that the equipment is disconnected from all sources of power including AC and DC. Once de-energized, check that the equipment is at a zero-energy state using the manufacturer’s approved method. Then verify the proper operation of the voltage measuring device against a known source and retest the equipment for zero energy. Testing for the presence of voltage requires a specific level of PPE per the applicable NFPA, OSHA, or CCOHS standards.

Mechanical Hazards

Electrical energy isn’t the only energy that requires LO/TO. Devices such as motor-operated switches and circuit breakers may contain mechanical energy. This energy must be dissipated before servicing the equipment or serious injury could occur. Once the energy has been discharged or dissipated, the source of the stored energy must be locked out/tagged out if feasible. Ensure that remote operating handles are tagged in a local or manual mode. This will prevent someone from inadvertently operating the equipment. 

Chemical Hazards

Some power transformers may pose a chemical hazard; caution must be taken with gases, chemicals, and liquids. Proper containment of liquids (e.g., spill containment pads) may be employed and additional environmental concerns may need to be addressed. Ensure compliance with all owner, state, and federal regulations. Beware of units containing polychlorinated biphenyls (PCBs) or other hazardous fluids. When working on such units, follow appropriate state and federal guidelines for fluid handling and disposal, and avoid skin contact. 

Some cleaners pose a respiratory and skin irritant if used in enclosed areas or if they contact bare skin. Read the chemical container’s label and check the safety data sheet (SDS) to identify proper use. Ensure the use of the correct PPE for the chemical used. Examples of this include nitrile gloves, safety glasses, and face shields. In some cases, respiratory protection may be necessary.

PHOTO: © stock.adobe.com/contributor/205797582/artboyshf142
Confined and Enclosed Spaces

When performing visual inspection, mechanical inspection, maintenance, or repairs on a power transformer, personnel may be required to enter the transformer’s tank. Entering into a confined space requires the entrant to be aware of the conditions that exist within that space. 

The entrant must first determine whether the confined space is located at a) a facility regulated by OSHA 1910.269, Electric Power Generation, Transmission, and Distribution, or b) in a commercial entity or space regulated under OSHA 1910.146, Permit-Required Confined Spaces. OSHA regulations provide different requirements for the hazards found at each type of location. OSHA 1910.269, Electric Power Generation, Transmission, and Distribution, Appendix A provides a flow chart to help make this determination at www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.269AppA.

The entrant must know whether the work to be performed falls under the scope of §1910.269, which applies to work conducted during the operation and maintenance of electric power generation, control, transformation, transmission, and distribution lines and equipment. Specific information on defining the work can be found throughout 1910.269(a)(1).

The entrant must also determine the potential hazards within the space that require control prior to entry. For more information regarding those hazards, see all of 1910.269(e).

To correctly apply the flow chart, the entrant must determine whether the space is a confined space or an enclosed space.

  • §1910.269, Permit-Required Confined Spaces, (b) Definitions:
  1. Is large enough and so configured that an employee can bodily enter and perform assigned work; and 
  2. Has limited or restricted means for entry or exit (for example, tanks, vessels, silos, storage bins, hoppers, vaults, and pits are spaces that may have limited means of entry.); and 
  3. Is not designed for continuous employee occupancy.
  • §1910.146(b) additionally defines which confined spaces require a permit to enter:
  1. “Permit-required confined space (permit space)” means a confined space that has one or more of the following characteristics: 
    • a. Contains or has a potential to contain a hazardous atmosphere; 
    • b. Contains a material that has the potential for engulfing an entrant; 
    • c. Has an internal configuration such that an entrant could be trapped or asphyxiated by inwardly converging walls or by a floor that slopes downward and tapers to a smaller cross-section; or 
    • d. Contains any other recognized serious safety or health hazard.
  • §1910.146(b) also defines an enclosed space:
  1. Enclosed Space: A working space, such as a manhole, vault, tunnel, or shaft, that has a limited means of egress or entry, that is designed for periodic employee entry under normal operating conditions, and that, under normal conditions, does not contain a hazardous atmosphere, but may contain a hazardous atmosphere under abnormal conditions.
  2. Note to the definition of “enclosed space”: The Occupational Safety and Health Administration does not consider spaces that are enclosed but not designed for employee entry under normal operating conditions to be enclosed spaces for the purposes of this section. Similarly, the Occupational Safety and Health Administration does not consider spaces that are enclosed and that are expected to contain a hazardous atmosphere to be enclosed spaces for the purposes of this section. Such spaces meet the definition of permit spaces in §1910.146, and entry into them must conform to that standard.
Other Physical Hazards

Gravity is an energy that may also need to be controlled and is often overlooked. The size and weight of panel covers and inspection plates may make them difficult to handle. Should gravity be a potential energy source, ensure that the energy is dissipated and mitigated from recurrence before performing the work. For panel covers, the procedure might include using another person to hold the cover in place or leaning against it while you remove it. Do bottoms first, then tops.

Figure 1: §1910.269 Appendix A-1 Application of §1910.269 and Subpart S of This Part to Electrical Installations
Improper PPE Hazards

After verifying that the power transformer is de-energized, the method to disconnect the equipment may require different PPE. Refer to CCOHS, NFPA, or OSHA to determine which PPE is required. Identifying the correct level of PPE and voltage-rated gloves will help prevent injury from a potential arc flash. 

PPE used must always be adequate for the task and energy levels and worn before entering within the applicable Boundary. 


Grounds are an excellent secondary means of protecting the worker from inadvertent energization. Refer to any applicable CCOHS, OSHA, NFPA, and ASTM standard for specific guidance on grounding locations and sizing of grounds required for the task. Grounds must be applied upstream and downstream of the equipment and as close to the work as possible unless utilizing equipotential grounding. Using correctly sized and applied grounds is an additional safeguard for employees if a form of electrical energy is introduced into the system or equipment the work is being performed on. Induced voltage or back-feed are just two forms of energy that may be inadvertently introduced into a system that has been properly locked out/tagged out.


Be aware of these factors when performing maintenance and testing on a power transformer: 

  1. Obtain all available service bulletins, maintenance documents, arc flash studies, and manuals before beginning work.
  2. Review all prints and one-line drawings associated with the equipment.
  3. Establish a safe work area and barricade it off.
  4. Perform a pre-job brief with all employees on-site.
  5. Identify and wear proper PPE.
  6. Disconnect the electrical feed and control circuit(s), verify mechanical interlocks are properly engaged, and test equipment before performing visual or mechanical inspections.
  7. If applicable, verify that there is zero energy (test, check, test) and discharge all stored energy, including pressurized gasses and gravity.
  8. If possible, lock out and tag out all energy sources.
  9. Connect grounds where and if applicable.
  10. Identify, visually mark, and/or flag the equipment being worked on.

Identifying and mitigating the hazards listed above can lead to a safer work environment while performing inspection, maintenance, and testing of a power transformer. 

Paul Chamberlain has been the Safety Manager for Asplundh Electrical Testing, LLC (formerly American Electrical Testing Co., LLC), a subsidiary of Asplundh Engineering Services, LLC since 2009. He has been in the safety field since 1998, working for various companies and industries. Chamberlain received a BS from the Massachusetts Maritime Academy.