U.S. Department of Labor

Occupational Safety and Health Administration

Directorate of Technical Support & Emergency Management

Office of Science and Technology Assessment

Hazards when Purging Hydrogen Gas-Cooled Electric Generators

Safety and Health Information Bulletin

Introduction

Many electric power plants now use 

hydrogen gas-cooled electric generators 

versus air- or water-cooled generators. To 

prevent injury or fatality, employers must 

ensure hydrogen gas is completely purged 

from generator housings and bushing boxes 

before conducting preventive, predictive, or 

emergency maintenance (29 CFR 1910.146). 

Employers should include all manufacturer 

recommendations in written purging 

procedures.

Infrequent procedures such as purging 

hydrogen gas from electric generators and 

bushing boxes call for employers to pay 

special attention to safety. Employers must 

provide adequate training and ensure that 

workers comply with safe work practices 

and written procedures before beginning the 

task. This SHIB highlights this need and 

includes a case study about a fatal flash fire 

that resulted from inadequately purging a 

hydrogen gas-cooled electric generator 

housing and bushing box.

Case Study: Fatal Flash Fire

In 2011, inadequate hydrogen gas purging 

resulted in a fatal flash fire during a turbine 

generator repair. Three electrical power 

generation units each used a boiler to deliver 

steam to a turbine generator. Hydrogen gas 

cooled both the generator housing and its 

bushing box through connecting paths. (See 

Figure 2)

This Safety and Health Information Bulletin is 

not a standard or regulation, and it creates no new 

legal obligations. The Bulletin is advisory in 

nature, informational in content, and is intended 

to assist employers in providing a safe and 

healthful workplace. Pursuant to the 

Occupational Safety and Health Act, employers 

must comply with hazard-specific safety and 

health standards and regulations promulgated by 

OSHA or by a state with an OSHA-approved 

state plan. In addition, pursuant to Section 

5(a)(1), the General Duty Clause of the Act, 

employers must provide their employees with a 

workplace free from recognized hazards likely to 

cause death or serious physical harm. Employers 

can be cited for violating the General Duty 

Clause if there is a recognized hazard and they do 

not take reasonable steps to prevent or abate the 

hazard. However, failure to implement any 

recommendations in this Safety and Health 

Information Bulletin is not, in itself, a violation of 

the General Duty Clause. Citations can only be 

based on standards, regulations, and the General 

Duty Clause.

Figure 1: Electrical power turbine generator set

Source/Copyright: iStock – Banks Photos

SHIB 01-22-2016
Terminology

Bushing box is an attached “box” or housing that 

contains generator lead terminals, often located at 

the generator casing’s base.

Flash fire occurs when a fuel diffused in air 

contacts an ignition source, creating a rapidly 

moving flame front for three seconds or less.

Minimum ignition energy is the minimum energy 

amount required to ignite a combustible vapor, 

gas, or dust cloud.

Terminals are devices attached to electrical 

conductors to make electrical connections.

One turbine generator was taken offline 

because workers detected hydrogen 

contamination and suspected a leak. They 

used a portable leak detector and located a 

leak where the terminal plate was bolted to 

the bushing box. Repair required workers to 

enter through a bushing box man-way. Prior 

to removing the bushing box man-way 

cover, workers purged the generator housing 

with carbon dioxide (CO2

) and then with 

ambient air. CO2 is used first to prevent 

explosions or flash fires that could occur 

with a hydrogen-air mixture.

After purging, the man-way cover was 

removed, the workers tested for hydrogen at 

a fixed location, and one worker entered the 

confined space with an electric drop light 

and an electric fan. A flash fire erupted, and 

the worker was fatally burned.

OSHA investigators identified the following 

unsafe work practices as potential flash fire 

causes:

• According to the employer’s written 

procedures based on internal volume, 

twelve CO2 gas cylinders are needed to 

fully purge the generator housing. Only 

six CO2 cylinders were released, not 

enough to fully purge the generator.

• A fixed gas detector was used to test for 

hydrogen after purging rather than a 

portable gas monitor, as required by the 

employer’s written procedures. As a 

result, hydrogen gas was measured from 

the generator housing base rather than 

from the generator housing roof where 

the hydrogen vented and would naturally 

accumulate since it is lighter than air. 

This measurement did not identify the 

residual hydrogen.

• The confined space assessment was 

conducted three days before the bushing 

box man-way cover was removed and 

the worker entered the space. The space 

was sealed and closed for those three 

days, which affected the original 

assessment.

• A fire hazard existed from using tools 

unsafe for this environment (e.g., electric 

drop light and electric fan) rather than 

tools rated as intrinsically safe.

Figure 2: Bushing box with 

man-way cover removed

Source: OSHA
OSHA Standards

• 29 CFR Part 1910.103, Hydrogen

(overview of OSHA standards pertaining to 

hydrogen: 

osha.gov/dep/greenjobs/hydrogen_standards)

• 29 CFR Part 1910.146, Permit-Required 

Confined Spaces

• 29 CFR 1910.146 Appendix B, Procedures for 

Atmospheric Testing

• 29 CFR Part 1910.147, Lockout/Tagout of 

Energy Sources

• 29 CFR Part 1910.269, Electric Power 

Generation, Transmission, and Distribution

• 29 CFR Part 1910.303, Electrical Standards

• 29 CFR Part 1910.307, Hazardous (classified) 

Locations

• 29 CFR 1910.1200, Hazard Communication

Hazards from Purging Hydrogen GasCooled Electric Generators

Work in the typical electric generator 

housing or bushing box usually involves 

permit-required confined space entry 

procedures including atmospheric checks for 

oxygen concentration and flammable gases 

such as hydrogen. If hydrogen gas remains 

in any area in a concentration from 4% to 

75% (the lower and upper flammability 

limits) in air, then the atmosphere in that 

area is flammable (see Figure 3). The wide 

flammability range increases susceptibility 

to flash fires in particular. OSHA defines an 

atmosphere with greater than 10% of its 

lower flammability limit as immediately 

dangerous to life and health, and entry is 

prohibited.

Hydrogen gas is lighter than ambient air and 

can concentrate in overhead areas, making it 

difficult to detect. Because hydrogen gas has 

a low minimum ignition energy, it can ignite 

with any small ignition source—even an 

electrostatic spark from a worker’s 

movement or a tool touching a surface. In 

addition, employers must continuously 

monitor the area where the workers are 

located because hydrogen gas is colorless, 

odorless, and tasteless, and worker 

asphyxiation may occur if this gas is not 

completely purged.

These hazards differ from the hazards 

associated with a hydrogen gas escape or 

leak, which could cause an explosion when 

it reacts with ambient air.

Workers must also take electric power 

generators offline and follow the established 

written safety procedures to prepare the 

equipment for entry.

Preventing Injuries when Purging 

Hydrogen Gas-Cooled Electric 

Generators

OSHA recommends taking the following 

actions to prevent worker injuries or 

fatalities when purging generator housings 

and bushing boxes:

• Train workers on hydrogen gas purging 

procedures and the implementation 

sequence, and the hazards associated 

with hydrogen gas (29 CFR 

1910.1200(h)).

• Develop and implement written 

procedures for working in confined 

spaces (29 CFR 1910.146(c)(4)-(5) & 

(g), as well as enclosed spaces related to 

Figure 3: Hydrogen flammability diagram

Source/Copyright: Swiss Hydrogen Association 

(http://hydropole.ch/en/hydrogen/abouth2/)
electric power transmission, generation 

and distribution (29 CFR 1910.269(e)).

• Review and update procedures for 

purging generators to ensure they 

completely purge hydrogen with carbon 

dioxide or other suitable inert gas, and 

then purge that gas with ambient air to 

create a safe, inert atmosphere.

• Follow the equipment manufacturer’s 

recommendations when establishing 

written purging procedures (e.g., 

housing capacity, purge gas volume).

• Place detectors in areas where hydrogen 

gas could accumulate. Follow a detailed 

and accurate piping and instrumentation 

diagram to identify areas where residual 

hydrogen gas may accumulate due to 

inadequate purging.

• Test atmospheric conditions in the 

permit space before authorizing entry 

and verify that acceptable entry 

conditions exist during the entire 

operation (29 CFR 1910.146(d)(3)(vi) & 

(d)(5)(i)). Verify that no hydrogen gas is 

present using fixed and portable 

hydrogen monitoring equipment and 

real-time direct reading instruments.

• Establish and implement ignition control 

procedures.

Employers must comply with the OSHA 

standards (see OSHA Standards box for 

applicable standards).

Industry consensus standards and additional 

resources include:

• National Fire Protection Association 

(NFPA) 2, Hydrogen Technologies Code

• NFPA 70, National Electric Code

• NFPA 55, Compressed Gases Code

• Department of Energy Hydrogen and 

Fuel Cells Program

• NASA Hydrogen Topics Page

How OSHA Can Help

OSHA has compliance assistance specialists 

located in most OSHA offices throughout 

the nation who can provide information to 

employers and workers about OSHA 

standards, short educational programs on 

specific hazards, or OSHA rights and 

responsibilities, as well as information on 

additional compliance assistance resources. 

Contact your local OSHA office for more 

information.

OSHA's Consultation Service offers free and 

confidential advice to private-sector 

employers to help them identify and correct 

hazards, with priority given to small 

employers with the most hazardous 

operations, or in the most high-hazard 

industries. Consultation services are separate 

from enforcement activities. To locate the 

OSHA Consultation program nearest you, 

call 1-800-321-6742 (OSHA) or visit 

OSHA's Small Business web page: 

http://www.osha.gov/dcsp/smallbusiness/ind

ex.html.

Workers' Rights

Workers have the right to:

• Working conditions that do not pose a 

risk of serious harm.

• Receive information and training (in a 

language and vocabulary they 

understand) about workplace hazards, 

methods to prevent them, and the OSHA 

standards that apply to their workplace.

• Review records of work-related injuries 

and illnesses.

• Receive copies of results from tests and 

monitoring done to find and measure 

hazards in their workplace.

• File a confidential complaint with 

OSHA to have their workplace inspected 

if they believe there is a serious hazard 

or that their employer is not following 

OSHA's rules.
• File a complaint with OSHA if they have 

been retaliated or discriminated against 

by their employer as the result of 

requesting an inspection or exercising 

any of their other rights under the OSH 

Act. 

Contact OSHA 

• For questions or to get information or 

advice, or to report an emergency, report 

a fatality or catastrophe, order 

publications, sign up for OSHA's enewsletter QuickTakes, or to file a 

confidential complaint, contact your 

nearest OSHA office, visit 

www.osha.gov, or call OSHA at 1-800-

321-OSHA (6742), TTY 1-877-889-

5627.

Twenty-eight states and territories operate 

their own occupational safety and health 

state plans approved by OSHA. State plans 

may have different or additional 

requirements. A list of state plans is 

available at: http://www.osha.gov/dcsp/osp/.