Safety Corner
by Lynn Hamrick, ESCO Energy Services,
Electrical Risk Assessment
Using Probabilistic Risk Assessment
T
he concept of risk assessment and mitigation is a central theme across a
wide range of industries. How can risks be reliably identified, how can
they be managed, under what circumstances should they be accepted or
rejected and, especially, how are they likely to be interpreted or ‘perceived’ by
different people? These questions arise in areas as diverse as health and lifestyle,
hazardous industries, pensions and investments, transport, climate change, and
environmental protection. For this article, we will provide some background on
risk assessment, with particular focus on utilizing probabilistic risk assessment
(PRA) to account for varying risk levels of tasks being performed. Then we will
focus on how we may apply PRA in making decisions associated with electrical
safety in the workplace.
Background
Methodologies for performing risk and reliability assessment in the early
1960s originated in the U.S. aerospace and missile programs. A risk, or reliability,
calculation of some sort was performed, and the result was a very low success
probability value. So disappointing was this result that NASA became discouraged
from performing quantitative analyses of risk or reliability until after the Challenger mishap in 1986. In the meantime, the nuclear industry picked up PRA to
assess safety, almost as a last resort, in defense of its very existence. This analytical
method was gradually improved and expanded by experts in the field and has
gained momentum and credibility over the past three or four decades, not only
in the nuclear industry, but also in other industries like petrochemical, offshore
platforms, defense, and even NASA after the Challenger incident. Because of
its logical, systematic, and comprehensive approach, PRA has repeatedly proven
capable of uncovering design and operation weaknesses that had escaped even
some of the best deterministic safety and engineering experts. This methodology showed that it was very important to examine not only low-probability and
high-consequence events, but also high-consequence scenarios which can emerge
as a result of multiple high-probability influences such as adequate maintenance
and the worker’s qualifications.
Risk assessment is widely recognized as a systematic process for quantitatively
or qualitatively describing risk. Risk is commonly described as a combination of
the likelihood of an undesirable event (accident) occurring and its consequences.
www.netaworld.org Another way of looking at risk is as a
mathematical combination of an accident’s event probability of occurrence
and the consequence of that event
should it occur. Therefore, determining
risk generally amounts to answering
the following questions:
1. What is the hazard?
2. What is the probability that something will happen?
3. What are the consequences of the
hazard?
What is the hazard?
The answer to the first question is
a set of accident scenarios or hazards.
For electrical safety in the workplace,
this translates into the three known
associated electrical hazards of electric
shock, electric arc flash, and electric
arc blast.
What is the probability
that something will
happen?
The second question requires the
evaluation of the probabilities associated with these scenarios. This is a much
more difficult question to answer. This
involves developing a probability distribution associated with the hazards.
For a PRA, answering this question
involves the development of a probability of occurrence based on the task
being performed and the influences of
other contributing factors. A form of
Summer 2010 NETA WORLD
1
task analysis should be provided for tasks to be performed.
To a degree, NFPA 70E has provided an example of a task
analysis with the provision of Table 130.7(C)(9), Hazard/
Risk Category Classifications and Use of Rubber Insulating
Gloves and Insulated Hand Tools.
Other contributing factors that come to mind for determining the probability of electrical hazards should include:
• Maintenance Program
• Preventive/predictive maintenance performed
• Maintenance frequency
• Electrical testing performed
• Equipment condition
• Equipment age
• Equipment design
• Equipment installation (i.e., per NEC, etc.)
• Worker qualification
• Worker skills
• Worker experience
• Electrical safety training
• Safety program
• Corporate safety culture
• Voltage-rated tools and equipment
• PPE requirements
• Electrical hazard analysis
This listing is not complete but should give you an idea
of the types of things that need to be considered when developing a probability distribution for a hazard. Quantifying
these probabilities is typically performed or supported by
experts within the industry.
What are the consequences
of the hazard?
The third question estimates the consequences or extent
of the hazard. As an example for arc-flash hazards, this can
be quantified by evaluating the incident energy, in cal/cm2,
associated with an arc-flash event at a given location within a
facility’s electrical infrastructure, IEEE 1584-2002, Guide for
Performing Arc-Flash Hazard Calculations. Based on an arcflash analysis, one can determine the consequences or extent
of the hazard and provide safety precautions accordingly.
For shock hazards, the consequence is electrocution and
is more difficult to quantify. Most requirements are provided
through OSHA requirements in the form of mitigation
through the application of energy source controls (i.e.,
de-energize to eliminate the hazard) or PPE requirements
based on the voltage levels. However, it is understood that
the extent of the hazard is a function of the voltage level,
where typically the higher the voltage, the higher the degree
of shock hazard.
2
NETA WORLD Summer 2010
Incorporating PRA into Electrical Safety
in the Workplace
Given the background above, a question that should
be asked is, can we use PRA in evaluating electrical safety
hazards in the workplace and if so, how can we use PRA
in developing work processes to accommodate performing
electrical work on energized circuits? A foremost strength of
a PRA is that it is a decision support tool. In safety applications, PRA helps managers and engineers find design and
operation weaknesses in complex systems and then helps
them systematically and efficiently uncover and prioritize
safety improvements. The mere existence of a PRA does not
guarantee that the right safety improvement decision will
be made. The results must be understood and appreciated by
the implementers so that the appropriate safety precautions
and techniques are correctly implemented.
There are many who suggest that no work should ever
be performed on an energized circuit. My view is that this,
although preferred, is unrealistic in today’s workplace. Some
level of voltage measurement, testing, and troubleshooting
will always be required to adequately maintain the complex
systems encountered in the workplace. Additionally, most
“best practices” in electrical maintenance include a predictive
maintenance component (e.g., IR surveys, oil sampling, etc.)
which is performed while the equipment is operating and
energized. I feel that a combination of PRA with hazard
mitigation requirements from OSHA and NFPA 70E is
the solution.
With the following discussion, I hope to exemplify how
an approach of incorporating probabilities and task assessments into risk assessment can be provided and implemented in performing various tasks in the workplace. Let’s
take the task of operating a switch with the door closed
(i.e., energized but not exposed circuit) when an arc-flash
analysis has calculated the arc-flash hazard as dangerous (>40
cal/cm2) when the circuit is exposed. In this case, there is
no shock hazard since there is no exposure to an energized
circuit. However, there is an arc-flash hazard even with no
exposure to the circuit. But how do we determine the extent
of the hazard with the door closed? In this case, it is not
unreasonable to assume that the hazard is reduced when
the door is closed. Even though a specific arc-flash hazard
analysis is not provided for a switching operation with the
doors closed, NFPA 70E does provide some insight into
these types of operations through Table 130.7(C)(9). Some
excerpts from this table are provided below for information.
This table was developed such that the Hazard/Risk
Category provided for exposed, energized work is representative of the analytically determined incident energy
associated with the assumed conditions for the circuits. It
is noted that, typically, the table enables a drop of up to
two Hazard/Risk Categories when performing switching
operations with the enclosure doors closed. The Hazard/
Risk Category for tasks, other than working on energized
circuits, were derived through a consensus of committee
members based on their experience and a thorough review
of reported incidents and associated injuries. They are not
www.netaworld.org
analytically determined or based; however, they are consistent with the process
of having experts establish a probability distribution within a PRA. This drop of
two Hazard/Risk Categories represents an effective reduction in incident energies
of ~ 80% (i.e., 40 to 8 cal/cm2 = 80% reduction, 25 to 4 cal/cm2 = 84% reduction,
8 to 1.2 cal/cm2 = 85% reduction). One could surmise from this observation that
the arc-flash hazard associated with a switch operation with the doors closed is
effectively 20% of the exposed hazard to the worker. This would suggest that if a
switch has an exposed arc-flash hazard of up to 200 cal/cm2, using PPE meeting
a Hazard/Risk Category of 4, or 40 cal/cm2, should be sufficient to protect the
unexposed worker.
NETA’s Safety Corner is a section of NETA
World devoted to issues that those working in
the electrical industry face on a daily basis.
These articles are written by experts in the
field, most of whom serve on NETA’s Safety
Committee. NETA thanks these individuals
for their commitment to safety and for helping
to further the education of each reader with
every issue of NETA World.
As Operations Manager
of ESCO Energy Services Company, Lynn
brings over 25 years of
working knowledge in
design, permitting, construction, and startup of
mechanical, electrical,
and instrumentation and
controls projects as well as experience in the
operation and maintenance of facilities.
Lynn is a Professional Engineer, Certified
Energy Manager and has a BS in Nuclear
Engineering from the University of Tennessee.
Correction to Winter
2009-2010 Safety
with Arc-Flash Article
For the electrical hazard discussed above, the reduction to 20% of the analyzed
hazard is similar to the implementation of a probability of 0.20 that the hazard
will occur based on a task assessment and contributing factors. Other contributing
factors that could or should be incorporated into this assessment are equipment
condition and design (i.e., proper maintenance, nonlouvered doors, etc.).
Similarly, this type of assessment could be performed for IR surveys, which are
presented in the table with a typical reduction of one Hazard/Risk Category, or
insertion and removal of equipment from a live bus, which has a typical increase
of one or two Hazard/Risk Categories.
Although some may feel that all risks to an electrical worker should be mitigated through the use of energy source controls, PPE requirements, and special
precautionary techniques, there is benefit in combining PRA-type analyses with
current OSHA and NFPA 70E requirements to accommodate less hazardous
work activities. All electrical tasks, as presented in NFPA 70E Table 130.7(C)(9),
do not have the same level of risk. Currently with shock and arc-flash hazards,
the standards assume a probability of “1” that an incident will occur, regardless
of the task being performed, the equipment condition, or the experience and
capabilities of the qualified worker. This inherent difference in risk levels could
be accounted for through the judicious application of PRA techniques while
allowing for adequate risk mitigation.
www.netaworld.org NETA would like to extend its
sincerest apologies for a misprinted
byline in the winter 2009-2010 issue
of NETA World. The Safety with ArcFlash feature article, “How do I get
my technicians to adequately protect
themselves from electrical hazards?”,
was written by Mose Ramieh III of
Power & Generation Testing, Inc.
Mose Ramieh,
III has over ten
years of experience
in the electrical
power field. He is
certified as a Level
II Thermographer,
a NICET Certified
Level III Electrical Testing Technician, and a NETA Certified Level IV
Technician. His expertise covers industrial and utility power systems from
480 volts to 161 kV and all controls
associated with these systems. Mose
has served as a member of the NETA
Safety Committee since 2008.
Summer 2010 NETA WORLD
3