The subject of electrical hazard analysis has been recognized by a small segment of the electrical industry for many years.   The petrochemical industry and many government institutions have performed research on this subject for over 20 years.   For the most part however, the electrical industry, at least at the user level, has largely ignored the subject, essentially reacting to catastrophic accidents, rather than proactively trying to predict and prevent them.   Recent changes in consensus standards, along with a better general understanding of the seriousness of electrical hazards have resulted in a renewal of interest in the subject.  

As the awareness of electrical hazards increase, many are puzzled by phrases like: "limited," "restricted," "prohibited approach Boundary," and "flash protection boundary."   Understanding these terms is important to understanding shock and arc-flash hazard protection.   Below is an abbreviated definition of these terms.   The full definition can be found in NFPA 70E-2004, Article 100.

  Limited Approach Boundary - "A distance from an exposed live part within which a shock hazard exists."    

Restricted Approach Boundary - "A distance from an exposed live part within which there is an increased risk of shock, due to electric arc over combined with inadvertent movement."

Prohibited Approach Boundary - "A distance from exposed live parts within which work is considered the same as making contact with the live parts."

Flash Protection Boundary - "A distance from exposed live parts within which a person could receive a second degree burn."

The NFPA 70E-2004, "Electrical Safety in the Workplace," addresses the requirements for conducting an "Electrical Hazard Analysis" with emphasis on the "Shock Hazard Analysis" and the "Flash Hazard Analysis" issues.   NFPA 70E-2004 tells us that if circuits operating at 50 volts or more are not deenergized (placed in an electrically safe work condition) then other electrical safety-related work practices must be used.   These work practices must protect the employee for arc flash as well as inadvertent contact with live parts operating at 50 volts or more.   These analyses must be performed before an employee approaches exposed live parts within the Limited Approach Boundary.

This article will provide an overview of the principal types of electrical hazard analysis, along with a discussion of the relevant standards and regulations pertaining to the subject.

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Shock Hazard Analysis

Each year several hundred workers are killed due to inadvertent contact with energized conductors.   Surprisingly, over half of those killed are not in traditionally electrical fields (i.e. linemen, electricians, technicians, etc.), but are from related fields such as painters, laborers and drivers.   Because of this, the 2004 edition of NFPA 70E established a new requirement for conducting a "Shock Hazard Analysis" in order to determine the voltage that a person would be exposed to, shock protection boundaries, and personal protective equipment requirements.  

Recent investigations into the causes of these fatalities point to three principal causal factors:

•  Failure to properly or completely de-energize systems prior to maintenance or repair work;

•  Intentionally working on energized equipment; and

•  Improper or inadequate grounding of electrical system components.

These three factors form the basis for hazard analysis of the electrical shock hazard.

To appropriately assess the electrical shock hazard associated with any type of maintenance or repair work, it is necessary to evaluate the procedures or work practices that will be involved.   These practices should be evaluated against both regulatory and standards requirements as well as recognized good practice within the industry.   These principles are summarized below.

 

OSHA Regulatory Requirements

•  All equipment must be placed in a deenergized state prior to any maintenance or repair work. (Limited exceptions exist).

•  The deenergized state must be verified prior to any work.

•  The deenergized state must be maintained through the consistent use of locks and tags, and in some cases, grounding.

•  When energized work is performed, it must be performed in accordance with written procedures.

 

NFPA 70E-2004 Standard Requirements

•  The Shock Hazard Analysis must establish the:

1. Limited Approach Boundary;
2. Restricted Approach Boundary; and
3. Prohibited Approach Boundary.

•  This applies to all exposed live parts operating at 50 volts or more.

•  Only qualified persons are permitted within these boundaries.

•  Unqualified person may not enter these boundaries unless the conductors and equipment have been placed in an electrically safe work condition.

Industry Recognized Good Practices

•  Plan every job.

•  Anticipate unexpected results and the required action for these results.

•  Use procedures as tools.

•  Identify the hazards.   Keep unqualified workers away from these hazards.

•  Assess employee's abilities.   Remember, there is a difference between 10 years of experience, and one year of experience repeated 10 times.

In addition to the assessment of work practices, shock hazard analysis must include an assessment of the physical condition of the electrical system.   The assessment must also identify the proper PPE for shock protection, which would include, but not be limited to, rubber insulating gloves with leather protectors, rubber blankets and mats, and insulated hand tools.

Although the continuity and low resistance of the equipment grounding system is a major concern, it is not the only one.   Of equal importance is to insure that equipment covers and guards are in place; that access to exposed conductors is limited to electrically qualified personnel; and overcurrent protective devices are operable and of appropriate interrupting rating.   Even the safest procedures, when performed on poorly constructed or maintained equipment represent a risk to employees.

Flash Hazard Analysis

Two industrial electricians began work in the basement electrical room one day.   They wanted to take some physical measurements and knew the switchgear was energized but were in a hurry to get started.   As they were taking measurements on the bus with a wooden ruler the metal tip of the ruler made contact with the bus and caused a massive electric arc.   The arc-flash only lasted a fraction of a second.   Although no one was electrocuted, one man died instantly from the arc-flash and the other man was badly burned.   The man that died was within 24 inches of the bus while the other man was about 10 feet away.

An estimated 75 percent to 80 percent of all serious electrical injures are related to electrical arcs created during short circuits and switching procedures.   In recognition of this fact, standards organizations such as the National Fire Protection Association (NFPA) have provided the industry with better techniques to evaluate both the magnitude of the electrical arc hazard and appropriate protective clothing and equipment.

When the insulation medium, between phases or phase and ground, whether air, porcelain, polymer, or other medium, can no longer support the applied voltage an electrical arc is formed.   A short circuit or insulation breakdown is a switching action that creates a bypass around a circuit that involves either phase-to-phase or phase-to-ground or a combination.   The heat generated by the high current flow may melt or vaporize the material and create an arc.   This arc-flash creates a brilliant flash, intense heat, and a fast moving pressure wave that propels the arcing products.

While commercial electricity has been around for over 100 years, the most common hazard of electricity has been electric shock or electrocution. As commercial electric systems grew, other hazardous effects such as arc-flash and arc-blast began to surface.   The initiation, escalation, effects and prevention of electrical arcs have been analyzed and researched since the early 1960's.   Human errors and equipment malfunctions contribute to the initiation of an electrical arc.   Engineering design and construction of arc resistant equipment as well as requirements for safe work practices are continuing to target the risk of electrical arc-flash hazard.    As the demand for electricity increases, transmission and distribution utility systems are being upgraded. Transformers are being upgraded or replaced with higher KVA ratings and lower impedances at both the utility and industrial/commercial level. In addition, as the demand for higher reliability also increases, transformers are being operated in parallel by closing a tiebreaker.   All of these modifications to the system can cause dramatic increases in the available fault current.   More electrical energy throughput is a result of these modifications; however, the downside is an increase in the electrical current to feed a fault to existing equipment in industrial and commercial facilities that may now be under-rated to interrupt available fault current.   This increase in available fault current can wreak havoc on under-rated and/or improperly maintained equipment.