Arc Flash: a Sudden and Deadly Killer

It almost sounds like something out of a science fiction novel; you’re walking through your facility and out of nowhere, within milliseconds, heat hotter than the sun occurs, everything surrounding you melts, including metal. The burns you suffer are horrific.

Is this the result of a laser cannon-shot from your arch nemesis? Nope. It’s an arc flash.

What is an Arc Flash?

An arc flash is the result of an arc fault and is the visible part of the fault, resulting in heat and light. It occurs when energy is rapidly released and travels from one phase Busbar to another phase Busbar, or from a phase Busbar to the ground. During an arc flash, the air acts as a conductor, basically becoming a short-circuit in the air.

An arc fault is one of the most devastating and deadly electrical problems experienced. The resultant discharge and electrical explosion cause significant damage to nearby equipment. The discharge is also a severe problem for anybody in the area as well.

This YouTube video shows just how quickly a flash event happens. This specific event occurred in Cudahy, Wisconsin at an outdoor substation.

Two Types of Shorting Faults

An electrical system can be subject to two types of shorting faults: There are two types of shorting faults an electrical system is subject to; bolted and arc. Fluke defines them both: (emphasis added)

Bolted Faults

A bolted fault is everyone’s idea of a short-circuit, such as energizing the circuit with a ground set in place. A bolted fault results in a very high current; it is a low impedance short because of the solid connection. Bolted faults behave predictably and so conductors can be rated to withstand the overcurrent for the time required for an interrupt device to operate. Bolted faults rarely result in an explosion.

Arc Faults

The second — and far more destructive — fault is an arc fault. An arc fault occurs when the insulation (or, more specifically, the air separation) between electrical conductors is no longer sufficient to withstand their potential difference. This can occur for many reasons. A dropped tool or any other conductive element (even rust), introduced between or near energized components may compromise the insulating clearances. Often, incidents occur when a worker mistakenly fails to ensure that equipment has been properly de-energized. Incidents can even occur when a worker is simply removing a cover from a piece of equipment. A significant proportion of arc faults occur simply due to some form of equipment failure and are not limited to human interaction alone.

In contrast to the low impedance required for a bolted fault, an arc fault is a high impedance short because the discharge occurs through the air. The current is, therefore “comparatively” low but the explosive effects are much more destructive and potentially lethal. Unlike a bolted fault, it is difficult to predict exactly how much energy will be released by an arc fault. In particular, it is difficult to predict the duration of an arc fault because this depends on many factors, feedback mechanisms, and the response of the overcurrent protection devices.

Not only does the blast itself knock a person off of their feet, but it also causes damage to internal organs, leads to blindness, deafness and burns the lungs. The high temperatures, which reach up to 35,000 °F (hotter than the sun!), vaporizes nearby metal components. It’s pretty apparent that these temperatures cause significant burns to the nearby victims.

Between five and ten times per day, an arc flash occurs. 2,000 workers each year are sent to burn centers because of flash injuries. Although injuries are dropping each year, arc flash training remains vital.

In addition to saving lives, proper training reduces or eliminates possible safety violations and fines. Not following proper safety procedures also opens your company up to:

• Potential litigation
• Lost workdays
• Employee death

With such a potential for damage, safety becomes paramount. Fortunately, in the United States, companies have rules to follow.

Industry Standards

In the United States, four different industry standards have been established that prevents flash incidents and ensure the safety of your employees.

OSHA 29 Code of Federal Regulations (CFR) Part 1910, Subpart S

Quoted from the OSHA website:

This subpart addresses electrical safety requirements that are necessary for the practical safeguarding of employees in their workplaces and is divided into four major divisions as follows:

• 1910.301(a) Design safety standards for electrical systems. These regulations are contained in 1910.302 through 1910.330. Sections 1910.302 through 1910.308 contain design safety standards for electric utilization systems. Included in this category are all-electric equipment and installations used to provide electric power and light for employee workplaces. Sections 1910.309 through 1910.330 are reserved for possible future design safety standards for other electrical systems.
• 1910.301(b) Safety-related work practices. These regulations will be contained in 1910.331 through 1910.360.
• 1910.301(c) Safety-related maintenance requirements. These regulations will be contained in 1910.361 through 1910.380.
• 1910.301(d) Safety requirements for special equipment. These regulations will be contained in 1910.381 through 1910.398.

NFPA 70–2002, National Electrical Code, Section 110.16

Jeffrey Sargent, writing at the NFPA Journal defines breaks it down.

Recently, provisions were added to NFPA 70®, National Electrical Code® (NEC®), and NFPA 70E® requiring that labels on electrical equipment convey an important safety message to employees who have to examine, adjust, service, or maintain energized electrical equipment. Reading those labels is akin to a consumer reading a product label: They tell you what you are getting, or, in the case of energized electrical conductors and circuit parts, what you are getting into.

Please read the entire thing.

NFPA 70E-2000, Standard for Electrical Safety Requirements for Employee Workplaces

An article from the Industrial Safety & Hygiene News illustrates the standard for workplace electrical safety.

Provisions of NFPA 70E® encompass safety-related work practices, safety-related maintenance requirements, and safety requirements for special equipment. The standard includes guidance for making hazard identification and risk assessments, selecting appropriate PPE, establishing an electrically safe work condition, and employee training. The NFPA 70E® gets updated every three years.

Kindly read the entire article.

IEEE Standard 1584–2002, Guide for Arc Flash Hazard Analysis

Again, the Industrial Safety and Hygiene News site explains:

Section 4 of IEEE 1584–2002, Guide for Arc Flash Hazard Calculations, states that the results of the arc flash hazard analysis are used to “identify the flash-protection boundary and the incident energy at assigned working distances throughout any position or level in the overall electrical system.”

Section 4 also suggests a nine-step approach to a short-circuit coordination study:

1. Collect system and installation data
2. Determine system modes of operation
3. Determine bolted fault currents
4. Determine arc fault currents
5. Find protective device characteristics and duration of arcs
6. Document system voltages and classes of equipment
7. Select working distances
8. Determine incident energy for all equipment
9. Determine flash-protection boundary for all equipment

Again, please read the complete article.

Arc Flash Safety

I wish I could write that by following the steps listed below you’ll never see an arc flash. That’s not possible. However, becoming proactive and learning all you can about a flash helps reduces the likelihood of an event occurring.

Arc Flash Study

The first and most important part of the process is to complete an arc flash study. An arc flash study identifies any problem areas to make any necessary adjustments. It also identifies areas within the facility that has an increased risk of arc flash. The results of the study offer recommendations about the personal protective equipment essential for entering the area and labeling found within those areas as well.

Education

Some of the issues associated with an arc flash stem from the lack of knowledge about the seriousness of the problem. Educate employees about what they need to wear if they are in an arc flash area. Also, teach them what to avoid in a flash area.

What Causes Arc Flash

Teaching employees the danger areas is one key to arc flash safety. Explaining the cause of a flash helps prevent some incidents. Some of these causes include:

• Poor maintenance
• Insulation failure
• Wear-and-tear
• Contamination

Human Error

Unfortunately, human error is also a cause of an arc flash. These include:

• Improper installation
• dropping a tool
• using the wrong tool
• Touching the wrong thing
• Being in the wrong place at the wrong time

Protection

Set Boundaries

The NFPA recommends protection boundaries and a safe arc flash approach. Establishing those boundaries helps ensure the safety of nearby personnel.

The flash protection boundary is the outermost area where an employee is at risk of at least a second-degree burn. Moving in, you reach the area of limited approach, and anyone inside of this area is subject to electrical shock. The restricted approach is an area of increased shock hazard. The prohibited approach is the boundary considered to be direct contact with the equipment.

Be Proactive

Following some extra measures help ensure protection for both employees and equipment. Some additional measures include:

• Signage warning of arc flash danger
• Proper grounding
• Establishing barricades
• Installing arc flash relays

Use Arc Flash Relays

There is one proactive step to avoid the damage from a flash event: installing arc flash relays.

Arc flash takes place so fast that nearby personnel is unable to react. An arc flash relay, yet, works to shut down the power upstream at a circuit breaker within 1 millisecond. It identifies the very beginnings of the arc flash by using light sensors. There are also other types of overcurrent safety devices available, but they do not react as fast to shut down the power at the upstream device. ABB explains the role relays play preventing a flash event:

Ultra-fast clearing of arc flash faults in medium voltage (MV) switchgear panels is essential in controlling arc flash hazards. Reducing the arcing time through faster detection is the most practical way of reducing incident energy levels and improving workplace safety.

The reduction of arc flash incident energy levels is a priority, and one way to do this is to detect and trip for an arc flash event in the shortest time possible.

In modern protection systems, the need to operate in a few milliseconds is typically met by detecting the light from an arc flash and initiating tripping action via solid-state tripping elements. This approach is recognized in the IEC standard 62271–200 (pdf).

The intensity of light instantaneously released by an arcing fault can be thousands of times higher than normal ambient light, and it is this phenomenon that is used in arc flash detection relays to achieve faster operating times than is possible with conventional relaying. Optical sensors detect the sudden increase in light intensity. Instantaneous over-current elements are used as fault detectors to supervise the optical system for security.

First-generation arc flash protection, dating from the early 1990s, uses only single-point light receptors called ‘lens sensors’. In this type of system, lens sensors are typically in each cubicle where an arc flash might occur. Each lens sensor is individually targeted, for more precise location of the arc flash fault, and radially connected to electronics via a clad fiber.

Electric Safety Principles

Common sense plays a role in arc flash safety. A significant part includes following safety principles as described by the Electronic Library of Construction Occupational Safety & Health (elcosh).

Plan every job. Decide on your approach and step-by-step procedures. Write down first-time procedures. Discuss hazards and procedures in a job briefing with your supervisor and other workers before starting a job. Your employer should already have or develop a permit system for working on live circuits if a circuit must be worked live.

• Identify the hazards. Do a job hazard analysis. Identify steps that could create an electric shock or arc-flash hazards.
• Minimize the hazards. De-energize the equipment or insulate or isolate exposed live parts so you cannot contact them. If this is impossible, get proper personal protective equipment (PPE) and tools.
• Anticipate problems. If it can go wrong, it might. Make sure you have the right PPE and tools for the worst-case scenario.
• Get training. Make sure you and everyone working with you is a qualified person with appropriate training for the job.

To De-Energize or Not to De-Energize

When working on something electrical, do you keep it connected to the power source? This answer is one scenario where common sense plays its role. De-energizing eliminates the electrical hazard. From the elcosh site:

The most important principle of electric safety is, assume electric circuits are energized unless you make sure they are not. Test every circuit and conductor every time you work on them.

Lockout/Tagout application. Exposing each person to electric energy must be involved in the lockout/tagout process.

Proper Personal Protective Equipment

When working on or around live circuits, be sure to wear the right PPE to protect against electric shock and arc flash. Never wear clothing made from synthetic materials, such as acetate, nylon, polyester, or rayon — alone or combined with cotton. Such clothing is dangerous because it can burn and melt into your skin.

The type of PPE worn depends on the type of electric work being done (see table below).

[table id=12 /]

In Sum

After reading all of this, I imagine you thinking that dodging death rays from bad guys is easier than understanding arc flash. Like most things, the more you study something, the better you understand it. Arc flash isn’t a simple topic to master. Rely on qualified service organizations, like L&S Electric, to help you understand your safe boundaries and teach you ways to protect people and equipment from such a destructive act.