Concerned About Arc-Flash and Electric Shock?
Electrocution is the obvious danger faced by anyone working on or near live electrical equipment and it is clearly important to understand shock hazards and wear appropriate protection. However, most electrical accidents are not the result of direct electric shocks. A particularly hazardous type of shorting fault—an arc fault— occurs when the insulation or air separation between high voltage conductors is compromised. Under these conditions, a plasma arc—an “arc flash”—may form between the conductors, unleashing a potentially explosive release of thermal energy.
An arc flash can result in considerable damage to equipment and serious injuries to nearby personnel. A study carried out by the US Department of Labor found that, during a 7-year period, 2576 US workers died and over 32,000 suffered injuries from electrical shock and burn injuries. 77 % of recorded electrical injuries were due to arc flash incidents. According to statistics compiled by CapSchell Inc (Chicago), every day, in the US alone, there are 5-10 ten arc flash incidents, some fatal.
NFPA 70E is the leading internationally recognized safety standard for electrical safety in the workplace. The Canadian Standards Association has developed its own set of standards based on NFPA 70E: CSA Z462.
These standards define a set of safe requirements for personnel working on electrical equipment. To comply with the standards, employers must carry out a hazard risk assessment and ensure that all employees working in a potential arc-flash hazard zone use appropriate equipment and wear the right protective clothing. Although it is not the responsibility of the thermographer to put in place the appropriate safety procedures, it is important to recognize and understand their need, and to ensure that the correct procedures, equipment and protective clothing are used.
The installation of IR windows, panes or ports allows a thermographer to inspect live electrical equipment without the removal of protective covers and the exposure of equipment. An arc-resistant window, unlike a port or pane, provides additional protection for the thermographer in the event of an arc flash resulting from unexpected component failures or work on other parts of the system. This substantially reduces the hazard rating for the inspections and, in most cases, may allow the thermographer to work more safely minimizing the need for excessively bulky and cumbersome protective clothing.
What is an Arc-Flash?
An electrical system can be subject to two types of shorting faults:
• Bolted faults
• Arc 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 over current for the time required for an interrupt device to operate. Bolted faults rarely result in an explosion.
Older switchgear which holds a “fault-rating” will usually be rated for its ability to withstand this high current for a particular time period. A bolted “fault-rated” piece of equipment will usually have a BIL (Basic Impulse Level) highlighted on the casing itself in the form of a fault current for a set duration. E.g 100 kA for 5 seconds.
The second—and far more destructive—fault is an arc-fault. This 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 component failure and is 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 air. The current flow 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 as this depends on many factors, feedback mechanisms and the response of the over current protection devices.
When an Arc Fault Occurs
NIOSH (National Institute for Occupational Safety and Health, Pittsburgh, USA) has published the results of a survey of electrical accidents reported by MSHA (Mine Safety and Health Administration, US Department of Labor) in the mining sector over the period: 1990-2001. In more than two-thirds of the cases of arc flash injuries, the victim was performing some form of electrical work such as troubleshooting and repair. More surprisingly, 19 % of the accidents arose from the direct failure of equipment during normal operation. Overall, 34 % of the accidents involved some form of component failure.
The key components involved in the accidents where: circuit breakers (17 %), conductors (16 %), non-powered hand tools (13 %), electrical meters and test leads (12 %), connectors and plugs (11 %). Of the cases that reported the arcing voltage, 84 % occurred with equipment at less than 600 V and only 10 % with equipment at more than 1000 V.
Arc Fault Make-up
When an arc-fault is triggered, a plasma arc—the arc flash—forms between the shorted components. Once established, the plasma arc has a virtually unlimited current-carrying capacity. The explosive energy release causes:
• A thermoacoustic (dynamic) pressure wave
• A high intensity flash
• A superheated ball of gas
The thermoacoustic wave is a dynamic pressure wave caused by the instantaneous expansion of gas local to the fault. It causes panels to rupture, flying debris and barometric trauma. The wave front travels outwards, away from the fault, and as it impacts surfaces it increases in energy: an effect known as “pressure piling”.
A common misconception is that an arc flash will always result in panel rupture. However, by incorporating high-speed interrupt devices and additional protection systems, an engineer can reduce the arc flash energy to a level where the thermoacoustic wave front does not have sufficient energy to rupture the panel.
Although the thermoacoustic wave resulting from an arc fault can be very destructive, it is not the only characteristic of an arc flash. Unlike a chemical explosion, the energy of an arc flash converts primarily to heat and light energy. Temperatures at the epicenter of an arc flash can reach 20,000 °C (four times hotter than the surface of the sun) within a millisecond. Such high temperatures are capable of explosively vaporizing metals such as copper. The presence of vaporized metal can then feed and sustain the plasma arc and exacerbate its power.
An arc flash essentially lasts until the over current protective devices open the circuit. A fast-acting fuse may open the circuit as quickly as several milliseconds.
The Consequences of an Arc Flash
Arc faults are potentially fatal to any personnel in the vicinity. The intense heat of the arc flash can severely burn human skin and ignite the clothing of anyone within several feet of the incident. Treatment for arc flash burns can involve years of skin grafts.
Without proper eye protection, projectiles and molten debris can cause eye damage. The intense UV radiation associated with the flash can cause retinal damage. Superheated vapors can injure lungs and impair breathing. The thermoacoustic blast can damage hearing with ruptured eardrums, cause collapsed lungs and damage other internal organs. The blast can knock personnel off their feet; falls may result in broken bones or lead to electrocution or further injuries on other parts of the system.
Inevitably, a serious arc flash will damage or even destroy the affected equipment. This leads to extensive downtime and expensive replacement and repair. An incident may also represent a failure on the part of the employer to comply with industry guidelines and regulations. This could result in a fine, litigation fees, increased insurance costs, expensive legal actions and accident investigations.
Standards and Guidelines
The potential dangers of an arc flash can be reduced by following the relevant safety guidelines and using personal protective equipment PPE. In the USA, the following OSHA and NFPA regulations apply to personnel working with energized electrical equipment:
• NFPA 70 (“National Electric Code”)
• NFPA 70E (“Standard for Electrical Safety in the Workplace”)
• OSHA Standards 29-CFR, Part 1910 (S) 1910 333
• OSHA Standards 29-CFR, Part 1926 Subpart K
• IEEE Standard 1584-2002, (“Guide for Performing Arc Flash Hazard Calculations.”)
Many other countries have their own broadly similar standards and regulations. For example, Canada’s regulations can be found in CSA Z462. In the UK, compliance with EAWR (Electricity at Work Regulations) 1989, section 5 is required.
JM Test Systems rents and sells Fluke Test Tools compliant with every NFPA 70E rating.
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