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By Eugene Williams
Troubleshooting electrical equipment can introduce many new safety concerns especially when inspecting equipment that is energized. Testing often requires the troubleshooter to temporarily connect test instruments to "live" terminals which may involve opening enclosures or cabinets that normally are locked or bolted closed to protect workers.
This introduces two main hazards:
This article deals with the second above listed item: the Flash Hazard.
If you are familiar with electric arc welding, then you are aware that the small arc created by the welding equipment can generate enough heat to melt metal as well as generate enough UV rays to burn your skin.
In the case when electrical equipment fails causing and electrical arc, the energy released during the arcing can be many many times greater than the welding arc and can cause severe flash burns. The burns fall into one of three categories:
First Degree: the outer skin layer is damaged, it is painful, but since the growth areas are not damaged, the skin is quickly re-grown and no scarring is left.
Second Degree: the outer skin layer is severely damaged and blistering usually occurs. Healing is much longer as it occurs from the deeper sweat glands and hair follicle areas. Scarring is often the result.
Third Degree: complete destruction of the skin and growth areas. If the burn is small healing may occur from the sides, however skin grafting is usually required.
In addition to the burns caused by the heat generated by an electric arc, another hazard also exists. When an electric arc is established the material and air around it are superheated causing a rapid expansion in the area. Pressures up to 200 psi can be produced. Many electrical enclosures are not designed to contain pressures of that magnitude. The result is molten metal and parts become shrapnel flying at great velocities toward anyone in the vicinity.
The greater the amount of energy the arc produces (measured in cal/cm2) the worse the burn. Any heat over 1.2 cal/cm2 can cause burns to unprotected skin.
In order to determine the energy that an arc could possibly produce in various situations, the following factors must be determined.
Using the above information the cal/cm2 value can be calculated using one of several free software programs. By determining the energy capable of being generated by an electric arc, you can then determine what type of precautions need to be taken in order to work safely on the equipment.
Many injuries have occurred as a result of being in the vicinity of electrical equipment failure. NFPA (National Fire Protection Association) recognized that most of these injuries could have been prevented or minimized simply by wearing proper flash protective equipment and developed a standards document called the NFPA 70E. Among other things, it identifies and classifies the hazard risk of each task an electrician would be expected to perform and details personal protective equipment required to perform these tasks.
Many companies/jurisdictions have adopted the safety measures contained in this document.
Before working on or even opening an electrical enclosure you should know what the hazards are and how to protect against them. NFPA has introduced documentation that has been included in legislation in many jurisdictions to label all enclosures with the level of protection required and distances to maintain when working on or near equipment contained in an enclosure. An example is shown below.
Some types of labelling also show the following information:
Hard hats, safety glasses, gloves and work boots with electrical insulation rating give the worker protection during normal work, however in the event of circuit or switchgear failure resulting in a thermal arc being created, much greater protection is required.
Clothing can be made from many different materials. These materials have an Arc Thermal Performance Exposure Value (ATPV) associated with them which is defined as the amount of heat energy that the fabric will handle deflect or absorb and is measured in calories/cm2. Some of these materials offer better protection against the heat caused from an arc than others. Let's look at some of them:
In order to provide proper flash protection, shirts and overalls should have long sleeves to protect the full arm. Flash Suits are constructed of heavier material, usually either NOMEX or PBI flame resistant material. They usually start at 10 ounces per yard and go up. The heavier materials contain material like 'Kevlar' to protect against impact as well as heat. Typical ratings begin at 7 calories/cm2 and go up to about 100 calories/cm2. The suit can be in the form of smock and hood or complete overall style with hood.
In general, all clothing including undergarments should be 100% cotton. Flame resistant clothing should then be worn over this when working on or near energized electrical equipment. When combined in layers, the fabrics gain significant rating from the air space between them, and multiple layers have much higher ratings than the sum of the individual ratings. For example, a pant and shirt made of Indura 301 (ATPV=8.2) that is worn under coveralls made of Indura 451 (ATPV=11.1) achieved test results of ATPV=32.7. Even though the test results proved greater than the addition of the ATPV values of each layer, your outer layer should always be a garment rated for the predetermined arc flash value of the installation.
For normal work clear lenses are adequate, however for flash protection like that required for live work, troubleshooting, switching and applying or removing grounds, then the flash rated eye protection is required. In some cases full face protection is required.
Other specialized personal protective equipment may be required when performing work where a flash hazard is present. Some examples are: fire resistant hard hat liner, leather gloves, hearing protection, and leather work shoes.
NFPA has identified tasks typically performed on energized electrical equipment and has given each a Hazard/Risk Category. The categories range from 0 to 4 with the higher the number the higher the Hazard Risk.
The document then describes the Protective Clothing and Personal Protective Equipment required for each category.
For Example: a Category 0 task may require long sleeve shirt and pants of untreated natural fiber and safety glasses where a Category 1 task may require Fire Resistant long sleeve shirt and pants/coveralls with hard hat, safety glasses, leather gloves and leather shoes.
Be sure to refer to NFPA 70E for the actual requirements for any task performed on energized equipment.
Good luck troubleshooting and stay safe!
About the Author: Eugene Williams has over 35 years experience in the electrical industry both in the field and as an electrical instructor. He currently consults and provides training for various organizations throughout North America and is a contributor to Simutech Multimedia's Troubleshooting Skills Training software. Eugene can be reached at eugenew@onlink.net.
