Troubleshooting electrical equipment can involve many 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 are normally locked to protect workers.
This introduces two distinct hazards:
Shock hazard. Contacting live equipment with your body or a even a tool you are holding allows the current to flow through your body, and can cause severe injury, burns and even death.
Flash hazard. If you are in the vicinity of equipment that fails and causes an electric arc, the ﬂash, heat and shrapnel caused by the arc can also be life threatening.
In this article, we will be discussing flash hazards.
Burns and Shrapnel
If you are familiar with electric arc welding, then you are well aware that the small arc created by the welding equipment can generate enough heat to melt metal and enough UV rays to burn your skin.
When electrical equipment fails (or a test probe is used incorrectly), electric current can flow through an air gap between conductors and cause an arc flash. The energy released during the arcing can be many, many times greater than the welding arc. It can cause severe ﬂash burns that 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 remains.
- Second degree: The outer skin layer is severely damaged and blistering usually occurs. Healing takes much longer, as this kind of burn affects the deeper sweat glands and hair follicle areas. Scarring is often the result.
- Third degree: The skin and growth areas are completely destroyed. If the burn covers a small area, 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, there is another hazard: shrapnel. During an electric arc flash, 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 withstand pressures of that magnitude. The sudden pressure can turn molten metal and other materials into shrapnel hurtling toward anyone in the vicinity.
Calculating the Arc Energy
Protective precautions can help keep troubleshooters safe from flash hazards, but it is necessary to know the amount of energy the arc may generate in a particular situation in order to determine what precautions are needed to work safely on the equipment.
For instance, the greater the amount of energy the arc produces (measured in cal/cm²), the worse the burn it can cause. Any heat over 1.2 cal/cm² can cause burns to unprotected skin.
You need the following information to determine the potential arc energy in a particular situation:
- Available fault current
- The voltage
- The clearing time for the protective device
- How close the body part is to the arc
Once you have this information, you can calculate the cal/cm² value using one of several free software programs.
Many injuries have occurred to workers in the vicinity of electrical equipment failure. NFPA (National Fire Protection Association) recognizes that most of these injuries could have been prevented or minimized simply by wearing proper ﬂash protective equipment. They developed a standards document called the NFPA 70E. Among other things, it identiﬁes and classiﬁes the hazard risk of each task an electrician may be expected to perform and details the personal protective equipment (PPE) required to perform it safely.
Many companies/jurisdictions have adopted the safety measures contained in this document.
Identifying Flash Hazards
Before working on or even opening an electrical enclosure, you should know what the hazards are and how to protect yourself against them. NFPA has introduced standards, which have been included in legislation in many jurisdictions, for labelling all enclosures with the level of protection required and the distances to maintain when working on or near equipment contained in an enclosure. (See “Hazard Risk Category Classification,” below.)
Some types of labelling also show the following information:
- Flash hazard boundary
- cal/cm² flash hazard at 18 inches
- PPE level (what type of personal protective equipment is required)
- Approach boundaries to live parts for shock protection
Protecting Against Flash Hazards
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 creating a thermal arc, much greater protection is required.
Flash Protection Clothing
Clothing can be made from many different materials. These materials have an Arc Thermal Performance Value (ATPV) associated with them, which is deﬁned as the amount of heat energy that the fabric will handle (i.e., deﬂect or absorb) and is measured in cal/cm². Some of these materials offer better protection against the heat caused by an arc than others.
Let’s look at some of them:
- Synthetic material such as nylon, rayon or polyester should never be worn when working on or near energized electrical equipment because it is ﬂammable and has a tendency to melt and stick to skin when exposed to high temperatures.
- Cotton/synthetic blends should not be worn near electrical equipment for the same reasons.
- Pure cotton provides a minimum barrier to arc temperatures, but can ignite quickly. It does burn and fall away rather than stick to the skin.
- Materials like cotton or cotton blends treated with a ﬂame retardant chemical provide a minimum level of ﬂame resistance. Some chemical treatment degrades with repetitive laundering. For proper protection, this material should be a minimum of 7 ounces per square yard.
Nomex, Indura or PBI clothing provide an excellent ﬂame protection and are recommended for all work around energized electrical equipment. The material should be a minimum of 4 ounces per square yard.
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 such as Kevlar to protect against impact as well as heat. Typical ratings begin at 7 cal/cm² and go up to about 100 cal/cm². The suit may take the form of smock and hood, or coveralls with hood.
In general, all regular clothing, including undergarments, should be 100% cotton. Flame resistant clothing should then be worn over these cotton garments when working on or near energized electrical equipment.
When combined in layers, the fabrics gain signiﬁcant rating from the air space between them, with much higher ratings than the sum of the individual layers’ 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.
However, even though the ATPV value of wearing multiple layers is greater than the sum of the individual layers, your outer layer should still always be rated for the predetermined arc ﬂash value of the installation.
For normal work, clear lenses are adequate. However, for live work, troubleshooting, switching and applying or removing grounds, 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.
Hazard Risk Category Classification
NFPA 70E identiﬁes tasks typically performed on energized electrical equipment and has assigned each a Hazard Risk Category (HRC). The categories range from 0 to 4; the higher the number, the higher the hazard risk.
The document then describes the protective clothing and PPE required for each category.
For example, a Category 0 task may require a long-sleeved shirt and pants of untreated natural ﬁber and safety glasses, whereas a Category 1 task may require fire resistant long-sleeved shirt and pants or 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.
What you need to do:
- Be informed. Be aware of the hazards. Make sure you know and understand all the rules and regulations that apply to the work you are doing. These can be governmental regulations or policies and procedures produced by your company.
- Follow all safety rules and procedures. These are designed to protect you. Follow them and don’t take short cuts.
- Wear all required PPE. If you are in the vicinity of equipment failure causing an electrical arc, your PPE may save your life.
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.