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70E defines a series of boundaries relating to electrical safety when working on energized equipment. Only “qualified” people can enter these boundaries and they are required to wear appropriate PPE within these boundaries.

The four protection boundaries are:

1. Flash Protection Boundary
2. Limited Approach Boundary
3. Restricted Approach Boundary
4. Prohibited Approach Boundary

Classification of Hazard/Risk Category:

NFPA 70E defines 5 levels of risk category for arc flash hazard based upon the calculated incident energy at the working distance, as shown in Table below.  Examples of typical protective clothing that cover the torso are also provided in this table.  Other PPE are also required to protect various parts of the body.

NFPA specifies the requirement of personal protective equipment (PPE) for workers within the flash protection boundary.  All parts of the body which may be exposed to the arc flash, need to be covered by the appropriate type and quality of PPE. The entire PPE set may be comprised of FR clothing, helmet or headgear, face shield, safety glasses, gloves, shoes, etc. depending upon the magnitude of the arc energy. The amount of PPE required and its quality needs to be determined on the basis of the calculated incident energy on the worker’s body.  The calculations need to be performed by a qualified person such as an engineer.  The protective clothing should limit the incident energy reaching the chest/face of the worker to less than 1.2 cal/cm^2.  FR clothing provides thermal insulation and is also self-extinguishing.  Protective clothing is rated in cal/cm^2.

Glow to arc discharge:

Dust and impurities: Dust and impurities on insulating surfaces can provide a path for current, allowing it to flashover and create arc discharge across the surface. This can develop into greater arcs. Fumes or vapor of chemicals can reduce the breakdown voltage of air and cause arc flash.

Corrosion: Corrosion of equipment parts can provide impurities on insulating surfaces. Corrosion also weakens the contact between conductor terminals, increasing the contact resistance through oxidation or other corrosive contamination.  Heat is generated on the contacts and sparks may be produced, this can lead to arcing faults with nearby exposed conductors of different phase or ground.

Condensation of vapor and water dripping can cause tracking on the surface of insulating materials. This can create a flashover to ground and potential escalation to phase to phase arcing.

Spark discharge:

• Accidental touching: Accidental contact with live exposed parts can initiate arc faults.
• Dropping tools: Accidental dropping of tools may cause momentary short circuit, produce sparks and initiate arcs.

Over-voltages across narrow gaps: When air gap between conductors of different phases is very narrow (due to poor workmanship or damage of insulating materials), arcs may strike across during over-voltages.

Failure of insulating materials.

Improperly designed or utilized equipment.

Improper work procedures.

The electrical arc is recognized as high – level heat source. The temperatures at the metal terminals are high, reliably reported to be 20,000   K (35,000 ° F). The special types of arcs can reach 50,000   K (about 90,000 ° F). The only higher temperature source known on earth is the laser, which can produce 100,000   K. The intermediate (plasma) part of the arc, that is, the portion away from the terminals, is reported as having a temperature of 13,000   K.

In a bolted three – phase fault, there is no arc, so little heat will be generated. If there is some resistance at the fault point, temperature could rise to the melting and boiling point of the metal, and an arc could be started. The longer the arc becomes, the more of the system voltage it consumes. Consequently, less voltage is available to overcome supply impedance and the total current decreases.

Human body can exist only in a narrow temperature range that is close to normal blood temperature, around 97.7 ° F. Studies show that at skin temperature as low as 44 ° C (110 ° F), the body temperature equilibrium starts breaking down in about 6 hours. Cell damage can occur beyond 6 hours. At 158 ° F, only a 1 – second duration is required to cause total cell destruction.

Apart from thermal burns, an arcing phenomenon is associated with other hazards too, namely:

• Electrical shock
• Molten metal
• Projectiles
• Blast and pressure waves
• Intense light
• Intense sound
• fire
• Effect of strong magnetic fields and plasma, of which not much is known

•     Toxic gases and vapors.

Electrical arcing signifies the passage of current through what has previously been air. It is initiated by flashover or introduction of some conductive material. The current passage is through ionized air and the vapor of the arc terminal material, which has substantially higher resistance than the solid material. This creates a voltage drop in the arc depending upon the arc length and system voltage. The current path is resistive in nature, yielding unity power factor. Voltage drop in a large solid or stranded conductor is of the order of 0.016 – 0.033   V/cm, very much lower than the voltage drop in an arc, which can be of the order of the order of 5 – 10   V/cm of arc length for virtually all arcs in open air (Chapter  3 ). For low voltage circuits, the arc length consumes a substantial portion of the available voltage. For high voltages, the arc lengths can be considerably greater, before the system impedance tries to regulate or limit the fault current. The arc voltage drop and the source voltage drop are in quadrature. The length of arc in high voltage systems can be greater and readily bridge the gap from energized parts to ground.

Under some circumstances, it is possible to generate a higher energy arc from a low voltage system, as compared with a high voltage system.  In a bolted three – phase short circuit, the arcing resistance is zero, and there is no arcing, and no arc fl ash hazard. Sometimes, when short circuit occurs, it can be converted into a three – phase bolted short circuit by closing a making switch or circuit breaker, which solidly connects the three – phases. The fault current is then interrupted by appropriate relaying. This method, however, will subject the system to much greater short – circuit stresses and equipment damage, and, is, therefore, not recommended.