The Hazards of Electricity
Electricity is a silent killer because it cannot be seen or heard. Our proximity to electrical appliances throughout our home and work environment make us take them for granted and we forget that if these are not handled properly they can kill. People fail to realize that even our domestic supply of 120 Volts is a potential hazard that can be deadly if certain conditions are met.
The hazards of electricity are of three kinds: electrocution, electric arc flash, and electric arc blast. Apart from these an electric shock can also kill or injure people working at heights through falls. Electric current passing through the heart can cause fibrillation of the heart resulting in a cardiac arrest. The electric arc generated by switch gears while loading and unloading equipment can cause deep tissue burns, and the pressure from an arc blast can make a household wall fall due to the impact. In view of all these hazards the NFPA 70E has stated regulations to reduce the risk of injury to workers due to shock and arc flash hazards
Electric Shock Hazard Boundaries
There are three electrical shock hazard boundaries depending on the voltage of the appliance and termed the limited approach boundary, the restricted approach boundary, and the prohibited approach boundary.
As per NFPA 70, the Limited Approach Boundary is a shock protection boundary that can only be crossed by qualified persons and unqualified persons under escort by qualified persons. No untrained person can approach closer than this boundary and a trained person must use PPE or personal protective equipment to cross this boundary. It is the minimum distance from the energized part any unqualified person can come. This boundary depends on the voltage of the equipment. It is the limit within which a shock hazard exists.
A Restrictive Approach Boundary is shock protection boundary that can be crossed only by qualified persons with knowledge of shock protection techniques and wearing PPE. Any unqualified person cannot cross this boundary, even with an escort or PPE. Within this limit there is an increased risk of shock due to electric arching and movement of the personnel while working.
A Prohibitive Approach Boundary can only be crossed by qualified persons and is considered the same as having contact with a live part. Suitable PPE must be worn. Working within this limit is considered as the same as working on an energized live part.
How Shock Hazard Boundaries are Calculated
NFPA 70E calculates electrical shock hazard boundaries depending upon the voltage and then specifies the distance for the various approaches. For example, referring to NFPA 70 E Table 130.2 ( c) for a voltage between 50 to 300 Volts. the limited approach boundary with an exposed movable conductor is 10 feet. Another example is for a voltage between 751 V to 15 KV the distance for the limited approach boundary for a circuit with fixed exposed circuit part is 5 feet 0 inch, whereas the restricted approach boundary is 2 feet 2 inches, and the prohibited approach boundary is 0 feet 7 inches. Please refer to the table for more voltages.
NFPA 70E Table 130.2 (C)
Who is a Qualified Person?
In various shock hazard analyses, the qualified person has been authorized to enter the various shock hazard boundaries. A qualified person is a person who has a knowledge of electrical hazards associated with working with electrical equipment. A qualified person is also aware of the limitations of PPE and can relate the hazards to the type of work performed. He is trained to recognize the hazard and ways of avoiding the hazard. For a person to be deemed qualified he must have training in the construction and operation of the equipment. A qualified person must be trained in the use of PPE in a dry run to make him familiar. An apprentice under direct supervision of a qualified person is also considered to be a qualified person.
Shock Hazard Analysis
Shock Hazard Analysis is required by NFPA 70E-2004 and is a study in which the maximum voltage the personnel may be exposed to is calculated and by which, based on the voltage, the electric shock hazard boundaries are specified. It also specifies the type of PPE to be worn by the personnel to reduce their likelihood of receiving an electric shock.
Shock Appraisal Chart
A shock appraisal chart summarizes the overall effect of voltage, current, and body resistance and rates the incidence of fatality based on them. The x coordinate gives the exposed voltage and the y coordinate gives the body resistance of the subject. The curve plotted is a series of straight lines which give the current passing through the body. The range of current is summarized from very slight possibility of fatality to possibly and probably fatal.
Shock Appraisal Chart
Arc Flash Hazard Analysis
The other hazards of electricity are arc flash and arc blast. Arc flash results in the production of heat and fire that can cause burns. Arc blast results in the production of pressure, shrapnel, and sound which can cause associated injuries. The temperature in an arc blast can reach up to 35000 degrees F and burn up to a distance of 10 feet. The copper in the circuit can vaporize and expand up to 67000 times its original volume, and the resulting blast can damage structures and knock out personnel.
Thus in view of all the above, NFPA 70 E 130.3 states that flash hazard analysis shall be done in order to protect personnel from the possibility of being injured by an arc flash. In the arc flash hazard analysis, the determination of the flash protection boundary and the PPE required is done. All parts of the body inside the flash protection boundary have to be protected by suitable PPE. For example for voltage between 50 to 600 Volts, the flash protection boundary is 4 feet.
The Hazards of Electricity: Do you know what they are? by Dennis K. Nietzel (PDF)
AVO Training Institute, Arc-Flash and Electrical Shock Hazards - How to Keep Your Facility Compliant by Dennis K. Neitzel (Microsoft Word Document)
Brainfiller, Approach Boundaries, Jim Phillips, PE (PDF)
NFPA 70E Table 130.2 (C) by Mohit Sanguri, Chief Engineer
Other Related Article
Ac and DC shock comparision by Prof. Sunita Sanguri at https://www.brighthubengineering.com/power-plants/89792-ac-and-dc-shock-comparison/