HV (High-voltage) Lines: Transmission lines having a voltage level between 100kV to 230kV lie under the category of HV lines.
EHV (Extra-high voltage) Lines: EHV lines are those transmission lines whose voltage level is between 230kV to 1000kV.
UHV (Ultra-high voltage) Lines: Those transmission lines whose voltage level is above 1000kV are categorized as UHV lines.
Safe Distance: Safe distance is the distance from the transmission tower to the end of the corridor defined by the right of way. It can also be named as a border zone.
Ground Clearance: The distance from the lowest point of the conductor to the ground is known as ground clearance.
Conductors: substances, such as metals, that have little resistance to electricity
Insulators: substances, such as dry wood, rubber, glass and Bakelite, that have high resistance to electricity
Grounding: a conductive connection to the earth which acts as a protective measure
Limited Approach Boundary: The Limited Approach Boundary (LAB) is the approach distance to exposed, energized electrical components within which a shock hazard exists. It is the approach limit for unqualified persons. Unqualified persons may only cross this boundary if they are under the direct supervision of a qualified person, and wearing the appropriate personal protective equipment for the hazards involved. Working within the LAB is only acceptable if an Energized Electrical Work Assessment has been completed and approved, or if the work is specifically exempt per NFPA 70E-2018 130.2(B)(3).
The Restricted Approach Boundary: (RAB) is the approach limit for qualified persons to exposed, energized electrical components where there is an increased likelihood of electric shock due to electrical arc-over combined with inadvertent movement. Only qualified persons may cross this boundary. A qualified person required to cross the RAB must be protected from unexpected contact with exposed energized conductors or circuit parts. Working within the RAB is only acceptable if an Energized Electrical Work Assessment has been completed and approved, or if the work is specifically exempt per NFPA 70E-2018 130.2(B)(3).
The Prohibited Approach Boundary: (PAB) was used in earlier versions of NFPA 70E, but is no longer referenced in the current version. The term, however, may still be used in electrical research applications. The PAB is the approach distance for qualified persons to exposed live parts within which work is considered the same as making contact with the live part.
The Arc Flash Boundary: (AFB) is the distance from exposed, energized electrical components within which a person could receive a second degree burn if an electrical arc flash were to occur. The AFB is determined either through calculation or through the use of the following tables. Personnel must be wearing the appropriate arc-rated protective equipment before crossing this boundary. Arc-rating is reported as either the Arc Thermal Performance Value (ATPV) or the Energy of Break-open Threshold (EBT). In order to determine if there is a likelihood of an arc flash occurrence, and whether protective equipment is required/recommended for a given task, identify the task in NFPA 70E-2018 Table 130.5(C). If there is a likelihood of an arc flash event occurring, NFPA 70E-2018 Table 130.7(C)(15)(a) for alternating current or table (b) for direct current can be used to determine the arc flash personal protective equipment category and the arc flash boundary based on the equipment involved. The category of personal protective equipment required is defined and detailed in NFPA 70E-2018 Table 130.7(C)(15)(c).
History of Electricity
600BC: Thales, a Greek, found that when amber was rubbed with silk it attracted feathers and other light objects. He had discovered static electricity. The Greek word for amber is elektron’, from which we get electricity’ and electronics’.
1600: William Gilbert, scientist and physician to Queen Elizabeth I, coined the term electricity. He was the first person to describe the earth’s magnetic field and to realize that there is a relationship between magnetism and electricity.
1752: Benjamin Franklin, famous U.S. politician, flew a kite with a metal tip into a thunderstorm to prove that lightning is a form of electricity.
1820: Hans Christian Oersted of Denmark found that when electricity flows through a wire, it produces a magnetic field that affects the needle of a nearby compass.
1821: Michael Faraday discovered that when a magnet is moved inside a coil of copper wire, a tiny electric current flows through the wire. This discovery later led to the invention of electric motors.
1870s: Thomas Edison built a DC (direct current) electric generator in America. He later provided all of New York’s electricity.