28-11-2012, 05:26 PM
Overhead and Underground Power Lines
1Overhead and Underground.ppt (Size: 994.5 KB / Downloads: 291)
Background
Power lines transmit electricity
Electrical current exposes workers to serious hazards
Most common hazard is electrocution
Most workers are unaware of potential electrical hazards
Power lines can be
Overhead
Underground
Some activities may cause interference with power lines
Overhead power lines
Operation of a crane, forklift, and other construction equipment
Changing street lights
Tree trimming
Underground power lines
Excavation
Types of Electrical Lines
Electrical Cables
High Voltage, Low Voltage
Supervisory and Signaling Cables
Cathodic protection (cabinets, cables, earthing conductors, cable shrouds)
Communication Cables
Phone lines
Coaxial cables (e.g. data cables)
Broadband cables
Tram traction cables and trolley wires
Railway supply cables and traction wires
Common Misconceptions
Birds land on power lines, so they must be safe to touch - NO
Main objective of electricity is to reach the ground using the most direct route
Birds do not get electrocuted when they land on wires because they do not represent a path to the ground
Common Misconceptions
Power lines are insulated, so they are safe to touch - NO
They are not always insulated
Insulation deals with placing nonconductive material around the wire
Insulation is a material that offers high electric resistance making it suitable for covering wires to prevent the possible future contact of adjacent conductors resulting in a short circuit
Just because a wire is weather protected does not mean that it is insulated
Weather protection prevents water and snow from reaching the wire (and insulation, if used)
Hazard Identification
Conduct site investigation to identify existing conditions of:
Voltages of lines and equipment
Presence of hazardous, induced voltages
Presence and condition of protective grounds and equipment grounding conductors
Locations of electrical circuits and equipment
Maximum switching transient voltages
Underground Power Lines
Locate and mark the perimeter of underground lines
Exercise caution when excavating near power lines
Some manual excavation may be required
Stay away from pad mount transformers
Green metal box