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Overhead Transmission Line Clearance Considerations

Robert E. Carberry Northeast Utilities

October 1, 2002

Safety Considerations

National Electrical Safety Code Requirements , Part 2, Safety Rules for Overhead Lines


232...Vertical Clearances...Above Ground, Roadway, Rail or Water Surfaces 233...Clearances Between Wires...Carried on Different Supporting Structures 234...Clearances of Wires...From Buildings, Bridges, Rail Cars, Swimming Pools, and Other Installations

Vertical Clearance

National Electrical Safety Code (NESC) Criteria

Blow Out Clearance


Wind Displacement Of Structure, Insulators And Conductors

NESC Criteria

Blow Out Clearance

Edge of ROW


Edge of ROW

Building/Structure (Existing or Future)

Critical Clearance

11 ft ­ 115kV 16 ft ­ 345kV

NESC Criteria

Related Safety Clearance Considerations

Vegetation Contact Line Reliability, Insulation Live-Line Worker Safety Construction Near Existing Lines

Blow Out Clearance

60 degree F Conductor 285 degree F Conductor

Edge of ROW

Vegetation Contact

10 ft

8.5 ft

Line Reliability, Insulation & Live-Line Worker Safety ­ 345 kV

Live-Line Work

10 ft 8.5 ft

Line Reliability, Insulation & Live ­ Line Worker Safety ­ 345kV

6 ft

Line Reliability, Insulation & Live-Line Worker Safety - 115 kV

6 ft 6 ft

Line Reliability, Insulation & Live-Line Worker Safety ­ 115kV

Delta Configuration

Electrical Environmental (EE) Effect Considerations

Audible Noise in Wet Weather AM Radio Noise Electric Fields Magnetic Fields

Edge of ROW

Edge of ROW

Example ProfileAM Radio Noise, Wet Weather Audible Noise Electric Fields and Magnetic Fields


Conductor Spacing Reductions

­ Reduced line reliability and safety ­ Loss of live-line maintenance capability ­ Increases in noise levels

Span Reductions to Reduce ROW

­ More line structures and cost ­ Reduced ability to span over wetlands ­ Higher EE levels at ROW edges


Space Factors

18 pages

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