Overhead Distribution Construction and Maintenance
This domain covers the physical plant above ground: poles, guys, crossarms, insulators, conductors and the hardware that holds them, plus the maintenance decisions that keep clearances legal. Questions mix CEC Section 36 clearance tables, pole setting depth, guying geometry, and stringing and sagging practice.
Poles and setting depth
- Common rule of thumb: setting depth = 10% of pole length + 0.6 m (2 ft), so a 13.7 m (45 ft) pole is set about 1.97 m (6.5 ft) deep. Utility specifications override the rule.
- Confirm pole class, length and species against the loading calculation: conductors, wind and ice, equipment, guy loads and unbalanced tension.
- Decay inspection means sounding and boring at and below grade and excavating the butt; a pole that sounds dull or shows shell rot is rejected, not reinforced.
Guys and hardware
- A down guy with a lead roughly equal to its attachment height (about 45°) is efficient. Short leads multiply tension sharply and require a larger anchor and guy.
- Anchor types include screw, plate, log and rock anchors; guy strain insulators isolate the anchor from the circuit.
- Pin insulators suit small angles and light spans; suspension strings carry heavier loads; dead-end hardware and preforms terminate full tension.
- Sag is set from stringing tables for the specific conductor, span and conductor temperature. Under-sagging leaves clearance violations; over-sagging overloads poles and reduces ground clearance.
Clearances
CEC Section 36 sets ground clearance over roads, fields and railways, and separation from other utilities and communication plant. Measure at the governing condition (maximum sag, highest temperature, ice load), not at installation sag.
Worked example
A 45 ft pole is to carry a 25 kV circuit over a road. Setting depth is 10% of 45 ft plus 2 ft, about 6.5 ft, leaving 38.5 ft above grade. If the design needs the conductor at 34 ft with the crossarm below the pole top, the 45 ft pole works. If the design calls for 40 ft above grade, the 45 ft pole cannot deliver it - a longer pole is required. The reasoning: setting depth is subtracted before clearance is checked.
Traps that cost marks
Checking clearance against pole length instead of above-grade height; adding a guy without checking anchor capacity or right-of-way; and using a sag table for the wrong conductor size or ice loading.