TP

1
New Clearance Rules NESC - Section 23
Jerry Reding Bonneville Power Administration
January 9, 2007 Orlando Towers, Poles
Conductors Meeting
2
Summary Clearances

• Section 23 Clearances
• Background and basis for changes
• What has changed in 2007 Edition
• Rational for changes
• What has not changed in 2007 Edition
• Potential future changes beyond 2007

3
Clearances Section 23

• Electrical clearances for safety
• Other Activities (Ground, roads, rails, etc 231
  232)
• Other wires on structures (233 235)
• Other Installations (Buildings, Bridges, etc 234)
• Qualified electrical worker clearances (236
  237)
• Clearance from other facilities
  (Communication, brackets, etc 238 239)

4
Wire Sags Section 23

• For wire location through out Section 23
• Vertical clearance largest final sag.
• 1) 50 C, no wind displacement
• 2) Maximum wire temperature, no wind
• 3) 0 C, no wind, radial ice thickness as
  specified in Rule 250B

5
Final Sag Rule 250B

• Wire final sag includes inelastic stretch from
  (which ever is greater)
• Combined ice and wind loading plus constant (k)
• Long-term creep under everyday tensions
• Final sag dependent on ice and wind loading for
  all three cases not just ice case (3)
• Changes in Rule 250B directly impacts final sags
  hence clearance calculations

6
Final Sag Rule 250B
ice (Table 250-1)
wind force (Table 250-1)
weight
wire
constant k (Table 251-1)
total load
_at_ specified temperature (Table 251-1)
7
Final Sag Rule 250B

• Subcommittee 5 has been considering (eventually)
  changing Rule 250B
• Previously three loading districts light,
  medium, and heavy loading
• New Rule 250D ice map is ultimate ice which
  generally increases ice thickness
• Final sags potentially increased as clearances
  are referenced back to Rule 250 ice map(s)

8
Final Sag Rule 250B
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Final Sag Rule 250B

• Combined ice wind
• 50-Year freezing rain with 3-second gust
• More maps to follow
• Loading is complex
• Ice is generally heavier

10
New Rule 250B Issues

• Ice map moves from working ice to ultimate ice
  (ASCE)
• Impacts clearances
• Increases final sag
• Increase iced sags
• Experience does not justify increase in clearance
  calculations
• Referencing Rule 250 from Section 23 complicates
  this Edition and future changes

11
Changes In Section 23

• Added Clearance Zone map and wire loading tables
  to Section 23
• Clearance Zone emulates Rule 250B Loading
  District map (Figure 230-1)
• Wire loading tables (Table 230-1 230-2)
  emulates Table 250-1 250-2
• Separates Section 23 from Section 25

12
Changes In Section 23
Figure 230-1 - Clearance zone map of the United
States
13
Rational for Changes

• Separates Section 23 and Section 25
• Section 23 specifies loadings for clearances
• Section 25 specifies loadings for wire and
  structure strength
• Ensures Subcommittee 4 controls clearances
• Ensures Subcommittee 5 controls strengths
• Facilitates future potential changes without
  problematic conflicts nor unintended impacts

14
Not Changed Section 23

• Retention of loading districts and component
  loading in Section 23
• Essentially NO CHANGE for clearances
• New Appendix B provides background and rational
  for new Clearance Zones

15
Potential Future Changes

• Additive k constant serves no purpose in
  Section 23
• Used for calculating tensions by effectively
  increasing wind from 4 lbs/sq ft to 8 lbs/sq ft
• Separates wire tension from vertical and
  horizontal loads applied to structure attachments
• New Clearance Zones could specify wind directly
  for clearances and eliminate k
• Simplify clearance rules and reduce confusion in
  Code for calculating clearances

16
Thank You

• Q/A