APPLICATION OF THE OPTIMAL CLEAR WIDTH CALCULATOR (OCWC) AND LINE STRIKE PROBABILITY CHARTS (LSPC)
Scenario 1
You’ve identified a section of distribution line running through a forested area as problematic. Every time there’s a stiff wind trees are failing and taking the line out. The hazard tree removal program has had limited success. You wonder if widening the right of way is the solution but you hesitate to make a major investment since you have no means forecasting the impact on reliability.
This is where the Optimal Clear Width Calculator comes in. You can use it to quantitatively assess the risk exposure the line currently has and changes in the level of risk, as clear width or pole height changes.
There is field information you need to assemble to make the assessment. Assume you find the following:
- Line height – 30 feet
- Tree height – 90 feet
- Trees/Acre – 120
- Current clear width – 10 feet
What would be the benefit of increasing the clear width to 20 feet?
Using the data in the Optimal Clear Width Calculator produces this Line Strike Probability Chart.
From the chart we see that at a 10 foot clear width the Risk Factor is about .68 while at a 20 foot clear width the Risk Factor is about .42. That information can then be put into a simple spreadsheet that shows increasing the clear width another 10 feet would result in a 37% improvement in line security. (Downloadable active spreadsheet file)
Scenario 2
Your company is building another transmission line. Due to siting difficulties, the most expedient approvals are likely if the line is added to an existing right of way. Applying to increase the easement may also result in delays so the company is favouring making do with the existing right of way. Before finalizing this decision they want your assessment of the impact this action will have on line security.
Much of the line runs through forest. The clear width will be reduced to 30 feet.
It’s time to go to the Optimal Clear Width Calculator to develop a Line Strike Probability Chart.
There is field information you need to assemble to make the assessment. Assume you find the following:
- Line height – 60 feet
- Tree height – 90 feet
- Trees/Acre – 120
- Current clear width – 65 feet
Using the data in the Optimal Clear Width Calculator produces this Line Strike Probability Chart.
From the chart we see that at a 65 foot clear width the Risk Factor is about .03 while at a 30 foot clear width the Risk Factor is about .26. (For improved accuracy you would run a second Line Strike Probability Chart noting that the clear width at 30 feet has fallen below a wide right of way and re-setting that entry field.) That information can then be put into a simple spreadsheet that shows the impact of decreasing the clear width by 35 feet would result in a 767% drop in line security. In other words you should expect 7 to 8 times the current number of tree-related outages. (Downloadable active spreadsheet file)