CFO Calculation and Arrester Application Guidelines

The DSTAR CFO Calculation and Arrester Selection Tool (CFOCAST) is a convenient tool for estimating the critical flashover voltage (CFO) for an overhead pole system and providing arrester application guidelines. Similar to the DSTAR Guy Tension Analyzer (GTA) software created in Program 7, CFOCAST allows the user to quickly input the layout of an overhead system using a convenient graphical interface. The software uses the CFO-Added methodology, developed in the late 1980’s and outlined in IEEE 1410-2004, to estimate the overall overhead system CFO.

In addition to calculating CFO, the software provides arrester application guidelines for shielded and unshielded conditions. The software estimates the number of flashovers per 100 miles per year for both direct and indirect lightning strokes. This estimate is based upon the chosen CFO level, the arrester discharge voltage characteristics, and the assumed lightning parameters. The flashover rate versus arrester span intervals is shown in a graphical format for easy decision making.

General CFO Calculator Structure

The CFO calculator is an Excel-based software tool that utilizes an installation process to enable advanced features such as un-installation and association of the *.cpd file extension with the CFO calculator software. The software is structured such that data may be entered via a wizard-style user interface that guides the user through the data forms that need to be filled in to calculate the overall CFO. After the overhead system data is entered, the user is brought to a screen displaying the overall pole system layout and detailed CFO results.


Like other DSTAR software tools, the CFO Calculator has a set of custom parameter data that is to be managed by the designated program administrator. Clicking on the File menu item and selecting “Administrative Options” accesses the custom data.

As shown in the figure below, the administrator has the ability to add/modify the insulation value of components, modify lightning parameters, specify arrester properties, and change the administrator password. This enables the program to be modified to meet utility's specific standards and geographic situation.


Building a Pole System

CFOCAST enables a user to quickly layout and configure a pole system for analysis. Using the Pole Design Wizard shown below, the user enters the physical dimensions and insulation characteristics of the pole and crossarm. The flexible design wizard and administrative options enable a user to create configurations that will closely match nearly any physical layout.


After entering the pole and crossarm data, the CFO calculator is ready for the phase and neutral conductor layout selection. As the user enters the location of the conductor, as shown in Figure below, the conductor will appear on the screen as a colored star. This designation will be used in the output report to aid in understanding the critical flashover problem areas. The wizard will guide the user through the process of entering similar data for all the phase conductors and neutral conductors.

After completion of the conductor information, the wizard requests information regarding other potential pole attachments that may impact the system CFO. Figure below shows the related wizard window.


The window in Figure 13 is used to specify all of the materials for each of the components that make up the insulation system. In this way, the CFO calculator may be used to understand the potential trade-offs between different insulation materials.


After specifying pole and crossarm material, the user will be required to enter the installation elevation and the wood reduction factors.The reduction factors may be applied for situations where there is a known degradation of the insulation quality of the wood due to age or some other factor. Figure 16 shows the results from a completed calculation. The CFO calculator determines the CFO value for every possible combination of path to ground.


After reviewing the calculation results or even before performing any CFO calculations, the user may click on the Arrester Guideline Wizard to determine the probability of flashover for shielded and unshielded configurations. Figure 17 and 18 show the data input and output windows. As shown in Figure 17, the CFO value used for the arrester guidelines can be modified as desired and does not need to match a value calculated via the pole design.


The CFO Calculator was created, modified, enhanced the following project(s): P10-10

An Office 2007 compatible version is available in: 11-0