Section 4/Lightning Shielding Methods
IEEE 998 Section 4 covers the three methods for designing lightning shielding of substations: empirical, rolling sphere, and electrogeometric model.
Lightning shielding for substations must be designed using one of three accepted methods. The empirical method uses fixed shielding angles based on historical performance data. The rolling sphere method models lightning as a sphere of a specified radius that rolls over the substation; any equipment touched by the sphere is unprotected. The electrogeometric model (EGM) is the most rigorous, using the striking distance as a function of the stroke current to determine whether a downward leader will terminate on a shield wire, mast, or the protected equipment.
Why this section exists
A direct lightning strike to substation equipment can cause catastrophic insulation failure, equipment destruction, and extended outages. Shield wires and masts intercept lightning strokes before they reach critical equipment by providing a preferred termination point. The design methods ensure that the shielding system provides adequate coverage for the selected design stroke current, typically corresponding to a stroke that would cause a backflashover at the station's basic insulation level (BIL).
What plan reviewers look for
Plan reviewers check the lightning shielding study for the method used, the design stroke current, and the resulting shield wire or mast layout. They verify that the shielding coverage shown on the plan and profile drawings matches the study results. They check that all critical equipment (transformers, breakers, buses) falls within the protected zone.
Common violations
Related IEEE 998 requirements
Section 5 covers the application of the electrogeometric model in detail. IEEE 80 covers the grounding grid that dissipates lightning current once it reaches the shield system. NESC covers lightning protection for overhead transmission and distribution lines.