Section 5/Electrogeometric Model Application
IEEE 998 Section 5 covers applying the electrogeometric model for substation shielding including striking distance calculations and shield wire geometry.
The electrogeometric model relates the striking distance to the stroke current through an empirical equation. For a given design stroke current (typically 5-10 kA for substations), the striking distance determines the radius of the arc that a downward leader can bridge to reach a grounded object. Shield wires and masts must be positioned so that their striking distance sphere overlaps the protected equipment, ensuring the leader terminates on the shield system rather than the equipment.
Why this section exists
The EGM provides the most physically accurate model of the lightning attachment process. Unlike the fixed-angle empirical method, the EGM accounts for the fact that smaller stroke currents have shorter striking distances and are harder to intercept. The design stroke current is selected based on the BIL of the protected equipment: only strokes large enough to cause a backflashover need to be intercepted, as smaller strokes are handled by the station's insulation.
What plan reviewers look for
Plan reviewers check the EGM analysis for the striking distance equation used, the design stroke current, and the geometric verification that shield wires and masts provide complete coverage. They verify the shield wire heights, sag at maximum temperature, and mast locations shown on the substation plan and elevation drawings match the EGM study inputs.
Common violations
Related IEEE 998 requirements
Section 4 introduces the three shielding methods. Section 6 covers shielding of GIS (gas-insulated switchgear) substations. IEEE 80 covers the grounding system that works with the shielding system. IEEE 1243 covers shielding of transmission lines.