The field problem
Twin High-Rise Towers — NAC
One tower was assigned a high protection level while the adjacent shorter tower was assigned a much lower level mainly because of assumed shielding by the taller structure.
why the obvious answer can be misleading
Formal lightning risk cannot be replaced by a visual shielding assumption. Occupancy, function, roof equipment, services and consequence of loss may make the shorter building more critical.
Why it failed
Formal lightning risk cannot be replaced by a visual shielding assumption.
Occupancy, function, roof equipment, services and consequence of loss may make the shorter building more critical.
Follow the engineering
Switch between the field condition and the engineering concept. Trace the path and examine what changes.
Inspect the engineering detail
What changes
Recalculate each structure using the applicable IEC 62305-2 risk methodology and then select protection measures from the resulting risk, not from relative height alone.
Correction & verification
Recalculate each structure using the applicable IEC 62305-2 risk methodology and then select protection measures from the resulting risk, not from relative height alone.
Verification recorded in the case
- Record assumptions and sensitivity of the risk calculation.
- Record as-built topology and connection points.
- Repeat the measurement or functional test under a defined condition.
- Confirm continuity and mechanical integrity of the corrected path.
- Document instrument/setup, environmental condition and test method.
- Close the NCR/case only when evidence supports the conclusion.
References & project context
References are reproduced from the PIS casebook. Select the governing edition and project acceptance criteria before using them for a design. The diagrams explain the concept, rather than define construction dimensions.
IEC 62305-2.
PIS reviewed field handbook · PIS-CS-026
View the original social cardThe engineering lesson
Adjacent height is an input to geometry — not a substitute for risk management.
Take the lesson into your next design review.