Equipment Interaction: Cabinet-to-Cabinet Dynamic Coupling

Case classification

Illustrative case study. This scenario is constructed from common interaction patterns observed in equipment arrays in control rooms and switchgear rooms.

Scenario description

Two electrical cabinets were placed adjacent to each other with a 10 mm gap. Each cabinet was individually qualified by seismic analysis. Cabinet A had a natural frequency of 8 Hz and cabinet B had a natural frequency of 9 Hz. During a seismic event, the cabinets displaced toward each other, closing the gap and impacting. The impact damaged the side panels of both cabinets and caused internal components to experience shock accelerations well above the seismic qualification level.

Analysis of the failure

Each cabinet was qualified independently, with the seismic displacement of each cabinet calculated from its own response spectrum analysis. Cabinet A had a peak displacement of 12 mm and cabinet B had a peak displacement of 10 mm. The gap between them was 10 mm. The analysis assumed that the cabinets would not interact. However, because the cabinets had different natural frequencies, their peak displacements did not occur simultaneously, but the combined displacement exceeded the gap at some point during the seismic event. The impact produced shock accelerations in the side panels that propagated to the internal components.

Root causes

• The cabinets were qualified independently without considering the adjacent cabinet
• The gap between the cabinets was smaller than the sum of the peak displacements
• The cabinets had different natural frequencies, so their peak displacements occurred at different times, but the combined displacement still exceeded the gap
• The impact (pounding) was not considered in the analysis — pounding produces high-frequency shock that can damage internal components
• The cabinets were not tied together or separated by an adequate gap

Lessons learned

• Adjacent equipment must be assessed for pounding — the gap must exceed the sum of the peak displacements from both items
• If the gap is insufficient, the equipment should be tied together (so they move in phase) or the gap increased
• Pounding produces high-frequency shock accelerations that can exceed the seismic qualification level for internal components
• Independent qualification of each cabinet does not guarantee adequate performance when cabinets are placed adjacent to each other
• The relative displacement between adjacent items, not the absolute displacement of each, governs the pounding check

Remediation

The gap between the cabinets was increased to 30 mm, exceeding the sum of the peak displacements (22 mm) with margin. Alternatively, the cabinets were bolted together at the top to force them to move in phase, eliminating the relative displacement. The internal components were re-assessed for the shock accelerations from the impact, confirming that the re-design eliminated the impact and the shock.