22.214.171.124 Control Panels, Motor Control Centers, and Switchgear
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This category includes tall, narrow floor-mounted electrical items in sheet metal cabinets such as electrical control panels, motor control centers, switchgear, and substations.
- Typical Causes of Damage
- Seismic Mitigation Considerations
Building Code Provisions
Electrical control panels, motor control centers, switchgear, and substations are designed using the provisions of Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) Chapter 13. The principal objective is to prevent the components from sliding or toppling. These components may also be sensitive to building displacements if they are both anchored to the floor and braced to the floor above.
- ASCE/SEI 7-10 requires anchorage design for all equipment in Seismic Design Categories D, E, and F if the equipment weighs over 400 pounds. Lighter components may be exempt if the component Importance Factor Ip = 1.0.
- Items that are exempt from the anchorage design requirements must still be positively anchored to the structure. The anchorage need not be designed or detailed on the construction documents. Exempt items must also be provided with flexible connections between the equipment and associated raceways, bus ducts, or conduits if there is a potential for damaging differential movement between the equipment and connected components.
- Electrical component supports and their attachment to the component must be designed for the appropriate forces and displacements. Supports include braces, frames, skirts, legs, pedestals, and snubbers, as well as elements forged or cast as a part of the mechanical or electrical component. There are prescriptive requirements for reinforcing the cabinets around anchor points.
- The seismic design must consider the loads imposed on the components by attached utility or service lines that are attached to separate structures.
- Control panels, motor control centers, switchgear, and substations with slide out components must have a latching mechanism to hold the components in place.
- The design of the component cabinet must comply with the applicable National Electrical Manufacturers Association (NEMA) standards.
- If cutouts are made in the lower panels of the cabinet that were not made by the manufacturer, the effects of the cutouts on the strength of the cabinet should be specifically evaluated.
- Attachments for additional external items not provided by the manufacturer that weigh more than 100 lb should be evaluated. The principal objective of the code provisions is to prevent the component from sliding or overturning.
Retrofit Standard Provisions
Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06) classifies electrical equipment, including panel boards, motor control centers, and switch gear as force controlled. These components are subject to the provisions of the standard when the performance level is Immediate Occupancy. The requirements also apply when the performance level is Life Safety in high and moderate seismicity areas, and the equipment is over 6 feet in height and weighs more than 20 pounds. When applicable, electrical equipment meeting any of the following criteria must comply with the requirements of ASCE/SEI 41-06:
- The item weighs more than 400 pounds.
- The item is unanchored, weighs over 100 pounds and is subject to overturning. These items may be exempt if they have a factor of safety greater than 1.5 when design loads are applied.
- The item weighs over 20 pounds and is mounted over 4 feet above the floor.
- Building operation equipment.
Acceptance criteria for electrical equipment focus on providing adequate anchorage for seismic forces. Prescriptive anchorage and bracing provisions may be used for smaller equipment.
Typical Causes of Damage
- Overturning or sliding due to lack of anchorage or inadequate anchorage.
- Loss of function due to failure of internal components caused by inertial forces.
- Damaged electrical equipment may cause electrical hazards and fire hazards.
Figure 188.8.131.52-2 Unanchored electrical cabinets overturned in a paper products plant during the 1999 magnitude-7.4 Izmit, Turkey earthquake (Photo courtesy of NISEE Izmit Collection, No. IZT-682, photograph by Halil Sezen).
- Working around electrical equipment can be extremely hazardous. Read the Electrical Danger Warning and Guidelines in Section 6.6.8 of this document before proceeding with any work.
- Many of these components can be supplied with shop welded brackets or predrilled holes for base or wall anchorage. For any new equipment, request items that can be supplied with seismic anchorage provisions by the manufacturer.
- When tall units are anchored to the floor, flexible connections between the equipment and raceways, bus ducts, or conduits that are braced to the level above will limit damage due to story drift.
- See Section 184.108.40.206 for additional base anchorage details. Refer to Installing Seismic Restraints for Electrical Equipment (FEMA 413) for general information on seismic anchorage of electrical equipment.