Seismic Testing

Seismic testing is considered destructive.  In fact, if a plant were ever subjected to a design-basis seismic event, after safe shutdown it would never be operated again.  NLI retains many seismic test samples as they can often be used for other tests.  For example, switchgear that has been tested may be used later to test a breaker installed within that switchgear.

There are four different ways to perform seismic testing - single axis, dependent biaxial, independent biaxial, and triaxial.  NLI has three dependent biaxial “sleds,” the largest is 12 feet square and capable of 50,000 pounds-force.  Biaxial test equipment requires multiple tests to be run and the specimen to be rotated 90 degrees between tests.

Seismic testing is performed at two basic levels – Operational Basis Earthquake (OBE) testing is performed five times followed by the Safe Shutdown Earthquake (SSE) test.  Seismic tests subject equipment to both high and low frequency forces.  High frequency motion can unseat or prevent reseating of relief valves and open breaker contacts.  Low frequency “rolling” motion can break welds and warp switchgear.  A person could not survive the G-forces of a typical seismic test.

IEEE Standard 344 is the mother standard for seismic qualification.  IEEE Standard 323 is also invoked as mother standard for qualification.  There are many daughter standards specific to various equipment types, such as IEEE Standards 382 for valves/actuators, 37.98 (501) for relays, and IEEE 535 for stationary batteries.  These standards provide the basic requirements and the plants invoke additional requirements such as floor response spectra, amplified response, plus functional, mounting, and monitoring requirements.  For example, a breaker mounted at the bottom of a panel at ground level is subject to different forces than one mounted high in a panel on an upper floor.