U.S. NRC Experience with Undervoltage Trip Attachments on Reactor Trip Switchgear

An overview of the failures of reactor trip switchgear to trip soley on loss of voltage is presented. There are three types of switchgear in reactor protection systems in pressurized water reactors in the United States. The review of the failures to date and the corrective actions taken suggest that proper maintenance and surveillance testing of existing undervoltage trip attachments on reactor trip switchgear should reduce substantially the number of failures.


INTRODUCTION
switchgear. Two units, Ft. Calhoun and Palisades, do not use any of the three circuit breakers General Design Criteria 23 of 10 CFR 50, employed in the reactor trip circuits of other Appendix A states, "The protection system shall be pressurized water reactors.
Ft. Calhoun and designed to fail into a safe state or into a state Palisades use contactors for automatic tripping demonstrated to be acceptable on some other defined and other types of circuit breakers for manual basis if conditions such as disconnection of the tripping.
system, loss of energy (e.g., electric power, instrument air), or postulated adverse environments (e.g., extreme heat or cold, fire, pressure, steam,   Two reactor trip breakers failed at Oconee organize and place in categories. Focusing on the corrective actions taken as opposed to the causes Selected GE-AK-2 type reactor trip breaker of failure, which in some cases were never really anomalies that have occurred through December 1983 conclusively determined and in other cases involved are presented in summary form in Table V. differing professional opinions, some conclusions can be drawn.

Generally the corrective action
In the GE-AK-2 type of switchgear the underinvolved making adjustments to bring the voltage device per se requires no lubrication.
undervoltage device and the switchgear into A mechanical latch keeps the switchgear armed once compliance with vendor specifications or the the switchgear has been closed. The mechanical application of a lubricant (including the revitallatch mechanism includes a roller bearing. Two izing of the lubricant). Adjustments were necesbearings support the trip shaft.
Switchgear sary due to either improper settings at the vendor, operability depends on the interaction between the improper settings from previous maintenance or time undervoltage device and the latch mechanism in service that caused acceptable initial settings remaining unchanged over time. For example, given to become unacceptable. Lubrication appeared that the undervoltaqe device produces a constant to be a remedy for any type of anomaly whose cause output force, the switchgear would not open if was not immediately obvious, particularly with the force required to collapse the latch mechanism regard to the undervoltage trip attachment of the increased beyond some threshold value. This could W-DB-50. Slow trip times, which seem to be occur due to degraded lubricant in the latch indigenous only to the GE-AK-2 type switchgear, roller bearing or some other age related were remedied by the revitalization of the phenomenon.
lubricant in the latch roller bearing in the switchgear. On more than one occasion the remedy The major contributing factors to the anomawas only temporary in that slow trip times were lies experienced in switchgear operation on loss observed in some instances within days of the of voltage as just described were insufficient or revitalization of the lubricant. Generally, degraded lubricant in the trip shaft and latch sticky or slow operation via the loss of voltage roller bearings, undervoltage device armature function resulted in lubrication as a remedy. pickup voltage being below the recommended voltage Teueso wtheraerligo an and clearances not within that specified by the Teueso wtheraerligo an manufacturer. Remedial actions were to perform tenance to achieve acceptable performance and to maintainence, in this case to revitalize the remedy many of the anomalies experienced. The conlubricant as oppose to appyling lubricant, adjust tent of that maintenance and the frequency of the pickup voltage, adjust the clearances and to application were early issues between the users Carnegie-Mellon gram. In the experience described above, its not University, Pittsburgh, the M.S. degree in Engithe switchgear, but the undervoltage trip attachneering Science from New Jersey Institute of ment and the relationship between the undervoltage Technology, Newark and the Ph. D. degree in Mechtrip attachment and the switchgear that has not anical Engineering from Rutgers University, New been sufficiently examined. The content of main-Brunswick, in 1963, 1965 and 1970, respectively. tenance, the frequency of maintenance and the life-He also received the M.B.A. degree from Duquesne time of the undervoltage trip attachment should University, Pittsburgh, PA in 1977. have been known definitively before this safety related equipment was placed in service. Instead In 1981 he joined the U.S. Nuclear Regulatory maintenance and surveillance of the switchgear Commission, Washington, D.C. His work has been in while it is in service is being used to determine the equipment qualification area. He is currently the maintenance interval, the content of mainte-Senior Equipment Qualification Engineer in the nance and the lifetime of the undervoltage trip Office of Nuclear Reactor Regulation. attachment. Trending of various switchgear parameters is being used to establish a means of pre-Dr. Wright is a member of Sigma Xi and Betta dicting either degradation in performance or fail-Gamma Sigma. He is a Registered Professional Engiure to trip of the switchgear on loss of voltage. neer in the states of Pennsylvania and Maryland.