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**UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001**

June 3, 1998

**Addressees**

All holders of operating licenses for nuclear power reactors.

**Purpose**

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert addressees to the potentialfor some General Electric (GE) Type SBM control switches with spring return function to fail to return to the normalposition because the manufacturer failed to account for shrinkage of the phenolic material as a result of post-mold curing.It is expected that recipients will review the information for applicability to their facilities and consider actions, asappropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRCrequirements; IN 98-19

**Description of Circumstances**

In December 1997, the licensee for LaSalle County Station, Commonwealth Edison Company (ComEd), was replacing theGE Type SBM control switches at both units during a prolonged dual-unit maintenance outage. The licensee was replacingmore than 1,100 SBM switches because the switches were reaching the end of service life and because of concerns aboutcracking of the Lexan cam followers, which was originally described in NRC IN 80-13, "General Electric Type SBMControl Switches Defective Cam Followers" (Accession #8002280650). During the replacement effort, the licenseediscovered that some of the new switches that had been installed for about a year would not spring return to the reset(normal) position. The licensee found that 21 out of the 246 installed replacement switches exhibited the binding.

In early January 1998, two of the failed switches were sent to GE-Nuclear Energy (NE) for evaluation to determine thecause of the switch binding. On January 23, 1998, GE-NE issued a transfer of information letter to affected customers inaccordance with 10 CFR 21.21(b) to alert them to the possible failure of the SBM switches to return to the normal positionafter operation (Accession #9801280338). Subsequently, ComEd determined that the reported deviation represented asubstantial safety hazard because various equipment important to plant safety could be affected, and submitted a 10 CFRPart 21 notification to the NRC on March, 16, 1998.

**Discussion**

The SBM control switch is manufactured as a commercial item by GE-Electrical Distribution and Control (GE-ED&C),Power Management Division, in Malvern, Pennsylvania. GE-NE dedicates the switches for nuclear safety applications.The switches are used extensively throughout the nuclear power industry as control board switches to actuate safety-relatedpump motors, motor- operated valves, and circuit breakers. Although GE-NE has informed its customers of record aboutthe binding of the SBM switches, some licensees may have procured the switches from third-party dedicating entities thatpurchased the switches as commercial-grade components. This information notice is intended to alert all licensees to thepotential binding issue.

The failure mode has been identified as binding caused by interference between the rear brass bearing and the phenolicrear bearing support. The exploded view of an SBM switch in the attached Figure 1 shows the rear bearing support plate(Item 30, also called the barrier) and the rear bearing (Item 32) on the switch shaft. The interference is caused by post-mold cure shrinkage of the phenolic material, which takes place for up to 2 years after the phenolic material is molded intothe bearing support. GE-ED&C found that the mold used to make the bearing support had worn over the years to the pointthat the diameter of the opening for the brass rear bearing was still within specifications but near the minimum allowablevalue. The original GE design did not account for the post-mold cure shrinkage of the phenolic material. The secondcontributing factor to switch failure is a brass rear bearing at its maximum allowable diameter. Only switches with both theundersized hole in the bearing supports and bearings at the maximum tolerance have experienced failures. Testingconfirmed that switches with bearings of nominal size functioned properly, even with undersized holes in the phenolicbearing support.

The failed switches were manufactured between November 1996 and February 1997. Although most of the post-mold cureshrinkage occurs within the first 12 to 18 months, GE conservatively assumed that switches manufactured after March1996 may be susceptible. These switches can be identified by the last two characters on the green quality controlacceptance sticker. The affected date codes are PL, RL, SL, TL, UL, VL, WL, XL, YL, ZL, NM, OM,PM, RM, SM, TM,UM, VM, WM, XM, YM, ZM, NN, and ON. The manufacturer has improved the SBM switch manufacturing process toallow for phenolic post-mold cure shrinkage, and switches manufactured as of March 1, 1998, should no longer besusceptible to the failure mode.

There are two safety concerns regarding the switch binding: (1) possible control circuit damage caused by energizing themomentary circuit for a prolonged period and (2) possible prevention of control circuits from performing their functionbecause of the failure of the switch contacts to return to the normal position. GE recommends that licensees with SBMswitches manufactured after March 1996 advise operators to return the switch to the normal position after operation topreclude any circuit malfunctions. Licensees should also note that susceptible switches most likely will fail after about 12to 18 months after the date of manufacture. Switches more than 2 years old are not thought to be susceptible to bindingcaused by post-mold cure shrinkage.

This IN requires no specific action or written response. However, recipients are reminded that they are required to considerindustry-wide operating experience (including NRC INs) where practical, when setting goals and performing periodicevaluations under Section 50.65, "Requirements for Monitoring the Effectiveness of Maintenance at Nuclear PowerPlants," of Part 50 of Title 10 of the Code of Federal Regulations. If you have any questions about the information in thisnotice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation(NRR) project manager.

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