UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

January 25, 2006

NRC INFORMATION NOTICE 2006-03: MOTOR STARTER FAILURES DUE TO

MECHANICAL-INTERLOCK BINDING

ADDRESSEES

All holders of operating licensees for nuclear power reactors, except those who have

permanently ceased operations and have certified that fuel has been permanently removed

from the reactor vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform

addressees about the recent motor starter failures due to the mechanical interlock binding

between the “open” and “close” contactors at Cooper Nuclear Station (CNS). 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; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

On September 23, 2005, during a forced outage at CNS, an operator attempted to close a

residual heat removal (RHR) Train C suppression pool isolation motor-operated valve (MOV) to

align the RHR system for shutdown cooling. The valve failed to close. The licensee inspected

the 480 V MOV starter and found that the starter close contactor coil remained energized,

overheated, and failed. The problem was attributed to a slightly deformed mechanical interlock

which prevented the energized contactor from engaging properly. Later the same day, as

operators attempted to open the RHR Train B shutdown cooling isolation valve to place RHR

into shutdown cooling, the valve did not immediately open when the control switch was taken to

the open position. The valve opened on the second attempt. The licensee inspected the motor

starter and found a similar problem where the mechanical interlock was slightly deformed and

was causing the open and close contactors of the reversing starter to bind intermittently.

ML053340032
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DISCUSSION

The mechanical interlock with a reversing starter prevents a phase-to-phase fault by preventing

simultaneous actuation of the open and close contactors. This device is a redundant protection

for the electrical interlock in a reversing starter. The licensee’s root cause investigation found

that an inappropriate mounting alignment of the mechanical interlock caused a slight

deformation (bowing) of the nylon mechanical interlock for the open and close contactors of the

MOV. The deformation caused the interlock to bind and prevented the energized motor starter

contactor from engaging properly. The interlock had been positioned too close to the open and

close contactors. The mounting hole was offset 1/8 of an inch or more from the center line. 

According to the manufacturer’s (Cutler Hammer’s) instructions for mechanical interlocks, the

mounting offset from the centerline should have been 1/16 of an inch. This problem was

attributed to a fabrication error by the vendor. The slight misalignment of the mechanical

interlock in combination with the creep of the nylon material of the interlock under stress over

time slightly deformed the interlock, causing the interlock to bind against the open and close

contactors. 

CNS’s root cause investigation identified seven such events (including this event) at CNS since

1988. These events included abnormally long MOV opening or closing times, failed contactor

coils, and blown power fuses to the starter control circuit. The licensee’s analysis did not

indicate any obvious precursor problems, leading to the conclusion that the failures occurred

unexpectedly. 

The root cause investigation by the CNS also found that similar interlock problems had

occurred at several other nuclear plants. NRC Inspection Reports 50-346/99-11 (Accession

No. ML993370095) and 50-361/99-15 (Accession No. ML993330092) document similar

mechanical interlock problems at Davis-Besse Nuclear Station and San Onofre 2 Nuclear

Station, respectively, in 1999.

To correct the problem at CNS, the licensee changed the mounting arrangement. Instead of a

drilled hole in the interlock support bracket, the licensee created a slot that allows lateral

adjustment to align the interlock and replaced the damaged components. The licensee also

scheduled preventive maintenance to inspect similar mechanical interlocks in the active MOV

starters.

Subtle fabrication or installation errors in mechanical interlocks in motor starters can lead to

unexpected failures over time. A thorough evaluation and trending of the MOV stroke time

during the surveillance can help prevent unanticipated failures.
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CONTACT

This information notice requires no specific action or written response. Please direct any

questions about this matter to one of the technical contacts listed below or the appropriate

Office of Nuclear Reactor Regulation (NRR) project manager.

Christopher I. Grimes, Division Director /RA/

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contacts: Vijay Goel, NRR Thomas Koshy, NRR

301-415-3730 301-415-1176

 E-mail: vkg@nrc.gov E-mail: txk@nrc.gov

Project Manager: Sheila Ray, NRR

 301-415-2018

 E-mail: sxr4@nrc.gov

Note: NRC generic communications may be found on the NRC public Website,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.