Information Notice 2006-03, Motor Starter Failures Due to Mechanical Interlock Binding
| ML053340032 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 01/25/2006 |
| From: | Charemagne Grimes NRC/NRR/ADRA/DPR |
| To: | |
| Thomas Koshy, NRR-DE, 301-415-1176 | |
| References | |
| IN-06-003 | |
| Download: ML053340032 (4) | |
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.
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 licensees 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 manufacturers (Cutler Hammers) 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
CNSs 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 licensees 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.
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.
ML053340032 OFFICE
EEEB:DE
Tech Editor
EEEB:DE
BC:EEEB:DE
D:DE
NAME
VGoel
PKleene
TKoshy
RVJenkins
MEMayfield
DATE
11/30/2005
12 /06 /2005
11/30/2005
12/01/2005
12/05/2005 OFFICE
OIEB:DIRS
TL:IOEB:DIRS
BC:IOEB:DIRS
LA:PGCB
BC:PGCB:DPR
D:DPR
NAME
SRay
EJBenner JWF for
MJRoss-Lee JWF for
CHawes
CJackson
CIGrimes
DATE
12/06/2005
12/06/2005
12/06/2005
01/24/2006
01/24/2006
01/25/2006