Information Notice 2010-25, Inadequate Electrical Connections
| ML102530012 | |
| Person / Time | |
|---|---|
| Issue date: | 11/17/2010 |
| From: | Mcginty T Division of Policy and Rulemaking |
| To: | |
| Russell Andrea, 415-8553 | |
| References | |
| IN-10-025 | |
| Download: ML102530012 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 November 17, 2010
NRC INFORMATION NOTICE 2010-25: INADEQUATE ELECTRICAL CONNECTIONS
ADDRESSEES
All holders of an operating license or construction permit for a nuclear power reactor issued
under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of
Production and Utilization Facilities, 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 (IN) to inform
addressees about operating experience involving loose electrical connections. The NRC
expects recipients to review the information for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems. The suggestions that appear in this IN are
not NRC requirements; therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
The Office of Nuclear Reactor Regulation reviewed operating experience related to inadequate
electrical connections and found that the following issues often caused connection problems:
- lack of work order documentation indicating that connections had been loosened or
removed
- failure to follow vendor- or industry-recommended torque requirements
- inadequate quality control verification of connection torque and resistance measurement
- incomplete or inappropriate maintenance practices
- improper washer installation
- lack of resistance verification to verify adequate torque
- inadequate tightening of connection after the removal of test leads
- failure to crimp and inspect the tightness of a lug connection
- marginal electrical connection during installation
- loose fuse holder retaining clips
- thermal cycling of electrical connections over time
- electrical connectors that are not fully seated
- worn or high-resistance electrical connector pins and sockets
The events described below are examples of recent electrical connection problems.
Columbia Generating Station
On August 5, 2009, an electrical fault occurred on a 6.9-kilovolt (kV) nonsegregated bus at
Columbia Generating Station while the plant was operating at 100-percent power. The fault
caused a main generator differential lockout, which resulted in a main turbine trip and the
subsequent actuation of the automatic reactor protection system. The nonsegregated bus
experienced a catastrophic failure, and the associated fire generated enough smoke in the
turbine building to require the declaration of a notification of an unusual event. Although the
extent of damage to the bus made it impossible for plant personnel to substantiate a direct
cause, the licensee postulated that the most probable cause was the failure of a link assembly
that allowed a short circuit between phase conductors. The failure of the link was attributed to
thermal cycling that caused the bolted connections on the central flexible link to loosen. The
licensee omitted steps in its preventive maintenance procedure for torque verification and high- potential testing without evaluating the scope and implications of these omissions. The licensee
could have prevented the bus failure by performing these steps. (See Licensee Evaluation
Report (LER) 397/2009-004-00, 6.9 kV Non-Segregated Electrical Bus Failure, issued 2009, Agencywide Documents Access and Management System (ADAMS) Accession No.
Waterford Steam Electric Station, Unit 3
In May 2008, workers at Waterford Steam Electric Station (Waterford), Unit 3, replaced a cell in
a safety related battery bank. The licensee failed to ensure that the scope of work was
adequate. Specifically, the electricians did not (1) torque all of the affected intercell connections
as recommended by the vendor, (2) obtain the required quality control inspector verification for
every connection torque, (3) ensure that all the resistance measurements for the electrical
connections were performed, and (4) ensure that the quality control verification for resistance
checks met technical specification (TS) limits. These errors resulted in an undetected loose
electrical connection that rendered the Train B battery inoperable. On September 3, 2008, Waterford operations declared the station battery 3B-S inoperable because of low voltage and
entered TS 3.8.2.1. Licensee personnel discovered loose bolts on an intercell connector
between two battery cells and immediately corrected the condition by tightening and verifying
intercell resistance. The licensee declared the battery operable on the same day. (See LER 382/2008-004-01, Loose Intercell Connecting Bolts on 125 vdc Station Battery (Revised),
issued 2010, ADAMS Accession No. ML101800330.) San Onofre Nuclear Generating Station, Unit 2 On March 25, 2008, plant personnel at San Onofre Nuclear Generating Station, Unit 2, were
performing TS Surveillance Requirement 3.8.4.1 of a Class 1E battery while the unit was
operating at 100-percent power and discovered that its voltage was below the required value.
Subsequently, the licensee observed signs of a loose connection on the breaker that provides
charging current for the battery in its normal configuration. The licensee declared the battery
inoperable, as required by TS Surveillance Requirement 3.8.4, Action A. The licensee torqued
the loose connection bolts and completed the required TS surveillance requirements with
satisfactory results. The licensee concluded in its root cause evaluation that it had most likely
not fully torqued the connection bolts on March 17, 2004, when it last replaced the breaker. The
bolts were not torqued because of an incomplete work plan and the lack of an effective second
check of the critical steps. (See LER 361/2008-006-00, Loose Connection Bolting Results in
Inoperable Battery and TS Violation, issued 2008, ADAMS Accession No. ML082660036.)
Dresden Nuclear Power Station, Unit 3
On October 29, 2008, with Unit 3 at approximately 85 percent power, licensee personnel at
Dresden Nuclear Power Station identified that the open indication light for the normally open
primary containment isolation valve was flickering. Troubleshooting on November 1, 2008, led
to the discovery of an intermittent high-resistance electrical connection for control power to the
valve. The intermittent electrical connection caused the indicator light to flicker, and the
intermittent connection would have prevented valve closure from the control room. The licensee
declared valve 3-3702 inoperable and took actions in accordance with the requirements of
TS 3.6.1.3. The licensee restored the valve to operable status on the same day. The most
probable cause of the high resistance connection was a marginal electrical connection during
original installation and equipment vibration over time causing the intermittent connection.
Additionally, the cause of not entering TS 3.6.1.3 on October 29, 2008, was a lack of proper
procedural guidance that led operations personnel to assume the problem was simply a faulty
light socket. (See LER 249/2008-002-00, Unit 3 Primary Containment Isolation Valve Declared
Inoperable, issued 2008, ADAMS Accession No. ML090060879.)
BACKGROUND
Related Generic Communications
- IN 2008-18, Loss of a Safety-Related Motor Control Center Caused by a Bus Fault, dated December 1, 2008
- IN 89-64, Electrical Bus Bar Failures, dated September 7, 1989
- IN 88-11, Potential Loss of Motor Control Center and/or Switchboard Function Due to
Faulty Tie Bolts, dated April 7, 1988 Related Operating Experience Smart Samples
- FY 2009-01, Inspection of Electrical Connections for Motor Control Center, Circuit
Breakers, and Interfaces, issued 2009
DISCUSSION
Inadequate electrical connections can lead to unanticipated plant transients and the failure or
unavailability of safety related equipment. They can also affect equipment important to safety or
can potentially challenge safety related equipment. A recent review of operating experience has
determined that the following items are important to ensuring the integrity of electrical
connections:
- Torque verification
- Visual inspections
- Periodic thermography measurements
- Resistance measurements
- Adherence to vendor recommendations
- Use of proper lubricants for switch contact surfaces
- Identification of single point connection vulnerabilities
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below.
/RA/
Timothy McGinty, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contacts: Joseph Giantelli, DIRS/IOEB Matthew McConnell, DE/EEEB
301-415-0504 301-415-1597 E-mail: Joseph.Giantelli@nrc.gov E-mail: Matthew McConnell@nrc.gov
Jesse Robles, DIRS/IOEB
301-415-2940
E-mail: Jesse.Robles@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below.
/RA/
Timothy McGinty, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contacts: Joseph Giantelli, DIRS/IOEB Matthew McConnell, DE/EEEB
301-415-0504 301-415-1597 E-mail: Joseph.Giantelli@nrc.gov E-mail: Matthew.McConnell@nrc.gov
Jesse Robles, DIRS/IOEB
301-415-2940
E-mail: Jesse.Robles@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
ADAMS Accession Number: ML102530012 TAC ME4481 OFFICE NRR/DIRS/IOEB BC/NRR/DIRS/IOEB D/NRR/DIRS Tech Editor NRR/DLR/RASB BC/DE/EEEB
NAME JGiantelli JThorp FBrown KAzariah-Kribbs CDoutt RMathew
DATE 10/26/10 10/28/10 11 /12 /10 09/16/10 e-mail 11/03/10 10/27/10
OFFICE D/DE/EEEB DPR/PGCB DPR/PGCB DPR/PGCB BC/DPR/PGCB D/NRR/DPR
NAME PHiland (GWilson for) ARussell CHawes TAlexion SRosenberg TMcGinty
DATE 11/04/10 11/15 /10 11/15/10 11/16 /10 11/17/10 11/17/10
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