Information Notice 2016-08, Inadequate Work Practices Resulting in Faulted Circuit Breaker Connection
| ML16104A214 | |
| Person / Time | |
|---|---|
| Issue date: | 06/17/2016 |
| From: | Cheok M C, Louise Lund Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking |
| To: | |
| Schwab A | |
| References | |
| IN-16-008 | |
| Download: ML16104A214 (6) | |
ML16104A214 UNITED STATES NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION OFFICE OF NEW REACTORS WASHINGTON, D.C. 20555-0001
June 17, 2016
NRC INFORMATION NOTICE 2016-08: INADEQUATE WORK PRACTICES RESULTING IN FAULTED CIRCUIT BREAKER CONNECTIONS
ADDRESSEES
All holders of an operating license or construction permit for a nuclear power reactor under
Title 10 of the
Code of Federal Regulations (10 CFR) Part 50, "Domestic Licensing of Production and Utilization Facilities," except t
hose that have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.
All holders of and applicants for a power reactor early site permit, combined license, standard
design approval, or manufacturing license under 10 CFR Part 52, "Licenses, Certifications, and
Approvals for Nuclear Power Plants." All applicants for a standard design certification, including
such applicants after initial issuance of a design certification rule.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is is
suing this information notice (IN) to inform addressees of operating experience related to circuit breaker overheating and fires caused by
inadequate and high-resistance connections. Information from these events may apply to the
design, installation, testing, inspection, and maintenance of circuit breakers. The NRC expects 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 IN are not NRC requirements; therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
Fort Calhoun Station, Unit 1
On June 7, 2011, at Fort Calhoun Station, Unit 1 a fire occurred in the feeder breaker for a
safety-related 480-volt bus. The fire resulted in significant damage to the breaker, bus, and an extended facility shutdown. The fire occurred in a low-voltage breaker that had been installed in 2009 as part of a design modification to replace obsolete breakers. The new breaker assembly
consisted of a Square D
breaker and Masterpact
cradle assembly that served as the interface between the breaker and cubicle. Licensee investigation revealed that the main cause of the
fire was inadequate engagement of the breaker cradle primary disconnect assembly (PDA)
fingers and the bus stabs. Licensee inspection of Masterpact
cradles installed in the other safety-related buses revealed that the fingers on the cradles were longer than the original breaker fingers. The point where the fingers of the new cradle engaged the bus extended past the silver-plated portion on the copper bus stab, in an area that contained hardened grease. This likely led to high resistance, overheating, and ultimately the fire. During the design change process associated with the new breaker configuration, the licensee did not consider new failure
modes caused by the new finger-stab connections. Additionally, the installation procedures did not require post-modification testing to verify the resistance of the connections between the
cradle PDA fingers and bus stabs.
The licensee took corrective actions as a result of this event that included repairing the affected
bus, inspecting and testing the other breakers and buses, silver-plating the affected 480-volt bus stabs, and revising maintenance procedures to enhance installation and post-installation
inspection.
Additional information regarding this event is available in Licensee Event Report (LER) 05000285-2011-008 Revision 1, dated October 27, 2011 (Agencywide Documents Access and
Management System (ADAMS) Accession No. ML113010208, and NRC Special Inspection Report 05000285/2011014, dated March 12, 2012 (ADAMS Accession No. ML12072A128).
Browns Ferry Nuclear Plant, Unit 3
On November 4, 2013, an operator at Browns Ferry Nuclear Plant, Unit 3 noted the smell of
smoke coming from a 480-volt bus. Further licensee inspection revealed hotspots on the bus
feeder breaker. This breaker was a Square D
breaker with Masterpact
cradle assembly (similar to the breaker assembly at Fort Calhoun Station, Unit 1 described above).
The bus was transferred to the alternate feeder source, and the normal supply breaker was removed for
inspection. The licensee found the cradle PDA fingers discolored from excessive heat. The
licensee sent the affected Masterpact
cradle and breaker to the supplier, Nuclear Logistics Inc. (NLI), for failure analysis. NLI determined the direct cause of the overheating primary disconnects was the loss of vertical movement (float) on the primary disconnects of the breaker cradle. The cradle primary disconnects are designed to account for vertical misalignment of the
stabs in the switchgear. The primary disconnect fingers have vertical float that maintains the
finger pressure on the bus stabs when the bus stabs are not completely aligned in the vertical
bus. The loss of vertical float was caused by an incorrectly specified tolerance in the PDA's fabrication drawing. The licensee's installation procedures did not include steps to inspect for freedom of vertical float of the primary disconnects. Additionally, the supplier did not provide guidance for testing freedom of vertical float of primary disconnects.
Because of this event, the licensee performed inspections on the installed Masterpact
cradle assembly PDAs for signs of overheating, and to verify freedom of vertical float of primary disconnects. NLI generated a technical bulletin, and alerted licensees to inspect the affected assemblies for the manufacturing defect, and submitted a report under 10 CFR Part 21,
"Reporting of Defects and Noncompliance."
Additional information regarding this event is available in 10 CFR Part 21 Report 2014-09-00,
dated February 26, 2014 (ADAMS Accession No. ML14069A467), and NRC Integrated Inspection Report 05000259/2012004, 05000260/2012004, and 05000296/2012004, dated November 13, 2012 (ADAMS Accession No. ML12319A182).
Nine Mile Point Nuclear Station, Unit 2 On August 21, 2015, Nine Mile Point Nuclear Station, Unit 2 used an incorrectly configured
grounding cart that caused damage to the line-side connections in a 4160-volt breaker cubicle. A grounding cart is a device used to apply grounds to switchgear assemblies during maintenance activities to ensure personnel protection. The grounding cart used in this case
was configured for-2000 amp stabs versus the 1200-amp stabs required for the breaker cubicle.
There was no procedure for ensuring that the stabs on the grounding cart were the proper size
for their intended application. As a result, the stabs that were installed on the cart were one
inch larger in diameter than the stabs required for the breaker cubicle. The larger stabs caused damage to the draw-out connections on the load side of the breaker cubicle. Workers did not observe the damage following maintenance and removal of the grounding cart, and proceeded
with breaker re-installation. When the breaker was re-energized, an arc flash occurred as a
result of the damaged connections. This led to loss of the switchgear, loss of an electric fire
pump, loss of a feedwater drain pump, and an unplanned 10 percent downpower transient on the reactor. The cause of this event was determined to be the lack of procedural guidance for using the grounding cart in the breaker cubicle.
As a result of this event, the licensee took corrective actions that included implementing
procedural guidance to compare the configured grounding cart to the respective breaker cubicle, rather than relying on "skill-of-the-craft" knowledge.
Additional information regarding this event is available in NRC Integrated Inspection Report 05000220/2015003 and 05000410/2015003, dated November 9, 2015 (ADAMS Accession
No. ML15314A506).
Palo Verde Nuclear Generating Station, Unit 2 On September 16, 2015, at the Palo Verde Nuclear Generating Station, Unit 2, a breaker arc
flash and rapid combustion occurred, resulting in the licensee declaring a Notification of
Unusual Event (under the NRC's emergency classification system for grouping off-normal events or conditions). When racking in a non-class 1E load center 480-volt circuit breaker, operators unknowingly caused damage to the breaker's internal connections. During
installation, vertical misalignment between the ground clip and ground stab damaged the clip, causing the clip to spread apart as the breaker was racked in. One side of the ground clip came
into contact with one of the phases of the line side of the breaker. When the operator locally
shut the breaker, an arc flash occurred, causing significant damage to the back door of the cubicle, and creating an occupational safety hazard to the operator. The supply breaker to the bus tripped, de-energizing the fault. This breaker supplied power to non-essential loads, and its
failure did not adversely affect the safe operation of the plant. The licensee declared a
notification of unusual event as a result of the explosion in the breaker cubicle; however, the
operator was dressed out for 4160-volt work, and avoided serious injury. The cause of this incorrect installation was a lack of procedural guidance to verify proper breaker alignment during racking of the breaker. Several months prior, a similar event had occurred on a different
breaker at Palo Verde. However, in this event, the ground clip was forced outward instead of
inward toward the breaker, and therefore did not come into contact with the phase on the line
side of breaker.
The licensee found that misalignment of the breaker ground clip to the cubicle ground stab was limited to ABB K-Line 480-volt circuit breakers. The
licensee took corrective actions as a result of the September 16, 2015 event, which included revising procedures to check alignment of the
breaker ground clip to cubicle ground stab while racking in 480-volt breakers. Additional information regarding this event is available in NRC Integrated Inspection Report 05000528/2015003, 05000529/2015003, and 05000530/2015003, dated October 22, 2015 (ADAMS Accession No. ML15295A435).
BACKGROUND
Related NRC Generic Communications
NRC IN 2002-27, "Recent Fires at Commercial Nuclear Power Plants in the United States,"
dated September 20, 2002 (ADAMS Accession No. ML022630147). The NRC issued this IN to
alert addressees of several fire events, one of which was caused by a poor electrical connection between the breaker PDA and the bus stabs.
NRC IN 2007-34, "Operating Experience Regarding Electrical Circuit Breakers," dated October 22, 2007 (ADAMS Accession No. ML072390061). The NRC issued this IN to alert
addressees of several circuit breaker problems including deficient fit-ups, inadequate maintenance practices, and issues with design changes.
NRC IN 2008-18, "Loss of a Safety-Related Motor Control Center Caused by a Bus Fault,"
dated December 1, 2008 (ADAMS Accession No. ML082540130). The NRC issued this IN to
alert addressees of an electrical fire caused by a bus fault, which resulted in the loss of
safety-related equipment. The fault was caused by inadequate contact of a motor control center's stab fingers to its bus bars.
NRC IN 2010-25, "Inadequate Electrical Connections," dated November 17, 2010 (ADAMS
Accession No. ML102530012). The NRC issued this
IN to alert addressees of operating experience involving loose electrical connection that resulted in unanticipated plant transients and failures or unavailability of safety-related equipment.
DISCUSSION
Circuit breakers are relied upon to provide electrical power to equipment credited in accident
analyses. Industry operating experience has shown that effective breaker maintenance procedures should include provisions to ensure proper alignment during installation, and steps to ensure that there are no high resistance connections in the interface between breakers and
electrical buses following breaker installation. The examples provided in this IN illustrate how
inadequate breaker connections can result in adverse impacts to safety-related equipment, introduce occupational safety concerns, and present fire hazards. These examples illustrate the importance of adequate breaker testing, inspection, maintenance procedures, and the proper licensee oversight of physical modifications to circuit breaker designs.
CONTACT
S This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation project manager.
/ra/ /ra/
Michael C. Cheok, Director
Louise Lund, Director Division of Construction Inspection Division of Policy and Rulemaking and Operational Programs Office of Nuclear Reactor Regulation
Office of New Reactors
Technical Contact:
Jesse Robles, NRR 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.
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