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 those 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 issuing 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 
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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. 
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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. 

 
IN 2016-08 

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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. 
IN 2016-08 

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CONTACTS 

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. 
IN 2016-08 

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CONTACTS 

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. 

ADAMS Accession No: ML16104A214 *concurred via e-mail TAC MF7575

OFFICE QTE* NRR/DIRS/IOEB/TL* NRR/DIRS/IOEB/BC* NRR/DE/EEEB/BC * NRO/DCIP/QVIB1/BC* NRO/DEIA/ICE/BC* NRR/DIRS/D 

NAME CHsu JRobles HChernoff JZimmerman TJackson DCurtis, acting SMorris 

DATE 04/13/16 04/20/16 04/20/16 05/02/2016 04/20/2016 04/25/2016 04/20/2016 

OFFICE NRR/DE/D* NRO/DEIA/D* NRR/DPR/PGCB/LA NRR/DPR/PGCB/PM* NRR/DPR/PGCB/BC* NRO/DCIP/D NRR/DPR/D 

NAME JLubinski MMayfield ELee ASchwab SStuchell MCheok LLund 

DATE 05/31/2016 05/31/2016 06/07/2016 06/01/2016 06/08/2016 06/16/2016 06/17/2016 

OFFICIAL RECORD COPY