Operating Experience Smart Sample (OpESS) FY 2009-01

Inspection of Electrical Connections for

(Motor Control Center, Circuit Breakers, and Interfaces)

Highlighted documents have an active hyperlink. 

OBJECTIVE: 

To support NRC inspector’s review of safety related Motor Control Centers (MCC). This 

OpESS provides additional information for inspection of the preventive maintenance 

programs at nuclear power plants. This OpESS is a voluntary inspection activity 

performed under the Reactor Oversight Process baseline program. 

APPLICABILITY: All licensed operating commercial nuclear reactors with safety related 

motor control centers, cubicles, circuit breakers, and interfaces. Use this OpESS if there 

are indications of circuit breaker issues, the licensee is in the process of replacing, or 

upgrading their safety related circuit breakers. 

BACKGROUND:

Based on the recent information and several electrical stab failures in motor control 

centers, the Operating Experience Branch recommended development of an aid to 

inspectors in the inspection of preventive maintenance programs, specifically for 

electrical connections like stabs and bolted electrical bus bars in motor control centers. 

This Operating Experience Smart Sample (OpESS) focuses inspector attention in the 

area of electrical connections. Additional information is included for interfaces between 

the MCC and circuit breaker (CB). The interfaces include circuit breaker stabs and 

mechanism operated cell (MOC) switch and electrical fingers that supply control power 

to the circuit breaker. 

NRR staff review of recent operating experience related to motor control center failures 

have identified failures of aluminum buses using electrical type stabs connections and 

circuit breaker interface issues. The stabs are typically part of the circuit breaker and 

buses are integral to the MCC. NRR staff continues to review MCC and CB interface 

issues. 

The recent MCC findings generally involved the failure to implement adequate corrective 

actions, the failure to ensure adequate procedures are available to perform 

maintenance, or the failure to follow procedures. 

LESSONS LEARNED: PREVENTIVE MAINTENANCE PROCEDURES 

Preventive Maintenance (PM) procedures should provide guidance for inspecting, 

cleaning, & appropriate lubrication of bus/stab connections for motor control centers. In 

Figure 1 shown below is simplified stab and vertical bus bar cross sectional view with the 

stab in contact the vertical bus. 

PM procedures should include visual inspection of the plating for stab and vertical buses 

at the point of engagement between the stab and vertical bus. If the plating has worn off 

exposing bare metal, then clear direction should be given in the PM procedure to correct 

the deficiency. PM procedures should also verify breaker stab tension. Stab tension 

should be sufficient to handle all rated and fault currents. Some licensees have replaced 

the existing stabs with longer or double stabs. The point of contact between the stab 

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and vertical bus is crucial for licensees to inspect. Oxidation of silver plating over time 

can reduce the electrical connection properties. Over heating and failure of the electrical 

connections can occur without adequate preventive maintenance. Based on a review of 

the design for existing stabs on high risk motor control center cubicles, at least two 

licensees have added another set of stabs for large motor loads (50 Hp and above). 

Figure 1 Sketch of Stab & Vertical Electrical Bus 

The stabs rating should adequately handle the short circuit rating for the motor control 

center. This rating varies due to several factors but a typical 480-volt motor control 

center short circuit rating can be 22,000 amperes. The circuit breaker stab design 

should be adequate to handle this short circuit rating as well as the inrush current rating 

of the connected motor load. A typical 50 hp motor full load current can be as much as 

65 amperes. The inrush current for this motor can exceed six times the full load current. 

The inrush current could exceed 390 amperes. ANSI standards for motor control 

centers provide short-circuit acceptance criteria that the “Stabs and vertical buses at the 

point of contact shall be in essentially the same mechanical and electrical condition as 

before the test”. This criterion highlights the importance of good electrical connection, 

allowing 22,000 amperes to pass thru the stab without damage. 

Some licensees have replaced aluminum bus bars with copper to minimize high 

resistance stab connections. Aluminum bus bars plating worn or scraped due to multiple 

insertions and removals of the circuit breaker will oxidize. 

LESSON LEARNED: ELECTRICAL CONNECTIONS 

A Special Inspection began at San Onofre Nuclear Generating Station on August 4, 

2008 in response to the identification of deficient electrical connections. The deficient 

electrical connections potentially could adversely affect the safety function of multiple 

safety systems used for accident mitigation. 

One example referenced in the inspection charter was EN 41798 - San Onofre 3- EDG 

INOPERABLE DUE TO LOOSE ELECTRICAL CONNECTION. The licensee later 

retracted this event notification because the emergency diesel generator (EDG) was 

considered operable. The licensee submitted a voluntary licensee event report to 

document this event and inform the NRC of the corrective actions taken. To review this 

LER, click on the following link: Emergency Diesel Generator (EDG) 3G003 Declared 

Inoperable due to Loose Wiring Connection on Emergency Supply Fan. 

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LESSON LEARNED: INTERFACES BETWEEN CIRCUIT BREAKER & MOTOR 

CONTROL CENTER 

The two events below give examples how CB and MCC cubicles need to interface 

properly to actuate the mechanism operated cell (MOC). The most recent example at 

Palisades indicates what can happen when the force exerted on the MOC is too large. 

The second example at Columbia Generating Station describes the effects of too little 

force on the MOC. 

On March 26, 2008, at Palisades, the licensee was conducting planned maintenance on 

train “A” high pressure safety injection pump. A plant operator had experienced difficulty 

racking the breaker out, and during a visual inspection of the pump breaker and cubicle, 

the licensee found that a bayonet on the MOC shaft was bent and interfering with the 

CB. In addition, another bayonet on the same shaft had cracks on the brazing 

connecting it to the shaft. The licensee determined that a new CB installed from 1999 to 

2004 exerted additional force on the MOC switch that may have exceeded the 

capabilities of the brazed joint. At the time, the licensee evaluated compatibility of the 

cubicle and breaker, but did not recognize that in the 1980s the manufacturer had 

strengthened the bayonet design to accommodate faster acting breakers. The stronger 

bayonet design was not installed in the site's existing breaker housings. The licensee 

inspected the MOC switches for the remaining safety-related breakers and identified no 

other examples that resulted in equipment inoperability. Until further upgrades are 

completed, the licensee will inspect safety-related breakers after operation to ensure the 

integrity of the mechanism. See LER 255/2008-02 Title: Breaker Cubicle Switch Failure 

Results in High Pressure Safety Injection Pump Inoperability

At Columbia Generating Station on February 14, 2002, Emergency Diesel Generator 

(EDG) 2 output circuit breaker closed but the MOC switch assembly failed to change 

state as expected. The licensee found the root cause of this problem was the original 

CB had provided more force to overcome the MOC linkage resistance. The upgraded 

CB did not provide enough force to overcome MOC linkage resistance. See LER 

397/2002-01 Title: Completion of Technical Specification Required Shutdown to Comply 

with Technical Specification LCO 3.8.1 Required Actions of Condition F

RECENT CIRCUIT BREAKER INSPECTIONS: 

PILGRIM NUCLEAR POWER STATION – NRC COMPONENT DESIGN BASES 

INSPECTION REPORT 293/2008-007

During a Component Design Bases Inspection (CDBI ) at Pilgrim, the CDBI team 

identified a finding of very low safety significance (Green). Elevated temperatures were 

identified on the ‘B’ 125 Volt direct current (Vdc) battery charger supply circuit breaker 

terminals during the performance of infrared thermography by the licensee on January 4, 

2006. Later testing by the licensee did not recognize lower electrical current flow would 

generate less heat and thermography would indicate the lower temperature results. On 

April 27, 2007, the licensee conducted a battery discharge test. The B’ battery charger 

480 Vac supply circuit breaker tripped on over-current and subsequently failed while 

recharging of the ‘B’ battery. 

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FORT CALHOUN STATION - NRC SPECIAL INSPECTION REPORT 285/2007-006, 

Fort Calhoun – LER 285/2007-002 Common Mode Failure of Medium Voltage (4160) 

Circuit Breaker 

Common Mode Failure of Medium Voltage (4160) Circuit Breaker occurred on January 

25, 2007. The 4160 volt circuit breaker for raw water (RW) pump AC-10B closed on 

demand, but the auxiliary contacts did not actuate. On February 8, 2007, a similar event 

occurred where the circuit breaker for RW pump AC-10C closed on demand, but its 

auxiliary contact switch did not actuate. The failure was determined to be a broken 

mechanical linkage rod. 

SOURCE DOCUMENTS: 

Review the Background and Source Document Sections of this OpESS and the related 

documents as needed to obtain a general understanding of some of the various MCC 

concerns (i.e., failures of MCC stabs and failure to take prompt corrective action, 

especially related to post-maintenance testing, etc.). 

NRC Information Notices and Circular: 

IN 2007-34: Operating Experience Regarding Electrical Circuit Breakers

IN 2002-27: Recent Fires at Commercial Nuclear Power Plants in the United States

IN 2000-14: Non-Vital Bus Fault Leads To Fire and Loss of Offsite Power

IN 88-11: Potential Loss of Motor Control Center and/Or Switchboard Function Due To 

Faulty Tie Bolts 

CIRCULAR 77-03: Fire Inside A Motor Control Center 

INSPECTION FINDINGS: ELECTRICAL CONNECTIONS 

The following link provides inspection findings to highlight previous electrical connection 

issues: Electrical Connection Findings (4 Green Findings, 12 Green NonCited 

Violations, and 1 White Violation) 

Docket Inspection 

Report 

Licensee Site Finding 

05000353 2008002 Limerick Green 

05000278 2007003 Peach Bottom Green 

05000275 2004004 Diablo Canyon Green 

05000390 2003002 Watts Bar Green 

Docket Inspection 

Report 

Licensee Site NonCited 

Violation 

05000293 2008007 Pilgrim Green 

05000277 2008007 Peach Bottom Green 

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05000498 2008003 South Texas Project Green 

05000368 2007009 Arkansas Nuclear One Green 

05000443 2007005 Seabrook Green 

05000255 2006009 Palisades Green 

05000440 2005009 Perry Green 

05000269 

05000270 

05000287 

2005004 Oconee Green 

05000354 2004002 Hope Creek Green 

05000443 2002005 Seabrook Green 

05000298 2001008 Cooper Green 

05000483 2000009 Callaway Green 

Docket Inspection 

Report 

Licensee Site Violation 

05000298 2008002 Cooper White 

BASELINE INSPECTION PROCEDURES: 

Baseline inspection procedures can be used for performing this OpESS and can include 

inspection procedures used for performing inspections during surveillance testing, post 

maintenance testing, or PI&R corrective action follow-up inspections related to CB and 

MCC issues. Links to these base line procedures are provided in the section below. 

IP 71111.12 - Maintenance Effectiveness

IP 71111.19 - Post Maintenance Testing

IP 71111.21 - Component Design Bases Inspection

IP 71111.22 - Surveillance Testing

IP 71152 - Identification and Resolution of Problems

INSPECTION GUIDANCE: 

Review several electrical preventative maintenance procedures focusing on safety 

related circuit breaker procedures, special attention should be given to removal of 

electrical connections. Here is a list of questions to consider in reviewing the PM 

procedures. 

1) Does the procedure inspect the cleanliness of the exposed connections and did 

the connection require anticorrosion lubricant? 

2) Does the preventive maintenance procedure verify that the connection is 

adequately tight for the application? 

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3) Does the procedure verify the resistance measurements of the connection? 

4) If the licensee has indications of higher than normal thermography readings, did 

they properly evaluate and take adequate corrective actions? See example 

finding documented under section 1R21.2.1.1 of Pilgrim Nuclear Power Station – 

NRC Component Design Bases Inspection Report 293/2008-007

5) Assess licensee effectiveness in identifying deficient electrical connection 

problems, evaluating the cause of these problems, and implementing corrective 

actions to resolve identified problems. 

6) Assess the adequacy of licensee processes (procedures, maintenance 

instructions, training, etc.) for maintaining proper electrical connections. 

7) If during a Component Design Bases Inspection using Appendix 2, Component 

Condition and Capability review circuit breaker modifications and consider if the 

modification included replacement of bus work for the circuit breaker? If existing 

bus work is in service, does the licensee inspect for wear? 

Example: New bus bars for safety related batteries might have a different lubricant 

applied for shipping or storage. The lubricant may have excellent anticorrosion 

properties but poor electrical properties. The removal of the shipping grease, cleaning of 

the bus bar and applying the correct electrical anticorrosion grease should be included in 

the preventive maintenance / modification procedures.

REPORTING INSPECTION RESULTS / TIME CHARGES / ADDITIONAL ISSUES: 

Document any inspection result findings, as applicable, in an integrated inspection report 

(i.e., quarterly inspection report) and reference the title and OpESS number (example: 

"Review of Operating Experience Smart Sample (OpESS) FY 2009-01 Inspection of 

Electrical Connections for (Motor Control Center, Circuit Breakers and 

Interfaces)". If no findings are identified document completion of the OpESS using the 

"OpESS number/ title" under the applicable inspection attachment (i.e., 1R12,) stating 

that no findings of significance were identified. 

Inspection time for this OpESS is to be charged to the normal baseline procedure under 

which it is being documented (along with any routine preparation and documentation 

charge times). 

Note: This completes the OpESS inspection guidance. This inspection tool is voluntary 

for consideration by inspectors and can be updated or revised by providing feedback to 

NRR/ADRO/DIRS/IOEB to reflect the lessons learned by inspectors for improvement in 

knowledge transfer. 

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