Information Notice 2008-18, Loss of Safety-Related Motor Control Center Due to Bus Fault: Difference between revisions

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| issue date = 12/01/2008
| issue date = 12/01/2008
| title = Loss of Safety-Related Motor Control Center Due to Bus Fault
| title = Loss of Safety-Related Motor Control Center Due to Bus Fault
| author name = Goel V K
| author name = Goel V
| author affiliation = NRC/NRR/DPR/PGCB
| author affiliation = NRC/NRR/DPR/PGCB
| addressee name =  
| addressee name =  
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| document type = NRC Information Notice
| document type = NRC Information Notice
| page count = 5
| page count = 5
| revision = 0
}}
}}
{{#Wiki_filter:ML082540130 December 1, 2008
{{#Wiki_filter:UNITED STATES


NRC INFORMATION NOTICE 2008-18: LOSS OF A SAFETY-RELATED MOTOR CONTROL CENTER CAUSED BY A BUS FAULT  
NUCLEAR REGULATORY COMMISSION
 
OFFICE OF NUCLEAR REACTOR REGULATION
 
WASHINGTON, DC 20555-0001 December 1, 2008 NRC INFORMATION NOTICE 2008-18:                 LOSS OF A SAFETY-RELATED MOTOR
 
CONTROL CENTER CAUSED BY A BUS FAULT


==ADDRESSEES==
==ADDRESSEES==
All holders of operating licenses for nuclear power reactors, except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel, as well as all holders of operating licenses or certificates of fuel cycle facilitie
All holders of operating licenses for nuclear power reactors, except those who have permanently
 
ceased operations and have certified that fuel has been permanently removed from the reactor
 
vessel, as well as all holders of operating licenses or certificates of fuel cycle facilities.


==PURPOSE==
==PURPOSE==
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform addressees of a recent event involving an electrical fire caused by a bus fault at the Arkansas Nuclear One Unit 2 (ANO 2) power plant, which resulted in the loss of a safety-related motor control center (MCC) and the associated loss of one division of some safety-related load The NRC expects recipients to review the information for applicability to their facilities and to consider actions, as appropriate, to avoid similar problem The suggestions contained in this IN are not NRC requirements; therefore, no specific action or written response is require DESCRIPTION OF CIRCUMSTANCES An electrical fire caused by a bus fault occurred at ANO 2 on October 23, 2007, and resulted in the loss of a safety-related MCC and the associated loss of one division of some safety-related load On this occasion, centrifugal charging pump 'A' was undergoing routine surveillance testing following mechanical maintenanc While operators were starting the pump remotely from the control room, a fire occurred in MCC Cubicle 2B-52A5, which caused the loss of power to the MCC 2B-52 because its upstream load center breaker trippe As a result, the licensee declared the associated safety-related Division 'A' loads inoperable, including the low-pressure and high-pressure safety injection pumps, the control room emergency chiller, and the containment spray pum The licensee declared an Alert on ANO 2 because the onsite fire affected one power train of engineered safety feature system Operators realigned the electrical equipment in accordance with plant procedures, and the plant exited the Alert 11/2 hours after it was declare The licensee's root-cause analysis indicated that the fire originated in MCC Cubicle 2B-52A5, which supplies centrifugal charging pump 'A'. This cubicle had a large starter, which was located at the bottom of the MCC in the A colum The licensee found that all three vertical aluminum bus bars in A column had melted (from bottom up) to the first horizontal bus brac Videoscope photographs behind the bus work showed molten metal slag at the outer C and B phases where the 2B-52A5 stab fingers engage the bus bar Although the stab fingers were aligned with the bus bars, the visual inspection following the removal of the starter cubicle and bus safety barrier indicated that the engagement and contact of the stab fingers on all the three phase bus bars were questionabl MCC Breaker 2B-52A5 did not trip during the event because the fault was on the source side of the breaker (bus bars) at the MC The fault event at the MCC occurred in the following sequence:  
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
 
addressees of a recent event involving an electrical fire caused by a bus fault at the Arkansas
 
Nuclear One Unit 2 (ANO 2) power plant, which resulted in the loss of a safety-related motor
 
control center (MCC) and the associated loss of one division of some safety-related loads. The
 
NRC expects recipients to review the information for applicability to their facilities and to consider
 
actions, as appropriate, to avoid similar problems. The suggestions contained in this IN are not
 
NRC requirements; therefore, no specific action or written response is required.
 
==DESCRIPTION OF CIRCUMSTANCES==
An electrical fire caused by a bus fault occurred at ANO 2 on October 23, 2007, and resulted in
 
the loss of a safety-related MCC and the associated loss of one division of some safety-related
 
loads. On this occasion, centrifugal charging pump A was undergoing routine surveillance
 
testing following mechanical maintenance. While operators were starting the pump remotely
 
from the control room, a fire occurred in MCC Cubicle 2B-52A5, which caused the loss of power
 
to the MCC 2B-52 because its upstream load center breaker tripped. As a result, the licensee
 
declared the associated safety-related Division A loads inoperable, including the low-pressure
 
and high-pressure safety injection pumps, the control room emergency chiller, and the
 
containment spray pump. The licensee declared an Alert on ANO 2 because the onsite fire
 
affected one power train of engineered safety feature systems. Operators realigned the
 
electrical equipment in accordance with plant procedures, and the plant exited the Alert 11/2 hours after it was declared.
 
The licensees root-cause analysis indicated that the fire originated in MCC Cubicle 2B-52A5, which supplies centrifugal charging pump A. This cubicle had a large starter, which was
 
located at the bottom of the MCC in the A column. The licensee found that all three vertical
 
aluminum bus bars in A column had melted (from bottom up) to the first horizontal bus brace.
 
Videoscope photographs behind the bus work showed molten metal slag at the outer C and B
 
phases where the 2B-52A5 stab fingers engage the bus bars. Although the stab fingers were
 
aligned with the bus bars, the visual inspection following the removal of the starter cubicle and
 
bus safety barrier indicated that the engagement and contact of the stab fingers on all the three
 
phase bus bars were questionable. MCC Breaker 2B-52A5 did not trip during the event
 
because the fault was on the source side of the breaker (bus bars) at the MCC. The fault event
 
at the MCC occurred in the following sequence:
(1)      Service-related conditions (i.e., large loads, frequent starts, repetitive starts, equipment
 
age, and environmental conditions) caused a high-resistance connection to develop at
 
the MCC bus/stab interface, leading to arcing at MCC Cubicle 2B-52A5.
 
(2)      The arcing at the connection caused a phase-to-ground fault at the stab/bus connection, which moved to the bottom of the MCC. As this fault progressed to a phase-to-phase
 
fault, it vaporized the bottom of each bus bar.
 
(3)      The heat from the fault caused the MCC plastic base pan and space heater wires in the
 
MCC to catch on fire.
 
(4)      The pressure from the fault caused the side wireway door to open.
 
(5)      The upstream 480-volt feeder breaker then tripped open and interrupted the fault. The
 
fire in the base of the MCC extinguished itself, ending the event sequence.
 
The MCC bus bars consisted of tin-plated aluminum, whereas the cubicle bus stabs consisted of
 
tin-plated copper alloy. Any loss of the tin plating could result in the oxidation of bare aluminum
 
and the development of a high-resistance connection at the stab/bus bar interface. The fault
 
and the resulting fire were primarily caused by the high-resistance connection at the stab/bus
 
interface, combined with the high starting current associated with large load.
 
The licensee identified the following two factors as contributing to high resistance at the stab/bus
 
bar interface:
(1)      Preventive maintenance was inadequate. The existing procedures did not require
 
lubrication, which conflicted with the industry practice and cubicle installation instructions
 
contained in the MCC technical manual. Lubrication protects against aluminum oxidation
 
and plate damage. The preventive maintenance procedure did not require bus
 
maintenance (i.e., visual inspection, cleaning and lubrication of the bus/stab contact
 
surface), allowing degradation to continue unrecognized.
 
(2)      High resistance developed in the connection because of the limited physical engagement
 
of the stab on the bus. The cubicle design allows each bucket some ability to flex.
 
However, this design made proper engagement depth difficult to achieve, especially on
 
large cubicles. A fully engaged stab would have only a 1/8-inch to 3/16-inch overlap on
 
the bus.
 
The MCC 2B-52 is an ITE 5600 Series MCC that was installed in 1975 at ANO 2 as original
 
plant equipment. The MCC model installed at ANO 2 had a history of problems, including brief
 
fires in the MCC, from 1984 to 2007. The preventive maintenance of the MCC was not
 
adequate. As a result of the October 23, 2007, fire in MCC 2B-52, the licensee took or planned
 
a number of corrective actions, including the following: *      Revise the preventive maintenance procedures to provide guidance for inspecting, cleaning, and lubricating the bus/stab connections, specifically regarding the following
 
actions:
        -      Visually inspect the plating on the stab fingers and on the vertical bus at the point
 
where the stab fingers engage the vertical bus. If enough plating has worn off
 
such that bare metal is exposed (copper or aluminum), replace both the stab
 
assembly and the vertical bus.
 
-      Perform a breaker stab tension inspection of each cubicle.
 
-      Apply a thin film of NO-OX-ID grease to the cubicle stabs and contact surface of
 
the bus.
 
*      For the high-risk ITE MCC cubicles, replace the existing stabs with longer ones or install
 
double stabs.
 
*      Replace the existing plated aluminum vertical bus bars with plated copper bus bars for
 
MCC columns with large loads.
 
Additional information related to the October 23, 2007, event is available in NRC Special
 
Inspection Reports 05000313/2007009 and 05000368/2007009, dated February 27, 2008, (Agencywide Documents Access and Management System (ADAMS) Accession
 
No. ML080590142).
 
The licensees root-cause analysis report also discussed another recent fault related to the ITE
 
5600 Series MCC, which occurred on January 31, 2007, at Calvert Cliffs Nuclear Power Plant. A


(1) Service-related conditions (i.e., large loads, frequent starts, repetitive starts, equipment age, and environmental conditions) caused a high-resistance connection to develop at the MCC bus/stab interface, leading to arcing at MCC Cubicle 2B-52A (2) The arcing at the connection caused a phase-to-ground fault at the stab/bus connection, which moved to the bottom of the MC As this fault progressed to a phase-to-phase fault, it vaporized the bottom of each bus ba (3) The heat from the fault caused the MCC plastic base pan and space heater wires in the MCC to catch on fir (4) The pressure from the fault caused the side wireway door to ope (5) The upstream 480-volt feeder breaker then tripped open and interrupted the faul The fire in the base of the MCC extinguished itself, ending the event sequenc The MCC bus bars consisted of tin-plated aluminum, whereas the cubicle bus stabs consisted of tin-plated copper allo Any loss of the tin plating could result in the oxidation of bare aluminum and the development of a high-resistance connection at the stab/bus bar interfac The fault and the resulting fire were primarily caused by the high-resistance connection at the stab/bus interface, combined with the high starting current associated with large loa The licensee identified the following two factors as contributing to high resistance at the stab/bus bar interface:
fault occurred on an ITE 5600 Series MCC because of a high-resistance stab connection on a


(1) Preventive maintenance was inadequat The existing procedures did not require lubrication, which conflicted with the industry practice and cubicle installation instructions contained in the MCC technical manua Lubrication protects against aluminum oxidation and plate damag The preventive maintenance procedure did not require bus maintenance (i.e., visual inspection, cleaning and lubrication of the bus/stab contact surface), allowing degradation to continue unrecognize (2) High resistance developed in the connection because of the limited physical engagement of the stab on the bu The cubicle design allows each bucket some ability to fle However, this design made proper engagement depth difficult to achieve, especially on large cubicle A fully engaged stab would have only a 1/8-inch to 3/16-inch overlap on the bu The MCC 2B-52 is an ITE 5600 Series MCC that was installed in 1975 at ANO 2 as original plant equipmen The MCC model installed at ANO 2 had a history of problems, including brief fires in the MCC, from 1984 to 200 The preventive maintenance of the MCC was not adequat As a result of the October 23, 2007, fire in MCC 2B-52, the licensee took or planned a number of corrective actions, including the following:
50-horsepower motor load caused by degradation of the bus-to-breaker stab connection. The
* Revise the preventive maintenance procedures to provide guidance for inspecting, cleaning, and lubricating the bus/stab connections, specifically regarding the following actions: - Visually inspect the plating on the stab fingers and on the vertical bus at the point where the stab fingers engage the vertical bu If enough plating has worn off such that bare metal is exposed (copper or aluminum), replace both the stab assembly and the vertical bu Perform a breaker stab tension inspection of each cubicl Apply a thin film of NO-OX-ID grease to the cubicle stabs and contact surface of the bu
* For the high-risk ITE MCC cubicles, replace the existing stabs with longer ones or install double stab
* Replace the existing plated aluminum vertical bus bars with plated copper bus bars for MCC columns with large load Additional information related to the October 23, 2007, event is available in NRC Special Inspection Reports 05000313/2007009 and 05000368/2007009, dated February 27, 2008, (Agencywide Documents Access and Management System (ADAMS) Accession No. ML080590142).


The licensee's root-cause analysis report also discussed another recent fault related to the ITE 5600 Series MCC, which occurred on January 31, 2007, at Calvert Cliffs Nuclear Power Plan A fault occurred on an ITE 5600 Series MCC because of a high-resistance stab connection on a 50-horsepower motor load caused by degradation of the bus-to-breaker stab connectio The corrective actions taken by the Calvert Cliffs licensee included the installation of double stabs for loads of 50 horsepower and larger and the periodic replacement of stab DISCUSSION The electrical fire at ANO 2 resulted in the partial loss of one power train of a Class 1E system; however, the opposite power train was still available to provide power to the redundant loads, which would allow for the safe shutdown of the plan Although the safety significance of this event was not considered high because it impacted only one Class 1E division MCC, the potential for a common-cause failure still existed without the implementation of the proposed corrective action NRC Office of Inspection and Enforcement Circular 77-03, "Fire in a Motor Control Center,"
corrective actions taken by the Calvert Cliffs licensee included the installation of double stabs for
dated February 28, 1977, documented two events associated with the ITE 5600 Series MC One event happened at Three Mile Island, Unit 2, where an MCC cubicle was not fully inserted and therefore provided only partial engagement of the stab The resulting high-resistance connection eventually led to a fire and loss of power from the MC The second event, which occurred at the Trojan Nuclear Power Plant, involved misalignment of the stab Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fir The root cause of this event was determined to be an inadequate MCC design that made it possible to misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a high resistance connectio The high resistance of stab connections at the MCC could lead to a fire when large loads are starte The following factors can cause high resistance in these connections:


* oxidation of the aluminum bus caused when the protective coating is worn off over a period of time from the repeated action of the stabs
loads of 50 horsepower and larger and the periodic replacement of stabs.
* poor maintenance practices, including a lack of lubrication at the stabbing contact area
 
* design deficiencies, such as misalignment of the stabs and insufficient stab contact surfaces In general, aluminum buses are more prone to oxidation as compared copper buses and therefore to developing high-resistance in stab-type connection Licensees are encouraged to evaluate the design and anticipated frequency of operations of the loads within MCCs within their facilities and develop preventive maintenance procedures that are appropriate for the application condition
==DISCUSSION==
The electrical fire at ANO 2 resulted in the partial loss of one power train of a Class 1E system;
however, the opposite power train was still available to provide power to the redundant loads, which would allow for the safe shutdown of the plant. Although the safety significance of this
 
event was not considered high because it impacted only one Class 1E division MCC, the
 
potential for a common-cause failure still existed without the implementation of the proposed
 
corrective actions.
 
NRC Office of Inspection and Enforcement Circular 77-03, Fire in a Motor Control Center, dated February 28, 1977, documented two events associated with the ITE 5600 Series MCC.
 
One event happened at Three Mile Island, Unit 2, where an MCC cubicle was not fully inserted
 
and therefore provided only partial engagement of the stabs. The resulting high-resistance
 
connection eventually led to a fire and loss of power from the MCC. The second event, which
 
occurred at the Trojan Nuclear Power Plant, involved misalignment of the stabs. Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession
 
No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fire. The root
 
cause of this event was determined to be an inadequate MCC design that made it possible to
 
misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a
 
high resistance connection.
 
The high resistance of stab connections at the MCC could lead to a fire when large loads are
 
started. The following factors can cause high resistance in these connections:
*       oxidation of the aluminum bus caused when the protective coating is worn off over a
 
period of time from the repeated action of the stabs
 
*       poor maintenance practices, including a lack of lubrication at the stabbing contact area
 
*       design deficiencies, such as misalignment of the stabs and insufficient stab contact
 
surfaces
 
In general, aluminum buses are more prone to oxidation as compared copper buses and
 
therefore to developing high-resistance in stab-type connections. Licensees are encouraged to
 
evaluate the design and anticipated frequency of operations of the loads within MCCs within
 
their facilities and develop preventive maintenance procedures that are appropriate for the
 
application conditions.


==CONTACT==
==CONTACT==
This IN requires no specific action or written respons Please direct any questions about this matter to the technical contact listed belo Timothy J. McGinty, Director Daniel H. Dorman, Director Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards  
This IN requires no specific action or written response. Please direct any questions about this
 
matter to the technical contact listed below.
 
Timothy J. McGinty, Director                   Daniel H. Dorman, Director
 
Division of Policy and Rulemaking             Division of Fuel Cycle Safety and Safeguards
 
Office of Nuclear Reactor Regulation           Office of Nuclear Material Safety and Safeguards


===Technical Contact:===
===Technical Contact:===
Vijay Goel, NRR (301) 415-3730 e-mail: vijay.goel@nrc.gov NRC generic communications may be found on the NRC public Web site, http://www.nrc.go To access this information, select "Electronic Reading Room" and then "Document Collections." Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fir The root cause of this event was determined to be an inadequate MCC design that made it possible to misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a high resistance connectio The high resistance of stab connections at the MCC could lead to a fire when large loads are starte The following factors can cause high resistance in these connections:


* oxidation of the aluminum bus caused when the protective coating is worn off over a period of time from the repeated action of the stabs
===Vijay Goel, NRR===
* poor maintenance practices, including a lack of lubrication at the stabbing contact area
                        (301) 415-3730
* design deficiencies, such as misalignment of the stabs and insufficient stab contact surfaces In general, aluminum buses are more prone to oxidation as compared copper buses and therefore to developing high-resistance in stab-type connection Licensees are encouraged to evaluate the design and anticipated frequency of operations of the loads within MCCs within their facilities and develop preventive maintenance procedures that are appropriate for the application condition
                        e-mail: vijay.goel@nrc.gov
 
NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov.
 
To access this information, select Electronic Reading Room and then Document Collections. Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession
 
No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fire. The root
 
cause of this event was determined to be an inadequate MCC design that made it possible to
 
misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a
 
high resistance connection.
 
The high resistance of stab connections at the MCC could lead to a fire when large loads are
 
started. The following factors can cause high resistance in these connections:
*       oxidation of the aluminum bus caused when the protective coating is worn off over a
 
period of time from the repeated action of the stabs
 
*       poor maintenance practices, including a lack of lubrication at the stabbing contact area
 
*       design deficiencies, such as misalignment of the stabs and insufficient stab contact
 
surfaces
 
In general, aluminum buses are more prone to oxidation as compared copper buses and
 
therefore to developing high-resistance in stab-type connections. Licensees are encouraged to
 
evaluate the design and anticipated frequency of operations of the loads within MCCs within
 
their facilities and develop preventive maintenance procedures that are appropriate for the
 
application conditions.


==CONTACT==
==CONTACT==
This IN requires no specific action or written respons Please direct any questions about this matter to the technical contact listed belo Timothy J. McGinty, Director Daniel H. Dorman, Director Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards  
This IN requires no specific action or written response. Please direct any questions about this
 
matter to the technical contact listed below.
 
Timothy J. McGinty, Director                     Daniel H. Dorman, Director
 
Division of Policy and Rulemaking                 Division of Fuel Cycle Safety and Safeguards
 
Office of Nuclear Reactor Regulation             Office of Nuclear Material Safety and Safeguards


===Technical Contact:===
===Technical Contact:===
Vijay Goel, NRR (301) 415-3730 e-mail: vijay.goel@nrc.gov NRC generic communications may be found on the NRC public Web site, http://www.nrc.go To access this information, select "Electronic Reading Room" and then "Document Collections."


Distribution: IN Reading File ADAMS Accession Number: ML082540130 *Concurrence by e-mail OFFICE TECH EDITOR EEEB:DE EEEB:DE BC:EEEB:DE D:EEEB NAME KAzariah-Kribbs* VGoel VKG RMathew RKM GWilson GW PHiland PH DATE 9/16/08 9/22/08 9/23/08 9/25/08 9/29/08 OFFICE LA:PGCB:DPR TSB:FCSS D:FCSS PGCB:DPR BC:PGCB:DPR D:DPR NAME CHawes CMH D. Rahn D. Dorman DBeaulieu MMurphy SStuchell for TMcGinty DATE 9/30/08 10/31/08 10/31/08 11/06/08 12/01/08 12/01/08 OFFICIAL RECORD COPY}}
===Vijay Goel, NRR===
                          (301) 415-3730
                          e-mail: vijay.goel@nrc.gov
 
NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov.
 
To access this information, select Electronic Reading Room and then Document Collections.
 
Distribution: IN Reading File
 
ADAMS Accession Number: ML082540130                     *Concurrence by e-mail
 
OFFICE       TECH EDITOR       EEEB:DE           EEEB:DE           BC:EEEB:DE         D:EEEB
 
NAME         KAzariah-Kribbs*   VGoel VKG         RMathew RKM       GWilson GW         PHiland PH
 
DATE         9/16/08           9/22/08           9/23/08           9/25/08             9/29/08 OFFICE       LA:PGCB:DPR       TSB:FCSS     D:FCSS         PGCB:DPR       BC:PGCB:DPR       D:DPR
 
NAME         CHawes CMH       D. Rahn       D. Dorman       DBeaulieu       MMurphy           TMcGinty
 
SStuchell for
 
DATE         9/30/08           10/31/08     10/31/08       11/06/08       12/01/08           12/01/08 OFFICIAL RECORD COPY}}


{{Information notice-Nav}}
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Latest revision as of 14:07, 14 November 2019

Loss of Safety-Related Motor Control Center Due to Bus Fault
ML082540130
Person / Time
Issue date: 12/01/2008
From: Vijay Goel
Generic Communications Projects Branch
To:
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References
IN-08-18
Download: ML082540130 (5)
v • d • e


UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001 December 1, 2008 NRC INFORMATION NOTICE 2008-18: LOSS OF A SAFETY-RELATED MOTOR

CONTROL CENTER CAUSED BY A BUS FAULT

ADDRESSEES

All holders of operating licenses for nuclear power reactors, except those who have permanently

ceased operations and have certified that fuel has been permanently removed from the reactor

vessel, as well as all holders of operating licenses or certificates of fuel cycle facilities.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform

addressees of a recent event involving an electrical fire caused by a bus fault at the Arkansas

Nuclear One Unit 2 (ANO 2) power plant, which resulted in the loss of a safety-related motor

control center (MCC) and the associated loss of one division of some safety-related loads. The

NRC expects recipients to review the information for applicability to their facilities and to consider

actions, as appropriate, to avoid similar problems. The suggestions contained in this IN are not

NRC requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

An electrical fire caused by a bus fault occurred at ANO 2 on October 23, 2007, and resulted in

the loss of a safety-related MCC and the associated loss of one division of some safety-related

loads. On this occasion, centrifugal charging pump A was undergoing routine surveillance

testing following mechanical maintenance. While operators were starting the pump remotely

from the control room, a fire occurred in MCC Cubicle 2B-52A5, which caused the loss of power

to the MCC 2B-52 because its upstream load center breaker tripped. As a result, the licensee

declared the associated safety-related Division A loads inoperable, including the low-pressure

and high-pressure safety injection pumps, the control room emergency chiller, and the

containment spray pump. The licensee declared an Alert on ANO 2 because the onsite fire

affected one power train of engineered safety feature systems. Operators realigned the

electrical equipment in accordance with plant procedures, and the plant exited the Alert 11/2 hours after it was declared.

The licensees root-cause analysis indicated that the fire originated in MCC Cubicle 2B-52A5, which supplies centrifugal charging pump A. This cubicle had a large starter, which was

located at the bottom of the MCC in the A column. The licensee found that all three vertical

aluminum bus bars in A column had melted (from bottom up) to the first horizontal bus brace.

Videoscope photographs behind the bus work showed molten metal slag at the outer C and B

phases where the 2B-52A5 stab fingers engage the bus bars. Although the stab fingers were

aligned with the bus bars, the visual inspection following the removal of the starter cubicle and

bus safety barrier indicated that the engagement and contact of the stab fingers on all the three

phase bus bars were questionable. MCC Breaker 2B-52A5 did not trip during the event

because the fault was on the source side of the breaker (bus bars) at the MCC. The fault event

at the MCC occurred in the following sequence:

(1) Service-related conditions (i.e., large loads, frequent starts, repetitive starts, equipment

age, and environmental conditions) caused a high-resistance connection to develop at

the MCC bus/stab interface, leading to arcing at MCC Cubicle 2B-52A5.

(2) The arcing at the connection caused a phase-to-ground fault at the stab/bus connection, which moved to the bottom of the MCC. As this fault progressed to a phase-to-phase

fault, it vaporized the bottom of each bus bar.

(3) The heat from the fault caused the MCC plastic base pan and space heater wires in the

MCC to catch on fire.

(4) The pressure from the fault caused the side wireway door to open.

(5) The upstream 480-volt feeder breaker then tripped open and interrupted the fault. The

fire in the base of the MCC extinguished itself, ending the event sequence.

The MCC bus bars consisted of tin-plated aluminum, whereas the cubicle bus stabs consisted of

tin-plated copper alloy. Any loss of the tin plating could result in the oxidation of bare aluminum

and the development of a high-resistance connection at the stab/bus bar interface. The fault

and the resulting fire were primarily caused by the high-resistance connection at the stab/bus

interface, combined with the high starting current associated with large load.

The licensee identified the following two factors as contributing to high resistance at the stab/bus

bar interface:

(1) Preventive maintenance was inadequate. The existing procedures did not require

lubrication, which conflicted with the industry practice and cubicle installation instructions

contained in the MCC technical manual. Lubrication protects against aluminum oxidation

and plate damage. The preventive maintenance procedure did not require bus

maintenance (i.e., visual inspection, cleaning and lubrication of the bus/stab contact

surface), allowing degradation to continue unrecognized.

(2) High resistance developed in the connection because of the limited physical engagement

of the stab on the bus. The cubicle design allows each bucket some ability to flex.

However, this design made proper engagement depth difficult to achieve, especially on

large cubicles. A fully engaged stab would have only a 1/8-inch to 3/16-inch overlap on

the bus.

The MCC 2B-52 is an ITE 5600 Series MCC that was installed in 1975 at ANO 2 as original

plant equipment. The MCC model installed at ANO 2 had a history of problems, including brief

fires in the MCC, from 1984 to 2007. The preventive maintenance of the MCC was not

adequate. As a result of the October 23, 2007, fire in MCC 2B-52, the licensee took or planned

a number of corrective actions, including the following: * Revise the preventive maintenance procedures to provide guidance for inspecting, cleaning, and lubricating the bus/stab connections, specifically regarding the following

actions:

- Visually inspect the plating on the stab fingers and on the vertical bus at the point

where the stab fingers engage the vertical bus. If enough plating has worn off

such that bare metal is exposed (copper or aluminum), replace both the stab

assembly and the vertical bus.

- Perform a breaker stab tension inspection of each cubicle.

- Apply a thin film of NO-OX-ID grease to the cubicle stabs and contact surface of

the bus.

  • For the high-risk ITE MCC cubicles, replace the existing stabs with longer ones or install

double stabs.

  • Replace the existing plated aluminum vertical bus bars with plated copper bus bars for

MCC columns with large loads.

Additional information related to the October 23, 2007, event is available in NRC Special

Inspection Reports 05000313/2007009 and 05000368/2007009, dated February 27, 2008, (Agencywide Documents Access and Management System (ADAMS) Accession

No. ML080590142).

The licensees root-cause analysis report also discussed another recent fault related to the ITE

5600 Series MCC, which occurred on January 31, 2007, at Calvert Cliffs Nuclear Power Plant. A

fault occurred on an ITE 5600 Series MCC because of a high-resistance stab connection on a

50-horsepower motor load caused by degradation of the bus-to-breaker stab connection. The

corrective actions taken by the Calvert Cliffs licensee included the installation of double stabs for

loads of 50 horsepower and larger and the periodic replacement of stabs.

DISCUSSION

The electrical fire at ANO 2 resulted in the partial loss of one power train of a Class 1E system;

however, the opposite power train was still available to provide power to the redundant loads, which would allow for the safe shutdown of the plant. Although the safety significance of this

event was not considered high because it impacted only one Class 1E division MCC, the

potential for a common-cause failure still existed without the implementation of the proposed

corrective actions.

NRC Office of Inspection and Enforcement Circular 77-03, Fire in a Motor Control Center, dated February 28, 1977, documented two events associated with the ITE 5600 Series MCC.

One event happened at Three Mile Island, Unit 2, where an MCC cubicle was not fully inserted

and therefore provided only partial engagement of the stabs. The resulting high-resistance

connection eventually led to a fire and loss of power from the MCC. The second event, which

occurred at the Trojan Nuclear Power Plant, involved misalignment of the stabs. Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession

No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fire. The root

cause of this event was determined to be an inadequate MCC design that made it possible to

misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a

high resistance connection.

The high resistance of stab connections at the MCC could lead to a fire when large loads are

started. The following factors can cause high resistance in these connections:

  • oxidation of the aluminum bus caused when the protective coating is worn off over a

period of time from the repeated action of the stabs

  • poor maintenance practices, including a lack of lubrication at the stabbing contact area
  • design deficiencies, such as misalignment of the stabs and insufficient stab contact

surfaces

In general, aluminum buses are more prone to oxidation as compared copper buses and

therefore to developing high-resistance in stab-type connections. Licensees are encouraged to

evaluate the design and anticipated frequency of operations of the loads within MCCs within

their facilities and develop preventive maintenance procedures that are appropriate for the

application conditions.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below.

Timothy J. McGinty, Director Daniel H. Dorman, Director

Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards

Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards

Technical Contact:

Vijay Goel, NRR

(301) 415-3730

e-mail: vijay.goel@nrc.gov

NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov.

To access this information, select Electronic Reading Room and then Document Collections. Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession

No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fire. The root

cause of this event was determined to be an inadequate MCC design that made it possible to

misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a

high resistance connection.

The high resistance of stab connections at the MCC could lead to a fire when large loads are

started. The following factors can cause high resistance in these connections:

  • oxidation of the aluminum bus caused when the protective coating is worn off over a

period of time from the repeated action of the stabs

  • poor maintenance practices, including a lack of lubrication at the stabbing contact area
  • design deficiencies, such as misalignment of the stabs and insufficient stab contact

surfaces

In general, aluminum buses are more prone to oxidation as compared copper buses and

therefore to developing high-resistance in stab-type connections. Licensees are encouraged to

evaluate the design and anticipated frequency of operations of the loads within MCCs within

their facilities and develop preventive maintenance procedures that are appropriate for the

application conditions.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below.

Timothy J. McGinty, Director Daniel H. Dorman, Director

Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards

Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards

Technical Contact:

Vijay Goel, NRR

(301) 415-3730

e-mail: vijay.goel@nrc.gov

NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov.

To access this information, select Electronic Reading Room and then Document Collections.

Distribution: IN Reading File

ADAMS Accession Number: ML082540130 *Concurrence by e-mail

OFFICE TECH EDITOR EEEB:DE EEEB:DE BC:EEEB:DE D:EEEB

NAME KAzariah-Kribbs* VGoel VKG RMathew RKM GWilson GW PHiland PH

DATE 9/16/08 9/22/08 9/23/08 9/25/08 9/29/08 OFFICE LA:PGCB:DPR TSB:FCSS D:FCSS PGCB:DPR BC:PGCB:DPR D:DPR

NAME CHawes CMH D. Rahn D. Dorman DBeaulieu MMurphy TMcGinty

SStuchell for

DATE 9/30/08 10/31/08 10/31/08 11/06/08 12/01/08 12/01/08 OFFICIAL RECORD COPY


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