Information Notice 1989-64, Electrical Bus Bar Failures
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UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.
20555
September 7, 1989
NRC INFORMATION NOTICE NO. 89-64: ELECTRICAL BUS BAR FAILURES
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
This information notice is intended to alert addressees to potential problems
resulting from the failure of electrical bus bars caused by cracked insulation
and moisture or debris buildup in bus bar housings. It is expected that reci- pients will review the information for applicability to their facilities and
consider actions, as appropriate, to avoid similar problems. However, sugges- tions contained in this information notice do not constitute NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances
(1) Palo Verde Unit 1 (Licensee Event Report No.88-010)
On July 6, 1988, a phase B to ground fault occurred on 13.8-kv non-Class lE
electrical bus E-NAN-S02. This fault ionized the air surrounding the bus and
caused all three phases to short to ground. Feeder breakers to non-Class 1E
buses E-NAN-SO1 and S02 did not open immediately, resulting in excessive
currents being supplied by the unit auxiliary transformer (UAT) and the sub- sequent rupture and ignition of the UAT.
This caused the supply breakers to
buses E-NAN-SO1 and S02 and the main generator output breaker to open on a
UAT sudden overpressure signal. As expected, a reactor trip on low departure
from nucleate boiling ratio occurred because the reactor coolant pumps were
powered from buses E-NAN-SO1 and S02. The reactor was subsequently stabilized
in hot standby on natural circulation.
Failure of bus E-NAN-S02, initiated by the phase B to ground fault, was attri- buted to cracked and brittle Noryl insulation and dirt that had accumulated in
the cubicles. These conditions led to arc tracking, which caused the single
phase to ground fault.
This fault subsequently propagated into a three-phase
to ground fault.
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1%11 III 89-64 September 7, 1989- (2) Kewaunee (Licensee Event Report Nos.88-001 and 87-009)
On March 2, 1988, when the plant was at 93.3 percent power, a reactor trip and
associated turbine trip were generated as a result of undervoltage (UV) condi- tions on 4160-volt electrical buses 1-1 and 1-2, which supply power to the
reactor coolant pump and main feedwater pump motors.
Investigations of the UV condition on the buses revealed that an electrical fault
had occurred on the bus from the "Y" winding of the main auxiliary transformer
to buses 1-1 and 1-2 because of insulation failure. The bus bar, which was a
1/2-inch by 4-inch flat copper bar, was rated at 4000 amperes per phase and was
manufactured by the Calvert Company.
The bus bar, which was encapsulated with
Noryl flame-retardant insulation, was enclosed in aluminum ducting with screened
ventilation slots on the top and bottom.
The licensee determined that the cause of the event was failure of the insulation
on the bus bar and the accumulation of water and debris around the bus, which
provided a tracking path for the fault.
The bus bar runs horizontally into the
auxiliary building underneath areas where debris can fall into the bus work.
Additionally, water from a plastic drain hose, located on the floor above the
faulted section of the bus, was suspected to have dripped onto the bus work.
A similar event had occurred at Kewaunee on July 10, 1987, when a reactor trip
and-assoc-i-a-ted-turb-ne-tri-p--ooccurred-as-a result-of- an-tJV-trawsient-on-the-same--
4160-volt buses. In this case, however, a phase to ground fault occurred on
the bus bar from the "X" winding of the main auxiliary transformer to 4160-volt
buses 1-3, 1-4, 1-5, and 1-6.
This bus bar was similar to the one identified
above, except that it was a flat aluminum bar that was rated at 3000 amperes.
The cause of this event was also failure of insulation on the bus bar compounded
by the accumulation of particulate debris.
The bus bar was located perpendicular
to the turbine building ventilation fans, which pulled dust-filled air through a
section of the bus bar.
Dust and metallic powder that had collected on the cracked
bus bar insulation provided a tracking path for the phase to ground fault.
(3) Millstone Unit 1 (Licensee Event Report.No. 87-001-01)
On January 13, 1987, when Unit 1 was at 100-percent power, a visual inspection
of the 4160-volt ac (VAC) load centers showed that horizontal Noryl bus bar
insulation was cracked.
The problem was attributed to a manufacturing defect
on General Electric Company (GE) metal-clad switchgear type M-26 (4160 VAC) and
type M-36 (6900 VAC).
Apparently, during the manufacturing process, "black"
bus bar joint compound (GE # D50H47) contaminated the Noryl insulation and over
several years caused the insulation to crack.
(4) Sequoyah Unit 1 (Licensee Event Report No.83-067)
On May 18, 1983, when Unit 1 was at 100-percent power, startup bus lB at Unit 1 failed because of a phase B to phase C fault, which propagated to ground.
The
failure was attributed to degradation of Noryl insulation on the Westinghouse
Model EN-265 bus.
Further investigation revealed several degraded areas in the
bus insulation at the support blocks, and startup bus 1B was declared inoperable.
.
- September
7, 1989 (5) Browns Ferry Unit 2 (Licensee Event Report No.89-008)
On March 9, 1989, a bus fault to ground occurred on the secondary (4160-volt)
side of the unit station service transformer, causing an engineered safety
feature actuation. This electrical fault also caused damage to the 4160-volt
bus, the bus duct, and the transformer bushing connections on all three phases.
The fault was attributed to (a) deterioration of the Noryl insulation above the
bus joint, (b) poor design of the bus duct, which allowed condensation to collect, and (c) inadequate implementation of the vendor-recommended preventive maintenance.
Discussion:
Failures of medium-voltage electrical bus bars, principally involving 4160-
and 6900-volt ac buses, have resulted in bus bar electrical faults and fires, electrical power system undervoltage conditions, plant transients, reactor
trips, and engineered safety feature actuations.
Failure of the bus bars has been attributed to cracked bus bar insulation (bus
sleeving) combined with the accumulation of moisture or debris in the bus bar
housings.
Insulation failure, along with the presence of moisture or debris, provided undesired phase to phase, or phase to ground, electrical tracking
paths, which resulted in catastrophic failure of the buses.
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Corrective actions taken by the involved utilities included replacing damaged
bus bar sections with bus bars that were covered with insulation of a different
type, substituting "yellow" bus bar joint compound (GE # D50H109) for the "black"
joint compound previously used, modifying bus bar enclosures-to restrict ingress
and accumulation of water and debris, and instituting enhanced periodic inspections
and cleaning of bus bars and their housings.
This information notice requires no specific action or written response. If
you have any question about the information in this notice, please contact one
of the technical contacts listed below or the appropriate NRR project manager.
!C~aPr es E.
i, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Mark Padovan, AEOD
(301) 492-4445
Argil Toalston, NRR
(301) 492-0831 Attachment:
List of Recently Issued NRC Information Notices
Attachnent
1.N 89-64
September 7, 1989
Page 1 of I
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
nDat
of
Inforrtion
Notice No.
39-63
89-62
69-61 ES-48, supp. 2
89-60
89-59
89-58
89-5 7 Subject
Possible Submergence of
Electrical Circuits Located
Above the Flood Level Because
of Water intrusion and Lack
of Drainage
Issuance
9/S/89 Malfunction of Borg-Warner
8/31/89
Pressure Seal Bonnet Check
Valves Caused By Vertical
Misalignment ofODisk
Failure of Borg-Warner Gate
8/30/89
Valves to Close Against
Differential Pressure
Licensee Report of Defective 8/22/89
Refurbished Valves
Maintenance of Teletherapy
8/18/89 Units
Suppliers of Potentially
8/16/89
Misrepresented Fasteners
Disablement of Turbine-Driven 8/3/89
Aux1liary Feedwater Pump Due
to Closure of One of the
Parallel Steam Supply Valves
Unqualified Electrical
7/26/89 Splices in Vendor-Supplied
Environmentally Qualified
Equipment
Issued to
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
or CPs for nuclear
power reactors.
All NRC Medical
Teletherapy Licensees.
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
All holders of OLs
or CPs for nuclear
power reactors.
Clwewlonslblt
r_.llst-swA
ss- F
§ .
..
rsTrTs<<non- x>ous
of Material Supplied to the
Oefense Department by Nuclear
Supp iers
Au noucers Or ULS
or CPs for nuclear
power reactors.
- Operating License
- Construction Permit
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
OFFICIAL BUSINESS
PENALTY FOR PRIVATE USE, $300
FIRST CLASS MAIL
POSTAGE & FEES PAID
PERMIT No. 0-67
IN 89-64 x
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September 7, 1989 (5) Browns Ferry Unit 2 (Licensee Event Report No.89-008)
On March 9, 1989, a bus fault to ground occurred on the secondary (4160-volt)
side of the unit station service transformer, causing an engineered safety
feature actuation. This electrical fault also caused damage to the 4160-volt
bus, the bus duct, and the transformer bushing connections on all three phases.
The fault was attributed to (a) deterioration of the Noryl insulation above the
bus joint, (b) poor design of the bus duct, which allowed condensation to collect, and (c) inadequate implementation of the vendor-recommended preventive maintenance.
Discussion:
Failures of medium-voltage electrical bus bars, principally involving 4160-
and 6900-volt ac buses, have resulted in bus bar electrical faults and fires, electrical power system undervoltage conditions, plant transients, reactor
trips, and engineered safety feature actuations.
Failure of the bus bars has been attributed to cracked bus bar insulation (bus
sleeving) combined with the accumulation of moisture or debris in the bus bar
housings.
Insulation failure, along with the presence of moisture or debris, provided undesired phase to phase, or phase to ground, electrical tracking
paths, which resulted In catastrophic failure of the buses.
Corrective actions taken by the involved utilities included replacing damaged
bus bar sections with bus bars that were covered with insulation of a different
type, substituting "yellow" bus bar joint compound (GE # D50H109) for the "black"
joint compound previously used, modifying bus bar enclosures to restrict ingress
and accumulation of water and debris, and instituting enhanced periodic inspections
and cleaning of bus bars and their housings.
This information notice requires no specific action or written response. If
you have any question about the information in this notice, please contact one
of the technical contacts listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Mark Padovan, AEOD
(301) 492-4445
Argil Toalston, NRR
(301) 492-0831 Attachment: List of Recently Issued NRC Information Notices
- SEE PREVIOUS CONCURRENCES
- C/OGCB:DOEA:NRR*RPB:ARM
(CuR~IsT~ 7 CHBerlinger
TechEd
08 //8
08/31/89
08/30/89
- OGCB:DOEA:NRR *SELB:DEST:NRR*C/SELB:DEST:NRR *ROAB:DSP:AEOD
- C/ROAB:DSP:AEOD
PCWen
ALToalston
FRosa
LMPadovan
JERosenthal
08/29/89
08/30/89
08/30/89
08/29/89
08/31/89
IN 89-XX
August xx, 1989
Y. vbus insulation at the support blocks, and startup bus 1B was declared in- operable.
(5) Browns Ferry Unit 2 (Licensee Event Report No.89-008)
On March 9, 1989, a bus fault to ground occurred on the secondary (4160-volt)
side of the unit station service transformer, causing an engineered safety
feature actuation. This electrical fault also caused damage to the 4160-volt
bus, the bus duct, and the transformer bushing connections on all three phases.
The fault was attributed to (a) deterioration of the Noryl insulation above the
bus joint, (b) poor design of the bus duct, which allowed condensation to
collect, and (c) inadequate implementation of the vendor-recommended preventive
maintenance.
Discussion:
Failures of medium-voltage electrical
6900-volt ac buses, have resulted in
electrical power system undervoltage
trips, and engineered safety feature
bus bars, principally involving 4160- and
bus bar electrical faults and fires, conditions, plant transients, reactor
actuations.
Failure of the bus bars has been attributed to cracked bus bar insulation (bus
sleeving) combined with the accumulation of moisture or debris in the bus bar
housings.
Insulation failure, along with the presence of moisture or debris, provided undesired phase to phase, or phase to ground, electrical tracking
paths, which resulted in catastrophic failure of the buses.
Corrective actions taken by the involved utilities included replacing damaged
bus bar sections with bus bars that were covered with insulation of a different
type, substituting "yellow' bus bar joint compound (GE # D50H109) for the
"black" joint compound previously used, modifying bus bar enclosures to re- strict ingress and accumulation of water and debris, and instituting enhanced
periodic inspections and cleaning of bus bars and their housings.
This information notice requires no specific action or written response. If
you have any question about the information in this notice, please contact the
technical contacts listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts:
Mark Padovan, AEOD
(301) 492-4445
Argil Toalston, NRR
(301) 492-0831 Attachment:
- SEE PREVIOUS
- OGCB:DOEA:NF
PCWen
08/29/89 List of Recently Issued NRC InformatioL
X ces
S CONCURRENCES
D/DOEA:NRR
C/
EA:NRR*RPB:ARM
CERossi
CHBerlinger
TechEd
08/ /89
08/31/89
08/30/89 RR *SELB:DEST:NRR*C/SELB:DEST:NRR *ROAB:DSP:AEOD
C/ROAB:DSP
ALToalston
FRosa
LMPadovan
JERosentha
08/30/89
08/30/89
08/29/89
08/3S/89
IN 89-XX
August xx, 1989
a
.
_j e5) Browns Ferry Unit 2 (Licensee Event Report No.89-008)
On March 9, 1989, a bus fault to ground occurred on the secondary (4160-volt)
side of the unit station service transformer, causing an engineered safety
feature actuation. This electrical fault also caused damage to the 4160-volt
bus, the bus duct, and the transformer bushing connections on all three phases.
Inadequate insulation and the collection of condensation led to the deteriora- tion of the Noryl sleeving at the factory-cut end. The deterioration extended
appropriately 1/4 inch above the bus flex connector.
The fault was attributed to (a) inadequate insulation above the bus joint, JbJ poor design of the bus duct, which allowed condensation to collect, and
c inadequate implementation of the vendor-recommended preventative main- tenance.
Discussion:
Failures of medium-voltage electrical bus bars, principally involving 4160- and
6900-volt ac buses, have resulted in bus bar electrical faults and fires, electrical power system undervoltage conditions, plant transients, reactor
trips, and engineered safety feature actuations.
Failure of the bus bars has been attributed to cracked bus bar insulation (bus
sleeving) combined with the accumulation of moisture or debris in the bus bar
housings.
Insulation failure, along with the presence of moisture or debris, provided undesired phase to phase, or phase to ground, electrical tracking
paths, which resulted in catastrophic failure of the buses.
Corrective actions taken by the involved utilities included replacing damaged
bus bar sections with bus bars that were covered with insulation of a different
type, substituting "yellow" bus bar Joint compound (GE $ D50H109) for the
"black' joint compound previously used, modifying bus bar enclosures to re- strict ingress and accumulation of water and debris, and instituting enhanced
periodic inspections and cleaning of bus bars and their housings.
This information notice requires no specific action or written response. If
you have any question about the information in this notice, please contact the
technical contacts listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Mark Padovan, AEOD
(301) 492-4445
Argil Toalston, NRR
(301) 492-0831 Attachment: List of Recently Issued NRC Information Notices
D/DOEA:NRR
C/OGCB:DOEA:NRR RPB:ARM
/
CERossi
CHBerlinger
TechEd RS
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08/
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08/ /89
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C/ROAB:DSP:AEOD
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