Information Notice No. 89-64: Electrical Bus Bar Failures
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 7, 1989
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
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems. However,
suggestions 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 1E
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-S01 and S02 did not open immediately, resulting in excessive
currents being supplied by the unit auxiliary transformer (UAT) and the
subsequent rupture and ignition of the UAT. This caused the supply breakers
to buses E-NAN-S01 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-S01 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
attributed 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.
8908310301
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September 7, 1989
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(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)
conditions 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 associated turbine trip occurred as a result of an UV transient 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 1B 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.
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September 7, 1989
Page 3 of 3
(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
. Attachment
IN 89-64
September 7, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
______________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to_________
89-63 Possible Submergence of 9/5/89 All holders of OLs
Electrical Circuits Located or CPs for nuclear
Above the Flood Level Because power reactors.
of Water Intrusion and Lack
of Drainage
89-62 Malfunction of Borg-Warner 8/31/89 All holders of OLs
Pressure Seal Bonnet Check or CPs for nuclear
Valves Caused By Vertical power reactors.
Misalignment of Disk
89-61 Failure of Borg-Warner Gate 8/30/89 All holders of OLs
Valves to Close Against or CPs for nuclear
Differential Pressure power reactors.
88-48, Licensee Report of Defective 8/22/89 All holders of OLs
Supp. 2 Refurbished Valves or CPs for nuclear
power reactors.
89-60 Maintenance of Teletherapy 8/18/89 All NRC Medical
Units Teletherapy
Licensees.
89-59 Suppliers of Potentially 8/16/89 All holders of OLs
Misrepresented Fasteners or CPs for nuclear
power reactors.
89-58 Disablement of Turbine-Driven 8/3/89 All holders of OLs
Auxiliary Feedwater Pump Due or CPs for PWRs.
to Closure of One of the
Parallel Steam Supply Valves
89-57 Unqualified Electrical 7/26/89 All holders of OLs
Splices in Vendor-Supplied or CPs for nuclear
Environmentally Qualified power reactors.
Equipment
89-56 Questionable Certification 7/20/89 All holders of OLs
of Material Supplied to the or CPs for nuclear
Defense Department by Nuclear power reactors.
Suppliers
______________________________________________________________________________
OL = Operating License
CP = Construction Permit
..
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