United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-64: Electrical Bus Bar Failures

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                September 7, 1989

Information Notice No. 89-64:  ELECTRICAL BUS BAR FAILURES


All holders of operating licenses or construction permits for nuclear power 


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.  

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                                                            Page 2 of 3

(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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                                                            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 

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.


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 

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. 

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. 
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Friday, May 22, 2015