Information Notice No. 91-29: Deficiencies Identified During Electrical Distribution System Functional Inspections

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                               April 15, 1991

                                   DISTRIBUTION SYSTEM FUNCTIONAL 


All holders of operating licenses or construction permits for nuclear power 


This information notice is intended to alert addressees to deficiencies 
identified during recently performed electrical distribution system 
functional inspections (EDSFIs).  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:

During multidisciplinary inspections, the U.S. Nuclear Regulatory Commission 
(NRC) has identified many deficiencies related to the electrical 
distribution system.  To address these deficiencies, the NRC has developed 
an inspection to specifically evaluate the electrical distribution system.  
During the last year, the NRC completed eight EDSFIs, performing at least 
one in each of the five NRC geographical regions.  During these inspections, 
the staff found several common deficiencies in the licensees' programs and 
in the electrical distribution systems as designed and configured at each 
plant.  These deficiencies included inadequate ac voltages at the 480 Vac 
and 120 Vac distribution levels, inadequate procedures to test circuit 
breakers, and inadequate determinations and evaluations of setpoints.  


Inadequate Voltage

During inspections conducted at the San Onofre Nuclear Generating Station 
(Inspection Reports 50-361/89-200 and 50-362/89-200), the Susquehanna Steam 
Electric Station (Inspection Reports 50-387/90-200 and 50-388/90-200), and 


                                                            IN 91-29
                                                            April 15, 1991
                                                            Page 2 of 4

Waterford Steam Electric Station (Inspection Report 50-382/90-23), the staff 
found that, under certain conditions, the voltage available at the safety 
buses would be inadequate to operate safety-related loads and associated 
equipment.  These conditions could occur when the plant's electrical 
distribution systems were being supplied from an offsite grid that had 
become degraded but that continued to supply voltages that remained above 
the setpoints at which the degraded grid relays would be activated.  

At San Onofre, the numerous voltage drops throughout the system could cause 
the voltage to 120 Vac contactors to drop below the voltage range for which 
these contactors were certified as acceptable by their manufacturers.  To 
resolve this problem the licensee has performed testing that established a 
lower acceptable voltage range for this equipment.  

At Susquehanna, the degraded grid relay setpoint was set at 84 percent on 
the 4160 Vac buses.  This setpoint could have allowed inadequate voltage to 
be delivered to numerous safety-related loads at both the 480 Vac and 
120 Vac levels.  The licensee calculated that setpoints of at least 
93 percent would be required to ensure adequate voltage to all 
safety-related equipment.  To resolve this problem, the licensee has raised 
the setpoints of the degraded grid relays and has initiated a modification 
to install 120 Vac regulating transformers.  

At the Waterford nuclear plant, the degraded grid relays were set at 
87.5 percent as sensed on the 4160 Vac buses.  Although this setting was 
found to be marginally adequate for equipment connected to the 4160 Vac 
buses, the setting was too low to ensure that adequate voltage would be 
maintained at the 480 Vac and 120 Vac levels.  The licensee is evaluating 
solutions to this problem and has instituted a procedure to manually 
separate from the grid if the voltage falls to unacceptable levels but 
remains above the relay setpoints.  

The function of the degraded grid relays is to ensure that adequate voltage 
is available to operate all Class 1E loads at all distribution levels.  In 
order to ensure that all required Class 1E loads will remain operable during 
degraded voltage conditions, some licensees are currently reanalyzing the 
basis for the degraded grid relay setpoints.  The new analyses consider all 
required Class 1E loads and include the assumption that the voltage on the 
buses being sensed by these relays can remain at a level just above the 
relay setpoints.  

Inadequate Circuit Breaker Testing Procedures

During recent EDSFIs and previously on other NRC inspections, the staff 
identified repetitive deficiencies in licensees' programs to test circuit 
breakers.  These deficiencies included inadequate procedures, inadequate 
test acceptance criteria, inadequate test equipment, and inadequate control 
of testing.  At the Susquehanna plant (Inspection Reports 50-387/90-200 and 
50-388/90-200), the staff found that the licensee was testing dc molded case 
circuit breakers with a procedure written for testing ac breakers.  The 
licensee had not established specific acceptance criteria for the dc 


                                                            IN 91-29
                                                            April 15, 1991
                                                            Page 3 of 4

At the Shearon Harris Nuclear Power Plant (Inspection Report 50-400/90-200), 
the staff identified that the licensee was testing molded case circuit 
breakers against National Electrical Manufacturers Association (NEMA) 
acceptance criteria, which were less stringent than the manufacturer's 
time-current curves for these breakers.  In addition, the staff identified 
that the licensee was testing 480 V air circuit breakers with test equipment 
that applied a test signal (secondary injection) to the solid state trip 
units in the circuit breakers.  Although this method adequately verified 
that the trip units functioned properly, it did not verify that the 
breaker's current transformer functioned properly or that the wiring and 
connectors were adequate between the current transformer and the solid state 
trip units.  Other licensees have performed testing by primary current 
injection or by individually checking each component of the circuit breaker, 
including the wiring and connectors to demonstrate that the circuit breaker 
functions properly.  

Inadequate Determinations and Evaluations of Setpoints

Many of the findings identified during recent EDSFIs were related to 
inadequate setpoint determinations.  Useful guidance for determining 
setpoints is provided by Instrument Society of America Standard 67.04-1982, 
"Setpoints for Nuclear Safety-Related Instrumentation used in Nuclear Power 
Plants," which the NRC staff has endorsed by Regulatory Guide 1.105 Revision 
2 - 1986, "Instrument Setpoints for Safety-Related Systems."  This guidance 
applies both to process instrumentation and, in part, to certain relays that 
perform a safety function.  

Some licensees have operated equipment outside of acceptable limits because 
they did not determine proper setpoints and did not evaluate and account for 
instrument drift.  Operating the equipment under these conditions could 
compromise the safety functions of the equipment.  The staff has identified 
these circumstances primarily for those instruments in which the licensee 
has determined the setpoints as opposed to those instruments for which the 
setpoints were determined by the architect/engineer or the nuclear safety 
system supplier.  Those setpoints not contained in the plant technical 
specifi-cations were also more frequently found to be deficient.  During 
recent EDSFIs, the staff identified deficiencies in setpoints for diesel day 
tank level indicators, diesel air start compressor controllers and alarms, 
inverter low voltage shutdown circuitry, degraded grid relays, and diesel 
overcurrent relays.  

In addition to identifying deficiencies in the setpoints themselves, the NRC 
has determined that not all the licensees are typically verifying that the 
magnitude of instrument drift that was assumed in the original setpoint 
calculation coincides with the magnitude of drift observed in the plant.  
Some licensees have corrected this deficiency by verifying the magnitude of 
instrument drift by trending, (comparing "as found" calibration data to 
previous "as left" data).  This is particularly important for the degraded 
grid relay setpoints, which often require very tight calibration tolerance 
bands, and for which very low drift values are often assumed.  


                                                            IN 91-29
                                                            April 15, 1991
                                                            Page 4 of 4

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
the technical contact listed below or the appropriate NRR project manager.  

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical Contact:  Jeffrey B. Jacobson, NRR
                    (301) 492-0996

Attachment:  List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Thursday, March 25, 2021