United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-01: Improper Electrical Grounding Results in Simultaneous Fires in the Control Room and the Safe-Shutdown Equipment Room

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                                January 8, 1997


NRC INFORMATION NOTICE 97-01:  IMPROPER ELECTRICAL GROUNDING RESULTS IN 
                               SIMULTANEOUS FIRES IN THE CONTROL ROOM          
                               AND THE SAFE-SHUTDOWN EQUIPMENT ROOM 

Addressees

All holders of operating licenses or construction permits for nuclear power
reactors. 

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential component grounding problems that
could result in simultaneous fires.  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 are not NRC requirements; therefore, no
specific action or written response is required. 

Description of Circumstances

At approximately 5:00 p.m. on April 4, 1996, during a refueling outage, a
licensee firewatch detected smoke in the back panel area of the control room
at the Palo Verde Nuclear Generating Station, Unit 2.  Licensee operators
observed smoke emanating from the Train B emergency lighting uninterruptible
power supply (ELUPS) panel in the control room.  

The fire in the control room resulted in loss of power to Train B control room
emergency lighting circuits, some general plant essential lighting and plant
fire detection and alarm system panels.  Lighting powered by the unaffected
Train A lighting system was sufficient to operate the unit.  The circuit
breaker supplying power to the ELUPS panel tripped open when wiring insulation
in the conduit supplying the power supply panel melted and caused various
conductors to short circuit.  The circuit breaker trip also deenergized power
to the fire detection and alarm panels in the auxiliary building.  When
operators checked the control room fire alarm annunciator monitor, they noted
that a large number of fire detector trouble alarms were alarming and that the
alarms were scrolling on the monitor screen as a result of the deenergized
fire detection and alarm panels.  

The control room staff dispatched auxiliary operators to inspect their
assigned areas to check for possible fire conditions and additional problems. 
An auxiliary operator discovered smoke and fire in the Train B dc equipment
room on the 100-foot level of the auxiliary building.  The fire was located in
the 480/120-volt essential lighting isolation transformer.  The trouble alarms
resulting from the lost power supply masked the actual fire alarm in the Train
B dc equipment room.  

9701030090.                                                            IN 97-01
                                                            January 8, 1997
                                                            Page 2 of 3


The licensee's onsite fire department responded and all fires were
extinguished within a short period of time.  The licensee established the
required compensatory firewatches in areas with disabled fire detectors. 
Actual equipment damage was limited to the components involved in the fires.

Discussion

The licensee's root-cause investigation indicated that the core of the
regulating isolation transformer (located in the Train B dc equipment room)
failed and made contact with the transformer coils, causing a short circuit
fault to station ground through the transformer's panel ground.  The purpose
of the subject transformer is to isolate nonsafety-related portions of the
circuit from the safety-related function of the circuit.  The investigation
also determined that the neutral leg of the transformer had not been grounded. 
An alternate source of power for the essential lighting uninterruptible power
supply panel is provided by an inverter.  The system had been designed with
ground connections on the neutral leg of the inverter instead of grounding the
neutral leg of the power supply (regulating transformer) in accordance with
industry practice.  The neutral wiring conductors within the inverter and from
the inverter to the essential lighting distribution panel became the return
fault path to the regulating transformer.  These conductors were not designed
to handle the high fault currents to which they were subjected.  As a result,
these wires ignited under these high fault currents.  These components are
depicted in Figure 1.  The licensee also determined that the fires were
related and were caused by a design error in the electrical grounding, which
dated back to plant construction.  The licensee found similar grounding
arrangements in the other two  Palo Verde units.

The final safety analysis report for the facility documented that grounding
was accomplished in accordance with Institute of Electrical and Electronics
Engineers (IEEE) Standard 142-1982, "Grounding of Industrial and Commercial
Power Systems."  The transformer and equipment supplied by the transformer was
designed as a grounded system.  IEEE 142-1982 defined a grounded system as a
system of conductors in which at least one conductor or point (usually the
middle wire or neutral point of the transformer or the generator windings) is
intentionally grounded.  The licensee also indicated that the grounding design
for Palo Verde was in accordance with Bechtel Drawing 13-E-ZVG-007, "Grounding
Notes, Symbols, and Details," Revision 20.  The document did not require
grounding of the transformer's neutral leg.

The licensee's corrective actions included grounding the neutral leg of the
isolation transformer and fusing the output of the transformer to limit fault
propagation.  The licensee also removed the ground from the control room ELUPS
panel.  These modifications did not impact the isolation function of the
transformer.

The event was of concern because a single electrical fault caused simultaneous
fires in the control room and the Train B dc equipment room which supports
alternative post-fire safe shutdown capability in the event of a control room
fire.  This electrical design error is important because it created a fire
vulnerability in two separate areas of the plant.  The fire could have
resulted in operational challenges which are outside of the plant's design
basis 
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                                                            January 8, 1997
                                                            Page 3 of 3       


and the scope of the NRC fire protection regulations (10 CFR 50.48).  This
vulnerability was caused by the inadequate design of the grounding circuitry
from the electrical power supplies, which have been in service since the
original construction.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation project manager. 


                                          signed by D.B. Matthews

                                    Thomas T. Martin, Director
                                    Division of Reactor Program Management 
                                    Office of Nuclear Reactor Regulation 

Technical contacts:  Chris Vandenburgh, Region IV
                     (817) 860-8161
                     E-mail:  cav@nrc.gov

                     Phil Qualls, Walnut Creek Field Office
                     (510) 975-0245
                     E-mail:  pmq@nrc.gov

                     Ronaldo Jenkins, NRR
                     (301) 415-2985
                     E-mail:  rvj@nrc.gov

Attachments:  
1. Figure 1
2. List of Recently Issued NRC Information Notices  
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