United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-86: Operating with Multiple Grounds in Direct Current Distribution Systems

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

                                October 21, 1988 

                                   DIRECT CURRENT DISTRIBUTION SYSTEMS


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to alert addressees to potential 
problems caused by plants operating with ground(s) in the vital direct current
(dc) distribution systems (which are normally ungrounded systems).  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 recent NRC maintenance inspections at Quad Cities (50-254/88011 and 
50-265/88012), Oconee (50-269/88-17, 50-270/88-17, and 50-287/88-17), and 
D.C. Cook (50-315/88016 and 50-316/88018) power reactor facilities, it was 
found that plants had been operating with multiple grounds in the dc distri-
bution systems for extended periods.  Specific examples are as follows:

(1)  Quad Cities Unit 2 operated for a significant period of time with known 
     grounds on the negative side of the 125-Vdc system.  During this period, 
     when a momentary ground occurred on the positive side of the circuit a 
     fuse in the auto-start circuitry of a diesel generator blew, disabling 
     the auto-start function of the diesel generator for approximately 6 
     months.  This condition was not properly recognized or evaluated by the 
     licensee; it was identified during an 18-month surveillance test.

(2)  Oconee Nuclear Station operated for approximately 4 months with the 
     125-Vdc system ground alarm activated.  However, during this period no 
     station procedure or shift surveillance requirements existed to verify 
     the operability of the ground detection system.  As a result of the 
     maintenance inspections, this ground detection alarm system was found to 
     be inoperable; it had not been calibrated since 1976.

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                                                            October 21, 1988 
                                                            Page 2 of 3 

(3)  D. C. Cook Unit 1 had a negative dc system ground for approximately 7 
     months before it was cleared.  A safety evaluation for continued 
     operation with the ground or other grounds that might be masked had never
     been performed until requested by the NRC.

The licensees of Quad Cities and D. C. Cook were issued notices of violation. 
Safety evaluations were performed to support continued operation while the 
grounds existed.  

A review of selected licensee event reports of dc system grounds (see Attach-
ment 1) indicates that plant equipment is frequently rendered inoperable or is
started unexpectedly because of grounds.


The dc power system provides control and power to safety-related valves, 
instrumentation, emergency diesel generators, and many other components and 
systems during all phases of plant operation, including abnormal shutdowns and
accident situations.   Because of its importance to plant safety, the dc 
system requires a high degree of reliability and availability.

Most nuclear power plant dc systems are two-wire ungrounded, combination 
battery/charger systems equipped with ground detection.  Typical ground 
detection system features include a remote annunciator and a local indicator 
and/or recorder.  Ground detectors are incorporated in the dc system so that 
if a single ground point does occur, immediate steps can be taken to clear the
ground fault from the system. 

Failure to respond to a single ground will mask subsequent grounds.  Multiple 
grounds can cause the indiscriminate operation of equipment, which may have 
safety consequences.  Grounds can cause control circuit fuses to fail and can 
render important safety equipment inoperable as previously described.  
Furthermore, batteries have a designed capacity to supply power during a 
station blackout condition, and this capacity can be affected by the presence 
of unanalyzed loads in the form of multiple grounds.  

It is recognized that troubleshooting and finding grounds on a dc system are 
difficult tasks that may affect plant operation.  The licensees previously 
mentioned have reviewed their designs and conditions for potential impact on 
safety system operability and have taken corrective actions to minimize the 
effect of grounds.  

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                                                            October 21, 1988 
                                                            Page 3 of 3

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the 
technical contacts listed below or the Regional Administrator of the 
appropriate regional office.  

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

Technical Contacts: Z. Falevits, Region III
                    (312) 790-5772
                    S. N. Saba, NRR
                    (301) 492-1052 

Attachments: 1.  Selected Licensee Event Reports
             2.  List of Recently Issued NRC Information Notices
.                                                            Attachment 1 
                                                            IN 88-86 
                                                            October 21, 1988 
                                                            Page 1 of 1 

                         Selected Licensee Event Reports

Event No.: 50-454/86-03 (Byron, Unit 1)

Abstract: A reactor trip and auxiliary feedwater actuation occurred.  The trip
          was caused by grounds on the positive and negative sides of the 125-
          Vdc bus.  The grounded circuit energized a fast-close solenoid on 
          the 1A main steam isolation valve (MSIV).  When the MSIV closed, a 
          reactor trip resulted from steam generator low-low level.

Event No.: 50-369/82-48 (McGuire, Unit 1)

Abstract: After the battery trouble alarm actuated, it was noticed that 
     neither the closed nor the open indication light was energized on the 
     pressurizer power-operated relief valve and the valve would not operate. 
     A ground that had blown a fuse in the control circuit was identified.

Event No.: 50-293/82-53/03L-0 (Pilgrim)

Abstract:  A ground fault alarm was observed on the 125-Vdc battery circuit. 
           The ground was traced to the limit switch circuitry for a motor-
           operated valve.  The high-pressure coolant injection system was 
           subsequently declared inoperable.

Event No.: 50-305/85-04 (Kewaunee)

Abstract:  A control room operator noticed the reactor coolant pump running 
           while the control switch indicated a green flag or off condition. 
           Subsequent investigation revealed that the pump had inadvertently 
           started because of a ground condition in the 125-Vdc actuation 
           circuitry associated with the 4.16 kV switchgear.
.                                                            Attachment 2 
                                                            IN 88-86 
                                                            October 21, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                             NRC INFORMATION NOTICES
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-85          Broken Retaining Block        10/14/88       All holders of OLs
               Studs on Anchor Darling                      or CPs for nuclear
               Check Valves                                 power reactors. 

88-84          Defective Motor Shaft         10/20/88       All holders of OLs
               Keys in Limitorque Motor                     or CPs for nuclear
               Actuators                                    power reactors. 

88-83          Inadequate Testing of Relay   10/19/88       All holders of OLs
               Contacts in Safety-Related                   or CPs for nuclear
               Logic Systems                                power reactors. 

88-82          Torus Shells with Corrosion   10/14/88       All holders of OLs
               and Degraded Coatings in                     or CPs for BWRs. 
               BWR Containments 

88-81          Failure of Amp Window         10/7/88        All holders of OLs
               Indent Kynar Splices                         or CPs for nuclear
               and Thomas and Betts                         power, test, and 
               Nylon Wire Caps During                       research reactors.
               Environmental Quali-
               fication Testing 

88-80          Unexpected Piping Movement    10/7/88        All holders of OLs
               Attributed to Thermal                        or CPs for PWRs. 

88-79          Misuse of Flashing Lights     l0/7/88        All holders of OLs
               for High Radiation Area                      or CPs for nuclear
               Controls                                     power reactors. 

88-69, Supp 1  Movable Contact Finger        9/29/88        All holders of OLs
               Binding in HFA Relays                        or CPs for nuclear
               Manufactured by General                      power reactors. 
               Electric (GE) 

88-78          Implementation of Revised     9/22/88        All holders of OLs
               NRC-Administered Requali-                    or CPs for nuclear
               fication Examinations                        power reactors. 
OL = Operating License
CP = Construction Permit 
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