United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-86, Supplement 1: Operating with Multiple Grounds in Direct Current Distribution Systems

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

                                 March 31, 1989


Information Notice No. 88-86, SUPPLEMENT 1:  OPERATING WITH MULTIPLE 
                                                 GROUNDS IN DIRECT CURRENT 
                                                 DISTRIBUTION SYSTEMS


Addressees:

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

Purpose:

This supplement to Information Notice No. 88-86, "Operating With Multiple 
Grounds in Direct Current Distribution Systems," (IN 88-86) is provided to 
alert addressees to a failure mode in which direct current (dc) grounds can 
cause equipment to fail in the energized state.  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.

Background:

IN 88-86 was issued October 21, 1988, to alert addressees to potential 
problems caused by plants operating with ground(s) in vital dc distribution 
systems (which are normally ungrounded systems).

Description of Circumstances: 

A recent NRC Region II Operational Safety Team Inspection at the Virgil C. 
Summer Nuclear Station (Inspection Report 50-395/88026) identified a pre-
viously unidentified potential failure mode.  In this failure mode, dc grounds
can cause equipment to fail in the energized state instead of the deenergized 
state necessary to satisfy protection system performance.

Attachment 1 shows a solenoid for an ASCO valve with a ground fault on the 
positive side.  The circuit for this solenoid contains only one actuation 
contact which is located on the positive side of the solenoid.  Also shown is 
the licensee's installed ground detection system which consists of indicating 
lights in the control room and at the battery chargers.  A complete circuit 
exists through the solenoid and the ground detector, assuming a ground on the 
positive side of the solenoid.




8903270348
.                                                       IN 88-86, Supplement 1
                                                       March 31, 1989
                                                       Page 2 of 3


These ASCO solenoids experience dropout at approximately 20 volts, pickup at 
approximately 40 volts, and require only milliamps of current for these ac-
tions.  A harsh environment in the area of ASCO solenoids that were not sealed
for environmental qualification (EQ) could result in steam intrusion into the 
solenoid.  A resulting decrease in solenoid insulation resistance could then 
generate an electrical leakage path between the positive side of the solenoid 
and the plant ground.  A ground fault on the positive side of the solenoid, or
anywhere between the solenoid and the actuation contact, coupled with a ground
on the positive leg of the dc buss of 4,000 ohms or less, could prevent the 
solenoid from dropping out or cause it to pick up.  Spurious, uncontrolled 
valve actuation could result.

Even with the ground detection system disabled and the dc buss resistance to 
ground maintained at greater than 5,000 ohms, the licensee determined that the
plant was still susceptible to this failure mode.  This conclusion was drawn 
because, in a high-energy line break/loss-of-coolant accident (HELB/LOCA) 
environment, the dc system was very likely to become grounded because of the 
many non-EQ portions of the system such as terminal blocks, limit switches, 
and solenoid valves.  Buss ground faults in the range of about 100 ohms to 
4,000 ohms on the positive leg could occur for some periods of time.  These 
ground faults would not be large enough to blow fuses or trip breakers, but 
would be large enough to accommodate ASCO solenoids failing energized.  Thus, 
the licensee determined that the EQ of additional solenoids was required.

Other equipment besides ASCO solenoids may have circuits that could fail 
energized.  The licensee identified the main turbine electro-hydraulic control
system as a potential example.  The licensee's policy of maintaining greater 
than 5,000 ohms of resistance to ground on the positive leg and ensuring 
adequate equipment EQ should prevent such failures.

Discussion:

The importance of the dc power system to plant safety is discussed in IN 
88-86.  The safe recovery from various accident situations is dependent on the
availability and reliability of the battery and the dc power distribution 
system.  Licensees may have performed safety evaluations that assumed an 
ungrounded, vital dc system and specific equipment failure modes during harsh 
environments that may no longer be valid because of possible multiple grounds.
In addition, the use of installed ground detectors and portable ground 
locating equipment and the policies on operating the facility with grounds on 
vital dc systems may not maintain a minimum threshold resistance to ground for
dc power systems above which predicted system/component operation is ensured.

As noted, the licensee's evaluation determined that a HELB/LOCA event could 
cause common mode failures of the type described.  As a result of this deter-
mination, the licensee

1.   Environmentally qualified approximately 90 solenoids that actuate 
     approximately 30 valves.  The affected valves included all pressurizer 
     power-operated relief valves, main steam isolation valves, feedwater 
     isolation valves, reactor building cooling unit bypass dampers, reactor 
     building purge valves, and other containment isolation valves.  These 
     items 
     .                                                       IN 88-86, Supplement 1
                                                       March 31, 1989
                                                       Page 3 of 3


     previously had been exempted from EQ requirements because they had been 
     assumed to fail safe (i.e., deenergized).

2.   Established a policy on operating the facility with grounds that requires
     ground faults on the vital 125-volt dc busses to be promptly cleared if 
     they have a resistance of less than 5,000 ohms.

3.   Disabled the installed ground detection system on the vital 125-volt dc 
     busses.  These systems, as designed, caused a permanent ground of 1,500 
     ohms on both the positive and negative legs of the dc busses.

4.   Began pursuing the installation of a better ground detection system and 
     the purchase of state-of-the-art portable ground locating equipment.

5.   Planned to conduct further evaluations and to consider additional modifi-
     cations, including converting to a grounded dc system.

6.   Issued Licensee Event Report (LER) 50-395/88-12 on this subject.

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 techni-
cal 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:  Robert P. Schin, Region II
                     (404) 331-3911

                     Frederick H. Burrows, NRR
                     (301) 492-0833

Attachments:
1.  Diagram Description of Failure Mode
2.  List of Recently Issued NRC Information Notices
.                                                       Attachment 2 
                                                       IN 88-86, Supplement 1 
                                                       March 31, 1989 
                                                       Page 1 of 1


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

89-35          Loss and Theft of Un-         3/30/89        All U.S. NRC 
               secured Licensed Material                    byproduct, source 
                                                            and special 
                                                            nuclear material 
                                                            licensees. 

89-34          Disposal of Americium         3/30/89        All holders of an 
               Well-Logging Sources                         NRC specific 
                                                            license 
                                                            authorizing well-
                                                            logging 
                                                            activities. 

89-33          Potential Failure of          3/23/89        All holders of OLs
               Westinghouse Steam                           or CPs for PWRs. 
               Generator Tube 
               Mechanical Plugs 

89-32          Surveillance Testing          3/23/89        All holders of OLs
               of Low-Temperature                           or CPs for PWRs. 
               Overpressure-Protection 
               Systems 

89-31          Swelling and Cracking         3/22/89        All holders of OLs
               of Hafnium Control Rods                      or CPs for PWRs 
                                                            with Hafnium 
                                                            control rods. 

89-30          High Temperature              3/15/89        All holders of OLs
               Environments at                              or CPs for nuclear
               Nuclear Power Plants                         power reactors. 

89-29          Potential Failure of          3/15/89        All holders of OLs
               ASEA Brown Boveri                            or CPs for nuclear
               Circuit Breakers                             power reactors. 
               During Seismic Event 

89-28          Weight and Center of          3/14/89        All holders of OLs
               Gravity Discrepancies                        or CPs for nuclear
               for Copes-Vulcan                             power reactors. 
               Air-Operated Valves 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013