United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-16: Excessive Voltage Drop in DC Systems

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

                                February 16, 1989



All holders of operating licenses or construction permits for nuclear power


This information notice is being provided to alert addressees to potential 
problems resulting from unexpected large voltage drops in cables between the 
batteries and circuit breakers.  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:

Nine Mile Point Unit 1 was shut down in December 1987 for refueling.  During 
this shutdown, the licensee made modifications to its 125-Vdc power system.  A 
sub-sequent review of these modifications led the licensee to conclude that 
there was a voltage supply problem associated with the operation of battery 
charger (MG set) circuit breakers.  The manufacturer of these circuit breakers 
stated that the minimum voltage for operation of the circuit breakers was 90 
volts.  The technical specifications require a minimum of 106 Vdc at the 
battery terminals for the batteries to be considered operable.  However, 
studies showed that with 114 Vdc at the battery terminals, the calculated 
voltage at the breaker would be less than 90 Vdc because of line losses due to 
current demand during breaker closure.  A battery voltage of less than 114 Vdc 
could exist at the time ac emergency power is required to be reconnected to 
the battery chargers.  Thus, if a loss of site ac power were to occur, the dc 
voltage at the battery charger circuit breakers may not be adequate for the 
battery chargers to be reconnected to emergency ac power.  This situation 
would cause the batteries to discharge.  The decreasing dc voltage would lead 
to degraded dc control power and a subsequent trip of the emergency diesel 
generators.  Therefore, a loss of normal ac power could lead to a loss of dc 
power and the subsequent loss of emergency ac power. 


Problems with electrical systems at commercial power reactors have been 
identified with increasing frequency.  These problems led to the issuance 

.                                                            IN 89-16
                                                            February 16, 1989
                                                            Page 2 of 2

of Generic Letter 88-15, "Electric Power Systems - Inadequate Control Over 
Design Processes," on September 12, 1988.  The above-described event is a 
unique illustration of one of the types of problems presented in the generic 

The Nine Mile Point Unit 1 licensee is not aware of any testing that could be 
used to establish the adequacy of the original design, nor has the licensee 
found documentation of the original design basis.  The voltage drop could be 
attributed to an inadequate original estimate of load, hence cable sizing; 
load creep over the years without assessment of the consequences; or use today 
of a potentially more conservative power profile as determined by current 
practice for battery sizing.  After identifying the problem, the licensee 
assessed the actual loads and plans to replace the undersized cable to improve 
the system's operability.

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:  J. Carter, NRR
                     (301) 492-1194

                     J. Lazevnick, NRR
                     (301) 492-0814

Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment 
                                                            IN 89-16
                                                            February 16, 1989
                                                            Page 1 of 1

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

89-15          Second Reactor Coolant Pump   2/16/89        All holders of OLs
               Shaft Failure at Crystal                     or CPs for nuclear
               River                                        power reactors.

89-14          Inadequate Dedication         2/16/89        All holders of OLs
               Process for Commercial                       or CPs for nuclear
               Grade Components Which                       power reactors. 
               Could Lead to Common Mode
               Failure of a Safety System

89-13          Alternative Waste Management  2/8/89         All holders of NRC
               Procedures in Case of Denial                 specific licenses.
               of Access to Low-Level Waste
               Disposal Sites

89-12          Dose Calibrator Quality       2/9/89         All NRC medical
               Control                                      licensees.

89-11          Failure of DC Motor-Operated  2/2/89         All holders of OLs
               Valves to Develop Rated                      or CPs for nuclear
               Torque Because of Improper                   power reactors.
               Cable Sizing

89-10          Undetected Installation       1/27/89        All holders of OLs
               Errors In Main Steam Line                    or CPs for BWRs.
               Pipe Tunnel Differential
               Temperature-Sensing Elements
               at Boiling Water Reactors.

89-09          Credit for Control Rods       1/26/89        All holders of OLs
               Without Scram Capability                     or CPs for test and
               in the Calculation of the                    research reactors.
               Shutdown Margin

89-08          Pump Damage Caused by         1/26/89        All holders of OLs
               Low-Flow Operation                           or CPs for nuclear
                                                            power reactors.

OL = Operating License
CP = Construction Permit 
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