United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-08: Component Failures Caused by Elevated DC Control Voltage

                                                            SSINS No.: 6835 
                                                            IN 83-08       

                                UNITED STATES
                           WASHINGTON, D.C. 20555
                                March 9, 1983

                                   CONTROL VOLTAGE 


All holders of a nuclear power reactor operating license (OL) or 
construction permit (CP). 


This information notice is provided as a notification of a potentially 
significant problem pertaining to premature degradation failure of 
equipment, caused by elevated DC control voltage in safety-related circuits.

Because of the potential safety significance and related generic 
implications of this problem, addressees are expected to review the 
information for applicability to their facilities. No specific action or 
response is required 

Description of Circumstances: 

The folloWing three events, covered in Licensee Event Reports (LERs), 
indicate problems in safety-related DC control circuits where equipment 
degraded prematurely and caused short circuits and control problems, 
apparently as a result of DC voltages that exceeded the design voltage. 

1.   On October 3, 1982, at the Trojan Nuclear Plant, the indicating lamp 
     socket associated with a control switch on a 125V DC system broke and 
     fell into the panel, shorting out associated control circuitry. This 
     caused the fuse to blow, resulting in the loss of control power for the
     startup of an emergency diesel generator, and thus the diesel startup 
     capability was lost. 

     This event occurred as a result of an excess voltage condition which 
     led to thermal breakdown of the lamp socket. The 125V DC system 
     continuously subjected the lamp socket to its maximum rated voltage of 
     130V DC. In this case a higher than nominal operating voltage caused 
     the lamp socket to become brittle because excessive heat was generated 
     and also necessitated more frequent lamp replacement. 

     On the basis of the results of an engineering evaluation, two cells 
     were removed from each station battery; this reduced the system voltage 
     and battery charger output voltage. The licensee then conducted a load 
     profile test to demonstrate adequate ampere-hour capacity for the 
     design load. The bus voltage did not drop below the minimum acceptable 


                                                            IN 83-08      
                                                            March 09, 1983 
                                                            Page 2 of 2   

2.   On August 30, 1982, during preoperational testing of hydrogen monitors 
     at the Fort Calhoun Nuclear Plant, a control room operator noticed that
     two associated containment isolation valves had no position indication.
     One of the solenoid valves had an internally shorted coil; the other 
     valve had a coil shorted to ground. These shorts blew the fuse. The 
     blown fuse caused the loss of valve position indication and allowed the
     valves to fail open. Followup evaluation indicated that the solenoid 
     valves were intended for service at 125V DC +/- 10 percent. During the 
     month preceding the failures, the station batteries had been placed on 
     an equalizing charge of 140V DC. The licensee concluded that this 
     elevated voltage caused the valves to fail since they remained 
     energized for the entire period of time. 

     The licensee intends to replace all solenoid valve coils with coils 
     designed to operate at a higher voltage. 

3.   On March 15, 1982, at the Zion Nuclear Generating Station, a relay coil
     in a safety related reactor trip relay burned up and failed in a 
     nonconservative mode. The failed relay coil was a replacement and had 
     been rated at 120V DC; whereas the original relay coil had been rated 
     at 125/130V DC. The licensee concluded that the replaced relay coil 
     failed as the result of overheating that had been caused by five years 
     of operation at elevated voltage. During a follow-up evaluation, five 
     additional relays with 120V DC coil rating in a 130V DC system were 
     identified. These relays were replaced. 

These events show that DC safety-related control components and indicating 
circuit components which operate for a sustained period of time at elevated 
voltages or voltages above their rated design voltage are subject to 
accelerated degradation which may have some impact on plant safety. A 
careful balance of rated voltage for components in DC systems must be 
maintained to assure maximum voltage during equalizing charging doesn't 
adversely affect components and that those components which are required to 
function in an emergency remain operable at minimum battery voltages at 
design ampere-hour capacity. 

No written response to this notice is required. If you have any questions 
regarding this matter, please contact the Regional Administrator of the 
appropriate NRC Regional Office, or this office. 

                                   Edward L. Jordan Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  W. Laudan

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