United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-65: Underrated Fuses Which May Adversely Affect Operation of Essential Electrical Equipment

                                                          SSINS No.: 6835  
                                                          IN 84-65         
                                                          August 16, 1984  

                               UNITED STATES  
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                           WASHINGTON, DC 20555  

                              August 16, 1984  

Information Notice No. 84-65:   UNDERRATED FUSES WHICH MAY ADVERSELY 
                                   AFFECT OPERATION OF ESSENTIAL ELECTRICAL 
                                   EQUIPMENT 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 

Purpose: 

This information notice is to inform OL and CP holders of a potential 
generic problem involving the use of certain fuses which have improper 
voltage ratings. Using improperly rated fuses may result in an electrical 
ground fault through the fuse. This ground fault condition could lead to 
losses of other essential electrical services needed for safe plant 
operation. The information contained in this notice does not constitute NRC 
requirements and no specific actions or written responses are required. 
However, recipients are expected to review the information for applicability 
to their facilities and take appropriate actions. 

Background: 

Nuclear power plants use dc power fuses to provide circuit isolation that 
protects vital load systems from faulted conditions in nonvital load 
systems. They also are used to provide circuit isolation between a faulted 
load and other subloads within a large power distribution system. 

Underwriters Laboratories (UL) Standard 198L (July 6, 1981) defines 
stringent test time constants to determine voltage and interrupting ratings 
of fuses for use in dc power circuits. Cartridge type fuses for industrial 
use at 125, 250, 300 and 600 volt dc and current ratings up to 600 amperes 
must be tested to the requirements of UL 198L to be UL listed. The 
acceptance values of these tests are described in the standard. UL 198L also 
requires that production fuses previously qualified to UL 198L be marked 
with the dc voltage rating and the dc interrupting rating. 

Coordination involving isolation and interrupting capabilities between 
electrical subsystems in a power plant can only be accomplished with proper 
circuit breaker and fuse applications. The time constant of fuses is an 
important parameter in this coordination. 



8408100021 
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                                                         IN 84-65         
                                                         August 16, 1984  
                                                         Page 2 of 2      

Discussion: 

On May 30, 1984, Washington Public Power System, Unit 2 (WNP-2) reported a 
deficiency in dc voltage ratings that could result in loss of vital buses 
because of an electrical fault in nonvital equipment. Identified fuse types 
FRN and TR-R were manufactured by Bussman and Gould-Showmut respectively. 
These fuses are rated for 250-V dc applications and were installed to 
isolate a vital 250-V dc bus from a nonvital electrical system under fault 
conditions. The licensee had found, through discussion with these vendors 
that in 1981, when testing these fuses to UL 198L Standard they determined 
that certain current ranges of the fuses failed to isolate (open) fault 
currents at 250V-dc in the time specified in the Standard. Fuses made by the
previously mentioned manufacturers having current ranges of 15-30 amperes 
and 70-100 amperes did not clear the circuit satisfactorily. 

The manufacturers have since revised their specification sheets to reflect 
new (lower) dc voltage ratings. (The voltage ratings were reduced from 250-V
dc to about 200-V dc for the affected fuses.) The fuses are of commercial 
grade and the manufacturers have not issued any other form of generic 
communication to inform the end-users of the change. 

The deficiency at WNP-2 was discovered by an engineer who was reviewing the 
specifications of the in-plant fuses against a revised specification sheet 
from the vendor. The concern as discussed, relates to a fault that occurs in
the nonvital portion of the circuit that could lead to failure of the fuse 
by arcing to ground, which may cause the vital bus supply feed breakers 
located "upstream" of the fuse to open resulting in loss of that complete 
vital bus. 

Because the fuses are used extensively by both PWR and BWR power plants, the
problem may have generic implications. 

The licensee of WNP 2 has replaced all affected fuses and fuse holders with 
proper rated components. 

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:  V. D. Thomas, IE 
                    (301) 492-4755 

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