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 . 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 Attachment: List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021