Information Notice No. 84-65: Underrated Fuses Which May Adversely Affect Operation of Essential Electrical Equipment
SSINS No.: 6835
August 16, 1984
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
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
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.
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.
August 16, 1984
Page 2 of 2
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
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
Attachment: List of Recently Issued IE Information Notices
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