Part 21 Report - 1997-111
ACCESSION #: 9701220315
CHARLES H. CRUSE Baltimore Gas and Electric Company
Vice President Calvert Cliffs Nuclear Power Plant
Nuclear Energy 1650 Calvert Cliffs Parkway
Lusby, Maryland 20657
410 495-4455
BGE
January 15, 1997
U. S. Nuclear Regulatory Commission
Washington, DC 20555
ATTENTION: Document Control Desk
SUBJECT: Calvert Cliffs Nuclear Power Plant
Unit Nos. 1 & 2; Docket Nos. 50-317 & 50-318
Failures of General Electric Type AMH-4.76-250 Circuit
Breakers
REFERENCE: (a) Letter from Mr. C. H. Cruse (BGE) to NRC Document
Control Desk, dated October 25, 1996, same subject
Enclosed please find a follow-up report to Reference (a). Calvert Cliffs
has experienced two failures of the General Electric type AMH-4.76-250
circuit breaker. Investigation revealed the breaker's manual trip
paddles had bent, thus preventing proper automatic or manual closure of
the breaker. We have concluded the bent trip paddles were caused by a
weak trip latch retaining spring. The weak spring allows the trip paddle
to impact against its buffer pad when the breaker trips. This impact
causes the trip paddle to bend after repeated breaker open/close cycles.
This problem is being reported in accordance with 10 CFR 21.21 (a)(2). A
defect has been identified with the trip latch reset springs for these
breakers that represents a potential challenge to the reliability of
these safety-related components to perform their intended safety
function.
Should you have questions regarding this matter, we will be pleased to
discuss them with you.
Very truly yours,
CHC/CDS/bjd
Attachment
Document Control Desk
January 15, 1997
Page 2
cc: D. A. Brune, Esquire H. J. Miller, NRC
J. E. Silberg, Esquire Resident Inspector, NRC
Director, Project Directorate I-1, NRC R. I. McLean, DNR
A. W. Dromerick, NRC J. H. Walter, PSC
ATTACHMENT (1)
10 CFR PART 21 INTERIM REPORT
CONCERNING BENT MANUAL TRIP PADDLES IN
GENERAL ELECTRIC MAGNE BLAST 4-kV CIRCUIT BREAKERS
Calvert Cliffs Nuclear Power Plant, Units 1 and 2
Docket Nos.: 50-317 and 50-318
(i) Name and Address of Individual Making Notification
C. H. Cruse, Vice President-Nuclear Energy
Baltimore Gas and Electric Company
Calvert Cliffs Nuclear Power Plant
1650 Calvert Cliffs Parkway
Lusby, MD 20657
(ii) Basic Component Affected:
General Electric Magne Blast Circuit Breakers Type AMH-4.76-250
(iii) Firm Supplying Components:
General Electric Nuclear Energy
6901 Elmwood Avenue
Philadelphia, PA 19142
(iv) Nature of the Potential Defect
During surveillance testing in June and July 1996, the plant
found two problems with General Electric type AMH-4.76-250
circuit breakers. In June, a Low Pressure Safety Injection
(LPSI) pump breaker failed to close during testing.
Investigation revealed that there was no gap between the manual
trip rod and the manual trip lever. The trip lever was found
bent. The bent trip lever prevented the trip latch from fully
rotating onto the stop pin; thus, there was a smaller than
normal trip latch "wipe" (i.e., area of contact). With this
reduced wipe, there is no guarantee of breaker closure. The
trip lever was replaced.
The function of the manual trip lever is to transmit the forces
from the manual trip rod to the trip shaft and facilitate
manual tripping of the breaker. The function of the manual
trip rod is to transmit forces from the breakers manual trip
push button to the manual trip lever.
In July, another LPSI pump breaker failed to close during
testing. In this case, investigation revealed that a support
"L" bracket attached to the manual trip lever had broken and
the trip lever was bent, with the same effect as in the first
event. The "L" bracket serves to provide additional support
and stiffness to the manual trip lever. As the breaker trips,
the manual trip lever quickly rotates until it comes in contact
with a buffer pad located on the breaker frame. This contact
creates a momentary bending stress in the manual trip lever and
the "L" bracket. Each manual trip lever has a reset spring
that is designed to minimize the force that the trip lever
contacts the
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ATTACHMENT (1)
10 CFR PART 21 INTERIM REPORT
CONCERNING BENT MANUAL TRIP PADDLES IN
GENERAL ELECTRIC MAGNE BLAST 4-kV CIRCUIT BREAKERS
buffer pad, thus reducing the bending stress on the manual trip
lever. The breaker was replaced with a spare breaker. The
broken parts were removed from the breaker and sent to the
Baltimore Gas and Electric Company metallurgical laboratory for
analysis.
We conducted metallurgical analyses of the broken "L" bracket
and bent trip levers. Each of these parts is made of low
carbon steel. They appear to have failed due to "strain-aging
embrittlement," a phenomenon that affects hot-rolled low carbon
steel parts fabricated by cold bending. We performed an
operability evaluation for the rest of the breakers based, in
part, on these results.
After the second failure, an inspection program was initiated
for all 4 kV safety-related breakers to look for the broken "L"
bracket problem. Just prior to the start of this inspection
program, cracks were found in the "L" brackets in two other
breakers. The inspection program was accelerated and all
breakers were checked for the cracking identified on the "L"
brackets. During this inspection, two other breakers were
found with no gap between their manual trip lever and manual
trip shaft (one was the breaker which had failed in June; its
replacement lever had also bent). A method was developed to
verify operability of the safety-related breakers by insuring a
gap existed between the manual trip lever and the manual trip
rod after each breaker operation. The failed breaker was sent
offsite to the vendor for detailed testing and root cause
analysis.
Based on testing performed, the vendor has concluded that the
most probable cause was insufficient trip latch reset spring
force caused by either incorrect or damaged springs originally
being installed. Baltimore Gas and Electric Company believes
that it is also possible the springs suffer from age-related
degradation.
We sent the breaker from the July 1996 LPSI pump breaker
failure to the breaker manufacturer to determine the cause for
the failure to maintain the gap between the manual trip rod and
the trip paddle. A new trip paddle assembly was installed on
the breaker, and the gap adjusted to 1/4 inch. The breaker was
then cycled electronically.
After 30 operations, the 1/4 inch gap had reduced to 1/16 inch.
Continued operation showed the gap varied to between 0.015 and
0.060 inches. The total number of operations was 200.
The causes of the gap reduction was determined to be bending of
the trip paddle. The cause of the trip paddle bending was its
impact against the buffer pad (breaker frame) in one direction,
and then rebounding against the trip rod in the other.
The trip latch was disassembled, and a new reset spring was
installed. The trip paddle to trip rod gap was adjusted to 0.
115 inches and the breaker was cycled 110 times with no
resulting change in the gap. The original spring was then
tested and found to develop less than the minimum specified
forces.
These breakers have no difficulty in opening. The problem is
that they could lose their closing ability. Since Calvert
Cliffs uses many of these breakers in safety-related
applications, this could challenge the reliability of safety-
related components to perform as designed.
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ATTACHMENT (1)
10 CFR PART 21 INTERIM REPORT
CONCERNING BENT MANUAL TRIP PADDLES IN
GENERAL ELECTRIC MAGNE BLAST 4-kV CIRCUIT BREAKERS
(v) Date on Which Potential Defect Was Identified
We determined on August 27, 1996 that this breaker problem
could potentially represent a challenge to the reliability of
safety-related components to perform their intended safety
function. We issued an interim Part 21 notification on October
25, 1996, because we suspected this problem was a challenge to
the reliability of safety-related equipment that would
ultimately be tied to a defect a non-conforming condition.
(vi) Number and Location of Components
Calvert Cliffs has approximately 120 of the General Electric
type AMH-4.76-250 circuit breakers on site in safety-related
and non-safety-related applications.
(vii) Corrective Actions Taken
We established a team to evaluate and determine the causes of
the breaker problems after the discovery of the second breaker
failure in July 1996.
The vendor has recommended a modification to the breakers to
prevent additional cases of trip lever bending and "L" bracket
failures. We have completed this recommended modification on
the plant's General Electric Magne Blast 4 kV breakers. The
modification includes replacing the trip paddles, support
bracket and the spring discharge link. The trip lever material
was changed from AISI 1005 Carbon Steel to AISI 1018 Carbon
Steel. The "L" brackets changed from AISI 1005 steel to
aluminum. In addition, the configuration of the parts was to
be changed. This modification was designed and tested by the
vendor to ensure it eliminated the bending problem prior to
recommendation. This modification was tested at the vendor,
and we believe the breakers will continue to be operable with
this modification installed for the long-term.
Replacement of the reset spring is not a normal preventative
maintenance or overhaul activity. The breakers were not
designed to have this spring replaced. Replacement requires a
modification to the breaker that would include cutting a V-
notch in an angle support to remove the trip shaft.
Over the next two to three years, we plan to either replace the
reset springs during overhauls or replace the breaker. We are
confident that the trip paddle modification will maintain these
breakers in an operable condition until the springs or breakers
are replaced.
We have not experienced a similar problem with any other
breakers at Calvert Cliffs.
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