United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-68: Diesel Generator Failure at Calvert Cliffs Nuclear Station Unit 1

                                                           SSINS No.: 6835 
                                                            IN 85-68        

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                               August 14, 1985

Information Notice No. 85-68:   DIESEL GENERATOR FAILURE AT CALVERT 
                                   CLIFFS NUCLEAR STATION UNIT 1 


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


This information notice is provided to alert licensees of a potentially 
significant safety problem involving cracked interpolar connecting bars that
connect the damper circuit of each rotor pole to the damper circuit of the 
adjacent rotor pole of the emergency diesel generator,. 

It is suggested that recipients review this information for applicability to
their facilities and consider actions, if appropriate, to preclude similar 
problems at their facilities. However, suggestions contained in this 
information notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

On May 14, 1985, Calvert Cliffs Unit 1 was shut down for refueling. An 
overspeed test on the No. 11 emergency diesel generator (EDG) was initiated 
with the generator not connected to its essential bus. During the test, 
plant personnel who were stationed in the proximity of the EDG* heard loud, 
metallic rapping sounds coming from the generator section of the EDG and 
immediately terminated the overspeed test. Examination of the generator 
revealed that insulation on the stator windings had been rubbed and abraded 
to the point where one stator winding had been exposed. Further examination 
determined the cause of the failure to be a broken interpolar connecting bar
on the rotor. The bar initially had broken free on one end and damaged the 
stator windings. Because the generator was not connected to its essential 
bus, electrical field 

*Calvert Cliffs is a 2-unit PWR station with three EDGs that are designated 
as follows: Unit 1 (EDG 11), Unit 2 (EDG 21), and shared (EDG 12). 


                                                            IN 85-68       
                                                            August 14, 1985 
                                                            Page 2 of 3    

excitation had not been applied and no electric arcing occurred when the 
stator windings were damaged. The damaged generator was replaced with a 
spare generator and a detailed investigation was undertaken involving the 
vendor, Louis Allis, and the licensee. 

A metallurgical analysis on the failed interpolar connecting bar determined 
the predominant cause of failure to be high stresses resulting from periodic
centrifugal loading. To a lesser degree, bending of the connecting bar, 
during initial installation, and thermal expansion also were considered to 
be contributing factors in the failure. The licensee reports that the 
analysis indicated the problem was of a design nature and not the result of 
a material defect. Radiographic testing on the damaged generator showed 
several cracks in the remaining interconnecting bars. 

The investigation into the generator design determined that the interpolar 
connections between rotor poles are not necessary (a) if the EDGs are not 
operated in parallel, which could cause power pulsations between units, and 
(b) if the plant is operating with a balanced three-phase electrical load. 
Calvert Cliffs does not operate its EDGs in parallel with each other. 
Analysis shows that the potential three-phase electrical load unbalance 
factor (i.e., single-phase load) does not exceed 10% of the emergency 
three-phase load; therefore, it is not considered a concern. Consequently, 
the licensee initiated a program to remove the interpolar connecting bars 
from the three EDGs in service at Calvert Cliffs Units 1 and 2. 

On the basis of these findings, the licensee removed EDG No. 21 from Unit 2 
service on May 26, 1985 to determine if the interpolar connections on that 
EDG were degraded in a similar manner. Test results indicated cracks were 
evident. On this basis, the EDG No. 12 (which applies emergency power to 
either Units 1 or 2) was declared inoperable. Under this set of conditions, 
Calvert Cliffs Unit 2, which was operating at 100% full power, started 
shutting down. In order to continue power operation of Unit 2, the 
replacement EDG from Unit 1 was connected and aligned to Unit 2 to provide 
the necessary emergency power source and the shutdown was terminated. 

The licensee's corrective action, to terminate the limiting condition for 
operation, discussed above, involved the immediate removal of the interpolar
connections from EDGs 12 and 21. Postmodification qualification testing, 
conducted on both EDGs, proved to be satisfactory. 

The licensee's longer term corrective action plans are to remove the 
interpolar connections from the replacement (spare) EDG. 


Typically, a diesel generator that employs the continuous damper circuit 
design (i.e. , using interpolar connecting bars) uses 16 connecting bars. 
These bars are installed so that each of the eight rotor poles on the 
generator has two connecting bars, one installed on the front and one on the
back of each rotor pole. Noncontinuous damper circuit design does not employ
interpolar connections between the damper circuits on the rotor poles. 

                                                            IN 85-68       
                                                            August 14, 1985 
                                                            Page 3 of 3    

The safety concern of this event is the common cause failure mechanism if 
similar cracked connecting bars exist on all diesel generators at a nuclear 
power plant (as was the case at Calvert Cliffs). Cracked connecting bars can
lead to a condition that adversely affects the operating voltages which are 
necessary to operate essential equipment during accident conditions. 

Because of the generic implications of the Calvert Cliffs event, the vendor,
Louis Allis, issued a 10 CFR 21 report on the potential problem to the NRC 
on May 21, 1985. A followup letter, dated May 29, 1985, from Louis Allis to 
the NRC, briefly discussed the problem and identified other facilities that 
use similar Louis Allis generator units. Colt Industries also forwarded a 
Louis Allis report dated June 3, 1985, that identified the major cause for 
interpolar connecting bars cracking and provided a basis for requesting 
removal of the interconnecting bars from similarly designed generators in 
service at other facilities. The June 3, 1985 report from Louis Allis to 
Colt Industries also was sent to end-users of the Louis Allis generator 

As a result of the early notifications discussed above, the licensees of 
TMI/1, Vermont Yankee, and Peach Bottom Units 2 and 3 all report that they 
have either removed the interpolar connections or have established plans to 
have them removed from the affected generator units in service at their 

Although no similar interpolar connecting bar failures have been reported, 
related to other generator units, generators supplied by manufacturers other
than Louis Allis may have similar design feature's. 

Additionally, licensees are reminded that if corrective actions are taken to
resolve observed degradation of interpolar connecting bars this action 
should be reported to the NRC using the existing reporting requirements 
specified in 10 CFR 50.72 and 10 CFR 50.73. 

No specific action or written response is required by this notice. If you 
have any questions about this matter, please contact the Regional 
Administrator of the appropriate Regional Office or this office. 

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

Technical Contact:  Vincent D. Thomas, IE
                    (301) 492-4755 

Attachment: List of Recently Issued IE Information Notices 
Page Last Reviewed/Updated Friday, May 22, 2015