United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 79-09: Failures of GE Type AK-2 Curcuit Breaker in Safety Related Systems

                               UNITED STATES 
                           WASHINGTON, D.C.  20555

                               April 17, 1979

                                                      IE Bulletin No. 79-09 


Description of Circumstances: 

Twelve failures of General Electric (GE) type AK-2 (i.e., AK-2A-15, 25, 50, 
75, or 100) Circuit Breakers installed in safety-related systems have been 
reported since 1975.  The failures occurred at the following facilities: 

     Date           Facility                 System

1.    9/16/78       Arkansas-1          Control Rod Drive System
2.    9/25/78       Arkansas-1          Control Rod Drive System
3.   10/17/78       Arkansas-1          Control Rod Drive System
4.    1/22/78       Crystal River-3     Control Rod Drive System
5.    8/7/75        Oconee Unit-3       Control Rod Drive System
6.    1/18/79       Oconee Unit-3       Control Rod Drive System
7.    1/22/79       Oconee Unit-1       Control Rod Drive System
8.    1/31/79       Oconee Unit-1       Control Rod Drive System
9.    4/25/75       TMI/1               Control Rod Drive System
10.  11/26/78       Oyster Creek-1      Containment Spray Pump
11.  11/30/78       Oyster Creek-1      Service Water Pump No. 1
12.  11/30/78       Oyster Creek-1      Service Water Pump No. 2

It is significant to note that during a loss-of-off-site power test on 
November 30, 1978, at Oyster Creek, both service water pump circuit breakers
failed to trip, as required.  The undervoltage relays which monitor voltage 
level on each emergency bus functioned properly but could not actuate the 
trip mechanism via the undervoltage trip device within each circuit breaker. 
These failures, in turn, created a potential overload condition on each 
emergency diesel generator unit by allowing simultaneous starting of 
multiple high horse power motors during sequential loading phase of the 

The causes for failure were attributed to either binding within the linkage 
mechanism of the undervoltage (UV) trip device and trip shaft assembly or 
out-of-adjustment conditions in the same linkage mechanism.  Babcock and 
Wilcox (B&W) and GE determined that the binding and out-of-adjustment 
resulted from inadequate preventive maintenance programs 

IE Bulletin No. 79-09                                       April 17, 1979 
                                                            Page 2 of 3 

at the affected operating facilities. In each case listed above, cleaning 
and relubricating the trip shaft mechanism within the circuit breaker was 
required to correct the problem. 

The enclosed GE Service Advice Letter (SAL) 175(CPDD) 9.3 is being sent to 
all power reactor facilities notifying them of the potential problems. The 
SAL identifies the causes for failure and provides their recommended 
corrective action. Similar notification of the problem has been issued by 
B&W to all specific B&W designed facilities. 

Action to be Taken by Licensees: 

For all power reactor facilities with an operating license or construction 

1.   Determine whether or not GE type AK-2 breakers are used or planned for 
     use in safety-related systems at your facility(ies). 

2.   If such circuit breakers are used or planned for use, identify the 
     safety system involved and provide in written form your plans for 
     developing a preventive maintenance program which will assure design 
     performance with the GE type AK-2 circuit breaker. 

3.   The program shall include as a minimum but not be limited to the 

     a.   Establish and adhere to a preventive maintenance schedule 
          regarding the subject circuit breakers. 

     b.   Have the maintenance performed by qualified personnel. The GE 
          power circuit breaker instruction manual should be used as 
          guidance in setting up the maintenance procedures. 

     c.   During the preventive maintenance, perform the recommended 
          corrective actions described in the enclosed GE Service Alert -
          Letter No. 175(CPDD) 9.3, dated April 2, 1979. A copy of this 
          letter is enclosed for your information and appropriate use. 

4.   For facilities with an operating license, a written report of the above
     actions, including the date(s) when they will be completed, shall be 
     submitted within 30 days of receipt of this Bulletin. 

5.   For facilities with a construction permit, a written report of the 
     above actions, including the date(s) when they will be completed, shall 
     be submitted within 60 days of receipt of this Bulletin. 

IE Bulletin No. 79-09                                       April 17, 1979 
                                                            Page 3 of 3 

Reports should be submitted to the Director of the appropriate NRC Regional 
Office. A copy of your report should be sent to the U.S. Nuclear Regulatory 
Commission, Office of Inspection and Enforcement, Division of Reactor 
Operations Inspection, Washington, D.C. 20555.  

Approved by GAO, B180225 (R0072); clearance expires 7/31/80. Approval was 
given under a blanket clearance specifically for identified generic 

Copy of Text of General 
     Electric Service Advice 
     Letter (SAL) No. 175(CPDD)  

                              NO. 175(CPDD)9.3

An undervoltage device is used on some AK type breakers to trip the breaker 
when loss of voltage occurs on the undervoltage device coil. The 
undervoltage device is an optional accessory for tripping the breaker and is 
used in some circuits in place of a shunt trip device because of the 
inference that it "fails safe" i.e., it trips the breaker when all normal 
sources of control voltage have become unavailable for shunt trip type 
operations. Because of the many constraints imposed on the undervoltage 
device, such as the need to withstand rated voltage continuously, to pick-up 
at 80 percent or rated voltage (industry standards now require pick-up at 85 
percent of rated voltage) and to drop out at 30 percent to 60 percent of 
rated voltage, both the undervoltage device and breaker must be maintained 
at a high level of performance to provide the assurance the breaker will 
trip when voltage is removed from the undervoltage device coil. The 
following is recommended and except where the breaker is in warranty, should 
be performed at the customer's expense. 

Refer To Figure 1 

1.   Check the undervoltage trip device to assure there is not excessive 
     clearance between the armature and the rivet and that there are no 
     binds as the armature moves through its travel when manually operated. 
     If excessive clearance or binding exists, it should be adjusted by 
     loosening screws (11) and moving the magnet (10) up or down as 
     necessary to obtain the proper freedom of the armature (3) with respect 
     to its pivot on magnet (10) and the guide provided by pivet (13). 

2.   Check (and adjust if necessary) the trip latch engagement. This 
     adjustment is described in the maintenance manual for that particular 

3.   Check (and adjust if necessary) the pick-up setting of the 
     instantaneous undervoltage device. In some instances, the standards for 
     nominal voltage ratings and pick-up settings have been raised since the 
     breakers were shipped from the factory. The new nominal voltages and 
     pick-up settings deviate substantially from these values, the device 
     should be readjusted while it is on the breaker. 

                                    - 2 -

     Nominal Voltage                              Pick-up Setting

          125V DC                                      106
          250V DC                                      213
          120V AC                                      102
          240V AC                                      204

     To adjust the pick-up of the instantaneous undervoltage trip device, 
     remove the Locking Wire (15) and turn the adjusting screw (14) 
     clockwise to raise the pick-up to the desired setting.  A suitable 
     voltmeter and adjustable voltage source should be connected to the 
     undervoltage device coil leads at the terminal board for stationary 
     breakers or the secondary disconnects for drawout type breakers.  On AC 
     devices, the pick-up voltage should be read immediately before the 
     armature closes its air gap.  After the pick-up is adjusted, locking 
     wire (15) should be installed. 

     No adjustment is necessary on time-delay undervoltage devices, since 
     the undervoltage device coil is switched by a relay in the time-delay 

4.   The torque required on the trip shaft to trip a closed breaker should 
     not exceed 1.5 pounds-inches.  This may be checked with the equivalent 
     of an open-end wrench fabricated from a piece of sheet metal to which 
     a spring scale can be fastened approximately 1 inch from the trip shaft
     centerline.  Where the torque required to trip the breaker exceeds 1.5 
     pound-inches, hardened grease in the trip shaft bearings and/or latch 
     bearing may be suspected or dirt may have accumulated on the trip latch
     surface.  Hardened grease must be replaced or revitalized with a 
     suitable solvent such as WD-40 or CRC 5-56 or the bearing must be 
     replaced.  Excessive grease and dirt should be removed from the trip 
     latch surface. 

5.   The adjustment between the undervoltage trip device and the trip paddle
     on the trip shaft should be checked to assure the required positive 
     trip is maintained (positive trip exists when approximately 1/32 inch 
     additional travel of the armature occurs after the undervoltage device 
     trips the breaker.)  This adjustment is made with adjusting screw (20).

6.   The frequency of inspection of the involved breakers should be 
     increased until it can be verified that the revitalization of the 
     grease is effective for the normal should be revitalized at normal 
     maintenance intervals. 

Figure 1
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