United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 77-01: Pneumatic Time Delay Relay Setpoint Drift

                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                          WASHINGTON, D. C. 20555  

                                                       IE BULLETIN 77-01  
                                                       Date: April 29, 1977 
                                                       Page 1 of 3 

PNEUMATIC TIME DELAY RELAY SETPOINT DRIFT 

DESCRIPTION OF CIRCUMSTANCES: 

Millstone Unit 2 and North Anna Unit 1 facilities experienced repeated 
diesel-generator starting failures several of which investigation revealed 
were caused by setpoint drift on the pneumatic time delay relays used in the
control circuitry for the diesel-generator. The relays involved are 
identified as ITE Imperial, Catalog Nos. J20T3/J13P20 and J20T3/J13P30. The 
affected diesel-generators are Fairbanks Morse Units by Colt Industries. 

These types of time delay relays are used in several different applications 
in the control circuitry for the diesel-generator. One of these relays is 
used to bypass the normal low oil pressure shutdown functions during 
diesel-generator startup. At Millstone Unit 2 the relay had drifted 
approximately 10 seconds from the required 20 second delay which allowed the
low oil pressure trip circuit to shut down the diesel-generator before the 
oil pressure had time to build up. At North Anna Unit 1, excessive drift of 
similar ITE time delay relays was also observed during preoperational 
testing of the diesel-generators. 

ITE Imperial has identified the time delay relays involved at Millstone Unit
2 as coming from the 1972 and 1973 production runs. The catalog 
specification for this vintage of relays requires a trip-point setting 
repeat accuracy of +- 15 percent. Units manufactured in 1974 or later have 
demonstrated a repeat accuracy of +- 3 to 4 percent, well within the catalog 
specification of +-
 15 percent. 
.

                                                       IE Bulletin 77-01  
                                                       Date: April 29, 1977 
                                                       Page 2 of 3 

Month and year of production for the time delay relays in question can be 
determined by the six or seven digit bold white number on the timer head. 
The first two or three digits indicate the month and year or production. 
(The last four digits provide other coded information.)  For example: 124056
indicates a production date of January 1972; 1234056 indicates a production 
date of December 1973. 

According to the time delay relay manufacturers, the potential for setpoint 
drift is a common characteristic of most pneumatic relays, irrespective of 
manufacturer. The magnitude of setpoint drift is related to the repeat 
accuracy specified for the device and the mode in which it will operate, 
that is, energized or de-energized. In most cases energized units tend to be
susceptible to greater deviations from setpoint because of the temperature 
effects on the internal parts of the unit. 

ACTION TO BE TAKEN BY LICENSEES AND PERMIT HOLDERS: 

For all power reactor facilities with an operating license or construction 
permit: 

1.   If you have been notified of the potential problem, describe the 
     actions taken regarding corrective measures to identify and resolve any 
     setpoint drift problems with the ITE time delay relays. 

2.   In addition to Item 1 above, pneumatic time delay relays intended for 
     use in safety related systems and specifying a repeat accuracy range of
     +- 15 percent or greater should be demonstrated to provide satisfactory
     operation. You are requested to provide your basis for concluding that 
     existing pneumatic time delay relays are functioning as required, or 
     provide your plans to assure satisfactory operation. 

Reports for facilities with operating licenses should be submitted within 30
days after receipt of this Bulletin, and reports for facilities with 
construction permits should be submitted within 60 days after receipt of 
this Bulletin. Your report should include the date when the above actions 
were or will be completed. 
.

                                                      IE Bulletin 77- 01 
                                                       Date: April 29, 1977 
                                                       Page 3 of 3 

Reports should be submitted to the Director of the NRC Regional Office and a 
copy should be forwarded to the NRC Office of Inspection and Enforcement, 
Division of Reactor Inspection Programs, Washington, D. C. 20555. 

Approval of NRC requirements for reports concerning possible generic 
problems has been obtained under 44 U.S.C 3152 from the U. S. General 
Accounting Office. (GAO Approval B-180255 (R0072), expires 7/31/77)    
Page Last Reviewed/Updated Tuesday, July 23, 2013