Information Notice No. 92-29: Potential Breaker Miscoordination Caused by Instantaneous Trip Circuitry

                               UNITED STATES 
                           WASHINGTON, D.C.  20555

                               April 17, 1992

                               INSTANTANEOUS TRIP CIRCUITRY 


All holders of operating licenses or construction permits for nuclear power 


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to potential breaker miscoordination involving 
instantaneous trip circuitry installed by the manufacturer in certain solid 
state trip units.  It is expected that recipients will review the 
information for applicability to their facilities and consider actions, as 
appropriate, to avoid similar problems.  However, suggestions contained in 
this information notice are not NRC requirements; therefore, no specific 
action or written response is required.

Description of Circumstances

On November 6, 1991, personnel at the Sequoyah Nuclear Plant (Sequoyah) 
calibrated a solid state trip unit in a 480 Volt power circuit breaker.  The 
licensee had recently revised the procedure used for the calibration and had 
increased the test current required for measuring the response time of the 
short time delay trip element from 10 to 13.5 times the rating of the 
current sensor.  When plant personnel applied the test current, the current 
caused the breaker to trip instantly rather than after the expected short 
time delay.  The instantaneous trip indicated that breaker miscoordination 
existed between the 480 Volt power circuit breaker and any downstream load 


The trip unit was an "Amptector Solid State Trip Device" (Amptector) which 
was installed in a type DS power circuit breaker.  Both the circuit breaker 
and the trip unit were manufactured by the Westinghouse Electric Corporation 
(Westinghouse).  The licensee knew that the trip unit was equipped with a 
long and a short time delay element.  However, the licensee did not know 
that the Amptector trip unit included a circuit which provided an 
instantaneous trip feature.  After the unexpected instantaneous trip, the 
licensee reviewed the vendor's manual, "Westinghouse Instructions for 
Low-Voltage Power Circuit Breakers Types DS and DSL."  In Section 8.3 of the 
manual the licensee found that Westinghouse had included a "making current 
release" feature in certain models of Amptector trip units not equipped with 
an instantaneous trip element.  The manual 


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referred to this feature as a "Discriminator."  The Discriminator is a 
circuit in the trip unit which determines at the time of a fault whether or 
not there was any current flow in the primary circuit previous to the fault.  
The trip unit would trip instantaneously if (1) there was "no measurable 
current" flow previous to the fault, indicating that the circuit is just 
being closed or that another switching device upstream of the breaker has 
been closed, and (2) the current in the primary circuit exceeded 
approximately 12 times the sensor's rating.  Otherwise, the short time delay 
element would function to delay the breaker tripping.  Westinghouse stated 
that, in the event of a fault, this feature allowed the breaker to provide 
selectivity and continuity in unfaulted sections of a system so that service 
to those sections could be maintained but if there was no previously 
operating load in the faulted circuit the trip unit would function 
instantaneously to limit damage.

Based on the information found in Section 8.3, the licensee determined that 
the instantaneous trip that occurred during calibration of the above circuit 
breaker resulted from the Discriminator circuit sensing that there was no 
measurable current flow previous to the test and that the test current was 
greater than 12 times the sensor's rating.  Previous calibrations of similar 
circuit breakers had not resulted in instantaneous trips because the test 
current was less than 12 times the sensor's rating.  The licensee noted 
that, for the Discriminator feature, "no measurable current" is current flow 
that is less than approximately 10 percent of the sensor's rating.

The licensee determined that the Discriminator feature could cause breaker 
miscoordination in other electrical systems.  This problem could occur in 
those systems in which a motor control center (MCC) serves both 
safety-related and nonsafety-related loads and current flow in the 
nonsafety-related portion of the circuit could be less than 10 percent of 
the sensor's rating.  In these circuits, a fault on a nonsafety-related 
portion of the circuit could cause the safety-related MCC to become 
deenergized and result in the loss of the safety-related loads.

After consulting with Westinghouse, the licensee issued a design change to 
disable the Discriminator feature on 41 circuit breakers.  Westinghouse 
stated that its DS type circuit breakers receive interrupting tests with the 
short and long time delay elements only (i.e. no instantaneous element) and 
disabling the Discriminator feature would not affect the interrupting 
ratings of the breakers.  However, Westinghouse also indicated that 
disabling the Discriminator feature could increase the potential for 
equipment damage and personnel injury in downstream portions of the circuit.

Westinghouse informed the licensee that the Discriminator circuit had been a 
part of the Amptector trip unit since its first installation in DS type 
circuit breakers.  However, Westinghouse did not address the Discriminator 
feature in the vendor's manual from the time of the manual's original issue 
in January 1971 until Revision C was issued in August 1976.

Licensees may have designed circuits without considering the effect of the 
Discriminator feature on breaker coordination because this feature was not 

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addressed in early revisions of the vendor's manual and because testing of 
the time delay elements with current less than approximately 12 times the 
sensor's rating may not reveal the presence of the Discriminator circuit.

The staff previously discussed the importance of proper breaker coordination 
in NRC Information Notice 88-45, "Problems In Protective Relay and Circuit 
Breaker Coordination," July 7, 1988. 

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate Office of 
Nuclear Reactor Regulation (NRR) project manager.

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical contacts:  Paul J. Fillion, RII
                     (404) 331-7288

                     Joseph Birmingham, NRR
                     (301) 504-2829

Attachment:  List of Recently Issued NRC Information Notices

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