Information Notice No. 95-13: Potential for Data Collection Equipment to Affect Protection System Performance

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                               February 24, 1995

                               AFFECT PROTECTION SYSTEM PERFORMANCE


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 a potential single or common mode failure when
using devices to collect data on protection system performance.  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

Description of Circumstances

During testing of the main turbine at Fermi 2 after extensive maintenance, the
licensee received spurious high reactor water level alarms on the sequence-
of-events recorder for the reactor core isolation cooling (RCIC) system.  On
February 10, 1995, a portable computerized data acquisition system (DAS) was
connected to four level instruments and one pressure instrument in each of two
divisional testability cabinets.  The DAS consisted of two separate computer
units, with one unit being connected to each testability cabinet so that both
divisions could be monitored simultaneously.

It was assumed that the DAS units were nonintrusive, and no evaluation was
performed in accordance with Section 50.59, "Changes, Tests, and Experiments",
of Title 10 of the Code of Federal Regulations (10 CFR).  No detailed testing
of the DAS units was performed to determine potential failure modes or effects
on the instruments to which they were connected.  No guidance was given to the
technicians on whether or not to leave the devices connected when they were

Operators in the control room noted that when the DAS units were turned off on
February 11, 1995, the wide range level indication decreased by 25 cm [10
inches].  Investigation by the licensee showed that when the DAS input
multiplexer was energized, the input impedance was in excess of 100K ohm, but
when it was deenergized, the impedance dropped to approximately 4K ohm.  This
phenomenon appears to be an inherent characteristic of the DAS overvoltage
protection circuit, rather than any internal component failure.  The vendor 

9502240037.                                                            IN 95-13
                                                            February 24, 1995
                                                            Page 2 of 3

manual does not contain a caution about leaving the inputs connected when the
unit is deenergized.

The drop in impedance allowed more current to be drawn from the wide range
level indication circuitry and resulted in the decreased level indication. 
Since both units were turned off at the same time, both divisions were


The DAS units used at Fermi 2 are manufactured by Intelligent Instrumentation
and utilize a Model No. PCI-20098C multifunction carrier board.  The
overvoltage protection circuit is a common design used in solid state
multiplexers sold by various manufacturers of data acquisition equipment. 
This equipment may be widely used in the nuclear power industry to collect
special plant data for analysis of various input parameters.

Although detailed testing of the DAS units was not performed before their use,
a routine preconnection test was performed.  A digital voltmeter was used to
test the units in both the energized position and the deenergized position,
but the voltmeter did not detect the change in input impedance.

At Fermi 2, the DAS units were plugged into test jacks in the testability
cabinets normally used for surveillance testing.  This practice is common in
the industry.  The bias inserted by the devices when deenergized affected the
autostart and high level trip signals for RCIC and high pressure coolant
injection (HPCI).  Additional testing indicated that RCIC and HPCI would
initiate at a level approximately 13 cm [5 inches] higher than expected on low
reactor level and would trip about 43 cm [17 inches] above the expected high
level trip setpoint.

The DAS units were powered by a nonsafety-related source.  Although the
technicians turned the units off, the same effect would have been achieved if
the unit suffered a loss of station or offsite power during testing.  In
either case, the possibility existed for a common mode error in the low level
initiation and high level trip setpoints for HPCI and RCIC.

Separation of redundant channels of protection systems during testing is
addressed in Regulatory Guide 1.118, "Periodic Testing of Electric Power and
Protection Systems."  It states that a temporary test setup shall be
considered a part of the safety system.  In addition, the Institute of
Electrical and Electronics Engineers (IEEE) Standard 279-1977, "Criteria for
Protection Systems for Nuclear Power Generating Stations," specifies that a
single failure in the protection system not cause a loss of function, and
redundant channels be independent and physically separated.  IEEE Standard
338-1975, "Criteria for the Periodic Testing of Nuclear Power Generating
Station Safety Systems," specifies that test equipment not cause loss of
independence between redundant channels.  The licensing basis of each plant
contains the specific applicable commitments for channel independence.

.                                                            IN 95-13
                                                            February 24, 1995
                                                            Page 3 of 3

Section 50.59 of 10 CFR requires licensees to provide a basis for the
determination that a test or experiment not described in the safety analysis
report does not involve an unreviewed safety question.

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.

                                    /s/'d by BKGrimes

                                    Brian K. Grimes, Director
                                    Division of Project Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Monte Phillips, RIII
                     (708) 829-9637

                     David Skeen, NRR
                     (301) 415-1174

Attachment:  List of Recently Issued NRC Information Notices


Page Last Reviewed/Updated Thursday, March 25, 2021