United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-80: Inadequate DC Ground Detection in Direct Current Distribution Systems

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                               November 25, 1994


NRC INFORMATION NOTICE 94-80:  INADEQUATE DC GROUND DETECTION IN DIRECT 
                               CURRENT DISTRIBUTION SYSTEMS


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for operating with undetectable
grounds in vital direct current (dc) distribution systems due to inadequate
ground-detection equipment or inadequate ground-alarm setpoints or both.  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 do not
constitute NRC requirements; therefore, no specific action or written response
is required.

Background

NRC Information Notice 88-86 and its supplement, "Operating with Multiple
Grounds in Direct Current Distribution Systems," alerted all addressees to
potential problems and failure modes caused by grounds on vital dc
distribution systems, which are normally ungrounded systems.  Specifically, 
several plants had operated for extended periods with grounds on their dc
systems.  The information notice also listed licensee event reports (LERs)
related to dc system grounds.  Supplement 1 described a specific failure mode
whereby grounds on a dc system could cause equipment to remain energized
during harsh environments instead of assuming the de-energized state needed to
comply with protection system requirements.

Description of Circumstances

Discussions between the staff and several licensees indicated that ground
detectors may not be capable of detecting certain grounds and that ground
detector alarm resistance-to-ground values may be inadequate.  In addition,
the discussions indicated that some licensee reviews and actions associated
with Information Notice 88-86 and Supplement 1 were limited to specific
examples cited, such as qualification of ASCO solenoids.  Other specific
examples follow.




9411170279.                                                            IN-94-80
                                                            November 25, 1994
                                                            Page 2 of 4


Point Beach

During a monthly surveillance test on June 4, 1990, the licensee noted a solid
ground on the dc distribution system.  Although the ground detector was
equipped with an alarm, the control room annunciator had not alarmed.  The
licensee determined that as a result of its design, the ground detector could
become magnetized when a ground of one polarity occurred.  Consequently, if a
solid ground occurred of the opposite polarity, the hysteresis or
magnetization of the ground relay coil would prevent actuation of the ground
detector.  The licensee concluded that the ground detector was insensitive and
incapable of detecting even solid grounds.  As corrective action, a new, more-
sensitive ground detector was installed with an alarm value set at       
20,000 ohms.

Palisades

On August 8, 1991, the licensee discovered a solid ground in a dc system.  The
associated control room annunciator had not alarmed.  The ground-detector-
alarm setpoint was based on a 5-milliampere (ma) current; however, the maximum
current that could flow in the circuit was determined to be only 4.2 ma. 
Also, if a ground occurred gradually, it would not be detected because of the
mechanical properties of the detector-alarm mechanism.  For corrective action,
the licensee is monitoring the battery voltage on the positive and negative
buses once each shift.

Fermi

In September 1991, an NRC inspection determined that the licensee 260-volt
battery ground detector could not consistently detect grounds since the
relative brightness of lights was being used subjectively to indicate the
presence of a ground.  To improve sensitivity, the licensee replaced the
existing 25-watt light bulbs with 10-watt bulbs.  This change raised the
threshold for detecting grounds from about 92 ohms to about 1000 ohms.

Davis-Besse

During a November 15-19, 1993 inspection, the NRC determined that the ground
detector on the new alternate station blackout diesel generator battery was
only capable of detecting solid grounds and that the detector manufacturer
could not quantify the setpoint.  In response, the licensee proposed to review
the adequacy of the present ground detector and investigate alternative
methods to detect the presence of grounds in the dc system.

Grand Gulf

On November 1, 1994, a reactor scram occurred due to a ground in the "A"
backup scram valve control circuitry while a half scram was already present
due to surveillance testing of the reactor protection system.  The hard
ground, in conjunction with the use of a newly upgraded 125-volt dc ground .                                                            IN-94-80
                                                            November 25, 1994
                                                            Page 3 of 4


detector, enabled enough current to flow to energize the backup scram valve's
solenoid and open the backup scram valve.  The modified detector circuitry
provided a 1000-ohm path through an annunciating relay (control room alarm) to
ground.  This upgraded detector circuitry replaced a circuit which originally
had a 100,000-ohm path to ground.  For corrective action the licensee has
removed the hard ground, will disable the 1000-ohm path and has de-energized
the detector circuit used for local ground indication except when periodic
ground checks are performed.

Discussion


In Information Notice 88-86, Supplement 1, the staff described the potential
for ASCO solenoid valves to inadvertently actuate or fail to drop out due to
grounds of approximately 4000 ohms or less at the Summer station.  The Summer
licensee has also removed the on-line ground detector because its inherent    
1500-ohm resistance-to-ground value was below the plant threshold value.

After Information Notice 88-86 Supplement 1 was issued, the staff met with
Commonwealth Edison on March 1, 1989, to discuss corrective action for the
ground problem at Quad Cities (discussed in Information Notice 88-86).  The
Commonwealth Edison review revealed the potential for inadvertent relay
operation at a much higher ground-resistance value for General Electric HFA,
HMA, and HGA 125-volt dc relays.  All Commonwealth Edison nuclear plants now
have ground-clearing action with priorities based on two ground-threshold
values, 125,000 and 20,000 ohms.  The 125,000-ohm value is based on the
minimum ground current required to keep an HGA relay from dropping out when
its normal power is removed and an additional ground exists between the relay
and its actuating switch contact (see Figure 1).  The 20,000-ohm threshold is
based on the minimum current needed to pick up the most sensitive relay
(General Electric HMA) under the same conditions.  (For further information on
determining a ground threshold for a dc system, see Annex C of IEEE Std 946-
1992, "IEEE Recommended Practice for the Design of DC Auxiliary Power Systems
for Generating Stations.")

Nuclear power plant dc distribution systems are typically two-wire ungrounded
battery/charger systems equipped with ground-detection/alarm circuitry
including features such as annunciation in the control room, local indication,
and recording.  Ground detectors are incorporated in the dc systems so that if
a single ground does occur, personnel are aware of the ground and can take
immediate steps to clear the ground fault from the system.

Failure to promptly eliminate a single ground could mask subsequent additional
grounds.  Multiple grounds could lead to unpredictable spurious operation of
equipment, inoperable equipment, unanalyzed loads on batteries, or unanalyzed
equipment failure modes that could be expected to occur during harsh
environments attendant to accidents.  In addition, installed ground detectors
and portable ground-locating equipment themselves may create a ground on the
dc system and may not maintain a minimum threshold resistance-to-ground value
above which predictable system/component operation can be assured..                                                            IN-94-80
                                                            November 25, 1994
                                                            Page 4 of 4


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 NRR project
manager.



                                    Original signed by B.D. Liaw for
                                    Brian K. Grimes, Director
                                    Division of Project Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Roger Mendez, RIII
                     (708) 829-9745

                     Fred Burrows, NRR
                     (301) 504-2901 

Attachments: (see file IN94080.WP1 for Figure 1)
1.  Figure 1:  Ground-Threshold Valves
Page Last Reviewed/Updated Friday, November 15, 2013