Information Notice No. 88-86: Operating with Multiple Grounds in Direct Current Distribution Systems
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 21, 1988
Information Notice No. 88-86: OPERATING WITH MULTIPLE GROUNDS IN
DIRECT CURRENT DISTRIBUTION SYSTEMS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems caused by plants operating with ground(s) in the vital direct current
(dc) distribution systems (which are normally ungrounded systems). 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.
Description of Circumstances:
During recent NRC maintenance inspections at Quad Cities (50-254/88011 and
50-265/88012), Oconee (50-269/88-17, 50-270/88-17, and 50-287/88-17), and
D.C. Cook (50-315/88016 and 50-316/88018) power reactor facilities, it was
found that plants had been operating with multiple grounds in the dc distri-
bution systems for extended periods. Specific examples are as follows:
(1) Quad Cities Unit 2 operated for a significant period of time with known
grounds on the negative side of the 125-Vdc system. During this period,
when a momentary ground occurred on the positive side of the circuit a
fuse in the auto-start circuitry of a diesel generator blew, disabling
the auto-start function of the diesel generator for approximately 6
months. This condition was not properly recognized or evaluated by the
licensee; it was identified during an 18-month surveillance test.
(2) Oconee Nuclear Station operated for approximately 4 months with the
125-Vdc system ground alarm activated. However, during this period no
station procedure or shift surveillance requirements existed to verify
the operability of the ground detection system. As a result of the
maintenance inspections, this ground detection alarm system was found to
be inoperable; it had not been calibrated since 1976.
8810170391
. IN 88-86
October 21, 1988
Page 2 of 3
(3) D. C. Cook Unit 1 had a negative dc system ground for approximately 7
months before it was cleared. A safety evaluation for continued
operation with the ground or other grounds that might be masked had never
been performed until requested by the NRC.
The licensees of Quad Cities and D. C. Cook were issued notices of violation.
Safety evaluations were performed to support continued operation while the
grounds existed.
A review of selected licensee event reports of dc system grounds (see Attach-
ment 1) indicates that plant equipment is frequently rendered inoperable or is
started unexpectedly because of grounds.
Discussion:
The dc power system provides control and power to safety-related valves,
instrumentation, emergency diesel generators, and many other components and
systems during all phases of plant operation, including abnormal shutdowns and
accident situations. Because of its importance to plant safety, the dc
system requires a high degree of reliability and availability.
Most nuclear power plant dc systems are two-wire ungrounded, combination
battery/charger systems equipped with ground detection. Typical ground
detection system features include a remote annunciator and a local indicator
and/or recorder. Ground detectors are incorporated in the dc system so that
if a single ground point does occur, immediate steps can be taken to clear the
ground fault from the system.
Failure to respond to a single ground will mask subsequent grounds. Multiple
grounds can cause the indiscriminate operation of equipment, which may have
safety consequences. Grounds can cause control circuit fuses to fail and can
render important safety equipment inoperable as previously described.
Furthermore, batteries have a designed capacity to supply power during a
station blackout condition, and this capacity can be affected by the presence
of unanalyzed loads in the form of multiple grounds.
It is recognized that troubleshooting and finding grounds on a dc system are
difficult tasks that may affect plant operation. The licensees previously
mentioned have reviewed their designs and conditions for potential impact on
safety system operability and have taken corrective actions to minimize the
effect of grounds.
. IN 88-86
October 21, 1988
Page 3 of 3
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the
technical contacts listed below or the Regional Administrator of the
appropriate regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Z. Falevits, Region III
(312) 790-5772
S. N. Saba, NRR
(301) 492-1052
Attachments: 1. Selected Licensee Event Reports
2. List of Recently Issued NRC Information Notices
. Attachment 1
IN 88-86
October 21, 1988
Page 1 of 1
Selected Licensee Event Reports
Event No.: 50-454/86-03 (Byron, Unit 1)
Abstract: A reactor trip and auxiliary feedwater actuation occurred. The trip
was caused by grounds on the positive and negative sides of the 125-
Vdc bus. The grounded circuit energized a fast-close solenoid on
the 1A main steam isolation valve (MSIV). When the MSIV closed, a
reactor trip resulted from steam generator low-low level.
Event No.: 50-369/82-48 (McGuire, Unit 1)
Abstract: After the battery trouble alarm actuated, it was noticed that
neither the closed nor the open indication light was energized on the
pressurizer power-operated relief valve and the valve would not operate.
A ground that had blown a fuse in the control circuit was identified.
Event No.: 50-293/82-53/03L-0 (Pilgrim)
Abstract: A ground fault alarm was observed on the 125-Vdc battery circuit.
The ground was traced to the limit switch circuitry for a motor-
operated valve. The high-pressure coolant injection system was
subsequently declared inoperable.
Event No.: 50-305/85-04 (Kewaunee)
Abstract: A control room operator noticed the reactor coolant pump running
while the control switch indicated a green flag or off condition.
Subsequent investigation revealed that the pump had inadvertently
started because of a ground condition in the 125-Vdc actuation
circuitry associated with the 4.16 kV switchgear.
. Attachment 2
IN 88-86
October 21, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-85 Broken Retaining Block 10/14/88 All holders of OLs
Studs on Anchor Darling or CPs for nuclear
Check Valves power reactors.
88-84 Defective Motor Shaft 10/20/88 All holders of OLs
Keys in Limitorque Motor or CPs for nuclear
Actuators power reactors.
88-83 Inadequate Testing of Relay 10/19/88 All holders of OLs
Contacts in Safety-Related or CPs for nuclear
Logic Systems power reactors.
88-82 Torus Shells with Corrosion 10/14/88 All holders of OLs
and Degraded Coatings in or CPs for BWRs.
BWR Containments
88-81 Failure of Amp Window 10/7/88 All holders of OLs
Indent Kynar Splices or CPs for nuclear
and Thomas and Betts power, test, and
Nylon Wire Caps During research reactors.
Environmental Quali-
fication Testing
88-80 Unexpected Piping Movement 10/7/88 All holders of OLs
Attributed to Thermal or CPs for PWRs.
Stratification
88-79 Misuse of Flashing Lights l0/7/88 All holders of OLs
for High Radiation Area or CPs for nuclear
Controls power reactors.
88-69, Supp 1 Movable Contact Finger 9/29/88 All holders of OLs
Binding in HFA Relays or CPs for nuclear
Manufactured by General power reactors.
Electric (GE)
88-78 Implementation of Revised 9/22/88 All holders of OLs
NRC-Administered Requali- or CPs for nuclear
fication Examinations power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021