United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #:  9309140175
                       LICENSEE EVENT REPORT (LER)

FACILITY NAME:  Maine Yankee Atomic Power Company         PAGE: 1
OF 3

DOCKET NUMBER:  05000

TITLE:  Multiple 480 Volt Breaker Trips Caused by RMS-9 Trip
        Devices

EVENT DATE:  08/04/93   LER #:  93-016-00   REPORT DATE: 
09/06/93

OTHER FACILITIES INVOLVED:                          DOCKET NO: 
05000

OPERATING MODE:  3   POWER LEVEL:  0

THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR
SECTION:
50.73(a)(2)(v) & OTHER

LICENSEE CONTACT FOR THIS LER:
NAME:  Danny P. McDougold, Nuclear Safety   TELEPHONE:  (207)
882-6321
       Engineer

COMPONENT FAILURE DESCRIPTION:
CAUSE:  B   SYSTEM:  ED   COMPONENT:  BKR  MANUFACTURER:  GE
REPORTABLE NPRDS:  Y

SUPPLEMENTAL REPORT EXPECTED:  YES   EXPECTED SUBMISSION DATE: 
10/22/93

ABSTRACT:

This report is being submitted in accordance with two other
reporting
requirements:

     1.   Written report for 10 CFR Part 21
     2.   Special report to the commission as required by
Technical
          Specification 3.23 remedial action for P-4,
Electric-driven
          fire pump, being inoperable for greater than 7 days.

On July 30, 1993, Maine Yankee (MY) was cooling down in
preparation for
refueling.  At 1022 hrs. while in the transthermal condition, a
480 volt
ground alarm was received and P-29D service water [SW] pump [P]
tripped.
The pump's air circuit breaker [ACB] had opened, causing a SW low
pressure alarm, and an operator immediately started a standby
pump.

Normally a single ground fault will not actuate the RMS-9 device,
which
serves as an overcurrent protection on the breakers in the
ungrounded 480
volt buses.  This device was designed with long-time/short-time
trip
functions for motor control circuit (MCC) loads and instantaneous
trip
functions for motor loads, chosen to coordinate with the upstream
and
downstream protective devices.  This coordination ensures that an
overcurrent condition on a load would not result in de-energizing
the
entire bus or any upstream load.  The RMS-9 trip device was
certified by
GE as a suitable replacement for the EC trip units that were
originally
installed in GE AK circuit breakers.  The RMS-9 trip devices were
installed in all AK-25 breakers at MY for both safeguards and
non-safeguards 480 volt buses.  There are eight AK-25 circuit
breakers
which control loads required to mitigate an accident.  These
include 4 (2
on each of two trains) service water pumps (which provide the
ultimate
heat sink for engineered safeguards), and 4 (2 on each of two
trains)
which feed various instrumentation, valves and other equipment
necessary
for engineered safeguards.  In addition one breaker feeds an
electric
driven fire pump and one is the output breaker of an auxiliary
diesel
generator which is used to meet 10 CFR Part 50 Appendix R
requirements.

END OF ABSTRACT

TEXT                                                         
PAGE 2 OF 3

This report is being submitted in accordance with two other
reporting
requirements:

     1.   Written report for 10 CFR Part 21
     2.   Special report to the commission as required by
Technical
          Specification 3.23 remedial action for P-4,
Electric-driven
          fire pump, being inoperable for greater than 7 days.

On July 30, 1993, Maine Yankee (MY) was cooling down in
preparation for
refueling.  At 1022 hrs. while in the transthermal condition, a
480 volt
ground alarm was received and P-29D service water [SW] pump [P]
tripped.
The pump's air circuit breaker [ACB] had opened, causing a SW low
pressure alarm, and an operator immediately started a standby
pump.

Normally a single ground fault will not actuate the RMS-9 device,
which
serves as an overcurrent protection on the breakers in the
ungrounded 480
volt circuits.  This device was designed with
long-time/short-time trip
functions for motor control circuit (MCC) loads and instantaneous
trip
functions for motor loads, chosen to coordinate with the upstream
and
downstream protective devices.  This coordination ensures that an
overcurrent condition on a load would not result in de-energizing
the
entire bus or any upstream load.  The RMS-9 trip device was
certified by
GE as a suitable replacement for the EC trip units that were
originally
installed in GE AK circuit breakers.  The RMS-9 trip devices were
installed in all AK-25 breakers at MY for both safeguards and
non-safeguards 480 volt buses.  There are eight AK-25 circuit
breakers
which control loads required to mitigate an accident.  These
include 4 (2
on each of two trains) service water pumps (which provide the
ultimate
heat sink for engineered safeguards), and 4 (2 on each of two
trains)
which feed various instrumentation, valves and other equipment
necessary
for engineered safeguards.  In addition one breaker feeds an
electric
driven fire pump and one is the output breaker of an auxiliary
diesel
generator which is used to meet 10 CFR Part 50 Appendix R
requirements.

On July 30, 1993, electricians did not identify any RMS-9 trip
flags
present on the P-29D breaker.  A 74 device flag on the 480 volt
bus 8 was
present; this flag indicates a system ground and sends a signal
to the
panalarm.  P-29D motor and breaker were meggered with
satisfactory
results.

At 1200 hrs., P-29D was restarted and experienced no initial
problems.
At 1345 hrs., the P-29D breaker again tripped open.  This time,
MCC-8B
feeder breaker also tripped open.  P-29D is powered from Bus 8,
which
also feeds MCC-8B.  MCC-8B subsequently feeds MCC-8B1.  The
MCC-8B feeder
breaker trip therefore resulted in loss of both MCC-8B and
MCC-8B1.  The
deenergized buses made several motor operated valves inoperable
including
containment integrity valves which required entry into a 4 hour
remedial
action of Technical Specifications.  Only one ECCS train was
required at
the time and the "A" train remained operable.

Investigation of the ground continued until 0130 hrs. on July 31,
1993.
The ground was located in the Boric Acid Mix Tank (BAMT) startup
heaters.

The ACB spurious trip problem is believed to be attributed to the
RMS-9
trip devices initiating breaker trips when short duration current
transients were induced by a ground fault spike.  This spike can
result
in "sympathetic" trips occurring on other 480 volt breakers
powered from
parallel circuits of a 480 volt ground.  There is no operational
experience of a ground fault affecting other buses unless
cross-tied.  MY
believes the delta-delta transformer connection isolates the
fault to the
affected bus.

On August 4, 1993, MY determined that during a Loss of Coolant
Accident
(LOCA) non-environmentally qualified (EQ) components or cable
could degrade
and cause grounds.  The resulting current spikes could result in
spurious
tripping and de-energizing of equipment needed to mitigate the
consequences
of an accident.

The safety significance of this postulated event is that some
systems
required to mitigate an accident could be spuriously
de-energized.
Operational experience has shown that the ACBs can be reclosed
within a
few minutes even with the ground fault present, if the RMS-9 trip
device
has no flags (indicating no true overcurrent condition).

MY is considering various alternatives to correct the RMS-9 trip
device
deficiency prior to plant startup.  A supplemental LER will be
issued to
state the corrective actions.

TEXT                                                         
PAGE 3 OF 3

P-4, electric driven fire pump, is considered inoperable until
the RMS-9
condition is corrected.  P-5, diesel fire pump, remains operable
to
supply water under all necessary conditions.

While shutdown, the engineered safeguards equipment that are
affected by
the RMS-9 devices, are either not necessary or do not need
automatic
action.  The Residual Heat Removal (RHR) system, cooled by SW,
can be
stopped momentarily, without consequence, until the breakers are
reclosed.

ATTACHMENT 1 TO 9309140175                                   PAGE
1 OF 1

                              Maine Yankee
                     RELIABLE ELECTRICITY SINCE 1972

          EDISON DRIVE o AUGUSTA, MAINE 04330 o (207) 622-4868

                              September 2, 1993
                              MN-93-81       JRH-93-178

UNITED STATES NUCLEAR REGULATORY COMMISSION
Attention:  Document Control Desk
Washington, DC 20555

Reference:     (a)  License No. DPR-36 (Docket No. 50-309)

Subject:       Maine Yankee Licensee Event Report 93-016,
Multiple 480
               Volt Breaker Trips Caused by RMS-9 Trip Devices

Gentlemen:

     Please find enclosed Maine Yankee Licensee Event Report
93-016-00.
This report is submitted in accordance with 10CFR50.73(a)(2)(v)
and 10
CFR 21.

     Please contact us should you have questions regarding this
matter.

                              Very truly yours,

                              James R. Hebert, Manager
                              Licensing & Engineering Support
Department

SJB/jag

Enclosure

c:   Mr. Thomas T. Martin
     Mr. Charles S. Marschall
     Mr. E. H. Trottier
     Mr. Patrick J. Dostie

L:\93MN\9381

*** END OF DOCUMENT ***



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