United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-75: Spurious Tripping of Low-Voltage Power Circuit Breakers with GE RMS-9 Digital Trip Units

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

                              September 17, 1993


NRC INFORMATION NOTICE 93-75:  SPURIOUS TRIPPING OF LOW-VOLTAGE POWER CIRCUIT
                               BREAKERS WITH GE RMS-9 DIGITAL TRIP UNITS


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 loss of safety-related buses
or individual loads because of spurious tripping of low-voltage circuit
breakers fitted with General Electric (GE) RMS-9 digital 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

In October 1992, the Tennessee Valley Authority (TVA) reported to the NRC
pursuant to Part 21 of Title 10 of the Code of Federal Regulations (10 CFR
Part 21) that the ungrounded, delta-connected, 480-volt distribution system at
its Browns Ferry Nuclear Plant (BFN) had experienced short-duration, high-
amplitude current transients (possibly high-frequency electrical noise spikes)
that caused unwanted tripping of some GE AK-type circuit breakers fitted with
GE RMS-9 solid-state digital trip units.  These units have a low-pass filter
(described by some as a "holdoff circuit") that is supposed to exclude most
transients of this sort.  However, TVA reported that testing of the trip units
revealed that the instantaneous trip function of the trip unit would respond
to current transients as short as 100 microseconds and trip the breaker when
the peak amplitude of the current pulse or spike was sufficiently above the
instantaneous trip setpoint of the RMS-9 unit.

On August 4, 1993, the NRC was informed that the Maine Yankee Nuclear Power
Plant had a similar situation that the licensee described as sympathetic
tripping of two safety-related, RMS-9-equipped breakers, one of which was a
load breaker (with longtime and instantaneous trip functions) and the other a
motor control center (MCC) feeder breaker (with longtime-shorttime trips). 
Both of the affected breakers were fed from the plant's delta-connected,  
480-volt, engineered safeguards buses which are of an ungrounded design like



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those at Browns Ferry.  The trips may have been initiated by current spikes
caused by an intermittent ground fault on the boric acid makeup tank startup
heaters, a nonsafety-related load on that bus; although their breaker did not
trip.  Also, the trips occurred during the process of ground isolation which
may have created or contributed to transients on the busses.  However, the
transients were not sufficient to trip other RMS-9-equipped breakers, some of
which, acting as Class 1E isolation devices, are meant to protect the safety-
related busses from faults and overloads on nonsafety-related circuits, which
are also not environmentally qualified.

Discussion

In both these cases, the digital RMS-9 units were installed in GE AK-type low-
voltage power circuit breakers as conversion kits to replace either earlier
versions of solid state units or the electromechanical EC-type series overload
trip units previously used.  According to the manufacturer, GE Electrical
Distribution and Control (ED&C) in Plainville, Connecticut, the RMS-9 digital
electronic units were first available in October 1986 and the last of the
previous models, the analog electronic "MicroVersatrip" MVT-9 units, were
supplied in about January 1987.

GE Nuclear Energy (GE NE) in San Jose, California, has informed the NRC that
it is developing a modification specifically for the affected RMS-9 units at
Browns Ferry to make them less susceptible to unwanted tripping due to these
transients.  In addition, ED&C is undertaking a systems study in cooperation
with GE NE and Maine Yankee to determine the conditions conducive to these
transients and to develop long-term or permanent solutions.  These may be
software and/or hardware modifications to the trip units and/or recommended
modifications to the plant systems to prevent the unwanted tripping.  Once
fully developed and tested, any trip unit updates would be included in new
RMS-9 units and GE switchgear service shops would install them in existing
units.  Trip units could then be interchanged in breakers in the field or
purchased with new or replacement breakers.

Neither GE NE nor GE ED&C has issued any Service Information Letters (SILs) or
Service Advice Letters (SALs), respectively, on this subject as yet.  However,
GE product literature indicates that the units can be set up with their
instantaneous trip functions disabled.  This may be more appropriate for
certain applications (typically, feeder breakers) which, for purposes of
breaker coordination, would rely on a short time trip function for fault
protection, keeping the instantaneous trip functions only on load breakers.

The NRC is attempting to obtain sufficient information to better define the
range of applications of these units, envelop the conditions likely to make
the equipment susceptible to the problem, and identify any common-mode
initiators that may exist.  GE has confirmed that its evidence suggests that
ungrounded systems (particularly if delta-connected) are apparently the most
susceptible to the kind of transients to which the RMS-9s can respond.  RMS-9
units, used only on low-voltage switchgear, are used in certain models of GE 
.

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molded-case circuit breakers, and "Power Break" circuit breakers in addition
to the metalclad breakers.  Although, none except those on metalclad air
circuit breakers have reportedly experienced this problem, other RMS-9s may
not have been exposed to the transients of concern.  Meanwhile, GE NE and the 
GE Switchgear Apparatus Service Shop in Atlanta, Georgia, are currently
assisting Maine Yankee in exchanging (at least temporarily) several critical
RMS-9-equipped breakers for ones fitted with EC-type series trip units.

In terms of safety significance, these incidents of spurious tripping,
although somewhat isolated, raise the concern that a common-mode initiator of
intermittent ground faults on nonsafety-related and unqualified circuits, such
as a loss-of-coolant accident or high-energy line break, could conceivably
cause propagation of current spikes to multiple portions of an electrical
distribution system.  They may not trip Class 1E isolation breakers not fitted
with RMS-9s, and may not trip some with RMS-9s, but could result in unwanted
tripping of other RMS-9-equipped breakers and the attendant loss of vital
loads, possibly in more than one train.

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 AEChaffee/for


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

Technical contacts:  Stephen D. Alexander, NRR
                     (301) 504-2995

                     Kamalakar R. Naidu, NRR
                     (301) 504-2981

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