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75

Information Notice No. 93­75: Spurious Tripping of Low­Voltage Power

Circuit Breakers with GE RMS­9 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

9309140046 .

IN 93‐75

September 17, 1993

Page 2 of 3

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

.

IN 93‐75

September 17, 1993
Page 3 of 3

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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