Information Notice No. 88-45: Problems in Protective Relay and Circuit Breaker Coordination
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 7, 1988
Information Notice No. 88-45: PROBLEMS IN PROTECTIVE RELAY AND CIRCUIT
BREAKER COORDINATION
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to a potentially
significant problem concerning the possible lack of protective relay and cir-
cuit breaker coordination. 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:
(1) Lack of Circuit Breaker and Protective Relay Coordination at McGuire
Units l and 2
On September 6, 1987, a reactor trip and turbine trip occurred at the
McGuire nuclear station. These trips resulted directly from a lack of
proper circuit breaker coordination on the plant's onsite electrical
distribution system. To facilitate component maintenance, the power
supply to an auxiliary power panel board was shifted to an alternate
source, a 600 V motor control center (MCC). This MCC also provides power
to a compressor in the plant's instrument air system. A ground fault
developed in the compressor's motor. This fault not only caused the
compressor motor's feeder breaker to open but also caused the feeder
breaker to the 600 V MCC to open. The interruption of power to the MCC
precipitated the loss of the panel board. As a result, the turbine
control system closed the main turbine throttle, governor, and intercept
valves causing the reactor to trip on high pressurizer pressure.
(2) Lack of Breaker and Protective Relay Coordination at Salem Units 1 & 2
In October 1987, in response to staff questions raised during a fire
protection audit, the licensee of Salem Units 1 and 2 determined that the
lack of circuit breaker coordination in the plant could compromise
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IN 88-45
July 7, 1988
Page 2 of 3
the protection for redundant equipment and/or associated circuits from
common mode failures. As a result, the licensee voluntarily shut down
Unit 2 which was the only unit operating at the time. The resolution of
this issue contributed significantly to the outage that lasted ap-
proximately 7 weeks.
(3) Lack of Coordination of Ground Fault Protective Devices at Monticello
In June 1986, the Monticello licensee installed electrical ground fault
detection devices on circuit breakers throughout the onsite electrical
distribution system. The installation of these devices altered the
coordination between circuit breaker, motor control centers, and con-
nected loads so that actuation of protective devices in the proper
sequence was no longer assured. In June 1987, at least two operating
events involving the loss of a train of emergency core cooling resulted
from the lack of coordination between protective devices. These problems
were caused by the June 1986 design change which had not adequately con-
sidered coordination of the protective devices in conjunction with the
design modification.
Background:
Coordination is the selection and/or setting of protective devices so as to
sequentially isolate only that portion of the system where the abnormality
occurs. To achieve this isolation, it is necessary to set protective devices
so that only the device nearest the fault opens and isolates the faulted
circuit from the system. It is obvious that such selectivity becomes more
important with devices that are closer to the power source, as a greater
portion of the system can be affected.
Backup protective devices are set to operate at some predetermined time
interval after the primary device fails to operate. A backup device is able
to withstand the fault conditions for a longer period than the primary device.
If a primary device fails to clear a fault and the backup device must clear
it, then the design of the protective system becomes suspect.
To optimize the coordination of protective devices, good engineering practice
requires that consideration be given to the following:
(1) available maximum short circuit currents;
(2) time interval between the coordination curves; and
(3) load current.
Discussion:
Other plants may also have problems with relay and breaker coordination. The
staff relies on the exercise of good engineering practice by the designers of
electrical power systems at nuclear power plants to provide for the proper
functioning of protective devices. Breaker coordination is a key fire pro-
tection feature if cables for redundant trains pass through a fire area. NRC
IN 88-45
July 7, 1988
Page 3 of 3
Generic Letter (GL) 81-12, "Fire Protection Rule (45 FR 76602, November 19,
1980)," dated February 20, 1981 required licensees to submit information with
regard to the design description of modifications necessary to meet Section
III.G.3 "Fire Protection of Safe Shutdown Capability," 10 CFR 50, Appendix R,
"Fire Protection Program for Nuclear Power Facilities Operating Prior to
January 1, 1979." This letter set forth general guidelines for protection of
safe shutdown capability from the fire-induced failure of associated circuits.
ANSI/IEEE Standard 242-1986, "IEEE Recommended Practices for Protection and
Coordination of Industrial and Commercial Power Systems" provides detailed
guidance on achieving proper co-ordination.
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: N. K. Trehan, NRR
(301) 492-0807
E. N. Fields, NRR
(301) 492-1173
Attachment: List of Recently Issued NRC Information Notices
Attachment
IN 88-45
July 7, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information Date of
Notice No.
88-44 Mechanical Binding of 6/24/88 All holders of OLs
Spring Release Device or CPs for nuclear
in Westinghouse Type power reactors.
DS-416 Circuit Breakers
88-43 Solenoid Valve Problems 6/23/88 All holders of OLs
or CPs for nuclear
power reactors.
88-42 Circuit Breaker Failures 6/23/88 All holders of OLs
Due to Loose Charging or CPs for nuclear
Spring Motor Mounting Bolts power reactors.
88-41 Physical Protection 6/22/88 All holders of OLs
Weaknesses Identified or CPs for nuclear
Through Regulatory Ef- power reactors.
fectiveness Reviews (RERs)
88-40 Examiners' Handbook for 6/22/88 All holders of OLs
Developing Operator or CPs for nuclear
Licensing Examinations power reactors.
88-39 LaSalle Unit 2 Loss of 6/15/88 All holders of OLs
Recirculation Pumps With or CPs for BWRs.
Power Oscillation Event
88-38 Failure of Undervoltage 6/15/88 All holders of OLs
Trip Attachment on General or CPs for nuclear
Electric Circuit Breakers power reactors.
88-37 Flow Blockage of Cooling 6/14/88 All holders of OLs
Water to Safety System or CPs for nuclear
Components power reactors.
88-36 Possible Sudden Loss of RCS 6/8/88 All holders of OLs
Inventory During Low Coolant or CPs for PWRs.
Level Operation
88-35 Inadequate Licensee Performed 6/3/88 All holders of OLs
Vendor Audits or CPs for nuclear
power reactors.
OL = Operating License
CP = Construction Permit
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