Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 11, 1994
NRC INFORMATION NOTICE 92-51, SUPPLEMENT 1: MISAPPLICATION AND INADEQUATE
TESTING OF MOLDED-CASE
CIRCUIT BREAKERS
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 supplement to clarify the original information notice and to provide
additional references and information. 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.
Background
On July 9, 1992, the NRC issued Information Notice (IN) 92-51, "Misapplication
and Inadequate Testing of Molded-Case Circuit Breakers" (MCCBs). This notice
informed addressees about problems that can cause certain MCCBs to trip when
starting their safety-related motor loads. The main problems mentioned were
(1) that the effect of the inrush transient current when starting a large,
highly inductive load was not always being properly accounted for in selecting
ratings and settings for replacement MCCBs and (2) that some replacement
MCCBs, although correctly selected and set, were tripping out of tolerance
during post installation testing or operation because inadequate bench test
methods had failed to detect this condition.
Description of Circumstances
The first two sentences of the second paragraph of IN 92-51 under "Description
of Circumstances" incorrectly stated the magnitude of inrush transient current
and how it can vary. The first sentence stated that the inrush transient
current could be as much as three times locked-rotor current. The second
sentence stated that, depending on the impedance of the motor when started,
the inrush transient current can increase to as much as approximately six
times the value of the locked-rotor current. However, the notice should have
stated that, for most industrial applications, the inrush transient current
should seldom reach much more than two times the locked-rotor current, but
that, within this limit, the magnitude of the inrush transient current can
vary widely depending on factors that will be described below.
9404060017. IN 92-51, Supp. 1
April 11, 1994
Page 2 of 3
IN 92-51 also commented on certain guidelines in National Fire Protection
Association Publication NFPA-70, "The National Electric Code," (NEC) in a
manner that could be misinterpreted as written. It stated that the level of
protection recommended by the NEC might be insufficient to prevent an unwanted
trip. However, this statement was not intended as a comment on the adequacy
of the NEC guideline because the guideline was not intended to prevent
unwanted trips, but rather to ensure circuit protection. Instead, the
statement was meant to explain that by adhering to the NEC guideline for
maximum overload trip settings or fixed trip points (e.g., 13 times full
load), unwanted tripping may result if the various rating factors used in
selecting and setting MCCBs are not correctly applied. Certain exceptions in
the NEC provide for the limited use of rating factors.
Finally, IN 92-51 cited references (such as National Electrical Manufacturers
Association Standard AB4-1991) that contain standard test methods for MCCBs.
However, the notice was originally intended also to highlight certain
recommendations in those references that the NRC had found were often not
being followed in field testing procedures and practice. The recommendations
were meant to ensure that the test methods prescribed could accurately and
conclusively demonstrate that an MCCB is not only tripping when required, but
also not tripping prematurely. These points were that (1) the pulse method is
generally more accurate than the runup method, (2) the asymmetrical current
error inherent in the pulse test method test could be minimized and/or
compensated for by using equipment capable either of capturing and recording
the instantaneous values of the inrush transient or capable of sensing the
phase angle of the source voltage and initiating current flow at the instant
the voltage waveform is at the peak, (3) initial test current pulses are
injected at values just at or below the lower limit of the manufacturer's
specified trip band (adjustable or fixed) with appropriate test tolerance
applied, and (4) the expected result at this current level is that no
instantaneous-magnetic trip will occur. In some cases, a thermal trip may
occur during the test, but the inherent, measurable time delay would indicate
the absence of an instantaneous trip and hence confirm no premature trip by
the magnetic trip function being tested.
Discussion
The peak amplitude of the inrush transient current depends on the reactance-
to-resistance (X/R) ratio of the load and decays exponentially during the
first few cycles after the starter contacts close. Within the boundaries
determined by the electrical properties of the circuit, the inrush transient
current also varies randomly with the phase angle of the supply voltage at the
particular instant that the starter contacts close. However, the point of
IN 92-51 remains the same: that is, sometimes, the peak amplitude and duration
of the inrush transient current may be sufficient for very sensitive (and
sometimes inaccurate, misadjusted, and/or misapplied) magnetic trip mechanisms
in the supply breaker to respond to and trip the breaker prematurely.
Examples of the rating factors used by the application engineer that could
lead to premature tripping with settings that are in compliance with the NEC . IN 92-51, Supp. 1
April 11, 1994
Page 3 of 3
would include load class (type/duty cycle), cable type and size, ambient
temperature and altitude, frequency, and safety margin. These factors, used
in specifying the nominal load rating of the breaker in relation to the
expected or actual steady-state full-load current of the load, are given in
manufacturers' application guides and other technical documentation, such as
published by the American National Standards Institute (ANSI), Institute of
Electrical and Electronic Engineers (IEEE), National Electrical Manufacturers
Association (NEMA), and Electric Power Research Institute (EPRI).
The available industry guidance is intended to aid in selecting appropriate
breaker types, ratings and/or settings that will support the safety function
of the load equipment (Class 1E circuit) by (1) providing power reliably and
not unnecessarily inhibiting normal starts, (2) providing adequate fault
protection for the affected circuit(s) (equipment damage/fire protection), and
(3) maintaining adequate protection for other safety functions or Class 1E
circuits by isolating only the affected circuit(s) on faults or sustained
overload conditions (breaker coordination). Attachment 1 gives examples of
the available industry guidance.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.
/s/'d by CIGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contact: Stephen Alexander, NRR
(301) 504-2995
Attachments:
1. Examples of Industry Guidance on
Application and Testing of
Molded-Case Circuit Breakers
2. List of Recently Issued NRC
Information Notices
. Attachment 1
IN 92-51, Supp. 1
April 11, 1994
Page 1 of 1
EXAMPLES OF INDUSTRY GUIDANCE ON
APPLICATION AND TESTING OF MOLDED-CASE CIRCUIT BREAKERS
1. ANSI/IEEE Standard 242-1986, "IEEE Recommended Practice for Protection
and Coordination in Industrial and Commercial Power Systems."
2. EPRI Report NP-7410, "Breaker Maintenance," Volume 3, "Molded Case
Circuit Breakers," September 1991.
3. EPRI Report EL-5036, "Power Plant Electrical Reference Series,"
Volume 8, "Station Protection," pp. 8-61, 8-64, 8-66, and 8-147.
4. NEMA Standards Publication AB 3-1984, "Molded Case Circuit Breakers and
Their Application."
5. Heberflein, G. Erich, Jr. (IEEE, Allen-Bradley Co.), "Addressing
Nuisance Tripping of Instantaneous Trip Breakers in High Efficiency
Motor Applications," Pulp and Paper Industry Conference, 89-07.
6. Bartheld, R.G., "Motor Inrush Currents," NEMA Presentation, SCI2 Motor
Control Center Committee, June 1985.
7. Hartman, C.N., "Understanding Asymmetry," "IEEE Transactions on
Industrial Applications," Volume IA-21, No. 4, July/August 1985.
8. NEMA Standard MG 1-1988, "Motors and Generators" (nominal values for
locked-rotor KVA per HP designated by a nameplate letter code).
9. NEMA Publication AB 4-1991, "Guidelines for Inspection and Preventive
Maintenance of Molded Case Circuit Breakers Used in Commercial and
Industrial Applications" (cited in original notice)
