IE Circular No. 77-03 - Fire Inside a Motor Control Center
CR77003
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555
MEMORANDUM FOR: J. P. O'Reilly, Director, Region I
N. C. Moseley, Director, Region II
J. G. Keppler, Director, Region III
E. M. Howard, Director, Region IV
R. H. Engelken, Director, Region V
FROM: Dudley Thompson, Acting Director, DFO, IE
SUBJECT: IE CIRCULAR NO. 77-03- "FIRE INSIDE A MOTOR CONTROL
CENTER"
Enclosed is IE Circular 77-03 for distribution to all nuclear power reactor
facilities with an operating license or construction permit for information
only.
This Circular should be forwarded by mail or other means within five working
days of receipt of this memorandum by your office. A draft copy of the
transmittal letter is enclosed also.
Dudley Thompson, Acting Director
Division of Field Operations
Office of Inspection and Enforcement
Enclosures:
1. Draft Transmittal
Letter to Licensee
2. IE Circular 77-03
.
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
IE CIRCULAR 77-03
Date: February 28, 1977
Page 1 of 2
FIRE INSIDE A MOTOR CONTROL CENTER
DESCRIPTION OF CIRCUMSTANCES
On November 22, 1976, a fire occurred in a safety-related ITE Imperial
Series 5600 Motor Control Center (MCC) at the Three Mile Island Unit No. 2
facility. The fire was caused by partial engagement of the plug-in connector
assembly of a combination circuit breaker/starter unit with the vertical
bus. This resulted in a high resistance contact between one connector stab
and a vertical bus bar with localized overheating of the stab. The hot stab
ignited a vertical insulation backwall which was made of a fiberglass
polyester. This backwall was provided as a personnel safety feature by the
vendor to provide a barrier between the normally energized vertical bus and
the breaker/starter unit compartments.
A similar fire occurred in an ITE Series 5600 Motor Control Center at the
Trojan Nuclear Station on March 4, 1976, as a result of a mis-stab of the
plug-in connector assembly to the vertical bus. Subsequent to this fire at
the Trojan facility, ITE reportedly issued a technical instruction letter to
nuclear power plant licensees using these motor control centers to advise
them of the potential connector misalignment problem and recommended
procedural steps to preclude future occurrences.
This instruction was received by the Three Mile Island Unit No. 2 licensee
and was in the process of being implemented; however, the breaker/starter
unit which caused the fire at that facility had not yet been checked as
recommended in the ITE Instruction Letter. A copy of ITE's technical
instruction letter is attached.
The MCC fire at the Three Mile Island Unit No. 2 facility essentially
destroyed the entire motor control center. The insulation backwall continued
to burn after the MCC was de-energized and the fire progressed upward and
ignited a similar plastic barrier near the top of the MCC which was provided
to separate the horizontal bus from the wire-way. The fire apparently
continued along the horizontal barrier igniting the other vertical backwall
barriers and destroyed most of the breaker/starter unit in the MCC.
.
Revised
IE Circular 77-03
Date: February 28, 1977
Page 2 of 2
RECOMMENDED ACTION TO LICENSEES:
If motor control centers similar to those described are in use in safety
related systems or in proximity to safety related systems, you should verify
that your maintenance procedures contain adequate provisions to insure that
the electrical stab connection in these units are properly aligned to their
respective vertical bus bars to assure that a positive and full engagement
exists between the connector stab and its associated vertical bus bar.
Specifically you should review the ITE technical instruction letter and
follow the recommended procedure outlined in this letter.
You should also determine if the materials which are used for bus insulation
and supports in these motor control centers have flame retardant ratings
equal to or better than a 94 V-O classification as covered in UL Standard
No. 94 entitled, "Tests For Flammability Of Plastic Materials."
No written response to this Circular is required. If you require additional
information regarding this matter, contact the Director of the appropriate
NRC Regional Office.
Attachments:
1. ITE's Technical
Instruction 1tr
2. Instruction Procedure for
ITE 5600 Motor Control
Center Units
.
COPY OF ITE TECHNICAL INSTRUCTION LETTER
Gentlemen:
All motor Control Centers, 5600 Series, are built and factory inspected to
Class IE specifications to insure that when shipped to the jobsite they will
function as intended.
In spite of the rigorous and thorough inspection of Class IE equipment by
I-T-E, as well as, the separate inspection group contracted by the buyer, a
malfunction occurred at the Trojan Nuclear Facility of Portland General
Electric on March 1976 - MCC #B56 shipped on December 15, 1972. The
malfunction appears to be due to an apparent mis-stab of the plug-in
connector assembly to the vertical bus on a Size 4 combination circuit
breaker starter.
All front mounted control centers having rear access are visually inspected
to insure proper engagement of stabs on the bus. While the thorough final
inspection made, just prior to shipment are documented the validity of such
factory inspections are destroyed whenever starter units are moved from one
compartment to another at the jobsite due to layout or wiring changes or
when starters are removed for ease of pulling load cables,or maintenance of
the unit.
Motor Control Centers, in accordance with the state-of-the-art, are designed
to have plug-in capabilities for ease of changes or maintenance. However, it
cannot be assumed that the plug-in units can be treated without regard to
strict attention to handling and protecting while it is removed from the
vertical sections. Deformation of stabs can occur if units are not protected
against mechanical damage. For your field inspection of units, we can supply
you with a bus stab guage and instructions which will insure that the units
are reinstalled correctly.
Due to the critical loads involved on Class IE equipment, we suggest that
stabs be visually inspected where possible. Motor Control Centers with front
mounted units only, 20" deep, can be visually inspected by merely opening
the two hinged, half height rear doors. However, with back-to-back mounted
units, or Motor Control Centers mounted against the wall, the inspection of
the stabs can only be performed with the use of special equipment.
If there is any doubt as to the thoroughness of such field inspection on
units now installed, an inspection should be performed at the earliest
possible time for possible mis-stab of the plug-in connector assembly to the
vertical bus.
.
-2-
With respect to the bus stab gauge referenced to in Paragraph 4, one of
these bus stab guages will be forwarded to you on a "no charge" basis as
soon as you can advise us the exact shipping address and to whose attention
is should be shipped.
.
Attachment 2
INSTRUCTION PROCEDURE FOR
FIELD VERIFICATION OF PROPER
BUS PLUG-IN FINGER ALIGNMENT
FOR I-T-E 5600 MOTOR CONTROL
CENTER UNITS
CAUTION: Whenever a starter plug-in unit has been removed from the Motor
Control Center the bus plug-in feature must be verified for
correct alignment of the bus plug-in fingers using I-T-E Gauge No.
FB-:2104-1C as follows:
(1) Slide alignment gauge over the rear flanges of the starter mounting
panel. Left to right orientation of the fixture must match the plug-in
assembly on the rear of the panel?
(2) Seat the stops on the inside of the ends of the alignment gauge against
the flanges of the starter mounting panel as illustrated.
(3) The 1/4" round bars welded to the gauge approximate the vertical bus
positions in the MCC. These bars should be within the flared ends of
the plug-in fingers, allowing the fingers to self-center themselves
lightly around the bars. If fingers are not deformed, there will be no
problem visually verifying correct self-centering engagement as
illustrated below.
(4) If the self-centering action is not properly achieved, the plug-in
assembly must be repaired or replaced.
(5) After correct gauging, the starter should be handled with care and
immediately installed into the Motor Control Center.
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021