United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular No. 77-03 - Fire Inside a Motor Control Center


                               UNITED STATES 
                          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 


Enclosed is IE Circular 77-03 for distribution to all nuclear power reactor 
facilities with an operating license or construction permit for information 

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

1.   Draft Transmittal
       Letter to Licensee
2.   IE Circular 77-03

                          WASHINGTON, D. C. 20555 

                                                  IE CIRCULAR 77-03 
                                                  Date: February 28, 1977 
                                                  Page 1 of 2 



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. 


                                                  IE Circular 77-03 
                                                  Date: February 28, 1977 
                                                  Page 2 of 2 


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. 

1.   ITE's Technical
       Instruction 1tr 
2.   Instruction Procedure for
       ITE 5600 Motor Control
       Center Units



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. 


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. 

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