Information Notice No. 91-20: Electrical Wire Insulation Degradation Caused Failure in a Safety-Related Motor Control Center

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                               March 19, 1991

                                   CAUSED FAILURE IN A SAFETY-RELATED MOTOR 
                                   CONTROL CENTER 


All holders of operating licenses or construction permits for nuclear power 


This information notice is intended to alert addressees to potential 
problems resulting from degradation of dielectric polyvinyl chloride (PVC) 
machine tool wire insulation used in Class 1E and non-Class 1E motor control 
center (MCC) equipment wiring.  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:

On January 4, 1991, during a refueling outage at the H. B. Robinson Steam 
Electric Plant, Unit 2, the "B" feedwater motor-operated isolation valve 
failed to close during surveillance testing.  The licensee investigated the 
incident and identified a clear, hardened coating on one of the contactors 
in one of the compartments in the associated Class 1E (safety-related) motor 
control center (MCC).  During further investigation, the licensee identified 
a green "liquescent" substance coming out of the wires that connect the 
associated MCC electrical breaker to its forward and reverse contactors and 
that was hardening on the contactors (see Attachment 1).

The licensee inspected 110 other compartments in four Class 1E 480 and 
208 volt (V) MCCs and in numerous balance-of-plant MCCs.  The same type 
dielectric machine tool wire was used in these compartments and various 
amounts of the green liquescent substance were present on the wires. 

A similar event had occurred at H. B. Robinson a week earlier when the 
isolation valve for a containment fan cooler return line radiation monitor 
failed to operate.  


                                                            IN 91-20
                                                            March 19, 1991 
                                                            Page 2 of 3 

The licensee determined that the same type of dielectric machine tool wire 
was used in this MCC as was used in that for the "B" feedwater isolation 
valve.  There was also a green liquescent substance on the forward and 
reverse contactors located in the associated MCC compartment.  The licensee 
now believes that this green liquescent substance caused the failure of the 
isolation valve for the containment fan cooler return line radiation 

The H. B. Robinson licensee performed a laboratory analysis and identified 
the green liquescent substance as a "vegetable oil plasticizer," which is 
present in the composition of the cable's PVC insulation.  The analysis 
indicated that the green liquescent substance is a conductor in liquid form, 
but when dried, may either be a conductor or an insulator, depending on the 
amounts of copper oxide salts present.  In addition, the color of the 
substance can vary depending on the amount of copper oxide salts present.  

In 1988, a similar event occurred at the San Onofre Nuclear Generating 
Station, Unit 1 (SONGS1), when the Southern California Edison Company (the 
licensee) was performing maintenance (i.e., cleaning) on several wires 
attached to the 480/120V control trans-formers, located in a Class 1E MCC.  
The licensee stated that after performing routine cleaning of the electrical 
wires with isopropyl alcohol, they found a green substance seeping from the 
wires attached to the primary side of the control transformer.  Subsequent 
investigation later identified that of approximately 96 compartments 
inspected, 31 showed evidence of this green substance.  This MCC contained 
both Class 1E and non-Class 1E equipment. 

The SONGS1 licensee found that a black and green sticky gel was present on 
the entire length of the wire inside the wire's insulation and identified it 
as an interaction between the corrosion products formed on the surface of 
the copper wire and the chlorine leaching from the wire's PVC insulation.  
The licensee stated that the sticky consistency of the material was a result 
of a slight depolymerization of the inner surface of the insulation and that 
moisture from the environment had been transported, by capillary action, 
into the wire strand bundles.  The licensee suspected that an electro-
chemical reaction contributed to the insulation degradation, which may have 
been accelerated by the high voltage differential across the control 
transformer.  The licensee described the wire as a dielectric 600V machine 
tool wire (AWG #14).

The H. B. Robinson licensee identified the wire used between the MCC breaker 
and the contactors as a dielectric (insulated) PVC machine tool wire (AWG 
#12) with a temperature rating of 105� C.  The cable is black in color, and 
the specific manufacturer has not been identified to date.  Westinghouse 
(the manufacturer of the MCCs) has stated that this wire was procured in the 
1962-65 time frame and records pertaining to this wire do not exist.  It is 
possible that several different vendors may have manufactured similar wire 
and that this wire was not limited to any particular components (i.e., MCCs) 
or for use by any particular nuclear steam supply system (NSSS) vendor.  The 
H. B. Robinson licensee has replaced all affected Class 1E MCC wiring with 
wiring identified as "Surprenant Wire and Cable CSA Type CL-1251 XLPE 600V, 
125� C."   

                                                            IN 91-20 
                                                            March 19, 1991 
                                                            Page 3 of 3 

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 NRR project manager.

                                  Charles E. Rossi, Director 
                                  Division of Operational Events Assessment
                                  Office of Nuclear Reactor Regulation

Technical Contact:  John Thompson, NRR
                    (301) 492-1171

1.  Diagram of A Typical MCC Compartment
2.  List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Wednesday, March 24, 2021