Part 21 Report - 1998-262

 
 

ACCESSION #: 9803250033







                                                        GE Nuclear Energy



                                   General Electric Company

                                   175 Curtner Ave. San Jose, CA 95125



March 19, 1998

98-01NR2.DOC

MFN 012-98



Document Control Desk

United States Nuclear Regulatory Commission 

Washington, DC 20555



Subject: Spring Return Binding in GE Type SBM Control Switches



This letter supersedes my previous letter (same subject) dated January

23, 1998.  Information is provided concerning the possible failure of

certain GE Type SBM Control Switches with the spring return function to

reset properly after operation.  The failure mode has been identified as

binding due to an interference between the rear brass bearing and the

phenolic rear bearing support.  GE Nuclear Energy (GE-NE) is

conservatively assuming that SSM Control Switches with the spring return

function manufactured since March 1996 may be subject to this failure

mode.



GE Type SBM Control Switches are manufactured by GE Electrical

Distribution and Control (GE-ED&C) Power Management, Malvern, PA as

commercial grade items.  GENE has dedicated these switches and supplied

them to several licensees as basic components for unspecified, safety

related applications.  Since the specific applications and associated

safety functions of the switches are not known to GE-NE, we have informed

known affected licensees pursuant to 10 CFR Part 21.21 (b) so that they

may evaluate the condition.  However, since additional licensees may have

obtained these devices through other dedicating entities, we cannot

assure ourselves that all end-users have been notified.  We are therefore

providing tills information to the NRC for appropriate action.



On January 6, 1998, GE-NE was advised by a licensee that a safety related

SBM Control Switch with the spring return function had failed to reset

(return to normal position) properly.  The switch (model

16SBMB3A02S1S2P1) was returned for evaluation by GE-NE with the support

of GE-ED&C Relay Product Engineering.  Additional failed switches were

subsequently identified and also returned for evaluation.



The failure mode has been identified as binding due to an interference

between the rear brass bearing and the phenolic rear bearing support The

root cause has been determined to be "post mold cure" shrinkage of the

phenolic material.  Post mold cure is normal for this material, and takes

place over a period of one-to-two years after molding.





GE's investigation has found that the mold used to make the phenolic rear

bearing support has worn over the years to a point where the diameter of

a part being produced, while still within specification, is near the

minimum allowable value.  It has also been determined that the existing

design does not address normal post mold cure shrinkage.  Thus, switches

that arc initially functional can exhibit sluggish return or binding

several months after assembly.  The switches identified by the licensee

as failing to reset properly had been operating correctly for

approximately one year.



The failed switches have been examined and, in each instance, the bearing

support hole was found to be undersized and the bearing was at, or near,

its maximum allowable diameter.  Testing has confirmed that switches with

bearings that are at their nominal diameter function properly, even when

the support hole is undersized.  Consequently, the failure is not seen in

every switch.  The failure is most likely preceded by a gradual increase

in the force required to operate the device, and a sluggish return.



Although the interference can exist in any SBM Control Switch, GE has

determined that this is only an issue in those switches that have the

spring return function.  In spring return switches, the reset spring does

not generate enough torque on the operating shaft to overcome the

additional friction resulting from the interference.  Thus, the operating

handle, and consequently the contacts, may not reset when the handle is

released from the momentary positions The switch contacts will function

when the switch is operated and, if the switch is manually returned to

the reset (normal) position, the contacts will also return to their

normal configuration.  Measurements indicate that the torque required to

return a switch to its normal reset position is less than two inch-

pounds.  During functional testing of several switches that exhibited

spring return binding, a licensee's operator was able to place the

switches in the reset (normal) position in all cases.



"Maintain position" SBM Control Switches may also have the interference,

but the additional force required to overcome the friction resulting from

the interference is not great enough to prevent proper function. 

Nonetheless, it is considered good practice to assure that these switches

are in the proper position, as indicated by the switch pointer, before

releasing the handle.



A loss of torque in the tie bolts that secure the switch components has

been observed in conjunction with the spring return binding.  The loss of

bolt torque is also the result of post mold cure shrinkage and, although

it has been noted in those devices that have failed, it is not the cause

of the binding and does not indicate impending failure.  Loss of tie bolt

torque due to shrinkage of the phenolic materials was considered in the

switch qualification, and requires no additional action.  There is no

need for inspection or retorquing of SBM Control Switch tie bolts.



All of the failed switches identified to date were manufactured in the

period from November 1996 through February 1997.  There have been no

reports of spring return binding in earlier SBM Control Switches, despite

the fact that these switches have been widely used in a variety of

applications for many years.



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Although test results demonstrate that most of the post mold cure

shrinkage occurs in the first twelve-to-eighteen months after molding,

GE-NE is conservatively assuming that SBM Control Switches with the

spring return function manufactured since March 1996 may be subject to

this failure mode.  These switches can be identified from the last two

characters (date code) shown on the green "QC Acceptance" sticker.  The

affected date codes are PL, RL, SL., TL, UL, VL, WL, XL, YL, ZL, NM, OM,

PM, RM, SM, TM, UM, VM, WM, XM, YM, ZM, NN and ON.



The SBM Control Switch design has been revised to address this

phenomenon, and switches manufactured beginning March 1, 1998 (date code

PN) are no longer susceptible to the failure mode.



There are two safety concerns with regard to spring return binding: (1)

possible damage to control circuitry caused by the circuit being

maintained in the momentary position for a prolonged period; and (2) the

possibility that control circuits will be prevented from performing their

proper function by the failure of the switch contacts to return to their

reset (normal) configuration.



The loss of the spring return action does not prevent the SBM Control

Switch from performing its function of providing manual circuit control

switching, and GE-NE's evaluation has determined that neither the need to

manually assist the reset action nor the loss of tie bolt torque will

prevent proper switch function or degrade the device qualification.



If SBM Control Switches with the spring return function, manufactured

since March 1996, have been installed in safety related applications, it

is suggested that station personnel who operate these switches be advised

to return the operating handle to the normal (reset) position after

operation.  This will preclude any control circuit difficulties.



It should be noted that all of the switches identified to date as failing

to reset properly had previously operated correctly for approximately one

year.  Control switches that are at least two years old are not subject

to the failure mode (binding due to post mold cure of the phenolic

material).  Any corrective action or preventive measure undertaken in

response to this Notification should not be necessary after the subject

switch is at least two years old.



If you have any questions, please call me at (408) 925-1019.



Sincerely,



Michael A. Smith, Program Manager

Safety Evaluations

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cc. S. D. Alexander (NRC-NRR/DISP/PSIB)

G.  C.  Cwalina (NRC-NRR/DISP/PSIB)

J.  F.  Quirk (GE-NE)

H.  J.  Neems (GE-NE)

G.  W.  Sanders (GE-NE)

J.  A.  Steininger (GE-NE)

J.  Teague (GE-ED&C/Malvern)

GE-NE PRC File



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