ML20034E329

Package Consisting of Draft Generic Ltr Issued to All Licensees of Operating Nuclear Power Plants & Holders of Const Permits for Nuclear Power Plants Re Premature Tripping & Inadequate Testing of Molded Case Circuit Breakers

ML20034E329
Person / Time
Issue date:	11/20/1991
From:	NRC
To:
Shared Package
ML20034E327	List: ML20034E326 ML20034E329
References
FOIA-92-466 NUDOCS 9302250309
Download: ML20034E329 (10)

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{{#Wiki_filter:, t fjpTT siL ys3 iJi' tid 2 NOV 2 01991 t l TO: ALL LICENSEES OF OPERATING NUCLEAR POWER PLANTS AND H0ll)ERS OF CONSTRUCTION PERMITS FOR NUCLEAR POWER PLANTS

SUBJECT:

PREMATURE TRIPPING AND INADEQUATE TESTING OF THE INSTANTANEOUS i TRIP FEATURE OF MOLDED-CASE CIRCUIT BREAIIRS l PURPOSE: t The purpose of this generic letter is to provide information and to define the staff position regarding the application of molded-case circuit. breakers (MCCBs) t and testing of their instantaneous trip feature.

1.0 BACKGROUND

The information and staff positions described herein are based on the following: 1.1 An assessment of current inspection, maintenance, and monitoring methods for detecting degradation in MCCBs conducted as part of the NRC's Nuclear Plant Aging Research (NPAR) program. 1.2 A survey of industry practices and visits to nuclear power plants to observe the implementation of test procedures, as a part of the NPAR program. 1.3 Information on MCCB test methods and practices obtained during inspec-tions of nuclear plants and obtained from three of the major MCCB manufacturers (General Electric, Westinghouse, and ITE-Siemens) and other vendors who perform MCCB testing. This information was gathered during NRC's inspections and assessments of licensee and vendor procurement and commercial grade dedication activities. 1.4 Licensee event reports (LERs) and licensee and vendor 10 CFR Part 21 notifications regarding instances of premature tripping (tripping at a current level lower than the MCCBs' instantaneous setting or range) of Westinghouse and I-T-E MCCBs at D.C. Cook, Catawba, and Shearon Harris. Information from these sources revealed (1) potential inadequacies in the manner in which the instantaneous trip feature of MCCBs is tested, (2) . problems in out-of-tolerance operation of instantaneous magnetic trip functions in certain MCCBs, and (3) problems in the determination of MCCB application and coordination requirements (particularly with motor loads) and in the analyses (particularly of motor starting transients) necessary to properly specify. replacement MCCB ratings and dete mine operating settings. On the basis of these observations, the NRC staff has concluded that, although adequate technical guidance has been available for some time in the form of NRC generic communications, industry standards, and manufacturers' technical documentation, erroneous interpretation of this information led 'to the problems observed. 0001.0.0 9302250309 910914 i PDR FOIA \\ M SON 92-466 PDR

i f OQgFl Unr8 } 2.0 Description of Circumstances: Instances of out-o f-tole rance tripping of MCCB instantaneous magnetic trip i functions have been reported in recent years. The following examples are typical: I L l 2.1 A newly purchased replacement Westinghouse MCCB Type HFB3125 (ambient j compensating) tripped upon. startup of a hydrogen skimmer ~ fan motor. at the Catawba Nuclear Power Plant during postinstallation testing. An old, nontrace-l able MCCB was being replaced pursuant to NRC Bulletin 88-10, " Nonconforming i j Molded-Case Circuit Breakers." The startup trip was not expected to occur since the inrush transient peak current value for the fan motor was thought' to - be well below the lower limit. of the manufacturer's published tripping charac- { teristic curve for the instantaneous trip applicable to that MCCB. I 2.2 A similar instance involved a turbine room sump pump breaker at the Donald C. Cook Nuclear Power Plant and similar Westinghouse MCCBs. l 2.3 The Shearon Harris nuclear power plant experienced some test failures'of I 480-VAC rated MCCBs manufactured by the I-T-E Circuit Protection Division of { Siemens Energy & Automation, Inc. (ITE-Siemens) in Wilmington, North Carolina. l The MCCBs were purchased as commercial-grade replacements for some older Class - l 1E (safety-related) 600-VAC I-T-E MCCBs pursuant to NRC Bulletin 88-10. I 3.0 Discussion. 3.1 Premature Tripping: .I 3.1.1 The instances of premature tripping of Westinghouse MCCBs prompted notifications under 10 CFR Part 21 from the affected licensees and ' from Westinghouse. Westinghouse initially attributed the prematu; trips to ' the effect of the ambient compensating feature 'of.the time delay overcurrent trip ] function of the MCCB, which was believed to make the~ MCCB more sensitive. Further testing and analysis by Westinghouse indicated that most Westinghcuse thermal-magnetic MCCBs with nonadjustable' magnetic instantaneous trips could be susceptible to tripping outside,,the published curves near the so-called " knee,"' i i.e., in the inverse-time or "I"t" region of the characteristic curve above 300 [ percent of the current rating and up to the lower limit of the vertical l instantaneous magnetic portion of the curve (see Attachment 1). .} I Westinghouse eventually attributed this behavior to interaction between the thermal and magnetic trip functions under overload conditions in the affected operating region. The interaction occurs when the thermal trip element has i deflected to the point of pressing on the MCCB's tripper bar at the same time that the magnetic armature is vibrating against the tripper bar. This J interaction causes a combination trip below the lower tolerance limit of the ~ design tripping rat.ge as shown on the published curves. Westinghouse notified nuclear utilities of the problem in 10 CFR Part 21 notifications and in one of its generic letters, which recommended testing these MCCBs in their intended circuits before releasing them for plant operation. This ' disclaimer and l recommendation has also been included on certificates of conformance provided with dedicated Westinghouse MCCBs sold as safety-related items by the Replace-1 ment Component Services Operation of Westinghouse Nuclear Services Division. l 0002.0.0 l

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i 3.1.2 .I-T-E MCCB testing by Shearon Harris staff resulted in what the manufac-i turer, ITE-Siemens, claimed were erroneously low magnetic trip current values, ~ but testing at the factory by ITE-Siemens showed that some of the MCCBs were ' indeed tripping out of the range indicated in the manufacturer's published specifications and time-current characteristic curves. The MCCBs were also tripping, apparently by their instantaneous magnetic trip function, upon star-ting of their induction motor loads. However, the older MCCBs, which had not -i experienced premature tripping, were found to trip significantly above the instantaneous trip current values on their applicable time-current curves. t As part of the information ITE-Siemens provided the NRC regarding the instance ) of premature tripping of ITE MCCBs at Shearon Harris, ITE-Siemens stated that like many coma.ercial grade thermal-magnetic MCCBs, ITE MCCBs with nonadjustable 1 magnetic trips do not normally undergo production magnetic calibration. l However, under an ITE-Siemens policy in effect for about three years, magnetic j calibration of MCCBs with nonadjustable magnetic ~ t rips is done for special l order, commercial-grade MCCBs for nuclear plants that request a certificate of [ compliance in their purchase orders. l Because some brands of commercial grade MCCBs with nonadjustable magnetic I trips do not normally undergo production magnetic calibration, addressees I should not assume that this testing is accomplished at the factory. Therefore, for applications in which premature tripping or circuit coordiuation is. a concern, field verification may be required. Such testing should produce-i reasonably reliable results using proper procedures and other technical docu-mentation along with the guidance in this generic letter and using suitable. test equipment and properly trained technicians. Note that the major MCCB j manufacturers contend that the design curves (which are composites of the l separate thermal and magnetic response curves) were intended to show predicted design performance characteristics for installed MCCBs with all poles loaded j under plant operating conditions for use in application and coordination analysis, not necessarily as acceptance criteria for field testing; although, 1 the data for the instantaneous trip curves is generally obtained in single-pole testing. Nevertheless, most manufacturers' curves do contain - single-' f pole maximum clearing-time curves around the 300-percent range for thermal-l time-delay testing; and the manufacturers concede that with appropriate l tolerances applied and proper test methods employed, the magnetic and thermal curves can reasonably be used for MCCB field performance verification tests. ), 3.2 Testing Problems: I i 6 3.2.1 One of the instances of out-of-tolerance MCCB tripping involving a l Westinghouse MCCB is illustrative of problems encountered with testing at l numerous licensee and vendor facilities. In - this case, the MCCB tripped prematurely upon motor starting during postinstallation tests. After t.be faulty MCCB was sent to a test facility for testing to NRC Bulletin 88 L requirements, the test facility incorrectly reported that the breaker was operating satisfactorily. Further testing by the utility and by Westinghouse proved that the breaker was not operating properly in that the instantaneous l trip occurred at significantly less current than shown on the manufacturer's published instantaneous trip curve. A review of the test facility's test [ report showed no evidence that each pole of the circuit breaker had been. [ tested below the lower limit of manuf acturer's trip curve (with appropriate i 0003.0.0 i I ~ -~,

? r tolerances applied). The test current applied by the test facility was 1000 [ amps, which was 800 percent of the 125-amp full-load current rating for the l MCCB in question. However, the Westinghouse trip curve showed that the lower j end of the trip current range for the nonadjustable instantaneous magnetic trip } of this MCCB was approximately 550 percent of the current rating. Therefore, i i by only injecting a test current above the Iower 1dmit of the' design trip l range, the testing failed to detect that the instantaneous magnetic trip of 1 this particular out-of-tolerance MCCB would have first occurred well below the lower limit of the design trip range. The instantaneous trip appeared to be I working properly when tested at current values above the minimum design trip current of 550 percent of rated current (approximately 690 amps). However, in subsequent testing, this MCCB tripped instantaneously (i.e., on the magnetic trip with no intentional time delay) when tested with current values below 517 amps (which is approximately 75 percent of the 550-percent lower limit of the design trip current range). i 3.2.2 Attachment I to NRC Bulletin 88-10, dated November 22, 1988, provided guidance for field testing of MCCBs for which licensees were not able to l establish verifiable documented traceability to the eriginal circuit breaker manufacturer (CBM) as an alte rnative to replacing them with traceable MCCBs under certain specified circumstances. Section 2.5.1 of the attachment prescribed testing each pole of fixed (nonadjustable) instantaneous trip i MCCBs for pickup of the instantaneous trip function (i.e., the overcurrent i n level at which the magnetic trip will first actuate). The stated acceptance j criterion was that "each pole must be between 75% and 125% of the instantaneous trip rating, and the trip time should not exceed 0.1 seconds (6 cycles)." This j j acceptance criterion was based in part on the guidance provided in the National { p Electrical Manufacturers Association (NEMA) standard for field performance l verification of

MCCBs, NEMA AB2-1984.

Note that NEMA AB2-1984 has now been superseded by NEMA AB4-1991, which includes t.he technical information in NEMA AB2-1984 and provides additional guidance, particularly regarding i nonadjustable magnetic trip MCCBs. Although the 125-percent tolerance i corresponded to the factory tolerance given in AB7 for the low setting of i adjustable instantaneous magnetic trips, it was considered a reasonable tolerance to apply generally for the purposes 'of the bulletia provided sound l test methods were employed. The intent of the acceptance criterion was to demonstrate that the NCCB first tripped at a current level above 75 percent of I the sett.ing (for adjustsble trips) or above 75 percent of the low end of the design range (for nonadjustable trips) and not below these values, but would trip before exceeding 125 percent of the setting or hiv end of design trip i j range as applicable. Therefore, this nonadjustable trip should have been tested to demonstrate that it would not trip for' current values below 413 (550 X 0.75) percent of rated current. This level is 75 percent of the lower trip curve value (lower end of design range) at. 550 percent of rated current. 1 The test should also have demonstrated that the MCCB would trip before excee-ding 2250 (1800 X 1.25) percent, which is 125 percent of the upper trip curve value of 1800 percent of rated current. 3.3 MCCB Application Concerns: 3.3.1 During the research and inspections discussed above, the NRC also obtained information indicating that motor start transients may sometimes be underestimated or overlooked entirely during the analysis done to determine the 0004.0.0

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'IC I 5 l correct replacement MCCB parameters for a given application. Specifically, the .t inrush current transient that occurs in the first cyc?es after starting the l induction motor is a function of locked-rotor current (LRC) and is typically 2 1.8 t'o 2.8 times the value of LRC (more for high-efficiency motors). LRC l nominally varies from 3 to 8 times rated full-load current (FLC) 3although LRC is functionally unrelated to FLC), and is typically of 0.5 to 2.5 seconds dura-tion after starting. Hewever, LRC may sometimes be confused with the-inrush i ~ transient, which is of sufficient amplitude and duration to cause a magnetic trip of the supply MCCB if its magnetic trip setting is chosen based on avoiding only LRC. Depending on the inductive reactance-to-resistance ratio. i (X/R) of the load circuit and the voltage phase angle upon closing the starter contacts, the value of the resultant asymmetrier1 current (with a DC offset component) can increase the peak inrush transient current by as much as-100 percent. The maximum instantaneous magnetic trip setting of 1300 percent of FLC prescribed in the Nat2unal Electric Code (NEC) for circuit protection may be less than the inrush transient peak and thus insufficient on random occasions to prevent unwanted tripping. However, the NEC does contair special exceptions that allow use of MCCBs under certain circumstances rated'at 400 percent of the expected full load current. In any case, the MCCBs' performance must be verified to meet the individual applicatica design requirements. 3.3.2 Licensees should exercise caution in evaluating replacement MCCBs for l i meeting the application requirements to ensure that considerations such as overload protection requirements and coordination requirements (such as those of 10 CFR Part 50, Appendix R) are not violated when attempting to make i adjustments to specifications to accommodate starting transients when using thermal-magnetic MCCBs-. To avoid this problem, most industry technical infor-mation and MCCB application guides recommend the use of a special class of i MCCBs that were designed specifically for motor loads. These MCCBs are fitted j only with instantaneous magnetic trips that are independent of ' thermal inter-action for rapid and precise fault or short-circuit protection. The trip setpoints are normally adjustable and more reliably accurate than those in 4 3 thermal-magnetic MCCBs. These

MCCBs, commonly called "moter circuit protectorv (Westinghouse term), are intended for use with thermal overload contactors or starters that provide the coordinated sustained overload and low-level fault protection.

3.3.3 Nevertheless, once the application requirements have been established 1 and the replacement MCCB has been successfully bench tested to verify its i conformance to its design performance requirements, the importance of thorough postinstallation/preoperation testing to prove satisfactory performance with l .ystem interactions cannot be overemphasized. i 4.0 Safety Significance: i i 4.1 A safety-related MCCB that trips below the lower tolerance limit of its l design instantaneous magnetic trip current range as shown by the vertical l portion of the manufacturer's published trip curves (see Attachment 1) could render its safety-related load equipment effectively inoperable when attempting l to start up the equipment. This condition could be particularly challenging to safety systems and operators upon starting the equipment from' a standby l condition following loss of its counterpart in the alternate train. l 4 0005.0.0 l I -i

.. ~. -. - _. - .mm m 1 i i 4.2 The failure to properly analyze the load circuit' (particularly the worst-l case peak inrush transient current upon motor starting) when determining the required ratings and settings and coordination requirements for replacement ) MCCBs in safety-related plant applications could result in specifying MCCBs_ j that are prone to premature tripping even when operating within specifications. ] 4.3 The failure to properly and conclusively test the instantaneous magnetic trip feature of safety-related MCCBs (including postinstallation testing to verify operability with system interactions) could result in failure to detect an out-of-tolerance condition of the magnetic trip (or a misapplied, underrated MCCIO, thus allowing use of the MCCB in a circuit in which it could prevent-( starting of safety-related equipment (particularly mot. ors) .by-tripping prematurely. l 5.0 Staff Positions: I The NRC staff has concluded that the standard test methods _ and practices summarized below are acceptable for field verification of MCCB instantaneous magnetic trip performance in assuring compliance with the Commission's regula-I tions. As previously noted, this information is included in existing industry standards and other technical documentation, including that provided by MCCB canufacturers. To ensure proper interpretation of existing - guidance, this generic letter provides explicit, detailed guidelines for field testing the j instantaneous magnetic trip feature of MCCBs. l 1 5.1 In testing the instantaneous magnetic trip function of MCCBs for safety-related service, the application of test current (either continuous or-short 'l duration pulses) should be such that an accurate determination can be made that the trip setpoint characteristic of the particular MCCB (pickup point or lowest l current level ta. which the instantaneous trip will actuate) falls within the design range. Determination that the trip setpoint is within the design range (with appropriate tolerances) provides assurance that the MCCB will not trip below the minimum acceptable current level, but will trip above this value and l at least before exceeding the maximum allowable current level. i However, in cases in which it is desired to trend the performance of MCCBs, it would be necessary not only to show that the instantaneous trip point falls within the acceptable range, but to determae _ the actual current-level within j that range where the trip first occurs. This would be necess:ry in order to establish a baseline value during acceptance testing that would form the basis for comparison with future results obtained durirg periodic surveillance tests that may be prescribed for the particular application. j 1 To determine trends, MCCP manufacturers such as Westinghouse _ and General Electric, and suppl >ers of !!CCB test equipment such as Multi-Amp, indicate that increasing curren: values in closely spaced increments should be applied to verify the instantaneous trip setpoint. However, NUREG/CR-4715, "An Aging Assessment. of nelays-and Circuit Breakers and System Interact. ions,". Volume 1, in which some of the results of the NPAR program were published, identified the i possibility that excessive. heating of-directly heated thermal overcurrent trip l binetal elements 'could occur during instantaneous trip testing (if not l correctly performed) and have the effect of annealing the bimetals, thus j potent.ially causing changes in the thermal response characteristics of the l 0006.0.0-j

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.j MCCB.. Although. measurable effects on thermal performance attributable to i magnetic testing have not been conclusively demonstrated in controlled studies, [ the potential-for damage can be minimized by use of test pulses of the shortest ( -practical duration to minimize heating consistent with manufa cture rs ' test procedures. However, for conclusive results, the test current must first be j applied at a value below the lower tolerance limit of the trip setting (or i lover trip curve) with a no-trip result and then increased until the MCCB trips. If t.he exact. point within the design range where the' trip first occurs does not need to be determined, a single, no-trip result at or just below the lower tolerance limit of the curve (or setting) and a trip result at a point within the' range should indicate acceptable performance with respect to manufacturers' specifications. 5.2 The two standard test methods for this test, known as magnetic calibra-tion, are the "run-up" (or " ramp") and the " pulse" methods. The techniques for j and the advantages and disadvantages of both methods are described in various technical publications from manufacturers and from industry and professional organizations. The pulse method, although requiring more sophisticated test [ equipment, is considered more accurate and conclusive. Note that the run-up l test may yield slightly lower trip currents than the pulse test. j In order to meet manufacturers' specifications, test conditions sho_uld be j established to simulate, to the extent possible, the conditions under which the j time-current curves for the particular MCCB were developed under the guidelines of Underwriters Laboratories (UL) Standard UL-489. The test current in the j run-up method is applied in an increasing continuous ramp until the breaker trips at a rate-of increase fast enough to avoid interference by-the thermal { element. However, the instrument that indicates and/or records the current i level and the timer must be sufficiently responsive to accurately capture the peak instantaneous current level at the tripping point. In the pulse method, -l the short duration test pulses (typically less than 100 milliseconds) are l applied in successively higher current increments, starting below the' lower j tolerance .mit of the trip -setting or design range with brief cooling i intervals. The required test equipment must be capable not only of capturing l the peak instantaneous current and trip time, but should also be capable of l automatically closing its output contacts precisely at the instant that the voltage waveform is at its peak in order to minimize the DC offset error (of as much as 100 percent) introduced by asymmetrical current if the contacts close at other points in the cycle. Specifically, the following practices will facilitate obtaining accurate results: 5.2.1 In order to simulate installed conditions to demonstrate conformance i to application requirements, MCCBs should be tested while mounted on steel backplates or in dummy enclosures to reproduce flux distributions in the vicinity of the magnetic trip armature representative of actual installations. I i ~ 5.2.2 Test leads should be of the proper size cable and routed away from the-vicinity of the ' magnetic structures that are located near the lower back of the breaker case. 5.2.3 For standardization and to provide conclusiva results, poles should l be tested individually. l 0007.0.0 i l

~ ENT l 5.2.4 For conclusive results, initial test current should start below the i Iower tolerance limit of the design or trip curve range (or setting as applicable). - A no-trip result should be expected until the test current is above the lower tolerance limit of the trip curve (or setting). Note that i NEMA AB4-1991 recommends testing adjustable trips at. both the low and high } settings, but pulses at high settings should be minimized and as short as. practicable. i 5,2.5 The pulse - method is the most accurate, but, shoold. be used with test equipment capable of compensating for DC offset error from. asymmetry and } have sufficiently fast response to accurately sense and record current level at l the point of tripping. Trip time measurement capability may also be required j to demonstrate MCCB conformance to standards. { 5.2.6 Sufficient time (at least 5 minutes) should be allowed between each I test for the thermal element (if any) to thoroughly cool in order to preclude i attempting to reset the MCCB by hand immediately after each. trip should_ thermal interaction influencing the magnetic trip point. Additionally, j indicate if a thermal trip has also occurred. i l~ 5.2.7 If added assurance of accurate readings is desired, the test can be { repeated later using the last no-trip current value and the first trip f current value. If these two current values repeat, the results were accurate. l t 5.2.8 A generic tolerance of '-30 percent of the current value at the low end of the nominal magnetic trip range (vertical portion of curve) and +40 percent f of the high end (consistent with NEMA AB-4 values for nonadjustable ' trips) could reasonably be applied to the magnetic trip band (design range) ~ on the curves for field acceptance testing. The MCCB should not trip ' when tested i below the -30 percent value, but may trip above it; and the MCCB must trip l before test current exceeds the +40 percent value. Note that individual system l design requirements may be more restrictive and would take precedence over these generic acceptance criteria. In all cases, the practices discussed above should be considered in light of I system requirements and the manufacturers' procedures and recommendations. l The MCCB manufacturer should be consulted when problems with testing or MCCB performance arise. { 6.0 Requested Actions: Consistent with the guidance in NRC Generic Letter 91-05, " Licensee Commercial-Grade Procurement and Dedication Programs," and NRC Bulletin 88-10, addressees are not expected to initiate a comprehensive retesting program to reverify the inst.antaneous trip feature of MCCBs with safety functions involving this function except for individual MCCB testing in those specific. cases in which (1) premature tripping ~ (or loss of circuit coordination) of installed MCCBs is experienced during testing or operations or (2) other specific information is l obtained that may impugn the operability of certain installed safety-related - MCCBs or the suitability of previously tested MCCBs. However, addressees are expected to assess the applicability of this - guidance to their circumstances and adjust testing procedures and training as required to implement it accor-- dingly. No other specific actions are requested. 0008.0.0 r wn-.

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-{ 7,0 Reporting Requirements: No reports are required. 8.0 Backfit Discussion: The Commission's regulations in 10 CFR Part 50 require that safety-related. 't structures, systems, and components be tested to quality standards. These 1 general requirer.ents are contained.in General Design - Criterion (GDC) 3. (and -; j GDC 18 with respect to electrical power systems) of Appendix A; : and in. .l Criterion III, " Design Control," Criterion VII, " Control-of Purchased Material, -l Equipment and Services," and Criterion XI, " Test Control," of Appendix B. j Operation and testing of MCCBs consistent with the staff positions presented i above are necessary to ensure compilance with these regulations. This is aL I backfit, but is covered by 10 CFR 50.109(a)(4). l 7 ii James G. Partlow l Associate Director for Projects .l Office of Nuclear Reactor Regulation Technical Contacts: S.K. Aggarval Office of Nuclear Regulatory Research l (301) 492-3829 S.D. Alexander Office of Nuclear Reactor Regulation (301) 492-0995 i A.S. Gill Office of Nuclear Reactor Regulation (301) 492-3316 i Attachments: 1. Typical MCCB Time-Current Characteristic Curves 2. References and Related NRC Ceneric Communications 3. List of Recently Issued Generic Letters i -l 5 + 0009.0.0 I a

3 I t TYPICAL MCCB TIME-CURRENT CHARACTERISTIC TRIP CURVE (FIGURE 5.3 OF NEMA AB3-1984, ANNOTATED) i l 10000 g BECOMES 5000 - ASYMTOTIC TO 1.0 X RATED j , CURRENT LINE - 1: 1000 -- 1 i SINGLE-POLE i i MAX AT 25*C I k.4,! 2 l 500 '1 t' ir ' i THERMALTIME DELAY !' i PORBON ~ i \\ MAXIMUM / T i VCLEARING I 99 _ i TIME M i j r E 50 -- \\ I I [ MIN MUM., N CLEARING f TIME F ' KNEE" 10 r - POSSIBLE THERMAL-MAGNETIC S E I INTERACTION DURING OPERATION 5 1 Q l C 0 j N. g N i j. +40% D i T: 1 / FIELD 1 TOLERANCE S 1 -: i/ (MUSTTRIP i !.30% .5 -'N: FIELD 4 BEFORE i

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- (NUTRIPS i i IN HERE) f DESIGN TRIP l i i i RANGE" - i .1 i i MAGNETIC i i ilNSTANTANEOUS .0c- -Ni i ' PORTION i !(NO INTENTIONAL USUALLY CUT OFF ' _i j DELAY) AT INTERRUPT RATING AS-p/ FUNCTION OF RATED OR i i k((////d,, a,ERVICEVOLTAGE l S iil s .01 4

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