UL 840-2000 Insulation Coordination Including Clearances and Creepage Distances

UL840

ISBN1-55989-391-5 Insulation Coordination Including Clearances and Creepage Distances for Electrical Equipment

This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

Underwriters Laboratories Inc.(UL)

333P?ngsten Road

Northbrook,IL60062-2096

UL Standard for Safety for Insulation Coordination Including Clearances and Creepage Distances for Electrical Equipment,UL840

Second Edition,Dated May20,1993

Revisions:This Standard contains revisions through and including September7,2000.

UL is in the process of converting its Standards for Safety to the Standard Generalized Markup Language (SGML),and implementing an SGML compliant document management and publishing system.SGML-an international standard(ISO8879-1986)-is a descriptive markup language that describes a document’s structure and purpose,rather than its physical appearance on a page.Signi?cant bene?ts that will result from UL’s use of SGML and these new systems include increased productivity,reduced turnaround times, and data and information consistency,reusability,shareability,and portability.However,the fonts, pagination,and general formatting of UL’s new electronic publishing system differ from that of UL’s previous publishing system.Consequently,when revision pages are issued for a Standard with the new publishing system,these differences may result in the printing of pages on which no requirements have been changed-these additional pages result from relocation of text due to repagination and reformatting of the Standard with the new publishing system.

Text that has been changed in any manner is marked with a vertical line in the margin.Changes in requirements are marked with a vertical line in the margin and are followed by an effective date note indicating the date of publication or the date on which the changed requirement becomes effective.

The revisions dated September7,2000include a reprinted title page(page1)for this Standard.

The revisions dated October6,1999were issued for editorial corrections.These corrections include any combination of the following:Updated Foreword(item D);Updated Scope;title changes to UL489and/or UL1950;Withdrawn standards,UL519and UL547,being replaced by UL2111;Removal of the“94”?ammability classi?cation.These revisions may also include other miscellaneous editorial corrections.

As indicated on the title page(page1),this UL Standard for Safety is an American National Standard. Attention is directed to the note on the title page of this Standard outlining the procedures to be followed to retain the approved text of this ANSI/UL Standard.

The master for this Standard at UL’s Northbrook Office is the official document insofar as it relates to a UL service and the compliance of a product with respect to the requirements for that product and service, or if there are questions regarding the accuracy of this Standard.

UL’s Standards for Safety are copyrighted by UL.Neither a printed copy of a Standard,nor the distribution diskette for a Standard-on-Diskette and the?le for the Standard on the distribution diskette should be altered in any way.All of UL’s Standards and all copyrights,ownerships,and rights regarding those Standards shall remain the sole and exclusive property of UL.

All rights reserved.No part of this publication may be reproduced,stored in a retrieval system,or transmitted in any form by any means,electronic,mechanical photocopying,recording,or otherwise without prior permission of UL.

Revisions of UL Standards for Safety are issued from time to time.A UL Standard for Safety is current only if it incorporates the most recently adopted revisions.

UL provides this Standard″as is″without warranty of any kind,either expressed or implied,including but not limited to,the implied warranties of merchantability or?tness for any purpose.

In no event will UL be liable for any special,incidental,consequential,indirect or similar damages, including loss of pro?ts,lost savings,loss of data,or any other damages arising out of the use of or the inability to use this Standard,even if UL or an authorized UL representative has been advised of the possibility of such damage.In no event shall UL’s liability for any damage ever exceed the price paid for this Standard,regardless of the form of the claim.

UL will attempt to answer support requests concerning electronic versions of its Standards.However,this support service is offered on a reasonable efforts basis only,and UL may not be able to resolve every support request.UL supports the electronic versions of its Standards only if they are used under the conditions and operating systems for which it is intended.UL’s support policies may change from time-to-time without noti?cation.

UL reserves the right to change the format,presentation,?le types and formats,delivery methods and formats,and the like of both its printed and electronic Standards without prior notice.

Purchasers of the electronic versions of UL’s Standards for Safety agree to defend,indemnify,and hold UL harmless from and against any loss,expense,liability,damage,claim,or judgement(including reasonable attorney’s fees)resulting from any error or deviation introduced while purchaser is storing an electronic Standard on the purchaser’s computer system.

If a single-user version electronic Standard was purchased,one copy of this Standard may be stored on the hard disk of a single personal computer,or on a single LAN?le-server or the permanent storage device of a multiple-user computer in such a manner that this Standard may only be accessed by one user at a time and for which there is no possibility of multiple concurrent access.

If a multiple-user version electronic Standard was purchased,one copy of the Standard may be stored on

a single LAN?le-server,or on the permanent storage device of a multiple-user computer,or on an Intranet

server.The number of concurrent users shall not exceed the number of users authorized.

Electronic Standards are intended for on-line use,such as for viewing the requirements of a Standard, conducting a word search,and the like.Only one copy of the Standard may be printed from each single-user version of an electronic Standard.Only one copy of the Standard may be printed for each authorized user of a multiple-user version of an electronic Standard.Because of differences in the computer/software/printer setup used by UL and those of electronic Standards purchasers,the printed copy obtained by a purchaser may not look exactly like the on-line screen view or the printed Standard.

An employee of an organization purchasing a UL Standard can make a copy of the page or pages being viewed for their own fair and/or practical internal use.

The requirements in this Standard are now in effect,except for those paragraphs,sections,tables,?gures, and/or other elements of the Standard having future effective dates as indicated in the note following the affected item.The prior text for requirements that have been revised and that have a future effective date are located after the Standard,and are preceded by a″SUPERSEDED REQUIREMENTS″notice.

New product submittals made prior to a speci?ed future effective date will be judged under all of the requirements in this Standard including those requirements with a speci?ed future effective date,unless the applicant speci?cally requests that the product be judged under the current requirements.However,if This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

the applicant elects this option,it should be noted that compliance with all the requirements in this Standard will be required as a condition of continued Listing and Follow-Up Services after the effective date,and understanding of this should be signi?ed in writing.

Copyright?2000Underwriters Laboratories Inc.

This Standard consists of pages dated as shown in the following checklist:

Page Date 1-3.........................................................................September7,2000

4................................................................................April1,1999

5...........................................................................September7,2000 6-6B..........................................................................October6,1999 7-8A........................................................................September7,2000 8B............................................................................October6,1999 9-15.............................................................................May20,1993 16-22B...........................................................................April1,1999

23...........................................................................January30,1995

24..............................................................................May20,1993

25...........................................................................January30,1995 26-27............................................................................May20,1993 28-29........................................................................January30,1995 32...............................................................................April1,1999

No Text on This Page This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

ANSI/UL840-2000 MAY20,1993

(Title Page Reprinted:September7,2000)

1

UL840

Standard for Insulation Coordination Including Clearances and Creepage

Distances for Electrical Equipment

First Edition–December,1984

Second Edition

May20,1993

The most recent designation of UL840as an American National Standard occurred on May12,2000.

This ANSI/UL Standard for Safety,which consists of the Second edition with revisions through September7,2000,is under continuous maintenance,whereby each revision is ANSI approved upon http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.htmlments or proposals for revisions on any part of the Standard may be submitted to UL at any time.Written comments are to be sent to the UL-RTP Standards Department,12Laboratory Dr.,Research Triangle Park,NC27709.

An effective date included as a note immediately following certain requirements is one established by Underwriters Laboratories Inc.

Revisions of this Standard will be made by issuing revised or additional pages bearing their date of issue.A UL Standard is current only if it incorporates the most recently adopted revisions,all of which are itemized on the transmittal notice that accompanies the latest set of revised requirements.

ISBN1-55989-391-5

COPYRIGHT?1989,2000UNDERWRITERS LABORATORIES INC.

SEPTEMBER7,2000 2

UL840

No Text on This Page

This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

SEPTEMBER7,2000UL8403 CONTENTS

FOREWORD (4)

INTRODUCTION

1Scope (5)

2General (6)

2.1Insulation coordination (6)

2.2Undated references (7)

2.3Units of measurement (8)

2.4Components (8)

3Glossary (8)

CONSTRUCTION

4Clearance A(Equivalency) (9)

5Clearance B(Controlled Overvoltage) (9)

6Creepage Distances (11)

7Measurement of Clearance and Creepage Distances (16)

7A Reserved.........................................................................22A 7B Use of Coatings to Achieve Insulation Coordination...................................22A PERFORMANCE

8Switching Test.....................................................................22A 9Recurring Peak Voltage Determination................................................22B 10Dielectric Voltage-Withstand Tests...................................................22B

10.1Testing in lieu of measuring clearances........................................22B

10.2Testing for controlled overvoltage (23)

11Printed Wiring Board Coating Performance Test (23)

11.1General (23)

11.2Dielectric voltage-withstand test (25)

11.3Environmental cycling test (26)

11.4Humidity conditioning test (26)

11.5Thermal conditioning test (26)

11.6Coating adhesion (28)

11.7Insulation resistance between conductors (28)

11.8Impulse withstand voltage (28)

11.9Resistance to soldering heat (29)

11.10Flammability (29)

11.11Comparative tracking index(CTI) (29)

FOREWORD

A.This Standard contains basic requirements for products covered by Underwriters Laboratories Inc.(UL)under its Follow-Up Service for this category within the limitations given below and in the Scope section of this Standard.These requirements are based upon sound engineering principles,research,records of tests and ?eld experience,and an appreciation of the problems of manufacture,installation,and use derived from consultation with and information obtained from manufacturers,users,inspection authorities,and others having specialized experience.They are subject to revision as further experience and investigation may show is necessary or desirable.

B.The observance of the requirements of this Standard by a manufacturer is one of the conditions of the continued coverage of the manufacturer’s product.

C.A product which complies with the text of this Standard will not necessarily be judged to comply with the Standard if,when examined and tested,it is found to have other features which impair the level of safety contemplated by these requirements.

D.A product employing materials or having forms of construction which con?ict with speci?c requirements of the Standard cannot be judged to comply with the Standard.A product employing materials or having forms of construction not addressed by this Standard may be examined and tested according to the intent of the requirements and,if found to meet the intent of this Standard,may be judged to comply with the Standard.

E.UL,in performing its functions in accordance with its objectives,does not assume or undertake to discharge any responsibility of the manufacturer or any other party.The opinions and ?ndings of UL represent its professional judgment given with due consideration to the necessary limitations of practical operation and state of the art at the time the Standard is processed.UL shall not be responsible to anyone for the use of or reliance upon this Standard by anyone.UL shall not incur any obligation or liability for damages,including consequential damages,arising out of or in connection with the use,interpretation of,or reliance upon this Standard.

F.Many tests required by the Standards of UL are inherently hazardous and adequate safeguards for personnel and property shall be employed in conducting such tests.

APRIL 1,1999

UL 8404This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

SEPTEMBER7,2000UL8405

INTRODUCTION

1Scope

1.1These requirements cover an alternate approach to specifying through air and over surface spacings for electrical equipment through the use of the principles of insulation coordination.

1.2The complete principles of insulation coordination involve the consideration of the combination of clearances,creepage distances,and the properties of solid insulation used to constitute the insulation system.The empirical data gathered thus far has been used to develop the requirements for clearances and creepage distances as presented in this standard.The data needed to develop the evaluation procedure for solid insulation is still being gathered.When available,this evaluation procedure will be added to this standard and is not expected to affect the requirements for clearances and creepage distances.

1.3These requirements may be used as an alternate to required spacing levels speci?ed in end-product standards.The end product standard spacing requirements may be based on use and systems where overvoltages are not controlled,or if controlled,the level of control is unknown.

1.4These alternate requirements are intended to be applied to a particular product category if the standard covering the product category speci?cally references UL840or any of the requirements therein.

1.5Users of these requirements may need to specify the overvoltage levels and the methods of control which will be utilized,and establish the pollution degree to which the product insulation system will be expected to be subjected.

1.6It is not intended that the test values in this standard be employed for production line testing. However,users of these requirements will need to establish a means to ensure that production controls applied to permit the spacing reduction remain in effect during the manufacture of the product.This could include sample testing or physical measurements or another equivalent means.

1.7Users of these requirements will additionally need to ensure that in?uencing factors not addressed in this standard,such as mechanical movement,?eld placement of conductive material,and product damage,will not affect the system for insulation coordination.Examples are the deformation of the enclosure,movement of the?ttings for conduit or armored cable,or the improper installation of?eld wiring. Clearances and creepage distances at those locations must be veri?ed for compliance by physical measurement in accordance with Section7,Measurement of Clearance and Creepage Distances.

1.8A product that contains features,characteristics,components,materials,or systems new or different from those covered by the requirements in this standard,and that involves a risk of?re or of electric shock or injury to persons shall be evaluated using appropriate additional component and end-product requirements to maintain the level of safety as originally anticipated by the intent of this standard.A product whose features,characteristics,components,materials,or systems con?ict with speci?c requirements or provisions of this standard does not comply with this standard.Revision of requirements shall be proposed and adopted in conformance with the methods employed for development,revision,and implementation of this standard.

1.8revised September7,2000

2General

2.1Insulation coordination

2.1.1This standard uses the principles of insulation coordination to provide requirements for clearances and creepage distances for electrical equipment.A design consideration is to prevent undesirable breakdowns in some areas due to their disruptive nature and potential for risk of ?re and electric shock.Insulation coordination also requires the awareness of the insulating material properties so that design of the product can utilize the reduced spacing concept.

2.1.2Insulation coordination,as evaluated and implemented through the use of this standard,can be achieved in a multiconcept process.

a)The ?rst concept detailed in Section 4,Clearance A (Equivalency),of this standard is an alternative approach to measuring spacings currently de?ned in various product standards.This approach uses a dielectric voltage-withstand test impulse to evaluate equivalence of designed spacings to the prede?ned dimension.An actual reduction in spacings can be permitted through a re?nement in manufacturing techniques while maintaining equivalent breakdown levels.b)The second concept detailed in Section 5,Clearance B (Controlled Overvoltage),of this standard,a step in the insulation coordination process,is the selection of clearances based on the level of overvoltage protection and pollution degree.

c)The third concept detailed in Section 6,Creepage Distances,of this standard is the selection of an appropriate set of creepage distances based on operating voltage,pollution degree and material tracking characteristics.

d)The last concept,currently anticipated though not described in detail in this standard,is the choice of solid insulation to coordinate with the clearances and creepage distances chosen.The solid insulation must have suitable tracking properties for creepage considerations and it must have a breakdown level exceeding that of the controlled overvoltage level provided.

2.1.3The database upon which these requirements are based is contained in the International Electrotechnical Commission (IEC)Publications for Insulation Coordination Within Low Voltage Systems Including Clearances and Creepage Distances for Equipment,664,and the Supplement,IEC Publication 664A.

2.1.4The data contained in these IEC documents relate the ability of clearances to withstand overvoltages based on the characteristics of the electrical ?eld.For a speci?ed overvoltage,an inhomogeneous ?eld condition of two electrodes,which is represented by a point and a plane,requires the largest clearance.As the condition of the electric ?eld is improved by control of electrode shape,the clearance required to withstand the overvoltage decreases.The limit for this decrease in clearance is that of the homogenous ?eld which is represented by two parallel http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.htmling this data it is possible to evaluate clearances by their ability to withstand overvoltages.It should be recognized that the use of this technique requires the regular rechecking of the ability of the clearance to withstand the prescribed overvoltage because of possible variations in the production process,the wearing of tools,or other factors that might in?uence the clearance or electrical ?eld condition.The database may also be used to specify,for a given overvoltage,the inhomogeneous ?eld clearance,and the continued provision of this clearance may be made by measurements of the clearance.

OCTOBER 6,1999

UL 8406This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

OCTOBER6,1999UL8406A 2.1.5The data contained in these documents relating to the selection of creepage distances is based on an evaluation of empirical data coming from experience,and the results of extensive testing of certain combinations of materials in differing pollution degrees.The major variables for the selection of creepage distances are the arc tracking characteristics of the material used and the conditions of pollution.The four pollution degrees are based on the levels of moisture and contaminants at the creepage distance and therefore,the pollution degree which may exist.

2.1.6The values for creepage distances presented in this standard are for equipment being subjected to long term stress.Many products,and circuits within those products,are only subjected to short term or intermittent stress.End-product standards can permit a reduction in minimum creepage distance based on this use factor.A single row reduction(in Table6.1)while maintaining the same pollution degree and material group,would be typical.

OCTOBER6,1999 6B

UL840

No Text on This Page

This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

SEPTEMBER7,2000UL8407 2.1.7These requirements are based on the electrical characteristics of the clearances and creepage distances,and the assumption that the spacings will be maintained over the life of the equipment.In those locations where the spacings may be in?uenced by other conditions,the end-product standard users will need to separately consider whether use of this standard is appropriate.Examples might be the spacings between terminals that may be reduced by protruding wires or strands,or the spacings to enclosures where possible indentations may reduce the spacings.Spacings at arcing parts may need to be additionally evaluated for conductive deposits or loss of material during arcing tests.

2.1.8Within this standard,clearance and creepage distance requirements are separate and may be individually referenced.While creepage distances cannot be less than the associated clearances,each spacing parameter is in?uenced by different conditions.The clearances may be measured or tested,and can be further reduced by controlling overvoltages and pollution degree.Creepage distances may be measured,and may be reduced by controlling the materials and pollution degree present in the equipment.

2.1.9Other considerations which must be employed when pursuing insulation coordination include the following:

a)Insulation systems consist of a set of series and parallel insulators(of air and solid and

liquid insulation for example)and control over the system and internal spacings and voltages;

b)Air is considered a renewable insulation material;

c)Transient overvoltages may occur due to both internal(circuit generated)and external

(lightning)events;and

d)Solid(nonrenewable material)insulating material should have a dielectric strength greater

than the associated creepages and clearances.

2.2Undated references

2.2.1Any undated reference to a code or standard appearing in the requirements of this standard shall be interpreted as referring to the latest edition of that code or standard.

2.2.1revised September7,2000

2.3Units of measurement

2.3.1Values stated without parentheses are the requirement.Values in parentheses are explanatory or approximate information.

2.3.1revised September 7,2000

2.4Components

2.4.1Except of indicated in 2.4.2,a component of a product covered by this standard shall comply with the requirements for that component.

2.4.1added September 7,2000

2.4.2A component is not required to comply with a specific requirement that:

a)Involves a feature or characteristic not required in the application of the component in the product covered by this standard,or

b)Is superseded by a requirement in this standard.

2.4.2added September 7,2000

2.4.3A component shall be used in accordance with its rating established for the intended conditions of use.

2.4.3added September 7,2000

2.4.4Speci?c components are incomplete in construction features or restricted in performance capabilities.Such components are intended for use only under limited conditions,such as certain temperatures not exceeding speci?ed limits,and shall be used only under those speci?c conditions.

2.4.4added September 7,2000

3Glossary

3.1For the purpose of this standard,the following de?nitions apply.3.2CLEARANCES –Through air spacing.

3.3CREEPAGE DISTANCES (CREEPAGES)–Over surface spacings.

3.4FUNCTIONAL OVERVOLTAGE –Deliberately imposed transient overvoltages necessary for the function of a device.

3.5GENERAL ENVIRONMENT –The overall area or space in which the equipment is http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.htmlmonly referred to as ambient conditions.

3.6IMPULSE WITHSTAND VOLTAGE –The highest peak value of impulse voltages,of prescribed form and polarity,that does not cause breakdown under speci?ed conditions of test.

3.7INSULATION COORDINATION –The correlation of insulating characteristics of electrical equipment:

a)With expected overvoltages and characteristics of overvoltage protective devices (a key consideration for clearances);and

SEPTEMBER 7,2000

UL 8408This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

SEPTEMBER7,2000UL8408A

b)With the expected micro-environment and pollution protective means(key considerations for

creepage distances).

3.8LEAKAGE CURRENT–The current that can be measured as?owing between test points during a dielectric voltage-withstand test.

3.9LIGHTNING OVERVOLTAGE–The transient overvoltage at a given location on a system due to a speci?c lightning discharge.

3.10MICRO-ENVIRONMENT–The conditions that immediately surround the clearance or creepage distance under consideration.The micro-environment of the creepage distance or clearance and not the general environment of the equipment determines the effect on the insulation.The micro-environment might be less severe or more severe than the general environment that the equipment is in.It includes all factors in?uencing the insulation,such as climatic,electromagnetic,and generation of pollution.

3.11OPERATING VOLTAGE–The voltage across two points occurring due to normal operation of the product when controls are set in any position.

3.12OVERVOLTAGE CATEGORY–Grouping of products based on typical installed location with respect to overvoltage protection and available energy.See note c of Table5.1.

3.13POLLUTION–Any addition of contaminants,solid,liquid or gaseous(ionized gases),and moisture that may produce a reduction of dielectric strength or surface resistivity.

3.14POLLUTION DEGREE–The level of pollution present at the location on or in a product where the clearance and creepage distance measurement is made,and can be controlled by design of the product.For example,enclosures can be used to achieve pollution degree3,heaters within enclosures can help achieve pollution degree2,and encapsulation can be used to achieve pollution degree1.See 6.3.

3.15RATED VOLTAGE–The voltage employed for test purposes by the end-product standard.

3.16RECURRING PEAK VOLTAGE–Deliberately or naturally imposed transient overvoltage that exceeds the steady state voltage,and is caused by operating a control or adjustable component.The voltage level will be consistent for the same operation under the same conditions.Insulating materials can be degraded by ozone generated by partial discharges due to recurring peak voltages.

3.17SWITCHING OVERVOLTAGE–The transient overvoltage at a given location on a system due to

a speci?c switching operation or fault.

3.18SYSTEM VOLTAGE–The rated supply or line voltage to which the product will be connected. 3.19TRANSIENT OVERVOLTAGE–Nonperiodic voltages that may be caused by switching,lightning, or function of a device.

OCTOBER6,1999 8B

UL840

No Text on This Page

This PDF Standard downloaded from http://m.wendangku.net/doc/542b23da5022aaea998f0fa4.html

CONSTRUCTION

4Clearance A(Equivalency)

4.1Other than as noted in1.6,the requirements of this section may be used to evaluate clearances less than those speci?ed for the end product while maintaining the same overvoltage withstand capability for the equipment.

4.2A clearance,less than the speci?ed minimum through air spacing,in the product standard,may be suitable if acceptable results are obtained when tested in accordance with Section10,Dielectric Voltage-Withstand Tests,using a voltage with a value in accordance with Table 4.1.For speci?ed minimum clearances between the values in Table4.1,interpolation may be used to determine the test voltage.

Table4.1

Test voltages for verifying through air spacings(clearances)

End-product

Standard

speci?ed minimum through air spacing,

Test voltages,kilovolts

AC impulse,AC peak,or DC AC rms

Altitude a,m or(air pressure,kPa)b Altitude a,m or(air pressure,kPa)b

Inches(mm)0(101.3)

200

(98.8)

500

(95.0)

1000

(90.0)

2000

(80.0)0(101.3)

200

(98.8)

500

(95.0)

1000

(90.0)

2000

(80.0)

1/64(0.4) 1.7 1.7 1.7 1.6 1.5 1.2 1.2 1.2 1.2 1.1 1/32(0.8) 2.2 2.1 2.1 2.0 1.9 1.5 1.5 1.5 1.4 1.3 3/64(1.2) 2.75 2.7 2.65 2.5 2.3 1.95 1.9 1.9 1.75 1.6 1/16(1.6) 3.3 3.3 3.2 3.0 2.7 2.4 2.3 2.3 2.1 1.9 3/32(2.4) 4.4 4.3 4.1 3.9 3.5 3.1 3.0 2.9 2.8 2.5 1/8(3.2) 5.3 5.2 5.0 4.8 4.3 3.7 3.7 3.6 3.4 3.0 3/16(4.8) 6.9 6.8 6.6 6.2 5.6 4.9 4.8 4.7 4.4 4.0 1/4(6.4)8.38.27.97.5 6.8 5.9 5.9 5.6 5.3 4.8 3/8(9.5)10.910.710.39.88.87.77.77.37.0 6.3 1/2(12.7)14.013.713.212.511.29.99.79.38.97.9 1(25.4)25.524.624.022.720.218.217.617.116.214.4

a Next lower speci?ed altitude to be used for intermediate altitudes.

b Values of air pressure in kilopascals are provided to permit testing at pressures simulating elevations different from the elevation of the test facility.

4.3The withstand capability of a clearance is related to air pressure,therefore,the selection of test voltage is to be based on the altitude of the test location.

5Clearance B(Controlled Overvoltage)

5.1The requirements of this section may be used to evaluate clearances where the levels of overvoltage are controlled.

MAY20,1993UL8409

相关推荐
相关主题
热门推荐