the product safety newsletter - ieee safety newsletter • page 1 the what's inside product ......

Product Safety Newsletter •

What's InsideTheProductSafetyNewsletter

Vol. 6, No. 2 March-April 1993

Chairman's Message

Chairman's Message .................................. 1

Officers of the PSTC's ............................... 2

News and Notes ......................................... 3

Product Safeness as a Design Parameter... 4

Technically Speaking ................................ 5

Coated Metal Substrate PWBs .................. 6

CSA Gains NRTL Status ........................... 7

Institutional Listings ................................ 22

Employment Wanted .................. Back Page

This March I had the opportunity to visit StPetersburg, Russia, (formerly Leningrad) aspart of a six-man team conducting a series of

Continued on page 20

business management seminars at various venues inthe city. We had been invited by the pastor of abilingual church in the city, whose contacts in thebusiness all academic communities had convincedhim that such sharing would be very helpful.

St. Petersburg itself is a beautiful city .The mostcosmopolitan of Russian cities and arguably itscultural and business center, it has been the meetingof east and west in Russia throughout its history.The site of the city was a swamp less than 300 yearsago when Peter the Great decided to build his citythere. Peter was greatly impressed with westernscience, art and technology and he forcefullyintegrated as much as he could into his Russia. Forinstance, he employed western European architectsto design and build his city, the core of which waslargely completed by 1800.

The timing of our hip was interesting; the backdropwas one of change. We arrived as the Russian


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Officers of the Product SafetyTechnical Committees

Central PSTC (TC-8)Chairman Brian Claes (408) 578 5035Vice-Chair Richard Pescatore (408) 447 6607Secretary-Treasurer John McBain (408) 746 5016

(408) 746 5258 (fax)Standards Jim To (408) 492 9101

(408) 492 7244 (fax)Paper Review Mike Harris (707) 258 1360Symposium Mark Montrose (408) 247 5715

Austin TexasChair Bob Hunter (512) 250 6878

Central TexasChair Pro Tem Vic Baldwin (512) 469 7289

ChicagoChair Dick Hagedorn (708) 505 5722Vice-Chair John Allen (708) 827 7520

Orange County/Southern California Chair Charlie Bayhi (714) 367 0919Vice-Chair Ercell Bryant (714) 966 3459Secretary/Treasurer Deborah Tinsley (714) 773 7977Program Co-Chair Michael Dastmalchian (310) 604 8739Program Co-Chair Ray Jimenez (619) 726 9303

PortlandChair Jim Pierce (503) 626 6694Secretary-Treasurer Scott Varner (503) 656 8841

San DiegoChair Gene Biggs (619) 592 8236Secretary-Treasurer Jo Ann Fowler (619) 592 8163Program Chair Tom Radley (619) 592 8104

Santa Clara ValleyChairman Mike Campi (408) 987 6527Vice/Program Chair Murlin Marks (408) 985 2400 X2353Treasurer Mark Montrose (408) 247 5715Secretary John Reynolds (408) 390 1344

SeattleChairman Walt Hart (206) 356 5177Membership Chair Heber Farnsworth (206) 356 6045


News and Notes

by Dave Edmundsfax: (716) 422-6449

UL 1950 2nd EDITIONUnderwriters Laboratories has issued 1950 2nd edi-tion dated February 1993. This edition is based uponIEC 950 including amendments 1 and 2 and 33 CentralOffice Documents of IEC TC 76.

UL PRESENTS WORKSHOPSUL has announced three 2-day workshops on topics ofinterest to Electronic and Electrical Product manufac-turers.1) “UL 1950” -a hands-on workshop to provide in

depth information on evaluating products to com-ply with UL 1950.

2) “Globality: The Key to International Compli-ance” -to help manufacturers gain certification oftheir products by learning how to identify andmeet requirements for products intended for salein other markets. The seminar will focus on Euro-pean countries, Canada, Mexico, Japan/Korea,Pac Rim and emerging markets.

3) “Plastic in Electronic and Electrical Products” -appliance manufacturers and others responsiblefor third party certification will learn how tostreamline testing, select materials and the ratingsystem for plastic materials. The focus will be onUL 94 and the UL 746 series.

For more information and details of dates, contactLaura Murphy at UL Northbrook Office, Phone (708)272 8800, ext. 3402 or fax (708) 272 0919.

UNDERSTANDING THE EMCDIRECTIVEThe US Department of Commerce International TradeAdministration, Washington DC 20230, has issued apaper explaining the EMC Directive status, conform-ance requirements, and what US manufacturers can doto comply.

An article in the Northeast Product Safety Society(NPSS) newsletter April 1993 also discusses the EN45 000 series of documents.

STANDARDS NEWSIn the “CSA Info and Update” publication, they an-nounced

Elec. Certification Notice No. 436E June 1, post-ponement of effective date to Gen. lnst. No. 5 forC22.2 No. 21-M1984, Cords sets and Powercords.

Elec. Certification Notice No. 521B Sept 30,publication of amendment to C22.2 No. 220-M1986, Information Processing and BusinessEquipment and revised direction of Elec. Certifi-cation Notice No. 521.

Continued on page 9


Product SafenessAs A Design Parameter

by Paul W. Hill & Associates, Inc.© 1990

The following material is condensed from the text of“Product Safeness As A Design Parameter”, 2nd Edi-tion, 1990 by Paul W. Hill. The text is a registeredcopyright of Paul w. Hill & Associates, Inc. and isreproduced with permission. The book may be ob-tained by caning (407) 368 2538 or Fax (407) 3680744.

The financial investment in products shipped,products in the distribution network and finished goods inventories simply can not be put

at risk by the surfacing of product safety issues. Fewprofit margins can support a product recall, unplannedfield fixes or basic redesigns to accommodate a safe-ness deficiency once tooling has been committed andproduction runs are underway. Latent safety issues ina product are real risks to its financial and marketingsuccess.

Products obtain inherent safeness attributes by inten-tionally incorporating health, environmental and safetyconsiderations in the product design process. Clearly,product safeness is a design parameter.

The material presented here is intended for those whodevelop or assist in the development of products,select the components and materials to be used, gen-erate specifications and similar activities collectivelycalled product development. Topics covered are thosewhich have been the most difficult for developers ofelectrical, electronic and electromechanical productsto appreciate and incorporate into the design, materi-als and components utilized in their products.

There are several reasons why product developerstend to have difficulty with product safeness require-ments. It is not likely that engineers responsible forproduct development encountered product safety intheir formal education or “on the job” activities in aninstructive way. Few colleges and universities haveorganized instruction in product safety or regulatoryagency equipment safety matters. In industry, someorganizations sponsor seminars on the subject but theopportunity for product developers to participant islimited. There are product safety professionals inindustry but their availability to serve in a teachingcapacity is limited.

Industry standards and regulatory agency require-ments for the safeness of products are generally theproduct developer’s first encounter with the elementsof safeness in equipment design. Well written as theymay be, standards are not design guides, they arerecipes for testing agencies to follow in the evaluationand certification of finished products. Standards donot offer much in the way of suggestions or viableoptions in the solution of safety design problems.Industry safety standards always lag behind technol-ogy and industry practices by some period of timewhich further diminishes their usefulness to productdevelopers.

What appears to be needed is a much wider under-standing of the reasonableness of safety requirementsand the soundness of the engineering principles under-lying the requirements being imposed. Experience hasshown that once engineers come to appreciate thereasonableness of safety requirements and engineer-ing principles supporting the safety requirements,they have much less difficulty incorporating safeness



Technically Speaking

During the 1930’s, 1940’s, and 1950’s, Underwriters Laboratories did basic research inthe field of safety and published the results

of that research in a series of “Bulletins of Research.”At least 58 Bulletins were published relating to fire,,explosion, and electric shock.

One of those Bulletins, “Measurement of ElectricShock Hazard In Radio Equipment,” is a classicdocument in the field of product safety. It was writtenby Karl S. Geiges, an Associate Electrical Engineer atUL, as a thesis for a Master of Science degree. ULadopted Geiges’ work into its “Standard for Power-operated Radio Receiving Appliances,” and publishedthe paper as a “Bulletin of Research, No. 33” in July,1945. Geiges ultimately ascended to a vice-presi-dency of UL.

Geiges’ paper is a classic because it originates theleakage current circuit and meter that many stan-dards, not just UL, still use today. Geiges’ circuit wasused in ANSI C101, American National Standard ForLeakage Current, until the 1992 edition.

INTRODUCTIONThe basis of Geiges’ study was the technological andcompetitive development of radio receivers follow-ing World War II. In 1945, all radios were two-wireproducts as grounding had not yet been implementedin the National Electrical Code. Leakage current wasnot drained by the ground wire as it is today. Anddouble-insulation had yet to be invented.

“The development of radio receivers has followed acourse somewhat parallel to that of other electricalequipment as far as shock hazard is concerned. Frombattery operation the change to a transformer designwas a logical development because operation fromalternating -current lighting circuits was desirable andthe radio circuits could remain practically unchanged.The metal chassis was solidly connected to one side ofthe transformer secondary and was insulated from theline circuit by the primary-secondary insulation of thetransformer. In some receiver designs a capacitor wasconnected between the chassis and the primary side ofthe transformer, but this filter arrangement employedcapacitors of such small size that the ground currentwas of the order of fractions of a milliampere.

“ As competition forced the development of new, lessexpensive receiver designs, new vacuum tubes werebrought out and the so called “ac-dc” type of receiverwas introduced. In this receiver, the transformer was

Copyright 1993 by Richard Nute

ELECTRIC SHOCK FROM RADIOSA REVIEW



Coated Metal SubstratePrinted Wiring Boards

by Lal Bahra, P. Eng.Canadian Standards Association

Coated metal substrate (CMS) printed wiringboards are currently being used in information technology equipment evaluated to the

requirements of CSA Standard CAN/CSA C22.2 No.950, “Safety of Information Technology Equipment,Including Electrical Business Equipment”, or IECPublication 950.

Ceramic coated metal substrate printed wiring (PW)boards and other coated metal (i.e. coated with insulat-ing compound such as epoxy resin, polyester, etc.)substrate PW boards are also being used in variousindustries. These PW boards, especially ceramic coatedmetal substrate boards, offer excellent performanceand other advantages over conventional fibreglass/epoxy boards.

Mechanical strength, excellent heat sinking capabili-ties, good ground path for fault currents when themetal substrate is connected to ground, and smallersizes of components are some of the advantages ofusing CMS PW boards.

“Ceramic” Coated Metal Substrate PW BoardsFor this type of CMS PW board a suitable metal oraluminum substrate is prepared by cleaning the metalthoroughly. A few thin layers of ceramic plus suitablebonding materials are then applied. The ceramic coat-ing may also be directly electto-deposited onto theprepared metal. The assembly is then subjected to redheat, fusing the ceramic material to the metal substrateto provide a uniform coating. Copper is deposited andthe assembly is again subjected to red heat. The PWboard can now be prepared by optically transferring

the pattern of the circuit and chemically etching awaythe unwanted copper. Components are secured to theboard by utilizing surface mounting technology.

This method of preparing the PW board gives goodhandling properties to the board. As an assembly, it iscapable of withstanding much higher temperaturesthan conventional PW boards.

“Insulating Compound” Coated Metal SubstratePW BoardsFor this type of CMS PW board a few thin layers ofcompounds such as polyester or kapton etc., are de-posited on the metal substrate by coating and thencuring the assembly at a temperature that is consider-ably lower than the temperature necessary for red heatcuring of ceramic coated boards. The manufacturermust employ quality control methods to control thethickness and temperature during the process. Pinholes may be present if the coating is applied onlyonce. Several coatings need to be applied to achieve auniform and suitable thickness, and to eliminate anypin holes in the coating material.

Hand Soldering and Repair of CMS PW BoardsCoated metal substrate PW boards must be carefullyinvestigated if hand soldering is employed duringassembly or repair .CMS PW boards are usually notsubjected to hand soldering or repair. Defective unitsare returned to the factory rather than attempting torepair them in the field. Service instructions shouldcaution the user against attempting repairs in the field.Any CMS PW boards having supplementary or rein-forced insulation must not be subjected to repair. ForCMS PW boards having only basic insulation, theacceptability of hand-soldering in making repairs re-



CSA Gains NRTL Statusby Tom Tabor, P. Eng.Canadian Standards Association

[CSA as a NRTL is news, just as UL acceptance inCanada is news, because it is another step on the pathtowards North American standards harmonizationsimilar to the European Norms. A bi-national com-mittee (including the PSTC Chairman Emeritus, RichPescatore) is presently working to harmonize UL1950and CSA950. We asked both CSA and UL to describewhat their new status will mean to regulatory engi-neers wanting certification for Canada and the U.S. -advantages and possible disadvantages, how submit-tal and certification processes would change, recog-nition by local authorities ( e.g. -Province of Quebec,City of Los Angeles), plans for resolving problems,and so on. CSA has their say this issue, and UL nextissue. Please write with your questions or comments!- Ed.]

The recent accreditation of the Canadian Standards Association (CSA) as a NationallyRecognized Testing Laboratory (NRTL)

means that customers can now obtain convenient“one-stop” service from CSA to satisfy all of theirNorth American certification needs.

Our formal accreditation by OSHA (the U.S. Occupa-tional Safety and Health Administration) on Decem-ber 24, 1992 makes CSA equal to other NRTL organi-zations with respect to regulatory acceptance: a prod-uct tested and approved by CSA to Canadian or U.S.standards has full regulatory acceptance throughoutthe U.S. and can be sold on both sides of the border.

For CSA customers, this eliminates the need for dupli-cate testing and evaluation, promotes a fast turn-

around time in obtaining certification, and dramati-cally reduces follow-up inspection and frequency andcost. Manufacturers can therefore significantly reducetheir costs and get their products to the North Ameri-can marketplace more quickly.

The scope of CSA's accreditation covers a wide rangeof ANSI/UL standards, including all electrical andelectronic products and equipment for the informationtechnology, medical and telecommunications indus-tries.

To further improve service and reduce duplication inNorth American product certification. CSA will ac-cept test data and reports prepared by UL and otherNRTLs. We will also accept components listed by ULand other recognized certification agencies.

BENEFITS OF CSA CONFORMITYASSESSMENTThe recognition of CSA as a NRTL undoubtedly addsto the competition for customers among certificationorganizations. We have shown that we are fully pre-pared to work co-operatively within this marketplace,and we are committed to continuous improvement inoverall quality, timeliness and cost of service.

Besides reliability and cost-effectiveness, we canemphasize CSA’s long experience and stature as aworld leader in both standards development and con-formity assessment. Due to celebrate its 75th anniver-sary in 1994, CSA has established a network of officesand test facilities across Canada and around the world.In addition, we have developed reciprocal ties withconformity assessment organizations around the world.To deliver full and localized service within NorthAmerica, we intend to follow our strategy of working


in partnership with local independent laboratories,using local people. To that end, CSA is currentlylooking at increasing partnerships with independentlaboratories in the U.S. and Mexico.

Our customers can also count on CSA ‘s reputation forboth integrity and innovation. We are known forupholding the integrity of the CSA mark, which nowappears on more than a billion products around theworld, and for providing flexible and innovative pro-grams that respond to client’s needs.

In supporting CSA’s application to OSHA, for ex-ample, CBEMA (the Computer and Business Equip-ment Manufacturers Association) comment that “CSAhas been innovative in its approach to testing andcertification without compromising its third partyposition of independence.”

Hewlett Packard, which also supported CSA’ s appli-cation, singled out CSA personnel as “knowledge-able, thorough and professional in their work,” Theaverage length of service for CSA staff is over 10years.

Data General, a leading computer manufacturer, haspublicly stated that CSA is “the most progressive,most helpful and most technologically advanced” ofthe many certification organizations it works with, aswell as “the most customer oriented.”

UNIQUE ADVANTAGES OF CSA SERVICEUnique advantages we offer to customers include: •just-in-time certification

• flat fee certification • concurrent follow-up inspections on products

and quality system through a co-operative ar-rangement with its subsidiary, the Quality Man-agement Institute (QMI),

•The broadest product testing authorization underthe CB Scheme of any testing laboratory in NorthAmerica (a key benefit to North American manu-facturers that export equipment).

ENHANCED RECOGNITION IN THE U.S.The OSHA accreditation of CSA provides nationalrecognition that enhances CSA’s existing state andlocal accreditation in the U.S. CSA has been activesince 1989, the year the Free Trade Agreement be-tween the U.S. and Canada was signed, in seekingrecognition at the state and local level through bothformal and informal accreditation programs.

States such as Washington, Oregon, and North Caro-lina and cities such as Chicago, Boston, New York,and New Orleans have formally accredited CSA. Interms of informal recognition, CSA has established avast network of regulatory acceptance through yearsof visits to all states, major cities and large municipali-ties. In areas such as New York City, the CSA markhas been recognized for over a quarter of a century.

COMMITMENT TO FREE TRADE“One standard, one test, one mark” is a concept thatCSA advocates wholeheartedly. CSA is committed topromoting the principles of freer trade, the develop-ment of harmonized standards, and the needs of cus-tomers on both sides of the border. t


Elect. Certification Notice No.671 Sept 30, pub-lication of amendment to C22.2 No. 950-M89,Safety of Information Technology Equipment,Including Electrical Business Equipment

Wires and Cable No.18, publication of C22.2No. 49-1992 Flexible Cord and Cables.

Power Supplies No.1 March 31, Publication ofC22.2 No. 234-M90 Safety Components PowerSupplies.

For additional information phone CSA at (416) 7474171.

IEC STANDARDS RELEASEDIn the “ANSI Standard Action” it was noted:The following IEC Standards have recently been re-leased:

IEC 227-1:1993, Polyvinyl chloride insulatedcables of rated voltages up to and including 460/750V - Part 1: General requirements; Part 3: Non-sheathed cables for fixed wiring.

IEC 799: 1984/Amendment 1: 1993, Cord Sets

IEC 352-3:1993, Solderless connections- Part 3:Solderless accessible insulation displacementconnections - General Requirements, test meth-ods and practical guidance.

IEC 512-8: 1993, Electromechanical componentsfor electronic equipment; basic testing proce-dures and measuring methods- Part 8: Connectortests (mechanical) and mechanical tests on con-tacts and terminations.

IEC 12204: 1993, Low-voltage power supply

devices, d.c. output - Performance characteristicsand safety requirements.

IEC 1010-2-031: 1993, Safety requirements forelectrical equipment for measurement, controland laboratory use - Part 2-031: Particular re-quirements for hand-held probe assemblies forelectrical measurement and test.

The following drafts on Acoustics have been sent toCEN members of enquiry and comment. Copies areavailable from ANSI at the price indicated.

prEN 24869-3, Acoustics - Hearing protectors -Part 3: Simplified method for the measurement ofinsertion loss of ear-muff type protectors forquality inspection purposes (ISO/TR 4869-3:1989) - July 17,1993. $22.00

prEN 31201, Acoustics - Noise emitted by ma-chinery and equipment - Measurement of emis-sion sound pressure levels at the work station andat other specified positions - Engineering methodin an essentially free field over a reflecting plane- July 25, 1993, $18.00.

prEN 31202. Acoustics - Noise emitted by ma-chinery and equipment - Measurement of emis-sion sound pressure levels at the work station andat other specified positions - Survey method insitu - July 25. 1993. $18.00.

prEN 31203, Acoustics -Noise emitted by ma-chinery and equipment -Determination of emis-sion sound pressure levels at the work station andat other positions - July 25, 1993, $18.00.

prEN 31204, Acoustics - Noise emitted by ma-chinery and equipment - Measurement of emis-sion sound pressure levels at the work station andat other specified positions - Method requiring

News and NotesContinued from page 3


eliminated, vacuum tube filaments were connected inseries, and plate voltages were lowered to permit theuse of rectified 120-v alternating current. Then thedesigner was faced with a problem. If the chassis wereused as the negative return it would be connecteddirectly to one side of the line. Any contact with thechassis that was made by the user, as in replacingtubes, would expose him to full line potential if thechassis were connected to the ungrounded side of theline.

“...The alternative that has been generally used is toemploy an insulated negative bus within the chassisand, for higher frequencies, to ground the bus to thechassis through a capacitor of 0.10 to 0.25 µF. Withsuch an arrangement, when the receiver is connectedto a 120-v ac line the leakage current from chassis toground is 4.5 to 11.3 mA.”

These values, 4.5 to 11.3 mA, were calculated usingthe capacitive reactance equation to find the reac-tance, and then Ohm’s Law to calculate the current.Geiges went on to say that leakage current could becontrolled simply by controlling the maximum valueof the line-to-chassis capacitance. However, to do sorequired:

• study of the schematic diagram to find theleakage paths;

• finding parallel leakage paths, if any;

• determining vacuum tube leakage currentfrom heater to cathode;

• controlling voltages at accessible parts.

Rather than calculation of leakage current based oncircuit analyses, Geiges believed that direct measure-ment of leakage current would be much more practi-cal. Such direct measurement could be applied to mostother electrical products, and would not be restrictedto radios.

Interestingly, Geiges mentions that previous attemptsto measure leakage current bad failed, that a measur-ing means was not available, and that serious difficul-ties would be encountered in any practical solution.

ELECTRIC SHOCK HAZARDGeiges defined electric shock hazard as a circuit inwhich the body is a part of the circuit. He next definedthe circuit parameters:

a. The voltage between the two points of bodycontact. This is important, he said, because thevoltage breaks down the high initial resistanceof the skin, and because the voltage determinesthe magnitude of the current in the body.

b. The frequency of the supply voltage. Geigessaid that at high frequency there is less muscu-lac reaction than at low frequency.

c. The capacitance of the source. Geiges said thatthe charge, in coulombs, is important. Hedidn’t say why it was important.

d. The resistance of the source. This resistanceserves to limit the current in the body.

Technically SpeakingContinued from page 4

environmental corrections (ISO/DIS 11204) -July 25, 1993, $18.00.

prEN 32001, Acoustics - Noise emitted by ma-chinery and equipment - Rules for the draftingand presentation of a noise test code (ISO/DIS12001) - July 25, 1993, $18.00. t


Geiges said that current through the body has threemajor effects:

(1) involuntary muscular reaction,(2) ventricular fibrillation, and(3) tissue burns due to heating. Such currents

are readily available from the operating volt-ages on equipment.

Geiges studied various research work to identify val-ues of current for the threshold of perception (0.2mA), let-go (8 mA), and fatality (100 mA).

Geiges especially noted Dalziel’s work which con-cluded that let-go current is a function of the peakvalue, 11.3 mA, not the root-mean-square (rms),8.0 mA, value.

From these parameters, Geiges concluded that thelimits of current and voltage that needed to be mea-sured were 1-30 mA ac, and 1-150 v, ac and dc,(household voltage) and 1-600 v, ac and dc (platecircuits of vacuum tubes).

BODY RESISTANCEGeiges stated that the body is analogous to a resistorand capacitor in parallel. He said that the capacitanceis very small and can be disregarded for power linefrequencies.

Geiges studied Whitaker’s work, and concluded thata body resistance of 1500Ω was “the best compromisefor this investigation.”

1500Ω “is slightly less than the lowest recorded valuewith dry contact surfaces, 1550Ω, and it comparesfavorably with average values obtained under condi-tions that existed in the case of the wet hands and feet.”

“If a higher value is chosen it is difficult to justify inview of the 1500Ω dry value. A lower value offers

equal difficulties in that a safety factor is introducedwhich will be added to the factor that should beconsidered in the determination of permissible leak-age current.”

“The body resistance was also considered from thelow voltage hazard standpoint in arriving at the1500Ω, value. A 15-v circuit would cause a currentflow of 10 mA. If a resistance value similar to thatused for the electric fence work by Whitaker (500Ω)were used in this case, a 5-v circuit would result in a10 mA current and a 15-v circuit would result in a 30-mA current. Where indoor use equipment is underconsideration, the 1500Ω, value appears to be lowerthan conditions would warrant, while the 1500Ωvalue can be justified in terms of conditions that itwould be reasonable to expect in actual service.”

SHOCK HAZARDS OF RADIO EQUIPMENTIn 1945, Geiges was faced with the same, classicoperator-serviceman dichotomy we still face today.Geiges clearly sets forth the parameters, but we havecontinued the argument without resolution for 45years!

“There is no definite line of demarcation between theclass of persons who should and should not haveaccess to live parts but a rather sharp distinction canbe made in the reasoning that is involved in the actionof exposing such parts. Repair and service workgenerally requires that parts be live while exposed fortests and adjustments. Qualified persons know thatlive parts will be exposed when covers held in place byscrews and similar fastenings are removed, and thatinterlocks must be blocked in the “on” position tomake the necessary tests and adjustments. However,an unqualified person knows that replacement oftubes, fuses, batteries, etc., is necessary and he ex-pects to make such changes without the necessity fortaking any precaution other than turning the single-pole power switch to the “off” position.”


Geiges further noted that dc leakage currents wereavailable from various parts of ac-dc radios. Manyvacuum tubes were provided with metal caps forconnection to the control grid element. The controlgrid was a source of dc leakage current. Even if thecontrol grid was in a base terminal rather than a capterminal, the stator of the tuning capacitor was nor-mally accessible and was connected to a vacuum tubecontrol grid and was therefore a source of dc leakagecurrent.

METHODS OF MEASURING SHOCK HAZARDNote that the values of leakage current that Geiges andothers were attempting to measure were quite highcompared to today’s values. Geiges was attempting tomeasure leakage currents in the range of 4 mA toalmost 13 mA, while today, we measure leakagecurrents in the range of 0.5 mA to, at most, 3.5 mA.With these high values of leakage current, the resis-tance of the measuring device, if significantly greaterthan zero, affects the measured value.

For example, if we assume a leakage current of 10 mAfrom a 120 volt source, then the resistance of thecircuit is:

ER = -------------

I

120R = ------------- 0.010

R = 12,000Ω

If we insert a 1,500Ω resistor in the circuit, represent-ing the body resistance, then the value of currentwould be:

E I = ------------

R

120 I = -------------------- 12,000 + 1,500

I = 0.089 amperes or 8.9 mA

On the other hand, if we insert a 500Ω resistor in thecircuit, then the value of current would be:

120I = ----------------- 12,000 + 500

I = O.O96A or 9.6mA

Therefore, Geiges reported lack of correlation of leak-age current data due to lack of a standard resistancefor the measuring device.

For reasons given in the body-resistance discussion,Geiges chose 1,500Ω as the standard meter resistance.

Geiges then said, “The basis for evaluation of theseverity of electric shock would be expected to in-volve muscular reaction.”

Because muscular reaction and let-go phenomena areproportional to the peak value of ac current, Geigeswanted a meter that would measure a steady-state peakvalue.

Geiges then initiated a search for a suitable measuringdevice. In 1945, these were the common instrumentsof the time:Electrodynamometer instrument.Thermocouple instrument.Rectifier (copper-oxide) instrument.


Vacuum Tube Voltmeter.

The “rectifier-feed, diode-peak” vacuum-tube volt-meter had the characteristics that Geiges had speci-fied. In particular, it was sensitive to the peak value ofvoltage.

“The sine wave a-c reading is the most commonreading made with a leakage current meter so that forsimplicity it is desirable to calibrate the (peak-read-ing) meter to read the root-mean-square value of thesine wave.”

With a resistor across its terminals, the scale could becalibrated in rms milliamperes, “thereby giving adirect measurement of leakage current.”

‘The result is a measuring device that is actuated bythe shock hazard criterionpeak currentand themeasurement is in terms of root-mean-square milli-amperes when the current source is limited to an a-csine wave. If a nonsinusoidal wave shape is to bemeasured, the scale reading is .707 of the maximumpeak and the reading is a measure of the shockhazard.”

The resulting leakage current measuring system wasa peak-reading, rms-calibrated vacuum-tube voltme-ter, which measured the voltage across a 1500Ω resis-tor. The value of voltage was divided by 1500 to givethe value of the current through the resistor. The meterscale could include a current scale to avoid thecalculation.

SHOCK HAZARD MEASUREMENT SPECIFI-CATION FOR RADIO EQUIPMENT“Shock hazard shall be considered to exist at a livepart in a circuit involving a potential of 125 v or lessin the following cases:

“(a) At an exposed live part, if the open-circuit

potential is more than 25 v and the current with a 1500-ohm load is more than 5 mA.

“(b) At a partially protected live part, if the open-circuit voltage is more than 35 v and if the currentwith a 1500-ohm load is more than 15 mA, with amaximum allowable a-c component of 10 mA in anycase.“A part is considered to be exposed if it is subject tohandling in normal use (not servicing)... A part isconsidered to be partially protected if it is locatedwithin the overall enclosure or beneath the applianceso that contact by persons is unlikely.”

DISCUSSION OF SPECIFICATION“The specific current and voltage limits, althoughnot directly connected with the design of the measuringdevice, are a part of the general problem and theirorigin is covered here for completeness.”

“Open Circuit Potentials Exposed live parts oper-ating at potentials of 25 v and less are not consideredhazardous unless the equipment is intended for use ina location that involves special conditions of moistureor body exposure. ...The higher voltage specified forpartially protected live parts is based on the decreasedlikelihood of most favorable shock conditions... Theuse of 30-32 v for automatic tuning motors... has beencommon for years.”

“Leakage Current The 5-mA value for permis-sible leakage current is based on recommendations ofshock investigators and past practice. It is well abovethe threshold of perception and is admittedly a currentthat will startle the person receiving the shock.”

“The 5-mA current is believed to be low enough topermit release by the person receiving the shock.”

“Exceptions The size of small parts and the


relative inaccessibility of the tuning capacitor may beused as justification for practices followed for a num-ber of years in the radio industry, but it is difficult toreconcile this practice with shock hazard current limi-tations.’

Clearly, this was a landmark work. Reducing leakagecurrent from the 10 mA range to less than 5 mA shouldhave been a major re-design effort on the part of radiomanufacturers.

To bring this into perspective 45 years later, oneindustry segment is proposing to reduce leakagecurrent from 3.5 mA to 0.5 mA. If this proposalbecomes effective, manufacturers again will face amajor redesign effort.

Today, we continue to use the 500Ω resistor tomeasure leakage current. However, because we nowwork with much lower leakage current values, thevoltage does not break down the high initial resistanceof the body, so the body resistance is almost never aslow as 1500Ω. (You can tell when the bodyresistance breaks down---it occurs simultaneouslywith the onset of the sensation of electric shock).

When Geiges was measuring 10mA leakage currentsfrom 120V, the source resistance was 12kΩ. The1.5kΩ resistor had 8 percent effect on the measure-ment.

With today’s relatively low values of leakage current,the effect of the 1500Ω resistance on the measurementis negligible. For example, when we measure 0.5milliampere leakage current from 120 volts, the sourceresistance is 240 kΩ. The 1.5 kΩ resistor has only 1.6percent effect on the measurement.We should not lose track of Dalziel’s finding that let-go (and sensation, as implied by Geiges) is propor-tional to the peak value of current. Today, true-rmsmeters are common. They are not peak-measuring

meters calibrated in rms. So, such meters when,measuring non-sinusoidal leakage Currents, will givea lower reading than would the peak-measuring meter.

ANSI C101-1992, Leakage Current for Appliances,reinforces Dalziel’s finding that the body’s physi-ological response to current is proportional to the peakvalue.

ACKNOWLEDGMENTSJim Pierce, Eng. Testing Laboratories, dropped a copyof this UL Bulletin on my desk, asking if I had read it.I had seen and read the Bulletin many years ago, so thecopy just sat on my desk for many months. Eventually,I picked it up and started reading. I was impressed withthe work, and thought I would review it for you.

Your comments on this article are welcome. Pleaseaddress your comments to the Product Safety News-letter, Attention Roger Volgstadt, c/o Tandem Com-puters Inc., 10300 N. Tantau Avenue, Location 56,Cupertino, California 95014-0708. t

factors into product designs. With any reduction ofthese difficulties product development costs will de-cline, development times are reduced, they get it righton the first pass and the inherent safeness of the endproduct will improve. Clearly, product safeness is adesign parameter.

There is a challenge in all this to practitioners of thesafety profession, namely, they must understand theseprinciples and the underlying engineering rationalewell enough to communicate them to those in theengineering and product development community.

Product Safeness

Continued on page 4


ELEMENTS DRIVING PRODUCT SAFETYDemands and requirements placed on a product, suchas product safety, should have some justification forthe costs to the product and for the resources requiredto implement them. In the case of product safetyseveral justifications can be made.

Safety related legal conditions.Most countries in the industrialized world have bind-ing legal requirements that mandate safety certifica-tion before a product can be imported or sold in thecountry. Safety compliance is demonstrated by evalu-ation by a recognized testing agency. If the product isacceptable it is authorized to display the test agencylogo or monogram. In the United States equipmentcertification is referred to as “listed”. Similarly, safetysensitive components and subassemblies are “recog-nized”.

Therefore, the first justification is to have a productwhich is legally permitted to enter into commerce.

Risk of safety related litigation.Litigation requires the expenditure of resources shoulda charge or claim against a product be made, regard-less of the merits of the claim. Time and expense isinvolved in investigations, retesting, simulating vari-ous faults, design reviews, legal fees and other tan-gible and intangible expenditures. Some litigationcosts may never be recovered such as lost revenue ifproduct distribution is curtailed, lost market share ascustomers react to litigation disclosures and expensesnecessary to recapture lost market position. It may notbe possible to avoid all safety related litigation actionsagainst a product. However, it is possible to design aproducts that incorporates recognized safeness at-tributes so as to readily defeat unreasonable safenessclaim against it.

It follows that the second justification is the protectionof the investment made in the product and its market

position.

Market forces.Once a level of safeness for a product is established inthe market place, consumer expectations are corre-spondingly set. Competing products must be per-ceived to be at least as safe as others in the samemarket. The level of safeness established in a marketmay well exceed that required by industry standards.Competing products must be perceived by consumersto be keeping pace. Clearly, consumer expectationsjustify at least a competitive product safety position.

Market expansion.Often an established product has the opportunity toexpand into new market areas. For example, a com-mercial device can be adapted for use in hospitals or aNorth American product can find profitable marketsin Europe. Most market movements will requiresafety certification activities and possibly designchanges to comply with safety requirements in thenew market area. Positioning a product for expansioninto wider regional or global marketing is yet anotherjustification for considering safeness in the design ofa product.

Other elements may also drive product safety consid-erations but these four are the most compelling rea-sons to commit resources for incorporating safenessattributes into the design of a product.

HOW SAFE IS SAFE ENOUGH?There are four basic parameters for determining anacceptable minimum level of safeness for a product.These parameters address the intended end use, elimi-nation of latent hazards, product certifications andacceptability of residual risks.

Intended use and foreseeable misuse.The first element of product safeness is reasonablyobvious, products must be safe for their intended use.


Intended use are those usages and conditions forwhich the product was designed and designated by themanufacturer. These include installation, operationand service functions. The manufacturer or importermust define these activities and the limits of each in theoperating and service instructions.

Misuse is considered to be those uses to which theproduct can be put, but go beyond the intent of thedesigner or manufacturer. Such usage which can rea-sonably be foreseen in advance is called foreseeablemisuse. For example, a screw driver is intended toprovide torque to a slotted and threaded fastenerdevice such as a screw, lug or the like. Yet screwdrivers are commonly used as a lever, pry, and wedgeall extensions of the original intent.

A word of caution is in order concerning misuse.Designers can not assume a “user beware” attitude.The designer is expected to consider foreseeable mis-use possibilities and provide for an acceptable level ofsafeness should foreseeable misuse occur.

Latent hazards.Consideration of latent hazards is important becauseof exposure in litigation actions in which the issuecenters on whether the manufacturer “knew or couldhave known” hazards were inherent in the product orits usage. Latent hazards are identified by a processcalled fault testing. These tests are conducted so as toexpose potential hazards in operation and servicingwhich may not have been considered in the designobjectives or detected by traditional product evalua-tions, quality control activities or manufacturing tests.The tests involve physical inspections, real time op-eration, servicing activities, and adequacy of instruc-tions provided with the device. The intent is to deter-mine the degree of protection provided by the equip-ment should a hazard be encountered.

Fault testing is not to be confused with worst case

testing. Worst case testing is primarily performanceoriented and addresses performance capabilities un-der extreme operating conditions. Fault testing exam-ines the various ways operators and service personnelmight encounter hazards, and the ability of the productto provide reasonable protection against such hazard-ous encounters. For example, utilization of poweroutlets with inadequate or no earth grounding, unau-thorized servicing of parts easily accessible with com-mon tools, disposal of consumables containing toxicmaterials, and similar situations. These hazards caneasily escape quality controls, worst case testing andeven testing by safety certification agencies.

Product certifications.Most industrialized countries require products to besafety tested. There are few exceptions. Even OSHArequires testing for safeness by the manufacturer evenif formal listing or approvals are not pursued, such asone of a kind, customized or very small volumeproduction of a product. As evidence of compliancethe testing agency authorizes the display of their logoor monogram on the product. It is generally viewed asa marketing necessary for widely distributed productsand might carry several monograms such as CSA, UL,TÜV, SEMKO, and others.

The safety standards used for certifications will de-pend on the product marketing areas. In the UnitedStates many major safety standards are generated byUnderwriters Laboratories (UL), some of these arealso American National Standards Institute (ANSI)publications as well. In Canada the Canadian Stan-dards Association (CSA) issues similar safety stan-dards. Outside North America the major productsafety standards are generated by the InternationalElectrotechnical Commission (IEC). In the less in-dustrialized countries the IEC standards are generallyrecognized and accepted. In South America, UL,standards are widely accepted. Major differencesbetween product safety standards generated by UL


and IEC are being reviewed in an attempt to harmo-nized standards. However, many differences are likelyto remain and standards from both organizations mustbe considered for products globally marketed.

Residual safety risks.Residual risks are those safety issues which remain inthe product when final product evaluations are com-plete and certifications obtained. These may be safetyconcerns not covered by safety standards, unsatisfiedsafety design objectives or certification oversights,(i.e. see introduction or forward of product safetystandards for disclaimers test agencies make for safetytesting of products). There may be inadequate safetymargins to compensate for manufacturing variancesand parts supplier variability. There may also be safetyconcerns discovered in fault testing.

Proceeding to market the product requires either cor-rective actions or acceptance of the risks in the productas it is. It is the responsibility of senior management toconsider all the factors involved and make the decisionto correct the safety concerns or accept the residualrisks. In many organizations this management pro-cess is a formal safety review by a group representingengineering, manufacturing, marketing, quality andlegal counsel. The importance of this process reflectsback to the concept of “knew or could have known” ofsafety concerns mentioned above.

These are the four major factors in determining theminimum safeness of a product. Many organizationgo further and use safety reviews by a third partyconsultant and use internal safety standards to supple-ment deficiencies in industry standards. Some includehealth and environmental considerations as well aspersonal safety and property damage in product safe-ness evaluations. These issues are discussed in latersections on toxic materials and the need to designbeyond the requirements of safety standards. t

quires investigation.

ComplianceMany end product standards do not contain require-ments for evaluating CMS PW boards. For the pur-pose of determining suitability of such PW boards inan end product, a sample assembly of a CMS PWboard used in the product should be subjected to thethermal cycling, thermal aging, dielectric strength andabrasion resistance tests of Clause 2.9.5 of CSA Stan-dard CAN/CSA-C22.2 No.950 or mc Publication950.

Requirement for Basic Insulation on CMS PWBoardsThere is no thickness requirement in IEC Publication950 for basic insulation. Acceptance is based oncompliance with dielectric strength test requirementsfollowing normal and abnormal tests, thermal cycling,thermal aging and abrasion resistance tests. If themetal substrate is connected to ground, clearances andcreepage distances on the CMS PW board must be incompliance with the requirements of the applicablestandard. See attached Fig 1 which illustrates a CMSPW board with the metal substrate connected to ground.

Bond impedance and limited short circuit tests shouldbe conducted, as applicable, in accordance with CSAStandard C22.2 No 0.4, Bonding and Grounding ofElectrical Equipment (Protective Grounding). If themetal substrate of a CMS PW board forms part of theproduct enclosure then it is subject to mechanical testssuch as impact and steady force that are applicable tothe enclosure in additional to normal, abnormal andthermal tests.

Servicing of products utilizing basic insulation onCMS PW boards should not result in the reduction of

Coated Metal...Continued from page 3


clearances, creepage distances and pollution degree.

Requirements for Supplementary and Reinforcedinsulation on CMS PW BoardsIn general, for the type of construction utilizing supple-mentary and reinforced insulation on CMS PW boards,the metal substrate is located inside an insulatingenclosure and must not be connected to ground (i.e.the metal substrate is floating).

The minimum thickness requirement for supplemen-tary and reinforced insulations in IEC Publication 950is 0.4 mm. It might first appear that CMS PW boardscannot meet these requirements for supplementaryand reinforced insulations as the insulation on a CMSPW board is only a few microns thick. In this case,however, there is the option of using two thin layers asthere is no thickness requirement in the Standard forinsulation in thin layers. Only electric strength testrequirements have to be met.

During the preparation of “insulating compound”

coated metal substrate PW boards, several coats ofinsulating material are applied to avoid pin holes andachieve a uniform thickness. Although these indi-vidual layers or coatings cannot be evaluated on anindividual basis as specified in the standard, they canbe evaluated collectively by considering the totalthickness as being equivalent to two layers and apply-ing an electric strength test for the appropriate grade ofinsulation (supplementary or reinforced) using a testvoltage that is twice that for the single layer.

Fig 2 shows the cross-section of a ceramic coatedmetal substrate PU board with a primary andsecond-ary trace, where reinforced insulation is re-quired. This assembly is acceptable provided the crite-ria described in (a) through (g), as applicable, are met.

a. Required clearances and creepage distances forreinforced insulation are maintained between primaryand safety extra-low voltage (SELV) secondary traceson the surface of the CMS PW board. From primarytrace to the edge of the metal substrate, the minimum


clearance and creepage distance required for basicinsulation must be maintained. From SELV secondarytraces to the edge of the metal substrate, the minimumclearance and creepage distance required for supple-mentary insulation must be maintained. The metalsubstrate is considered to be intermediate metal andclearance and creepage distances are still required forsupplementary insulation from SEL V secondary tracesto the metal substrate.

b. Insulating material meets flame test classificationV-1 or better and is heat and moisture resistant.

c. As illustrated in Fig 2, two thin films of insulationare provided between primary and SELV secondarycircuits the first thin film being the insulation betweenthe primary trace and metal substrate and the secondthin film being the insulation between metal substrateand the SEL V secondary trace. The insulation be-tween all traces and metal substrate must be able towithstand the dielectric voltage withstand test forreinforced insulation after normal and abnormal tests,

thermal cycling, thermal aging and abrasion resis-tance tests.d. The raw metal substrate PW board with copperdeposit before processing (i.e. chemical etching) meetsthe dielectric strength test for basic insulation, as aproduction test.

e. For components mounted to the CMS PW board bysurface mounting technology, the effect of compo-nents coming off and reducing any clearances orcreepage distances during abnormal operating condi-tions is determined. For example, a component mountedby only 4 pins or less, must be investigated by conduct-ing fault testing which would result in highest tem-perature.

f. Servicing instructions clearly indicate that CMS PWboards must be replaced and not repaired.

g. Periodic testing of the CMS PW boards is conductedwhen it is necessary to ensure compliance on a con-tinuous basis. t


winter was beginning to turn to spring. The NevaRiver, which divides the city, was still coveredwith ice but clearly was beginning its thaw (when we leftit was mostly free of ice). Boris Yeltsin was facingthe most severe test of his leadership and there wasan air of nervous caution. Even though mostpolitical events were being staged in Moscow, thebroad evidence of change was clearly displayed inSt. Petersburg.

My personal goal for the trip was to spend as muchone-on-one time with Russians as our seminarcommitments would permit. We worked in asmany private meetings with businessmen, educa-tors, civic leaders and technologists as we could.We stayed in a typical apartment rather than at ahotel. Our logistics were coordinated by youngRussians, all in their early and mid-twenties, withwhom we spent almost all of our time. Mytranslator, an engineering student in his final year inhis university studies, was with me 12 or morehours a day, as were our other translators. Weformed close friendships there, friendships which Ibelieve will be lasting.

Probably the most important observation I canshare is that one must be very careful in makinggeneralizations about Russians and their country;the “average Russian” can be just as difficult tocharacterize as the “average American”. WhileRussians are more culturally homogeneous thanAmericans, there is much more individual diversitywithin each of our societies than there is betweenthe two. I found this particularly true of youngerRussians and is especially apparent when individualRussians are in more private settings and awayfrom their usual circles.

In some ways. the overall environment could becompared to “post-war reconstruction” without thebenefit of either a clear victor or evidence ofphysical “devastation”. I found some characteris-

tics that brought to mind our own “wild west” orChicago in the twenties with a little post-WW1Germany thrown in for good measure. Inflation isquite severe and many people will immediatelyexchange their rubles for almost anything...bustokens, clothing, anything...their buying power isseemingly being eroded each day. Many people,especially among the young, are running all sorts ofbusinesses on the side in attempts to improve theirsituations.

The impact of Russia’s conscious change indirection from a centrally controlled, defense- andgovernment-oriented economy is enormous.Seventy years of socialist tradition has clearly leftits impact and the change to a market economy hasmore than its share of challenges.

Not surprisingly, the most resistance to changeoften seems to come from those who believe theyhave the most to lose. And while the Russianleadership is grappling with monumental changessuch as decentralization and privatization ofproperty and industry, the old infrastructure andbureaucracy are struggling to keep up, sometimeswithout great success. Put yourself in the place of amiddle manager with twenty years in governmentservice or in a state-owned industry. You’ve spentyour career responding to central control, focusedon the supplier side, where higher authorityestablished much of your work reality. Yoursociety and economy has seventy years of thisinertia. Now, your focus must abruptly andradically shift to the market side even whileownership and the very nature of your business andits future is being decided by others. I imagine thatmany of us, including those in the US automobileor defense industries, in spite of our traditions ofthe free market, decentralized control and anadvanced infrastructure, can empathize.

The exisiting infrastructure in Russia is not wellgeared to both support these changes and facilitaterapid progress. For instance, consider the state of

Chairman’s MessageContinued from page 1


communications. We recognize our need tocontinually push the practical technological limitsto enhance efficiency of our communicationinfrastructure to survive in the global economy. Incontrast, while citizens of St. Petersburg can getMTV on their televisions and there’s evidence ofcellular phone networks in operation, the basicphone system is quite dated and communicationwith those outside the country can be very frustrat-ing and requires considerable patience. Mail sentinto Russia from outside the country takes weeks oreven longer to be delivered, if it arrives at all.

There are also “intangible” aspects of the infra-structure that don’t readily support the change to afreer, global market-oriented economy. In the USand elsewhere there has been a prevailing attitudeof trust, fairness and respect for a fair profit incommerce. In spite of embarrassing instances ofwhite-collar crime in business, we are able toconduct our business efficiently based on a founda-tion of expected ethics and standards of conduct.However, to many Russians, profit is immoral andis evidence of crookedness; even the term “busi-nessman” can evoke a negative connotation. Also,I found acknowledgment that basic trust in others(outside your small circle) was much less commonthan here; in fact, personal trust has been a preciouscommodity.

While I don’t want to minimize the challenges thatface Russia and the frustrations in trying to dobusiness there, I found St Petersburg exciting andfascinating. Conventional Western wisdom saysthat China is the emerging super-economic powerand the better choice for future ventures, at leastfor the next two decades. However, the potential inRussia appeals to my senses. The young people arebetter educated and probably better balanced thanwe are, all things considered. They recognize therelative spiritual and ethical vacuum in theircountry better than we do ours. Though they arebecoming more materialistic, they are not nearly asself-absorbed and selfish as we tend to be. In our

country, we squander talent by individual choice;in Russia, the talent has historically been restrainedand rechanneled by others. As a people, they haverepeatedly demonstrated their incredible survivalinstincts in spite of extreme hardships.

With regard to their individual and collectivefuture, I found quite a diversity of opinion andindividual differences among the Russians we metI imagine that many will become impatient withslow progress and be tempted to leave their areasof technical and professional expertise to go intocommerce, where more money can be made today.There was evidence that enterprising youngbusinesspersons can make more money hawkingtourist trinkets than top university graduateengineers could ever hope to make in their profes-sion, at least in the present system. So how manywill stick it out in the hope of being rewarded forthe important long-term contributions they canmake?

Still, there is incredible potential and the issue ishow to help them harness their talent and resourcesfor mutual benefit. We have some answers forthem and the West does still control the game inmany ways. However, we certainly don’t have allthe answers; we as a society have often demon-strated our short-sightedness and unwillingness tosacrifice now for future gain. However, I believethose who invest part of themselves in the Russiathat I saw will benefit more than they give,especially if they’re interested in more than simpleeconomic return.


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