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1. Tools Of The Trade 3 In response to regular requests for information from our customers on the fundamentals of EMC and safety compliance we have assembled a series of informational brochures These brochures are intended to aid design engineering professionals with the basics in many areas from design features to international compliance to terminology we intend to cover them all To receive other brochures in the series or for more information give us a call at 1 800 839 1649 There are literally thousands of safety standards in use around the world The good news is that harmonization allows the manufacturer to design a single product that complies with the requirements of numerous markets The intent of this Tools of the Trade installment is to provide criteria that will assist designers working with the more common products on the market Computers Medical Devices Machinery Household Appliances and Home Entertainment Equipment While this guide provides general and sometimes more detailed information the reader is advised to refer to the standard that is applicable to his her product for specific requirements What are Hazards Many of us think of hazards as harmful to people Hazards to the environment animals and sometimes damage to the work in process are also considered The following are generally considered when evaluating the hazards associated with equipment Conditions of normal use service and single faults o2. Should it fail it must fail in the safe mode i e hazard locked out Other general considerations are e that all hazards must be removed before the cover can be opened e the interlock switch can not be defeated by hand or without a tool e the door or cover can not be closed with the switch defeated Enclosures Enclosures serve to prevent access to hazards as well as to contain fire The standards outline a variety of requirements for example UL1950 EN60950 Information Technology Equipment specifies limits for sizes and location of openings plus the flammability ratings and thickness requirements for materials Enclosures intended to prevent access to hazards must be strong enough to withstand anticipated environmental conditions Hand held products should not break open when dropped and other equipment should not create hazards if tipped over Further the enclosure must not bend or bow and reduce spacings or cause short circuits Various tests are employed to evaluate the strength of enclosures among them steady forces and impacts are applied and drop tests are performed Following such tests the product must remain safe in accordance with the standard Plastic or other non metallic parts must not present a fire hazard Most often UL 94 ratings are acceptable for determining compliance with this requirement Generally parts with enough material to support and sustain a fire must not be flammable PC boards may also ha
3. must ensure that the component manufacturer does not make changes that affect safety Get the certificate from the manufacturer in the US it is a copy of the UL yellow card and elsewhere it would be a copy of an agency license or similar certificate Labeling and Instructions The standards identify the information that must appear on the product and or in the manuals Generally e the manufacturer s name or rated voltage registered trademark e current or power e model number requirement and frequency e serial number or date code must be on the product Additionally inputs and outputs indicators and controls and operator replaceable fuses should be marked All markings should be identified and described in the manuals Installation User and Service Instructions should be provided in manuals The Installation Manual should include all information necessary to install the equipment For example supply connections power air water etc disconnect switches over current protection ventilation requirements and other information appropriate for the particular equipment The User Manual should contain all warnings and cautions along with all information for proper operation cleaning etc The User Instructions MUST NOT direct the user to expose him herself to hazards even when replenishing consumables or making adjustments Language of the User Instructions as well as labeling have always been a difficult topics Gener
4. place to meet these safety objectives Protection may consist of some or all of the following Fuse protection will help prevent hazardous voltages from being accessible to the user during a fault or large currents being drawn by the equipment Fusing and circuit protection are determined during the design phase as well as during fault testing where shorts and or overloads will be imposed to ensure that hazards can not be created gt o gt O n m w E l an a wy Z w J Se i zZ Barriers or covers over hazardous parts can be used to reduce risk Such guards can prevent access to high voltages energy levels and mechanical hazards The standards generally identify the requirements for guards a tool must be required to remove the guard e sizes of openings do not allow access and e the guard must be attached in some way so that it can not be completely removed and misplaced Interlocks operate to remove hazards prior to access Interlocks disable electrical sources and or mechanical hazards Means for interlocking must be reliable often the switch or device must be cycle tested The interlock should consist of electro mechanical components only and should not rely on logic circuits or semiconductors An analysis should show that a single fault can not render the interlock circuit inoperable D o Z les D 6 ow a O 9 O E a0 ERATO EANET e Nog inro we lt aom Wowe Wee l foes 4
5. the branch circuit protection and Category III covers low voltage products such as keyboards mice etc For example a notebook computer with a brick power supply the power supply would be Category I and the computer itself Category III 4 4 Pollution Degree Yet another consideration for determining spacings is Pollution Degree This addresses where the product is to be used and what environmental conditions it will be subjected to Circuits components or products that are likely to be exposed to moisture or Page Seven particulates in the air will require larger spacings than items that are encapsulated used in a clean room etc For example take a look at the ventilation slots on a desk top computer power supply they re likely to have dust bunnies stuck to them The build up of dust and other solid matter is likely to cover the power supply board as well and may become moist on a humid day Proper creepage distances are intended to reduce the possibility that a conductive track and short circuit could occur Proper clearances help assure that arcing between live parts will not occur Protection It is important to protect people animals and the environment from harm Electric shock energy fire radiation and mechanical hazards are among the issues that are considered in setting safety design criteria While the protection provided should not impair the function of the equipment adequate safeguards should be in
6. ally they should be in a language appropriate for the intended market User Instructions for some product categories such as household goods must always be translated while others like those for machinery laboratory equipment or medical products may remain in English Service Instructions should include all information required to maintain the equipment replacement parts should be identified and any other special considerations should be explained Conclusion Presented here are merely fundamental issues affecting a product s compliance with various safety standards The reader is urged to procure the standard s appropriate for his or her equipment Remember too that there may be more than one applicable standard quite often the primary standard will list a number of others for components or other aspects of the equipment and declaring or certifying compliance with the base standard implies compliance with all It is also feasible to create one product model that meets the requirements for a number of markets Of course labeling and language will change but the basic construction of the equipment may not have to just apply the more stringent requirements and look for ways to meet the intent of all standards Remember that the standards generally tell you what to do and not how so with a little effort your design can be deemed to comply with all standards relevant to your product In addition to an evaluation of a pro
7. circuit breaker Some equipment may include a Residual Current Device RCD for Europe or a Ground Fault Circuit Interrupter GFCI for the US These devices detect fault currents and open the circuit as a fuse or circuit breaker would do These are often used in damp areas and in medical devices where small but lethal leakage currents can flow There are many requirements for PE A primary requirement is that double securement be used The protection of PE connections must be taken care of at the terminations themselves That is a cable tie or sleeving is not an option Double crimp terminals lock washers or hooking wires prior to soldering must be used There are fast on connectors available which meet the requirements these connectors provide a double crimp which secures the insulation and conductor independently pe Page Four and have a positive locking mechanism which prevents the terminal from sliding off by means of a tab and mating hole A tool may be required to remove such a connector PE must make first and break last when provided in a connector for example where an auxiliary outlet is available to plug in an accessory Approved plugs and sockets will meet this requirement and many connector manufacturers now provide pins of different lengths to satisfy this requirement The current path must be robust PE conductors and connections must provide a low impedance path from all conductive sur
8. duct s physical characteristics a number of tests are performed among them Dielectric Strength Humidity Preconditioning Insulation Resistance Enclosure Strength Leakage Current Sound Level Ground Impedance And others as specified in Heating the relevant standard Faults Abnormals These tests will be addressed in another Tools of the Trade 500 NBER CTD RIVES GAT HER SRS MDO 2007S Pre Oe oe ee kee i Ge Page Ten Page Eleven info wll com www wll com TTI iT e Washington Laboratories Ltd
9. faces to the supply earth Bare metal to metal mating surfaces should be provided this may mean that surfaces may need to be masked prior to painting Also attention should be given to the types of metals being used dissimilar metals are subject to corrosion due to galvanic action or differences in electro potentials SMOKE WoA aaa LUG GROUND WIRE ww D CHASSIS EMI FILTER CHASSIS TO OTHER CONNECTION GROUNDS GROUND GREEN YELLOW WIRE SAME GAUGE AS OR BIGGER THAN SUPPLY MECHANICALLY SECURED 3 4 Terminal Markings Mark the terminals for the supply connections with L and N for single phase systems and with L1 L2 L3 for the US or with U V W for Europe in three phase equipment PE is generally identified by the symbol and or by the letters PE Other terminals should be identified in the product and should correlate with the documentation for use and service Alpha numeric designations are usually acceptable 3 5 Wire Colors and Markings In the United States black gt e d Page Five i and white are generally used to identify Line and Neutral respectively Brown and blue are the preferred colors in Europe for Line and Neutral respectively Wire colors will be correct in a cord which bears approvals for the intended market Protective Earth is usually green US or green yellow International however green yellow can generally be used so that one model wi
10. ll meet most market requirements A question that often arises is What is the ratio of green to yellow Some but not all standards answer this 30 of one color and 70 of the other Further it is recommended and specified in some standards that green OR yellow is not used where there is a possibility of confusion no this does not apply to multi conductor signal or other cables the intent is to reduce confusion with the AC supply wiring Some standards recommend specific colors for wires in other circuits but there are few actual requirements beside the above Do note that the customer may demand wire color standards to provide consistency for service personnel or for other site standards The machinery standard mandates that all wires must be marked or identified in the product and the service documentation Other agencies require replaceable cables to be identified as well Isolation for Electrical Protection 4 1 Separation of Circuits One of the fundamental means of preventing hazards is to minimize the possibility of hazardous circuits coming in contact with other parts or circuits which in turn may be accessible to the user of the equipment The means may consist of Creepage Distance i e distance between two parts over a surface or Clearance distance through air Creepage and Clearance are also known as spacings Spacings are measured by the shortest distance between bare conductive parts Mounting hardware u
11. r abnormal operation are reviewed as well Energy The common PC power supply is very happy sourcing 25 amps or more at 5V Even though the voltage will not hurt anyone there is enough current available to melt insulation or start a fire under certain conditions Current limiting is provided to reduce the risk of this hazard Shock This is the most widely recognized hazard Studies have been done to determine the voltages and currents which are hazardous to people As a rule voltages exceeding approximately 30Vrms 42 2 Vpeak or 60Vdc are considered hazardous Currents that may cause harm are in the 10 s of microamps to 10 s of milliamps and the particular standard provides the limits The frequency of the waveform of interest is also important lower frequencies are more dangerous because they tend to travel deeper through human tissue and are more likely to affect the heart Higher frequencies present greater danger of burns Mechanical Moving parts required for correct function of the equipment or ancillary to it fans etc present a physical hazard to the user of the equipment Guarding interlocking or other means should be provided to minimize the risks Also the likelihood that a product will tip over is considered during the evaluation Heating Excessive temperatures can cause burns start a fire or degrade insulation within the equipment The standards generally provide temperature limits for various inte
12. rnal and external parts Installation or mounting methods are considered when evaluating the product under counter installed in a cabinet etc Radiation Lasers RF emissions audible noise X rays and other radiation sources are all potential hazards The equipment design must reduce the risk of human exposure to these sources Other Hazards There is a tremendous variety of products on the market each with unique characteristics and potential hazards The applicable standards will usually address the hazards associated with the particular product or the risks will usually be recognized and addressed by product safety specialists Getting Power to the Unit 3 1 Plugging In There are essentially three ways to get power to your product Permanent wiring fixed cord and appliance inlet or plug socket There are specific requirements for each of these 1 For permanent wiring often found in machinery terminals are usually provided within the equipment The terminals should be located close together and should be sized to carry the rated and anticipated fault currents for the particular product Installation is often performed by an authorized individual in accordance with the electrical codes in force at the site 2 Fixed cords non detachable are often used in Page Three appliances There may be several different options allowed by the standard depending upon the complexity of cord replacement These options may range f
13. rom user replaceable to non replaceable that is throw the device away if the cord is damaged Requirements for fixed cords may include the type s of strain relief allowed the type of cord and wire gauge required for a particular application and or the maximum length of the cord 3 Appliance inlets or plugs sockets provide distinct advantages over fixed cords appliance inlets are available with integral fuse holders and means for voltage selection and they can have built in EMC filters Inlets and plugs sockets allow easy changeover to cord sets for different countries Any inlets plugs and sockets must bear approvals for the countries of intended use and must be rated for the particular application 3 2 Sizing the Wires Phase Connections Terminals and or connections for Line and Neutral must be adequately sized often dictated by the standard and must provide adequate protection against loosening Methods for providing such protection often referred to as double securement include double crimp terminals shrink sleeving cable ties lock washers looping and hooking soldered connections etc The goal is to prevent a loose conductor from shorting against other parts or circuits 3 3 Protective Earthing grounding for electrical safety Earthing or bonding often abbreviated PE for Protective Earth offers a level of protection by providing a path for fault currents which by design will blow a fuse or trip a
14. sed to install PC boards metal screws and standoffs can reduce the spacings below acceptable values Insulation may be used either alone or in conjunction with spacings to provide the separation between circuits The standard will identify the ways in which the separation may be provided as well as specific requirements for each method PRIMARY N DOUBLE OR REINFORCED INSULATION 4 2 Working Voltage One of the considerations for determining spacing requirements is Working Voltage That is the maximum voltage that the relevant insulation will be subjected to The voltage is measured between any two points in the circuit including voltages to ground It is recommended that the derived Working Voltage should never be less than the supply voltage in a primary circuit regardless of the circuit configuration or the measured values Further periodic spikes that exceed the nominal value e g switching spikes are considered but transient over voltages are not 4 3 Installation Category Another factor that may be used to determine spacings is Installation Category Where is the product installed in relation to the power source Is it before or after the branch circuit protection Is it connected to a low voltage power supply There are four Installation Categories that cover most products Of these four the vast majority of products fall under two Category II is for equipment that is powered by 100 240V after
15. ve to be adequately rated many boards are 94V 0 which meet most if not all requirements The required ratings are based on size weight or mounting of the equipment along with quantities and locations of materials present Be aware that the flammability rating is often based on material thickness and an enclosure may not meet the requirements because it is too thin UL 94V 0 Least flammable UL 94V 1 UL 94V 2 UL 94HB Most flammable Components Safety related components must meet their relevant standards and must be used in accordance with epee bee their ratings Such components include e fuses and fuse holders e power supplies and transformers e circuit breakers e relays e supply connectors e switches in hazardous circuits and terminals e wiring and terminals The component vendor will usually provide the information needed to use the part correctly These parts will be identified and evaluated during the certification process and changes that involve these critical components must be evaluated to ensure continued compliance It is always better to procure parts that already bear the required approvals The components should meet the requirements in force in the intended market That is UL for the US IEC or EN for Europe etc It is possible to use unapproved components and many times evaluate them in the end product Be aware however that you are responsible for the continued compliance of those parts and
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