IEC_60950_For_Tt.pdf

Applying the IEC and UL 60950 Standardsto Telecommunication TransformersTechnical PaperTable of ContentsForeword.1Introduction .1Knowing the Application.1Voltage and Earthing.2AC Mains Supply Voltage .2Working Voltage .2Protective Earthing .2Circuit Type Definitions .3Insulation Type Definitions .3Insulation Requirements.4Insulation Requirements in Norway and Sweden .5Requirements for Transformers .5Functional Insulation Exception.5Basic Insulation Exception for US and Canada .5Electric Strength .5Insulation Between Windings .6Clearance Distances.7Creepage Distances .10Summary.11Appendix-A Definitions .12IntroductionWhen designing a transformer for telecommunications appli-cation safety agency issues are often a concern and can bedriving factors in the size,performance and even cost of thetransformer.Confusion as to what,exactly,is required in adesign is not uncommon and a thorough knowledge of theend application,voltage levels of the circuit,how the circuitis grounded and country/countries of final installation arepertinent at the very onset of the design.If this information islacking,the final design may be completed to incorporate themost stringent requirements;however this may result in adesign that is larger in size,higher in cost,and/or limited inperformance in comparison to what is required.Knowing the ApplicationTo optimize a design for size,performance and cost whilemeeting the necessary safety agency requirements,CoEvneeds to know what circuit types are on each side of thetransformer and whether or not the circuits provide protec-tive earthing.(Any reference to earthing in this documentrefers to protective earthing in contrast to functional earthingunless otherwise noted.)The highest nominal mains voltageand peak working voltage seen by the transformer are need-ed in order to determine creepage and clearance distances.CoEv must also know whether or not the end product will beused in Norway,Sweden and/or Australia.The right most column of Table 1 lists the defaults applicationinformation that CoEv uses if this information is not providedwhen a UL 60950 or IEC 60950 compliant device is requested.The blanks in the center column can be filled out and sentalong with design inquiries.This will allow CoEv to optimizethe design.Abbreviated circuit type and voltage type defini-tions are given in this document but reference to the 3rd Edi-tion of the UL 60950 standard must be made for completedefinitions.Applying the IEC and UL60950 Standardsto Telecommunication TransformersTyco Electronics Power ComponentsCoEv MagneticsDean HuumalaSenior Design Engineer1ForewordThis application note is intended to aid CoEv Magnetics in the process of determining which type of insulation is required indesigning a telecommunications transformer and is to be used in conjunction with the 3rd Edition of UL 60950,which was usedin the creation of this application.All references to the UL documents in this application note are referring to the 3rd Edition ofUL 60950,dated Dec.1,2000,with much of the text,charts,and graphs taken directly from the standard.The UL 60950 standard,3rd Edition,is jointly issued by CSA International and Underwriters Laboratories Incorporated.The UL60950 standard is based on the IEC 60950 third edition.As stated by UL 60950 the“standard adopts the IEC text with deviation.This standard is published as an equivalent standard.”While this application note is written as a guide for those designing to meet the UL 60950 and/or IEC950 standards,it is by nomeans meant to replace either standard.This guide is limited in nature to designs for the most common telecommunicationtransformers.Customer ApplicationCoEv DefaultTypeSELVCircuit 1Protective Earthed?NoTypeTNV-3Circuit 2Protective Earthed?NoHighest Nominal Mains Voltage250VPeak Working Voltage250VMarketed in Norway or Sweden?YesMarketed in Australia?YesTable 1.Shows default application assumptions if the information is not provided.Voltage and EarthingAC Mains Supply VoltageThe nominal voltage of the AC mains supply that the telecomequipment will be using needs to be known in order to prop-erly determine the clearance distance.As defined in section2.10.2.1 of UL 60950,the AC mains supply voltage is the line-to-neutral voltage in all countries except for Norway.In note4 of the same section it states that in Norway the AC mainssupply voltage is line-to-line which is 230V.(The highest nom-inal line to neutral voltage that we are aware of is 250V in cer-tain cities in South Africa.)In order to cover most worldwide applications,CoEv designstransformers using 250V as the AC mains supply voltage.There may be cases where a lower or higher voltage is moreapplicable for the equipment being designed,but such anapplication would be an exception to the usual designrequest and would need to be specifically noted whenrequesting samples.Working VoltageIn order to define creepage and clearance as well as theelectric strength test requirements,the working voltage ofthe transforme