Technical Paper Applying Applying the IEC and and UL 60950 Standards Standards to Telecommunication Telecommunication Transformers Transformers
Table of Contents Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Knowing the Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Voltage and Earthing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 AC Mains Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Working Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Protective Earthing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Circuit Type Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Insulation Type Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Insulation Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Insulation Requirements in Norway and Sweden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Requirements for Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Functional Insulation Exception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Basic Insulation Exception for US and Canada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Electric Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Insulation Between Windings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Clearance Distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Creepage Distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Appendix-A Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Applying the IEC and UL 60950 Standards to Telecommunication Transformers Tyco Electronics Power Components
Dean Huumala
CoEv Magnetics
Senior Design Engineer
Foreword This application note is intended to aid CoEv Magnetics in the process of determining which type of insulation is required in designing a telecommunications transformer and is to be used in conjunction with the 3rd Edition of UL 60950, which was used in the creation of this application. All references to the UL documents in this application note are referring to the 3rd Edition of UL 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 UL 60950 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 no means meant to replace either standard. This guide is limited in nature to designs for the most common telecommunication transformers.
needs to know what circuit types are on each side of the transformer and whether or not the circuits provide protec tive earthing. (Any reference to earthing in this document refers to protective earthing in contrast to functional earthing unless otherwise noted.) The highest nominal mains voltage and peak working voltage seen by the transformer are needed in order to determine creepage and clearance distances. CoEv must also know whether or not the end product will be used in Norway, Sweden and/or Australia.
Introduction When designing a transformer for telecommunications application safety agency issues are often a concern and can be driving factors in the size, performance and even cost of the transformer. Confusion as to what, exactly, is required in a design is not uncommon and a thorough knowledge of the end application, voltage levels of the circuit, how the circuit is grounded and country/countries of final installation are pertinent at the very onset of the design. If this information is lacking, the final design may be completed to incorporate the most stringent requirements; however this may result in a design that is larger in size, higher in cost, and/or limited in performance in comparison to what is required.
The right most column of Table 1 lists the defaults application information that CoEv uses if this information is not provided when a UL 60950 or IEC 60950 compliant device is requested. The blanks in the center column can be filled out and sent along with design inquiries. This will allow CoEv to optimize the 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 complete definitions.
Knowing the Application To optimize a design for size, performance and cost while meeting the necessary safety agency requirements, CoEv
Customer Application Type Protective Earthed? Type Circuit 2 Protective Earthed? Highest Nominal Mains Voltage Peak Working Voltage Marketed in Norway or Sweden? Marketed in Australia? Circuit 1
Table 1. Shows default application assumptions if the information is not provided.
1
CoEv Default SELV No TNV-3 No 250V 250V Yes Yes
• Electric strength: Section 5.2.2 describing the test procedure for electric strength does not give any exclusion for ringing voltages but refers to section 2.10.2 and states that “DC values of the working voltage shall be used for DC voltages and peak values for other voltages.” CoEv thereby assumes that ringing voltages must be considered in determining the working voltage for the electric strength test. • Clearance Distance: Because of requirements to treat secondary circuits in Norway and Sweden as primary circuits noted in section 6.1.2.1 as well as unearthed secondary circuits noted in section 2.10.3.3, CoEv assumes that clearances for primary circuits are required. As noted i n section 2.10.3.2 NOTE 4 the working voltage to be used must be equal to the AC mains supply voltage. • Creepage Distance: Section 2.10.4 specifies that for creepage distances the working voltage shall be the RMS or DC. Value without ripple and short-term conditions and disturbances such as cadence ringing signals and transients shall not be taken into account.
Voltage and Earthing AC Mains Supply Voltage The nominal voltage of the AC mains supply that the telecom equipment will be using needs to be known in order to properly determine the clearance distance. As defined in section 2.10.2.1 of UL 60950, the AC mains supply voltage is the line to-neutral voltage in all countries except for Norway. In note 4 of the same section it states that in Norway the AC mains supply voltage is line-to-line which is 230V. (The highest nominal 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 designs transformers using 250V as the AC mains supply voltage. There may be cases where a lower or higher voltage is more applicable for the equipment being designed, but such an application would be an exception to the usual design request and would need to be specifically noted when requesting samples.
Unless otherwise specified, the peak working voltage used is 250V. This voltage is used not only because of the AC mains supply voltage requirement for clearance distances but also because some telecom designs use line powering voltages up to 250V. For an optimal design, the working voltage must be defined on the inquiry.
Working Voltage In order to define creepage and clearance as well as the electric strength test requirements, the working voltage of the transformer will also need to be known. The working voltage is the highest voltage to which the transformer is subjected to while operating under normal conditions of use. According to section 2.10.2 of UL 60950 when determining the working voltage in transformers, the highest voltage possible between any point in a winding and any other part or winding of the transformer will be used. Also, floating transformer windings, or parts, are assumed to be earthed at the point that obtains the highest working voltage.
Protective Earthing It must be known whether or not SELV and TNV circuits are protective earthed in order to determine not only the type of insulation required, but also the clearance distances required. Unless specified, the default is that neither side is earthed. The provisions for protective earthing are described in Section 2.6.
The working voltage definition varies depending on whether it is being used to determine clearance distances, creepage distances or electric strength requirements for transformers. Occasionally, specific exclusion is made for ringing voltages. In other instances, the working voltage is defined to be equal to the mains voltage. CoEv determinations of the working voltage in the three different cases are based on the following cri teria: electric strength, clearance and creepage distances:
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which circuit types are in use, the designer must have intimate knowledge of the voltage levels and environment of the circuit. Unless otherwise specified, the transformer designer will assume the most stringent TNV conditions (TNV-3), and SELV to be the circuit types.
Circuit Type Definitions For most telecommunications applications the transformer will bridge the barrier between a SELV circuit, a low voltage circuit, and a TNV circuit, a telecommunications circuit. Abbreviated definitions are shown for all different circuit types available in Table 2 below. To accurately determine Primary Circuit Secondary Circuit Hazardous Voltage ELV Circuit
SELV Circuit TNV Circuit
TNV-1 Circuit TNV-2 Circuit TNV-3 Circuit
Connected to the AC Mains supply. No connection to a primary circuit. Exceeds 42.4V peak or 60VDC and does not meet limited current nor TNV requirements Secondary circuit not exceeding 42.4V peak or 60VDC under normal operating conditions, separated from hazardous voltages by basic insulation and does not meet limited current nor SELV requirements. Secondary circuit not exceeding 42.4V peak or 60VDC under normal operating and single fault conditions. Secondary circuit with limited accessibility, does not exceed 42.4V peak or 60VDC under normal operating conditions, and under single fault conditions may see a voltage spike of 1500V which degrades to 400V peak or DC within 2ms (see section 2.3.1 for complete definition of single fault maximum voltages). TNV circuit which meets SELV voltage requirements, but on which overvoltages from the telecommunications network are possible. TNV circuit which exceeds SELV voltage requirements and is not subjected to overvoltages from the telecommunications network. TNV circuit which exceeds SELV voltage requirements and on which overvoltages from the telecommunications network are possible.
Table 2. Abbreviates the circuit definitions from UL 60950 Edition 3.
and whether or not these circuits are connected to earth. Table 3 provides abridged definitions for each of the insulation categories.
Insulation Type Definitions UL 60950 Edition 3 defines five different categories of insula tion in section 2.9.5. The category of insulation to be used is dependant upon the type of circuit on each side of the barrier
F - Functional Insulation
Insulation necessary for correct operation of equipment (referred to as Operational Insulation in previous editions).
B - Basic Insulation
Insulation to provide basic protection against electric shock.
S - Supplementary Insulation
Insulation applied in addition to basic insulation in order to reduce risk of electric shock in the event of failure of the basic insulation.
D - Double Insulation
Insulation comprising both basic insulation and supplementary insulation.
R - Reinforced Insulation
Single insulation system to provide a degree of protection against electric shock equivalent to double insulation.
Table 3. Gives abridged definitions of the insulation categories defined in UL 60950 Edition 3.
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and the SELV circuits are un-earthed. With this scenario the insulation requirement is basic insulation. The requirements for basic insulation are more stringent than the requirements for functional insulation. The only time that functional insula tion is the appropriate insulation is if the barrier is betw een a SELV circuit and a TNV-1 circuit and one or both circuits are earthed.
Insulation Requirements For most telecom applications the circuits being isolated are a SELV circuit from a TNV circuit. Table 4, for selecting the appropriate insulation category, was derived from section 2.9.5, Figure 2F in UL 60950 Edition 3. For all of the possible SELV to TNV scenarios, whether earthed or unearthed, the insulation requirement is either functional insulation or basic insulation.
While basic insulation will suffice for all TNV to SELV circuits in most of the world, there is an exception for Norway and Sweden. The following section explains this exception.
When the exact nature of the telecom circuit is unknown, the default is that the TNV circuit is TNV-3 and that both the TNV
Primary Circuit Unearthed Hazardous Voltage Secondary Circuit Earthed Hazardous Voltage Secondary Circuit ELV Circuit Unearthed SELV Circuit Earthed SELV Circuit Unearthed TNV-1 Circuit Earthed TNV-1 Circuit TNV-2 Circuit TNV-3 Circuit
Primary Circuit
Unearthed Hazardous Voltage Secondary Circuit
Earthed Hazardous Voltage Secondary Circuit
ELV Circuit
Unearthed SELV Circuit
Earthed SELV Circuit
Unearthed TNV-1 Circuit
Earthed TNV-1 Circuit
TNV-2 Circuit
TNV-3 Circuit
F
B
B
B
R
B
R
R
R
R
B
F
-
B
R/S
B
R/S
R/S
R/S
R/S
B
-
F
B
R
B
R
R
R
R
B
B
B
F
-
F
S
S
S
S
R
R/S
R
-
F
F
B
F
B
B
B
B
B
F
F
F
F
F
B
B
R
R/S
1
R
S
B
F
F
-
B
B
R
R/S
1
R
S
F
F
-
F
B
B
R
R/S
1
R
S
B
B
B
B
F
B
R
R/S
1
R
S
B
B
B
B
B
F
1
1
Table 4. Shows which insulation category is required between circuit types.
1
By default Reinforced Insulation unless otherwise specified. Supplementary Insulation applies only when the working voltage is equal to the voltage between the secondary circuit at hazardous voltage and another secondary circuit at hazardous voltage (or a primary circuit).
4
1
1
1
1
a) ringing signals do not exceed the limits for TNV-3 b) the transformer is located on the disconnect side of the switchhook c) the wire complies with the component requirements for magnet wire, and d) the transformer is subjected to a 1000 VAC electric strength test as a routine test on 100% of production without evidence of dielectric breakdown (see section 6.2.1)
Insulation Requirements in Norway and Sweden In Sweden and Norway supplementary insulation for a primary circuit is required for any equipment connected to the telecommunications network. This is in part due to soil condi tions in that region of the world as well as to the power distribution systems used in Norway. The exceptions for Sweden and Norway are noted in UL 60950 Edition 3 section 6.1.2.1. This exception is noteworthy because, by default, the transformer designer works under the assumption that the end product will be marketed in Norway or Sweden unless con trary information is provided. The result is a transformer that is designed to meet supplementary requirements for a primary circuit, which may result in a larger, more expensive transformer than what is truly needed.
This exception is not utilized when designing unless the equipment manufacturer provides information that a) the ringing signals do not exceed the limits for TNV-3 and b) the transformer is located on the disconnect side of the switchhook. If this information is not provided the design might result in a larger, more expensive transformer than is necessary. Electric Strength The electric strength test that the transformer must meet is defined in section 5.2.2. with the levels specified in Table 5B. A copy of Table 5B Part 1 is shown as Table 5; a copy of Part 2 is also shown. Per section 5.2.2 the voltage applied in the strength test must be held for 60 seconds except for routine tests that must be held for 1 second as stated in NOTE 1. In addition section 6.2.2.2 requires that equipment connected to the telecommunications network is subject to the dielectric steady-state strength test of 1000VAC in accordance with section 5.2.2. When specifying transformer dielectric strength the greater of the two values is typically utilized.
Requirements for Transformers In keeping in compliance with the UL 60950 standards, a transformer must be designed with four parameters in mind. These parameters (i.e. the electric strength test, the insulation between transformer windings, the creepage distance and the clearance distance) all vary depending on the insulation category being designed to as well as the working voltage and mains supply voltage in the circuit. A special exception for basic and functional insulation in the US and Canada allows the transformer to be built with magnet wire only, negating the need for insulation between the windings and creepage and clearance. Special requirements apply, as these transformers must be subjected to electric strength tests. The following are exceptions for both cases along with the appropriate section in UL 60950 Edition 3 that list the exception Functional Insulation Exception Per section 5.3.4 one of the alternatives to meeting the clearance and creepage distances listed is to withstand the electric strength tests for functional insulation in section 5.2.2. Interleaved insulation for winding separation in wound components, defined in 2.10.5.4 does not men tion functional insulation. By default magnet wire is acceptable. When CoEv designs to Functional Insulation this exception is followed and an electric strength test is met using magnet wire insulation only for isolation. Basic Insulation Exception for US and Canada In section 2.3.2 a less stringent deviation for the US and Canada states that enamel coating on signal transformer winding wire may be used as an alternative to basic insulation only if:
Table 5. Taken from UL 60950 Table 5B defines the test voltages for electric strength test.
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Electric Strength in Australia: Section 6.2.2.2 notes that a steady-state electric strength test of 1.5kVAC must be met for telecom equipment in Australia. This is done to simulate lightning surges on typical rural and semi rural network lines. Australia also requires the 10/700(S impulse test described in section 6.2.2.1. Unless otherwise specified, the default is that the transformer will be used in Australia and the transformer will be designed to these requirements. Electric Strength in Sweden and Norway: Sec tion 6.1.2.1 notes that for both Sweden and Norway supplementary insulation for a primary circuit is required for any equipment connected to the telecommunication network. The required electric strength test voltage for a primary circuit, with supplementary insulation and an assumed working voltage of 250V, is 1500VRMS. Electric Strength in Secondary Circuits: The rest of the world does not have the supplementary insulation for a primary circuit stipula tion that Norway and Sweden do. The voltage for the electric strength test in Table 5 Part 2 for an assumed working voltage of 250V the voltage level is 1261VRMS. It should be noted that for products sold in Australia, Norway and/or Sweden the minimum required voltage strength is 1500VRMS and this is the default value used by CoEv. Insulation Between Windings In complying with UL 60950 Edition 3 requirements for distances through insulation in wound components, one must refer to section 2.10.5.4. When basic, supplementary or reinforced insulation is required between windings (the exception for BASIC INSULATION does not require this) there are three transformer construction methods possible; tape, specialty wire or void-free/cemented-joint construction. Table 5 (continued). Taken from UL 60950 Table 5B defines the test voltages for electric strength test.
Tape: Thin sheet materials, such as tape, are permitted as a means of insulation in section 2.10.5.2. This same section specifically states that, “Solvent-based enamel coatings are not considered to be insulation.” In other words the insulation on magnet wire is not considered insulation. As stated in section 2.10.5.2 the requirements for thin sheet material layers are:
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• SUPPLEMENTARY INSULATION comprises at least two layers of material, each of which will pass the electric strength test for SUPPLEMENTARY INSULATION; or • SUPPLEMENTARY INSULATION comprises at least three layers of material for which all combinations of two layers together will pass the electric strength test for SUPPLEMENTARY INSULATION; or • REINFORCED INSULATION comprises at least two layers of material, each of which will pass the electric strength test for REINFORCED INSULATION; or • REINFORCED INSULATION comprises three layers of material for which all combinations of two layers together will pass the electric strength test for REINFORCED INSULATION.
• BASIC INSULATION: two wrapped layers or one extruded layer • SUPPLEMENTARY INSULATION: two layers, wrapped or extruded • REINFORCED INSULATION: three layers, wrapped or extruded Void Free Impregnation and Cemented Joint: Section 2.10.8 allows for products to be built with void free impregnation and/or cemented techniques providing they meet the dis tance through insulation requirements of Section 2.10.5.1. Inherent with this method of construction are high development costs and long development lead times. While this method of construction has been used on a custom basis, CoEv Magnetics does not utilize this type of construction as a primary means of manufacturing compliant transformers, instead they opt for more industry standard methods.
While no specific requirements for thin sheet material layers are defined for basic insulation in section 2.10.5.2, the standard does specifically state in section 2.10.5.4 that windings requiring basic insulation “shall be separated by interleaved insulation”. Based on this, the conclusion can be made that, at a minimum, the layer requirements for a supplementary insulation apply.
Clearance Distances Clearance distances defined in section 2.10.3 are determined by insulation category required, peak working voltage, nominal AC mains supply voltage (both of which are assumed to be 250V if not specified), pollution degree of the environment, and whether or not the circuit is earthed.
Specialty Wire: Specialty wire is an acceptable means of insulation between windings as defined by section 2.10.5.4. The wire must have multi-layer spirally wrapped or extruded insulation where the layers comply with 2.10.5.2 and pass the tests of annex U (where the layers can be individually tested for electric strength). The minimum number of constructional layers applied to the conductor is defined in section 2.10.5.4 to be:
Pollution Degree: The pollution degrees defined in section 2.10.1 are shown in Table 6. Unless otherwise specified, Pollution Degree 2 is the default as it typically covers equipment within the scope of this standard (UL 60950).
Pollution Degree 1
For components and assemblies which are sealed so as to exclude dust and moisture
Pollution Degree 2
Generally for equipment covered by the scope of this standard (UL 60950)
Pollution Degree 3
Where a local internal environment within the equipment is subject to conductive pollution or to dry non-conductive pollution which could become conductive due to expected condensation
Table 6. Shows the definitions for Pollution Degree from UL 60950.
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Minimum Clearance in Primary Circuits: Exceptions in sec tion 6.1.2.1 require supplementary insulation for a primary circuit for Norway and Sweden. Unearthed secondary circuits shall also be subjected to the requirements for primary circuits as stated in section 2.10.3.3.
With the requirement of a 250V peak working voltage and nominal mains voltage the clearance required for Pollution Degree 2 is 1.5mm for 100% electric strength tested transformers. The 1.5mm distance is used as part of the quality control program and routine electric strength test requirements, which are met by CoEv. This 1.5mm distance is the clearance distance that CoEv uses for basic and supplemen tary insulation by default. For functional insulation the excep tion in section 5.3.4 for an electric strength withstand test is exercised and the creepage distance is not required.
Unless otherwise specified by the customer, an unearthed circuit as well as a requirement for compatibility for Norway and Sweden is used as the default because their criteria requires selection of clearance from Table 7 from UL 60950 Edition 3 Table 2H.
Table 7. A copy of UL60950 Table 2H, shows clearance distances for primary circuits.
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Minimum Clearance in Secondary Circuits: The clearance in Table 8, a copy of UL 60950 Edition 3 Table 2K, only applies for earthed secondary circuits and is not used by CoEv unless specific details are supplied stating that both secondary circuits are earthed.
Table 8. A copy of UL 60950 Table 2K, shows clearance distances for secondary circuits.
9
Creepage Distances Creepage distances for UL 60950 are defined in section 2.10.4. When determining the creepage distance, the working voltage (actual RMS or DC value without ripple, short-term condition or disturbances), the pollution degree, and the material group are taken into account. It is also noted in this same section that if the creepage distance is less than the applicable clearance distance then the creepage distance shall be at least as great as the minimum clearance distance. Material Group: The definitions for material groups shown in Table 9 are defined in section 2.10.4. If the material group is not known, Material Group IIIb is to be assumed. However, CoEv uses Material Group I material by default for standard designs. Material Group I
600<= CTI (comparative tracking index)
Material Group II
400 <= CTI < 600
Material Group IIIa
175 <= CTI < 400
Material Group IIIb
100 <= CTI < 175
Table 9. Shows Material Group definitions taken from UL 60950 Edition 3.
Minimum Creepage Distances: Using the CoEv defaults of a 250V working voltage, Pollution Degree 2 and Material Group I, the creepage distance of 1.3mm can be found from Table 10 (Table 10 is a copy of UL 60950 Edition 3 Table 2L which is located in Section 2.10.4). However, because this value is less than the 1.5mm clearance distance determined for the same conditions, the creepage distance must be at least 1.5mm. CoEv, as a default value for basic and supplementary insula tion, designs uses the 1.5mm creepage distance. For func tional insulation designs the exception in section 5.3.4 for an electric strength withstand test is again exercised and the clearance distance is not required.
Table 10. A copy of Table 2L in UL 60950, shows creepage distances.
10
All assumed parameters for supplementary insulation are listed in Table 11 along with the basic and functional insula tion assumed parameters. Table 12 summarizes build requirements for both specialty wire designed and taped-shelf designed transformers when the assumptions in Summary Table 11 are made.
Summary Since typical design inquiries do not specify the safety agency requirements, CoEv must make basic assumptions when designing transformers. For example, if only IEC 60950 or UL 60950 requirements are requested for telecommunica tions transformers, the transformer will be designed to meet supplementary insulation for most worldwide applications.
Neither
Peak Working Voltage 250V
Nominal Mains Voltage 250Vrms
TNV-3
Neither
250V
250Vrms
TNV-1
Either or Both
250V
250Vrms
Insulation
Circuit 1
Circuit 2
Earthed
Supplementary
SELV
TNV-3
Basic
SELV
Functional
SELV
Applicable Countries Worldwide All except Norway and Sweden All except Norway and Sweden
Table 11. Summarizes all application assumptions made by CoEv Magnetics if unknown.
Insulation Category Supplementary Basic Functional
Tape between Windings 2 Layers 2 Layers None
Special Wire Extruded Layers 2 1 None
Clearance
Creepage
1.5mm 1.5mm None
1.5mm 1.5mm None
Table 12. Summarizes the build requirements for default transformers.
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Electric Strength Voltage 1500VAC 1500VAC 1500VAC
Appendix-A Definitions
TNV CIRCUIT [1.2.8.9]: A circuit which is in the equipment and to which the accessible area of contact is limited and that is so designed and protected that, under normal operating conditions and single fault conditions (see 1.4.14), the voltages do not exceed specified limit values. A TNV CIRCUIT is considered to be a SECONDARY CIRCUIT In the meaning of this standard.
Appendix-A is a list of helpful definitions from the UL 60950 Edition 3 document.
AC MAINS SUPPLY [1.2.8.1]: The external a.c. power distribution system supplying power to the equipment. These power sources include public or private utilities and, unless otherwise specified in the standard (e.g. 1.4.5), equivalent sources such a as motor-driven generators and uninterruptible power supplies.
NOTE 1 —The specified limit values of
voltages under normal operating conditions and single fault conditions (see 1.4.14) are given in 2.3.1. Requirements regarding accessibility of TNV CIRCUITs are given in 2.1.1.1. (2.1.1.1 defines the limits as to not exceed SELV CIRCUIT limits under normal operating conditions and defines a single fault limit of 1500V peak down to 400 V peak or d.c. after 2ms.)
PRIMARY CIRCUIT [1.2.8.2]: A circuit which is directly connected to the AC MAINS SUPPLY. It includes for example, the means for connection to the AC MAINS SUPPLY, the primary windings of transformers, motors and other loading devices.
TNV CIRCUITS are classified as TNV-1, TNV-2 and TNV-3 CIRCUITS as defined in 1.2.8.10, 1.2.8.11, and 1.2.8.12. NOTE 2 —The
voltage relationships between SELV and TNV CIRCUITs are shown in table 1 A.
SECONDARY CIRCUIT [1.2.8.3]: A circuit which has no direct connection to a PRIMARY CIRCUIT and derives its power from a transformer, converter or equivalent isolation device, or from a battery. HAZARDOUS VOLTAGE [1.2.8.4]: A voltage exceeding 42.4 V peak, or 60 V d.c., existing in a circuit which does not meet the requirements for either a LIMITED CURRENT CIRCUIT or TNV CIRCUIT.
Normal operating voltages
Overvoltages from TELECOMMUNICATION NETWORKS possible? Yes No
Within SELV CIRCUIT Exceeding SELV CIRCUIT limits limits but within TNV CIRCUIT limits TNV-1 CIRCUIT TNV-3 CIRCUIT SELV CIRCUIT TNV-2 CIRCUIT
Table A1. Shows voltage limits of SELV and TNV circuits.
ELV CIRCUIT [1.2.8.5]: A SECONDARY CIRCUIT with voltages between any two conductors of the circuit and between any one such conductor and earth (see 1.4.9), not exceeding 42.4 V peak, or 60 V d.c., under normal operating conditions, which is separated from HAZERDOUS VOLTAGE by BASIC INSULATION, and which neither meets all of the requirements for an SELV CIRCUIT nor meets all of the requirements for a LIMITED CURRENT CIRCUIT.
TNV-1 CIRCUIT [1.2.8.10]: A TNV CIRCUIT: Whose normal operating voltages do not exceed the limits for an SELV CIRCUIT under normal operating conditions, and on which overvoltages from TELECOMMUNICATION NETWORKS are possible. TNV-2 CIRCUIT [1.2.8.11]: A TNV CIRCUIT whose normal operating voltages exceed the limits for an SELV CIRCUIT under normal operating conditions, and which is not subject to overvoltages from TELECOMMUNICATION NETWORKS.
SELV CIRCUIT [1.2.8.6]: A SECONDARY CIRCUIT which is so designed and protected that under normal operating condi tions and single fault conditions, its voltages do not exceed a safe value. [2.2.2] In a single SELV CIRCUIT or in interconnected SELV CIRCUITS, the voltage between any two conductors of the SELV CIRCUIT or CIRCUITS and, between any one such conductor and earth (see 1.4.9), shall not exceed 42.4 V peak, or 60 V d.c., under normal operating conditions.
TNV-3 CIRCUIT [1.2.8.12]: A TNV CIRCUIT whose normal operating voltages exceed the limits for an SELV CIRCUIT under normal operating conditions; and on which overvoltages TELECOMMUNICATION NETWORKS are possible. FUNCTIONAL INSULATION [1.2.9.1]: Insulation that is necessary only for the correct operation of the equipment.
LIMITED CURRENT CIRCUIT [1.2.8.7]: A circuit which is so designed and protected that, under both normal operating conditions and single fault conditions, the current which can be drawn is not hazardous. (see 2.4.2 for current limit values)
NOTE —FUNCTIONAL
INSULATION by definition does not protect against electric shock. It may, however, reduce the likelihood of ignition and fire.
HAZARDOUS ENERGY LEVEL [1.2.8.8]: A stored energy level of 20 J or more, or an available continuous power level of 240 VA or more, at a potential of 2 V or more.
BASIC INSULATION [1.2.9.2]: Insulation to provide basic pro tection against electric shock.
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SUPPLEMENTARY INSULATION [1.2.9.3]: Independent insula tion applied in addition to BASIC INSULATION In order to reduce the risk of electric shock in the event of a failure of the BASIC INSULATION.
REQUIRED WITHSTAND VOLTAGE [1.2.9.8]: The peak voltage that the insulation under consideration is required to withstand.
DOUBLE INSULATION [1.2.9.4]: Insulation comprising both BASIC INSULATION and SUPPLEMENTARY INSULATION.
MAINS TRANSIENT VOLTAGE [1.2.9.9]: The highest peak voltage expected at the power input to the equipment, arising from external transients on the AC MAINS SUPPLY.
REINFORCED INSULATION [1.2.9.5]: A single insulation system which provides a degree of protection against electric shock equivalent to DOUBLE INSULATION under the conditions specified in this standard.
TELECOMMUNICATION NETWORK TRANSIENT VOLTAGE [1.2.9.10]: The highest peak voltage expected at the TELECOMMUNICATION NETWORK connection point of the equipment, arising from external transients on the network.
NOTE —The term “insulation system” does not imply that the
CLEARANCE [1.2.10.1]: The shortest distance between two conductive parts, or between a conductive part and the BOUNDING SURFACE of the equipment, measured through air.
insulation has to be in one homogeneous piece. It may com- prise several layers which cannot be tested as SUPPLE- MENTARY or BASIC INSULATION .
CREEPAGE DISTANCE [1.2.10.2]: The shortest path between two conductive parts, or between a conductive part and the BOUNDING SURFACE of the equipment, measured along the surface of the insulation.
WORKING VOLTAGE [1.2.9.6]: The highest voltage to which the insulation or the component under consideration is, or can be, subjected when the equipment is operating under conditions of normal use.
BOUNDING SURFACE [1.2.10.3]: The outer surface of the ELECTRICAL ENCLOSURE, considered as though metal foil were pressed into contact with accessible surfaces of insulating material.
PEAK WORKING VOLTAGE [1.2.9.7]: The highest peak or d.c. value of a WORKING VOLTAGE, including repetitive peak impulses generated in the equipment, but not including external transients.
This material is reproduced, with permission, from Underwriters Laboratories Inc. Standard for Safety, 3rd Edition of UL 60950, Copyright 2000 (by Underwriters Laboratories Inc.), copies of which may be purchased from: comm 2000 1418 Brook Drive Downers Grove, IL 60515 USA 1-888-853-3503 in U.S. and Canada or 415-352-2168, outside the U.S. and Canada Fax: 1-888-853-3512 in U.S. and Canada Fax: 1-630-932-7387 outside the U.S. and Canada http://www.comm-2000.com UL shall not be responsible to anyone for the use of or reliance upon a UL 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 a UL Standard.
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