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61010-1 has provided examples of circuit types that can be used to determine when basic, reinforced or double insulation is needed.
The definitions are the same as other international standards except that functional insulation is not specified (although a note is provided that basic insulation may serve for functional purposes). As such, there is no minimum spacing specified between line and neutral conductors on a PC board.
Cat 4 / 300v pollution degree 2
Creepage Distance. Creepage is 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. A proper and adequate creepage distance protects against tracking, a process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of the electric discharges on or close to an insulation surface. The degree of tracking required depends on two major factors: the comparative tracking index (CTI) of the material and the degree of pollution in the environment. Used for electrical insulating materials, the CTI provides a numerical value of the voltage that will cause failure by tracking during standard testing. IEC 112 provides a fuller explanation of tracking and CTI.1 Tracking that damages the insulating material normally occurs because of one or more of the following reasons:
- Humidity in the atmosphere.
- Presence of contamination.
- Corrosive chemicals.
- Altitude at which equipment is to be operated.
Clearance Distance. Clearance is the shortest distance between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured through air. Clearance distance helps prevent dielectric breakdown between electrodes caused by the ionization of air. The dielectric breakdown level is further influenced by relative humidity, temperature, and degree of pollution in the environment.
When designing a switch-mode power supply for use in information technology (IT) equipment, a typical rule of thumb is to allow an 8-mm creepage distance between primary and secondary circuits, and a 4-mm distance between primary and ground. If these dimensions are allowed for during the design stage, there is a high probability (95%) that no failure will occur with respect to creepage or clearance when the final product is submitted for test.
Working Voltages. A working voltage is the highest voltage to which the insulation under consideration is (or can be) subjected when the equipment is operating at its rated voltage under normal use conditions. The appropriate creepage and clearance values can be determined from the figures provided in the relevant tables in EN 60950.2 These values must sometimes be calculated. To use Tables I–IV (2H, 2J, 2K, and 2L of the standard), the following factors must be considered: determination of working voltages, pollution degree of the environment, and the over-voltage category of the equipment's power source.
When measuring working voltages, it is important to measure both peak and root-mean-square (rms) voltages. The peak value is used to determine the clearance, and the rms value is used to calculate creepage. For example, if one measures a peak voltage of 670 V between two pins of a switching transformer in a switch-mode power supply, the clearance distance between primary and secondary circuits must be calculated using Table I. If the unit is powered via 240 V mains and has a pollution degree of 2, the figures in the center row (marked 300 V rms sinusoidal) and center column (since the mains voltage is >150 V and < 300 V) are used to establish the required clearance distance. In this case, the value for reinforced insulation is 4 mm. One then turns to Table II (Table 2J of EN 60950), which provides additional clearance based on the working voltages and pollution degree. (The middle column was used for calculating this example.) The appropriate row in that column covers the actual repetitive peak insulation working voltage. In this example, the value would be 0.8 mm for reinforced insulation. Adding the two figures together gives a total of 4.8 mm clearance distance. Similarly, if a voltage of 337 V rms was measured between the two pins of the switching transformer, Table IV (2L of the standard) must be used to calculate the creepage distance between the primary and secondary circuits. Assuming pollution degree 2 and material group IIIb, the required creepage distance for basic insulation would be 3.5 mm using linear interpolation. For reinforced insulation, the values for creepage distances are double the values provided in the table for basic insulation. In this case, the required creepage for reinforced insulation would be 7 mm.
|Functional, Basic, and Supplementary Insulation
Rms or Dc
|I, II, IIIa, or IIIb
||IIIa, or IIIb
||IIIa, or IIIb
from the appropriate
|Linear interpolation is permitted between the nearest two points, the calculated spacing being rounded to the next higher 0.1-mm increment
|Table IV. Table 2L of the standard provides minimum creepage distances (creepage distances in millimeters)
The use of these tables is explained in sections 2.10.3–2.10.4 of EN 60950. Measurements should be accurate and repeatable and should also consider the end application.
Pollution Degrees and Over-voltages
Pollution degree is divided into four categories. The following definitions are based on those in IEC 60664.3
- Pollution degree 1. No pollution or only dry, nonconductive pollution occurs. The pollution has no influence (example: sealed or potted products).
- Pollution degree 2. Normally only nonconductive pollution occurs. Occasionally a temporary conductivity caused by condensation must be expected (example: product used in typical office environment).
- Pollution degree 3. Conductive pollution occurs, or dry, nonconductive pollution occurs that becomes conductive due to expected condensation (example: products used in heavy industrial environments that are typically exposed to pollution such as dust).
- Pollution degree 4. Pollution generates persistent conductivity caused, for instance, by conductive dust or by rain or snow.
The over-voltage, also known as installation, category is also divided into four categories according to IEC 60664.
- Over-voltage category I. Signal level (special equipment or parts of equipment), with smaller transient over-voltages than over-voltage category II.
- Over-voltage category II. Local level (appliances and portable equipment), with smaller transient over-voltages than over-voltage category III.
- Over-voltage category III. Distribution level (fixed installations) with smaller transient over-voltages than over-voltage category IV.
- Over-voltage category IV. Primary supply level (overhead lines, cable systems, etc.). This category is not relevant to most product standards.
Typically, most standards are based on conditions being pollution degree 2 and over-voltage category II. It is important to note that as working voltage, pollution degree, over-voltage category, and altitude increase, the creepage and clearance distances also increase. The altitude is particularly important when testing to EN 61010.4
Creepage and Clearance in Practice
Each part of a circuit must be studied to determine the necessary insulation grade. Table 2G in EN 60950 describes common applications of insulation. For example, establishing the required creepage and clearance between a primary circuit and an ungrounded safety extra low voltage (SELV) circuit requires reinforced insulation. By measuring and establishing both the working voltage and the pollution degree, the appropriate row and column in Table 2H (and if necessary Table 2J) determine the minimum clearance distance needed. For one test, the internal components and parts in both primary and secondary circuits are subjected to a steady force of 10 N, and certain minimum clearance distances must be maintained during the test.
Because the primary circuit is an internal circuit connected directly to the external supply mains, this circuit typically contains hazardous voltage. A secondary circuit, which has no direct connection to primary power, may or may not be hazardous. Non-hazardous circuits are classified as SELV.
Dc input products, however, can be treated in one of two ways. They can be considered as being fed by an extra-low-voltage circuit, or as hazardous secondary voltages. This would mean that the clearances could be calculated using Table III rather than Table I, requiring slightly smaller clearance distances. Dc input products can also be considered as being fed by SELV secondary circuits, depending upon the end application. If isolation is needed, then Table III of the standard is used. However, if isolation is not required in the end application, then clearances are waived, and only operational insulation is required.
Simple Design Tips
As IT products continue to get smaller, it is more important than ever to have a good and calculated PCB design that not only reduces electromagnetic interference emissions, but that also reduces creepage and clearance problems. Where shortage of space on a PCB is an issue, especially between primary and SELV circuits, techniques such as slots or grooves can be used to attain desired creepage distance. Slots must be wider than 1 mm; otherwise, they are not considered acceptable. For a groove (>1 mm wide) the only depth requirement is that the existing creepage plus the width of the groove and twice the depth of the groove must equal or exceed the required creepage distance. The slot or groove should not weaken the substrate to a point that it fails to meet mechanical test requirements.
Another solution is to design the PCB so that components are mounted flat on the board rather than positioned vertically. This layout overcomes problems that might arise from the 10- N push test required in EN 60950. A minimum of 8 mm separation between primary and secondary circuits also prevents problems. When semiconductors operating at hazardous voltages are mounted on grounded or floating heat sinks, certain precautions must be taken to ensure compliance with EN 60950. If heat sinks happen to be live (and they can be), they should be marked accordingly to warn service personnel. Generally, a semiconductor's plastic enclosure is considered as operational (necessary for correct operation of the equipment) or, in some cases, as only basic insulation. Therefore, depending on the heat sink's grounding arrangement, the semiconductor requires either basic or reinforced insulation.
It is equally important to consider creepage and clearance even when using UL-recognized power-switching semiconductors. Although these products carry a recognition mark, the manufacturer's data sheets must be examined to ensure that the components are suitable for the intended application.
The working voltages of the circuit must be taken into account. Transistors with built-in reinforced insulation (body thicker than 0.4 mm) must also still meet the spacing requirements at their legs. Some designers mistakenly assume that UL certification eliminates the need for further examination.
EMC stands for electromagnetic compatibility and is defined as the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable disturbances to anything in that environment. EMC requirements concern two basic concepts: emissions and immunity or susceptibility.
Electromagnetic disturbance is any phenomenon that may degrade the performance of a device, equipment, or system, or adversely affect living or inert matter.
Electromagnetic interference (EMI) is the degradation of the performance of a device, transmission channel, or system caused by an electromagnetic disturbance. In certain parts of the world, "EMI" is used to characterize emissions. This may lead to confusion when it comes to the characterization of immunity, which is sometimes called EMC!
Electromagnetic disturbances can be conducted or radiated, and the emissions and immunity requirements are referred to in military standards (MIL-STD 461/462) as CE/RE (conducted emission/radiated emission) and CS/RS (conducted susceptibility/radiated susceptibility), respectively.
Disturbances may represent low-frequency (LF) and/or high-frequency (HF) phenomena, as well as broadband and/or narrowband. Broadband disturbances can originate from commutator motors, ignition systems, arc welding equipment, etc.; narrowband from digital electronic circuitry, switched-mode power supplies, and radio communication equipment. Computers have often been reported to cause interference with radio services, including police, aeronautical and broadcast services. On the other hand, radio transmission by a high-frequency carrier, such as a 900 MHz cellular or a 1.8 GHz DCS, can cause problems in computers and all electrical circuits because the carriers are easily picked up by cables and apertures functioning as antennas and are demodulated in electronic circuits by different nonlinear electromagnetic phenomena.
To really understand how electromagnetic disturbances emanate, propagate, and influence electrical systems, one must have a thorough knowledge of electromagnetic field theory and high-frequency phenomena. EMC represents a broad field of technology and may represent a new subject for many manufacturers of electrical products. However, if one follows certain rules of thumb regarding wiring, PCB layout, grounding and shielding, and usage of RFI suppression components, it is not difficult to cope with different EMC standards and regulations.
In industry, it is assumed that electronic control systems can be used in conjunction with interfering switching operations, motor drives, high-frequency ovens, welding equipment, etc. In a car, electronic automatic systems must function when we use our mobile phone or meet other vehicles (with interfering ignition systems). An electronically controlled wheelchair is presumed to function normally even when the person sitting in the chair uses a mobile phone or a portable PC. We demand that life-supporting electromedical apparatus in a hospital function safely even near high-frequency—radiating surgical equipment.
The general EMC requirements in the U.S. are set by the Federal Communications Commission (FCC), while the Food and Drug Administration (FDA) regulates medical products. Mandatory FCC requirements primarily concern computing devices, defined as any electronic device or system that generates and uses timing pulses at a rate in excess of 9000 cycles per second and uses digital techniques. FCC Part 15 covers radio frequency devices capable of emitting RF energy in the range of 9 kHz—200 GHz. Testing should be done according to ANSI C63.4-1992.
Part 18 covers industrial, scientific, and medical (ISM) equipment (Figure 1), defined as any device that uses radio waves for industrial, scientific, or medical purposes and is not intended for radio communications. While most FCC regulations only concern emissions, FDA also requires immunity for certain life-support equipment. FCC Parts 15 and 18 include regulations as well as technical aspects and limits. FCC Part 68, which governs the technical requirements for registration of telecom terminal equipment, includes lightning surge tests (surge immunity).
Figure 1. Required frequency range of radiated measurements—ISM
FCC Part 15 currently has three different procedures for showing conformance:
- Verification, where the manufacturer or the importer files a test report showing compliance.
- Certification, which requires a review of the application by the FCC, and the use of a unique FCC identification number.
- Declaration of Conformity (DoC), which requires that tests are performed by a test lab accredited by A2LA or NVLAP (other accredited labs may also be accepted).
- The following represents an example of the FCC's classification system, where the correct administrative procedure is listed according to product type (Class A refers to non-consumer products, Class B to consumer products):
- Personal computers and peripherals (Class B): Certification or DoC.
- Cpu boards and internal power supplies used with personal computers (Class B): Certification or DoC.
- External switching power supplies (Class A and B): Verification.
The FCC is currently reviewing these procedures with the aim of simplifying them by reducing the number of categories.
The European EMC Directive, 89/336/EEC, sets out the legal requirements on EMC for principally all electric/electronic equipment to be placed or used in the Common Market/
European Economic Area. The European legislation covers emissions as well as immunity.
The EMC requirements are valid for apparatus and systems placed on the market as complete units. Components such as resistors, transistors, or display units are not included. However, components with a direct function to the end-user, like plug-in PC boards, are regarded as equivalent to apparatus and have to follow the same rules (see Table I).
Scope of Regulation — Device Types
|Appliances and/or digital circuitry inside
|Automotive digital electronics
||Yes, Auto EMC Directive
|Devices with power
<6 n W
||No, except cpu boards and power supplies
||Yes, if it has a direct function
||Yes, except for induction motors
||No, except TV interface device
|Fluorescent lamps, fixtures
||No (unless RF ballast)
|Marine digital electronics
||Yes, Marine Equipment Directive
|Medical digital devices
||Yes, Active Implantable, or Medical Devices Directives
|Controls in industrial plants
|Battery operated clock <1.705 MHz
|Digital devices, PCs, peripherals
|Amateur radio transmitters
||No, except commercial power amplifiers
||No, except EMC for commercial transmitters
||Yes, Part 15
||ETS 300, 220, 328, 330, 440, 445, or national rules
|High power transmitters
||Yes, other FCC Parts
||ETSI or national rules
|Table I. Scope of FCC and EU EMC regulations for unintentional and intentional radiators
New EEC Directives only set out essential requirements and legal aspects (New Approach). Technical aspects are dealt with in specific standards, the application of which is voluntary. These standards are developed by specific bodies, such as CENELEC or ETSI, and are harmonized to the directives by the action of the European Commission. The EMC Directive itself, however, is based on a presumption principle, which means that a product that meets the requirements of the harmonized standards is also presumed to meet the essential requirements of the EMC Directive.
Under the General Agreement on Tariffs and Trade (GATT) and its successor, the World Trade Organization (WTO), member countries are obliged to adopt international standards for national use wherever possible. International standards concerning EMC are primarily developed by the International Electrotechnical Commission (IEC) and the International Special Committee on Radio Interference (CISPR). The new extensive series developed by IEC includes
- IEC 61000-1—Introduction, terms, and conditions.
- IEC 61000-2—Classification of electromagnetic environments.
- IEC 61000-3—Limits and disturbance levels.
- IEC 61000-4—Testing and measurement techniques.
- IEC 61000-5—Installation and mitigation guidelines.
- IEC 61000-6—Generic standards.
Some specific EMC standards have also been published by the International Organization for Standardization (ISO). North American EMC standards are published by the FCC, the American National Standards Institute (ANSI), and the Institute of Electrical and Electronics Engineers (IEEE). Private standards are submitted by Bellcore (for telecom equipment), the Society of Automotive Engineers (SAE) and automotive manufacturers, and the U.S. Department of Defense.
European EN standards concerning EMC are developed by the European Committee for Electrotechnical Standardization (CENELEC).
Regulations and standards concerning telecom and radio transmitting equipment are published by the International Telecommunications Union (ITU) and the European Telecommunications Standards Institute (ETSI).
International and European EMC standards are to a great extent becoming harmonized, due to the fact that many EN standards are based on IEC and/or CISPR standards. There are also similarities between international and U.S. standards, though they are not equivalent. Figure 2 shows some intercompatibility problems between similar emission standards (FCC Part 15 J and CISPR 22 limits, respectively, measured at a distance of 10 m). FCC, however, accepts conformity to CISPR 22: 1985 as an alternative to FCC Part 15 (sections 15.107 and 15.109).
Figure 2. Emission limits compared at a measuring distance of 10 m
Figure 3. Required frequency range of conducted measurements
EMC standards are continuously being developed and revised, and the standards "jungle" sometimes seems a bit difficult to get through. It is therefore important to keep track of standards' publication dates, in addition to knowing if a new standard is to be expected in the near future and when an old standard is no longer valid. The following dates are of specific interest in regard to a standard: date of publication (dop) and date of withdrawal (dow) of a conflicting (earlier) standard. Draft standards are sometimes called preliminary, for example prETS or prEN. Temporary EN standards are called ENV.
Standards are principally divided into the following main groups:
Generic standards refer to the electromagnetic environment in which the apparatus/system is to be used. General standards concern groups of equipment used for general applications in a specific environment, like general telecom equipment and medical and laboratory equipment.
Basic standards describe measuring methods and, in some cases, interference levels as well as limits. One of these is the standard series IEC 801-X for process industrial applications, which has been converted into a general series of basic standards, IEC 61000-4-X, which were then translated into European standards as EN 61000-4-X.
Product and Product Family Standards
These are applicable for specific product types, which are specified within the scope of the standard.
In addition to these standard documents, there are also standards offering guidance on installation techniques, or a code of practice, for example the IEC 61000 series, Part 5 (IEC 61000-5-X).
EMC Standards Classification
Generic standards have two environmental classes:
1. Residential, commercial, and light industrial environments, including domestic, office, laboratory, and light industrial environments where the apparatus or system is connected to the public mains.
2. Industrial environments, meaning "heavy" industrial environments with separate transformer stations for mains supply, usually with equipment spread over some distance.
This simplified view of the world as one of two categories does not hold true in all cases. There are so-called gray zones such as laboratory or hospital environments where equipment may be connected either to the public mains or to an in-house power net, through a separate transformer station.
CISPR standards concerning emissions, and to some extent corresponding EN standards, have a classification according to the following:
Class B/CISPR 22
Information Technology Equipment (ITE) intended primarily for use in domestic environments and may include portable equipment, telecommunications terminal equipment, personal computers, and auxiliary connected equipment.
Class A/CISPR 22
ITE, which satisfies Class A but not Class B limits.
The following warning shall be included in the instructions for use: This is a Class A product. In a domestic environment this product may cause radio interference, in which case the user may be required to take adequate measures.
Class B/CISPR 11
ISM equipment suitable for use in domestic establishments and in establishments directly connected to a low-voltage power supply network.
Class A/CISPR 11
ISM equipment suitable for use in all establishments other than domestic and those directly connected to a low-voltage power supply network.
The emission limits differ 10 dB between Class A and Class B equipment, as well as between the generic emissions standards for the "light" and "heavy" environments, respectively, when referred to the same measuring distance. When recalculated from 10 to 3 meters distance, there is a difference in radiated emissions of 9.5 dB. It should also be noted that the classification of computing devices according to FCC 15, and ISM equipment to FCC 18, is very similar to that of CISPR 22 and CISPR 11 Group 2.
Which Standards Apply?
A product standard is one that covers all EMC requirements for a certain product type. In some cases, product standards also cover electrical safety requirements. A product standard takes preference over all other standards. Once it is determined that a product is within the scope of an applicable product family standard concerning emissions and/or immunity, then that standard should be followed. Some examples of family standards include
- CISPR 11/EN 55011 Emission standard for industrial, scientific, and medical (ISM) radio RF equipment (see Figure 3).
- CISPR 12 Emission standard for vehicles, motorboats, and spark-ignited engine-driven devices.
- CISPR 13/EN 55013 Emission standard for broadcasting equipment like radio, television, etc.
- CISPR 14/EN 55014 Emission standard for household apparatus and portable tools.
- CISPR 15/EN 55015 Emission standards for luminaires.
- CISPR 20/EN 55020 Immunity standard for broadcasting equipment, such as radio and television.
- CISPR 22/EN 55022 Emission standard for information technology equipment (ITE).
- CISPR 24/EN 55024 Immunity standard series for ITE.
Newer product family standards also tend to appear as complementary emissions and immunity standards:
- EN 55014-1, EN 55014-2 Emissions and immunity requirements for household apparatus and portable tools.
- EN 55103-1, EN 55103-2 Emissions and immunity requirements for professional audio and video equipment.
Specific types of product family standards transferred into general standards include:
- IEC 555-2/EN 60555-2 LF emissions standard concerning harmonics for household products, etc.
- IEC 555-3/EN 60555-3 LF emissions standard concerning flicker and voltage variations for household products, etc.
- IEC 61000-3-2/EN 61000-3-2 General LF emissions standard concerning harmonics.
- IEC 61000-3-3/EN 61000-3-3 General LF emissions standard concerning flicker and voltage variations.
Today, however, very few pure product standards exist that cover all requirements. Therefore one has to look for an applicable product family standard. An additional complication is that, for the time being, a product can sometimes belong to different product family standards. For example, most household devices must fulfill emissions requirements according to EN 55014-1, as well as EN 60555-2 and EN 60555-3 or EN 61000-3-2 and EN 61000-3-3.
If no product family standard is applicable, one must follow the suitable generic or general standard, which in turn refers to different basic standards. Some of the product family standards are also referred to in other standards, which consequently gives them characteristics of basic standards.
The generic standards include:
- EN 55081-1 Emissions standard for residential, commercial, and light industrial environments.
- EN 55081-2 Emissions standard for industrial environments.
- EN 55082-1 Immunity standard for residential, commercial, and light industrial environments.
- EN 55082-2 Immunity standard for industrial environments.
The IEC is also developing corresponding generic standards, which will be numbered IEC 61000-6-X. A general EMC standard that covers emissions and immunity for medical equipment is EN 60601-1-2, the collateral standard for medical equipment. In addition to this collateral standard, there are a number of product standards covering safety and EMC for specific medical equipment, like EN 60601-2-24, which covers infusion pumps and controllers.
A general EMC standard that covers emissions and immunity for TTE equipment is ETS 300 339 (general standard for radio transmitting equipment). In addition to this standard, there are a number of ETS/prETS standards covering EMC for different telecom and radio transmitting equipment.
As far as the emissions requirements are concerned, the generic standard is more rigorous in regard to light industrial environments than on heavy industry, which as a rule is already rather electromagnetically contaminated. As far as the immunity requirements are concerned, the situation is the opposite. Interference immunity must be hardier in heavy industrial environments.
What then is applicable in mixed or special environments? When using the generic standards it is recommended to begin with the most strict requirements, which means that the equipment should be classified according to the "worst" combination, such as EN 50081-1/EN 50082-2. This scenario has sometimes been used for equipment in hospital environments.
Basic Immunity Standards
The IEC standardization committee for industrial processing techniques, TC 65, drafted the first basic standards for immunity to electrical disturbances. This responsibility has been taken over by the committee that works with general EMC standards, TC77 (IEC) and TC 210 (CLC). With this, the publication numbers changed from IEC 801 to IEC 1000/61000 and EN 61000, and thus far the disturbance types that have been dealt with have gone through the following changes:
1. Electrostatic discharges (ESD).
- IEC 801-2, ed. 1.
- IEC 801-2, ed. 2=IEC 61000-4-2=EN 61000-4-2.
2. Radiated, radio frequency electromagnetic fields (RF fields).
- IEC 801-3.
- ENV 50140~IEC 61000-4-3=EN 61000-4-3.
3. Electrical fast transients/burst (EFT).
- IEC 801-4.
- IEC 61000-4-4=EN 61000-4-4.
4. Surges (1, 2 µs/50 µs).
- ENV 50142~IEC 61000-4-5=EN 61000-4-5.
5. Immunity to conducted disturbances induced by
radio frequency fields.
- ENV 50141~IEC 61000-4-6=EN 61000-4-6.
6. Immunity to magnetic fields.
- IEC 61000-4-8, IEC 61000-4-9, and IEC 61000-4-10=
EN 61000-4-8, IEC 61000-4-9, and IEC 61000-4-10.
7. Voltage dips, short interruptions, and voltage variations.
- IEC 61000-4-11=EN 61000-4-11.
Because some generic standards and some product standards have been published at the same time as the aforementioned revised standards, we will have to live with references to new, as well as old, basic standard versions for some time.
EMC regulations will be successively refined. We are going to see a whole series of new standards and amendments during the next few years. In many cases new standards will mean increased testing requirements. And while these testing requirements might not be welcomed by all manufacturers, they will benefit the end-users of electrical equipment.