basic emc problems

1.1 definitions

Electromagnetic compatibility (EMC) is defined as:

“The ability of device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbance to anything in that environment”

Sources of EMI and EMC has been concerned principally with ensuring the proper operation of electrical and electronic apparatus used in proximity of source of interference- i.e other equipment or physical phenomena like lightning- described as environment which is.

´-the totality of electromagnetic phenomena existing at a griven location.

When considering systems and installations, these both create each own environment within their boundary ,and they also exist within a larger environment . It can be made distinction between EMC internal to a system , and EMC external to it modifying the definitions to.

Internal EMC “The ability of components of the system to function satisfactorily within the electromagnetic environment created by that system , without introducing intolerable electromagnetic disturbance to other component of that system

External EMC . The ability of whole system to function satisfactorily within the electromagnetic environment created by external sources , without introducing intolerable electromagnetic disturbance to other component of that system

2 EMC means that are fulfilled following criteria. Device, equipment or system does not introduce intolerable disturbances

to other device ,equipment or system. Device equipment or system perform satisfactorily in its environment and

is not susceptible to disturbances generated by other device , equipment or system.

Device equipment or system does not disturb performance of itself This implies that it is necessary to know what is tolerable disturbance and decide what is satisfactory performance more of this later.

IEY does give definition of electromagnetic disturbance

Any electromagnetic phenomenon, which might degrade the performance of a device , equipment or system , or adversely affect living or inert matter. An electromagnetic disturbance might be electromagnetic noise , an unwanted signal or change in the propagation medium itself .But it is not detailed enough for practical use The most useful listing of disturbance phenomena is given in IEC 61000-2-5 classification of electromagnetic environments which will be discussed further.

1-2 The need for EMC Electromagnetic disturbance is defined as intensity of electromagnetic field

or electrical signal which independent on its source origin and mechanism of coupling can cause malfunction or disturb performance of a device equipment or system . For purpose of analyses and optimization of a product design there is used simplified model of electromagnetic disturbance described as follow .

Level of disturbance emission level of interference

DE EMI S C V

Source of disturbance Coupling Victim receiver --emitter- -mechanism- --equipment system-

Compatibility means compatibility in both directions . equipment –system—Electromagnetic environment –prevention of interference to other equipment and correct operation of the equipment itself – It covers both complementary aspects of EMC.

1. Suppress the emission at its source.

2. Make the coupling path as inefficient as possible.3. Make the receiver less susceptible to the emission.1.3 Control of emission

Nowadays there is a wide spectrum heavily used many kinds of services ---broadcasting and telecommunication, telemetry , radar , radio navigation , etc .-which require continued reliability .It is necessary to regulate spread of types of apparatus dominated by broadband sources –motor driven equipment , fluorescent lights , pulsed ignition petrol engines ,etc – which have potential to disrupt these services . Electronic equipment has also potential to generate radio frequency interference as by –product of its operation –digital electronic incorporating microprocessors, fast electronic switches , transistors triacs and so on-

For decades emission limits have been placed on these kinds of equipment and national and international EMC control has been established.

4. With the expansion of the concept of EMC to include compatibility with the supply network , control of other types of emissions became necessary .These are power frequency harmonic currents flicker , load current disturbances transient due switching .

1.4 Control of immunity

The immunity aspect of EMC is a complex issue , which need to be separated and examined.

----the need for continued safe operation of safety related systems in the face of external interference .---the question of fitness in requirements for given electromagnetic environment .----the desire to establish a baseline for product quality and safety

If a particular product is intended to be used in a given environment , the user ought to be assume that it will that it will cope with any disturbances that might be expected in that particular electromagnetic environment . It the product suffers degradation in performance due to interference , to what degree is this acceptable . The question of performance is fundamental to immunity compliance.

The effect of regulating the electromagnetic immunity of products is to set baseline for product quality in EMC terms . In European Union countries the EMC Directive has been introduced describing which products have too meet certain minimum standards for immunity before it can be marked .One of the valid arguments minimum standards for immunity before it can be marked . One of the valid arguments was that good EMC design and installation practices will save many problems and costs in commissioning and operational reliability.

1.5 Safety aspects

The levels of electromagnetic disturbances indicated in various standards must be considered as regards safety implications in particular.---the disturbance levels vary according to a statistical distribution and given values can be significantly exceeded in some circumstances , I . e . infrequently or on particular sites.

---the standard levels and performance criteria are generally related to functional requirements and not safety .

The safety approach allows to matching flexibility of protection measures against the electromagnetic theat. For instance , the critical functions need to consider foreseeable environmental extremes .---lightning proximity to high power transmitters –misuse –ignoring the turn –off your call-phone sign – and faults ---a blown fuse—

1.6 Identification and sourcing critical parts

The effects and consequences of the different electromagnetic phenomena on electronic systems and installations are directly related to the different functions carned out by the various electronic equipment and control systems and to processes involved . In the evaluation of these effects and consequences it is useful to identify the main functions of the apparatus.

In electrical engineering there are two fundamental tasks , distribution and conversion of two parameters energy and information realized by the system, which assures cooperation of those quantities.

Power system communication system

Hierarchy of priorities 1 Energy 1 Information

2 Information 2 Energy

System are composed of interconnected modules .There are two aspects to the criticality of a module in this context. Functional criticality and EMC criticality.

Table 1 Functional criticality

Functional category Description

Safety critical EMC problems could result in loss of life , property of the system

Mssion critical EMC problems could result in injury , damage to the system ,loss of or delay to operation , or performance degradation which unacceptably reduces operational effectivess

Non critical EMC problems could result only in annoyance minor discomfort or acceptable and temporary loss of performance

EMC criticality ---table 2- is a classification of whether and to what extent EMC issues need to be considered .

Table 2 EMC criticality

Functional category Description

EMC benign Will not cause or suffer from interference , has no effect the EMC performance of the systemEMC relevant Will not of itself cause or suffer from interference ,but may have an effect on the EMC of related items , e. g . cables connectors and enclosures

EMC critical Active electronic equipment , interference sources such as motors lighting equipment power systems an apparatus

---class of apparatus ,the inherent qualities of which are neither liable to cause , nor is its performance liable to be degraded by electromagnetic interference called passive EM equipment

3 SOURCES OF NATURAL AND MAN-MADE ELECTROMAGNETIC NOISE AND INTERFERENCE –EMI AFFECTING ELECTRONIC EQUIPMENT.

2.1 Description of EMC phenomena

Sources of electromagnetic noise are numerous and have both natural and man- made origins

Natural sources are. Electrostatic discharges ---ESD ´Lightning discharges and radiated lightning electromagnetic impulses ---

LEMP which are one of most powerful disturbances –high and steep currents –

Cosmic noise and solar radiation with high frequency –above 10 MHzMan –made noise may be divided into intentional and unintentional sources--Intentional sources are those which must radiate to perform their tasks .They include AM, FM ,TV. Police , and other broadcast transmitters , pagers,diathermy machines and mobile phones. In this group are also classified nuclear electromagnetic impulses—NEMP.---Unintentional sources include computing devices , relays , switches, motors, appliances, power lines , fluorescent lights arc welders , auto ignition systems cable TV, and many others.

Electromagnetic disturbances according to EMC standards can be classified into four types of phenomena as shown in table 3 depending on frequency range and width of frequency band

TABLE 3 Disturbance phenomenaElectromagnetic disturbance phenomena Coverage in EMC standards

Emission Immunity Standard related To immunity Conducted low frequency phenomena

Harmonics and interharmonics Increasing None IEC 61000-3-2

Signaling voltages Existing None EN 50065-1 IEC 61000-2-2

Voltage fluctuation , dips and interruptions Increasing Increasing EN 50160 IEC 61000-4-11

Voltage unbalance None None

Power frequency variations None None

Induced low frequency voltages None Rare

DC in AC networks None None

Radiated low frequency phenomena

Magnetic fields Rare Rare EN 50082-2

Electric fields None None Conducted high frequency phenomena

Induced voltages and currents widespread Increasiang IEC 61000-4 -4 IEC 61000-4-5 Unidirectional and oscillatory transients None widespread IEC 61000-4-4 IEC 6100-4-5

Radiated high frequency phenomena Magnetic fields Rare None

Electric fields None None

Electromagnetic fields widespread Widespread IEC 61000-4-3Transient electromagnetic fields None Rare IEC 61000-4-9 IEC 6100-4-10

Electrostatic discharge N/A Widespread IEC 6100-4-2

In respect to these phenomena EMC is considered as compatibility with the power supply network which in general mean low voltage AC mains supply . Most of these disturbances appear between phases of the supply though earth potentials can be widely distributed.

Non-linear loads draw current at harmonics of power frequency ´--up to 203 kHz- which when flowing in supply network impedance cause harmonic voltage distortion The main threat is from large number of products i. e. electronic power supplies personal computers TV sets etc . Large industrial loads .---I,e. variable speed drivers can also cause harmonics in low ,medium and high voltage supplies .Interharmonios are due to other loads which are modulated in the same frequency range , but not related to the supply frequency .

Signaling voltages can exists in power networks which secondary use is for information transfer via mains signaling –frequency range 3kHz- 148,5 kHz –bottom of the long wave broadcast wave .

Power system fluctuations and frequency variations --- occurring from once a minute to 25 per second –caused usually by fluctuating industrial loads – arc furnaces motor switching or welding equipment known as flicker voltage dips and short interruptions due to fault clearance ,voltage unbalance due to asymmetrical loads of three phase supplies ---there are features which gives acceptable parameters of power quality

LF voltages induced by power supply cables at power frequency and its harmonics –possibly up 20 kHz –in adjacent signal cables . The effect is usually significant in audio systems studio or telecom applications –and under power system fault conditions

Local magnetic fields existing around components of the power network as result of current flow i. e overhead and buried lines power transformers –stray magnetic fields –during normal operation –or during fault conditions –can reach up tens of T A/m – depending on proximity and configuration .Display screens and TV sets will suffer perceptible effects on – screen at a threshold of 0,5—2,5T -0-4—2 A/m and themselves generate magnetic fields at the screen –scan frequencies .

High level electric fields of 10 kV/m or more may occur in proximity of overhead lines and substations but the magnitude is reduced by a factor of 10-30 within buildings due to shielding effect of the structure –no such effect occurs for magnetic fields –Display screens and TV sets may generate electric fields perpendicular to the screen if no design precautions are taken .

High frequency disturbances can be either transient or continuous They are conducted if principal coupling occurs in differential mode or common mode via cables , either power or signal , which are connected to the affected or disturbing equipment .

In the EMC test standards is made distinction based on the basis of frequency but in reality both conducted and radiated coupling can occur over wide and overlapping frequency range .

Induced continuous wave voltages or currents on any conductor exposed by fields from nearby radio –or tele –transmitters ,whose amplitude depends on the conductor length its separation form ground reference , loops formed by stray capacitance and any resonance effect

Transients are classified into a number of subsets depending on their source and nature Surges can be oscillatory or unidirectional and have relative high energy . Typical sources are lightning ,power system faults clearance or capacitor loads switching or inductive loads switching –fast transients burst –EFT/B.

Continuous radiated interference due to nearby radio transmitters and other radio—frequency generating equipment ---i. e . RF heating – which may affect equipment in different ways depending on the source impedance of the field . Three cases can be distinguished .

-----Magnetic field interference ,low source impedance

-----Electric field interference high source impedance

-----Electromagnetic wave interference , free space impedance –plane wave

The first two cases occur in the near field of transmitter while the last occurs in the far field .

Transient radiated field interference tends to arise from nearby lightning strikes and from high power switching events in substations or near other parts of the electrical supply infrastructure .

Electrostatic discharge is specific phenomenon ,which is hardly to classify in the same way as others treated here .It occurs as the result of equalization of charge between two objects carrying different levels of charge ,one of which often is a person .

133 COUPLING MODELS OF EMI

EMC has two complementary aspects Any situation of incompatibility must have source of interference emissions and victim ,which is susceptible to this interference . If either of these is absent there is no EMC problemsThere is the third important which expresses a coupling path between source and victim which can be trough a direct connection ,through proximity or through radiated energy . The same equipment may be source in one situation and victim in another

14. There are recognized following type of coupling mechanisms .

----Direct coupling ---conducted ---via power or signal lines or common impedance

---Capacitive coupling ---electric coupling –due to near electric field

-----Inductive coupling –magnetic coupling –due to near magnetic field -----Radiated coupling from propagating electromagnetic

15. 4-SITUATION OF EMC IN CASE OF LIGHTNING STRIKE

Actual situation of Electromagnetic Compatibility –EMC –to be considered when lightning strikes in the structure with lightning protection zones is shown in Fig . 12.The sensitive electronic equipment –i. e. information system – is installed inside LPZ 2.

The primary electromagnetic source of interference to the electronic equipment is the lightning current i. and magnetic field H . Along incoming lines –services –flow partial currents i. The currents i. and i. as well as the magnetic field H. have the same ware shape . According to IEC 1312 first stroke of lightning current is considered 10/350 s and subsequent strokes are 0. 25/1000. s . The current of the first stroke i. generates magnetic field H and the current of subsequent stroke generates the magnetic field H. The rise period of H. can be characterized by damped oscillating field of 25kHz with the time to maximum value T. of 10 s. In the same way the rise period of H can be characterized by a damped oscillating field of 1 MHz with the time to maximum value T of 0. 25. s

BY installation of LPZ with their electromagnetic shields and SPD at interfaces of LPZ s the original lightning effect defined by H. I and I , is reduced to the immunity level of the equipment installed in actual electromagnetic environment . As shown in Fig 11, the sensitive equipment has to withstand the surrounding magnetic field H, and conducted lightning transients u and I respectively

4 LIGHTNING ENVIRONMENTAL AND IMMUNITY REQUIREMENTS FOR SENSITIVE EQUIPMENT

Immunity levels of electronic equipment according to IEC standards are required for conducted and radiated lightning transients.

The standard IEC 61000—4-5 describes requirements and immunity levels of voltages current surges which are generated by combination wave –hybrid generator –and are applied using coupling –decoupling network to input/output ports of the equipment .

The selection of test levels is based on the installation conditions of equipment which are described in Table 9. Test levels for conducted surge immunity tests are given in Table 10.

Table 9 Installation classification of equipment according to IEC 1000-4-5

Class 0Well-protectedElectricalEnvironment i. e LPZ 2

All incoming cables are provided with overvoltage protection---primary and secondary SPD s installed—The units of the electronic equipment are bonded and connected with well designed earthing system . The electronic equipment has a dedicated power supply Surge overvoltages may not exceed 250 V

Class 1Partly protectedElectrical EnvironmentI .e. LPZ 1. All incoming cables to the room are provided with overvoltage protection –primary SPD installed . The units of the electronic equipment are bonded and connected with well –designed earthing system .The electronic equipment has its power supply separated from other equipment .Switching operations can generate interference voltages within the roomSurge overvoltages may not exceed 500V.

Class 2Electrical Environment whereCables are well separated Even at short runs The installation is earthed via a separate earth line to the earthing system of the power installation . The power supply to the electronic equipment is separated from other circuits mostly by a special transformer for the power supply ,Surge overvoltages may not exceed 1 kV

Class 3Electrical environmentWhere power and signal Cables run in paralled

The installation is earthed to common earthing system of power installation . Current due earth faults switching operations and lightning may generate overvoltages with relatively high amplitudes in

the earthing system .Protected electronic equipment and less sensitive electronic equipment are connected to the same power supply network ,The interconnection cables can be partly outdoor cables but close to the earthing network Unsuppressed inductive loads are in installation and usually there is no separation of different field cables .Surge overvoltages may not exceed 2 Kv.

Class 4Electrical environments Where the interconnections Are running as outdoor cables Along with power cables , and cables Are used for both electronic and Electric circuits

The installation is connected t to the earthing system of power installation .Current in the kA range due earth faults , switching operations and lightning may generate overvoltages with relatively high amplitudes in the earthing system .The power supply network can be the same both for electronic equipment and electric equipment .The interconnection cables are running as outdoor even to the high –voltage equipment .A special case of this environment is when the electronic equipment is connected to the telecommunication network within a densely populated area .There is no systematically constructed earthing network outside the electronic equipment , and the earthing system consists of pipes cables .etc only .Surge overvoltages may not exceed 4KV

Class 5.Electrical environment for Electronic equipment connectedTo telecommunication cables andOverhead power lines in nondensely Populated area All these cables and lines are provided with overvoltage protection –primary SPDs--.Outside the electronic equipment there is no widespread earthing system –exposed plan --.The overvoltages due to

earth faults –currents up to 10 k A and lightning –currents up 100 kA—can be extremely high .

The requirements of this class are covered by the test level 4 see table 7.

Class X Special conditions specified in the product specifications

Table 10 Selection of the test levels of electronic equipment --depending on the installation –

Test levels

Installation power supply --- unbalanced ---balanced ---SDB DBClass Coupling mode ---operated ---operated

Circuits/lines ---circuits lines---coupling mo-

LDB --Coupling mode

Coupling mode

Line Line Line Line Line Line Line Line To line to earth to line to earth to line to earth to line to earth KV KV KV KV KV KV KV KV

NA NA NA NA NA NA NA NANA 0.5 NA 0,5 NA 0,5 NA NA0,5 1,0 0,5 1,0 NA 1,0 NA 0,5 1,0 2,0 1,0 2,0 NA 2,0 NA NA2,0 4,0 2,0 4,0 NA 2,0 NA NA 2,0 4,0 NA 4,0 NA NA

Limited distance , special configuration special layout , 10 m to max 30 m , no test advised at .Interconnection cables up to 10 m . only class2 is applicable

Depends on the class of the local supply system .Normally tested with primary protection Explanation DB --data bus data line LDB -long –distance bus SDB -short- distance bus NA- not applicable

The test procedures for verifying the immunity or damage of equipment in case of pulse magnetic field .—simulation of the magnetic field of first stroke are given in IEC 61000-4-9 and in case of damped oscillatory magnetic field –simulation of the magnetic field of subsequent stroke are described in IEC 61000-4-10.

In both cases test levels are ranging from 1 to X Table 11. Selection of the test levels shall be done based on common installations practices .A guide for the selection of test levels for pulse and/or damped oscillatory fields testing are give in Table 12.

Table 11. Test levels according magnetic fields – IEC 61000-4-9and damped oscillating field IEC 61000-4-10.

Peak value of magnetic field strength A/ m

Level Simulation of Simulation of Magnetic field of first magnetic field of Stroke subsequent stroke 8/20 s 8/20 s

1 not applicable not applicable

2 not applicable not applicable 3 100 104 300 305 1000 100X According to product specification

Table 12,. Installation classification of equipment according to IEC 61000-4-9 And IEC 61000-4-10

CLASS 1 Test not applicable to this environment where sensitive device using electron beam can be used monitors , electron microscopes etc .

Class 2Well protected environment Test not applicable to this environment because of areas concemed are not subjected to the influence of lightning and initial transient fault current residential , office hospital protected areas far away from earthed conductors of lightning protection systems

Class 3Protected environment The environment is characterized by the proximity of earth conductors of lightning protection systems and metallic structures –commercial areas , control building ,field of not heavy industrial plants provided with lightning protection system or metallic structures in the proximity , computer room of H. V substations etc.Class4 Typical industrial environments

The environment is characterized by the ground conductors of the lightning protection system or structures field of heavy industrial and power plants control room of HV substation etc .

Class 5Severe industrial environment The environment is characterized by the attributes -------conductors , bus –bars or M.V .H. V lines carrying tens of Ka -----ground conductors of the lightning protection system or high structures like line towers carrying whole lightning current .Switchyard areas of heavy industrial plants.M V H V . power stations etc

Class X Special Special conditions specified in the product specifications

The manufacturer of sensitive electronic equipment shall give to the user or describe in technical documentation the data concerning the immunity against

damage or the devices according to EMC standards i. e . lightning surge and magnetic fields of impulses . If however the immunity against damages is not known as a basis should be taken immunity to interference . If the immunity of equipment to interference is not known ,it is common practice to take a basis the smallest value of the immunity to interference given in tables above .

List of related IEC Publications