emc fact sheets 1 - 7

8/8/2019 EMC Fact Sheets 1 - 7

1/24

EMC Fact Sheet 1

This fact sheet is one of a series. If you would like to receive otherfact sheets or mini guides on a regular basis please contact us on:-T 01588 673411 F 01588 672718 E [email protected]

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The small print: Every effort has been made to ensure the integrity of the information in this data sheet, which has been provided in goodfaith and the authors do not accept liability for any loss or damage caused by omissions, errors or the interpretation of the reader.

What is EMC (ElectroMagnetic Compatibility) all about ?

The proliferation of smaller and faster electronic equipment has made it necessary tointroduce regulations to ensure that equipment continues to function correctly, even though

it might be exposed to electromagnetic disturbances. Also equipment should not createexcessive electromagnetic disturbance.

How are electromagnetic disturbances generated ?

The answer is that they are all around us and the electromagnetic spectrum compriseseverything from gamma rays to long wave radio. This includes X rays, ultraviolet and visiblelight, infrared rays, microwaves and radio waves. In the case of EMC we are mainlyinterested in radio waves or RFI (radio frequency interference). If electronic equipment isnot designed and installed correctly it can function in the same way as a radio transmitter,or receiver and send out or receive RFI.

How is EMCtransmitted ?

How is EMCPrevented ?

CE Marking

The CE mark on equipment means that it conforms to various safety standards and alsothat it has been tested for RF immunity and emissions. These test are usually carried out ina controlled environment and there is no guarantee that the equipment will still conformwhen the equipment is used in a different manner or modified.

Radiated interferencefrom around doors

Radiated interferencefrom glands

Radiated interferencefrom holes

Conducted interference

Coupled interference

Radiated interferencefrom unscreened

signal cables

Use screenedcables and useEMC glands for

360 degreeearth bondingon both ends

Fit an EMCfilter on theincoming

power side

Separateincoming and

outgoing cables

Use shieldedgaskets to seal

doors andflanges

Keep signal cablesaway from power

cables
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EMC Fact Sheet 2


www.reo.co.ukThe small print: Every effort has been made to ensure the integrity of the information in this data sheet, which has been provided in good

faith and the authors do not accept liability for any loss or damage caused by omissions, errors or the interpretation of the reader.

Good practice Installation and Cabling

Screen ConnectionsTo ensure that the lowest possible levels of radiated

emissions occur in the >10 MHz region, supply cableswithin electronic equipment should be kept as short aspossible. Especially those cables, connecting EMC filtersto the mains supply.Output cables from switched power supply modules,Inverter Frequency Drives and other similar equipment,should be both screened and kept as short as possible.Where cables are screened, it is important that screeningis carried out in the correct manner.

Power cable routing for high frequency switching controllersIn a control panel there are always areas where the interference fromcables is greater than in other areas. Special care must be taken toprevent interference from being generated because of incorrect cablerouting. A common fault is the crossing of power input and outputconnections to drives or filters, or running them alongside each other intrunking. This can cause high switching losses in the semiconductors,leading to component failures and should be avoided. As a generalrule the level of interference increases as the clearance betweencables reduces, as a squared relationship. Conversely, doubling thedistance between cables quarters the level of interference.

Cable runsIt is important to ensure that signal, control and power cables do not

run alongside each other e.g in trunking. By separating and providingsufficient clearance, or by laying signal cables at right angles to powercables, cross-coupling can be reduced considerably.

Out and return cables

Return current carrying cables for either power orsignals should be laid in very close proximity totheir corresponding outgoing cable. Even if it is oneout and several returning, they should be bundledor twisted together. The better this is done, thegreater the reduction in radiated emissions andsusceptibility from interference.

Bad

Good

>20 cm

Bad

Good

Bundling is the optimum
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EMC Fact Sheet 3


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Filter Installation Guidelines

Position: Normally the filter is positioned as close as possible to the point where the supply enters the control

panel. However; with more complicated layouts, where there is a mixture of control and power circuits, thenthe filter should be located close to the source of interference. Sometimes a piece of equipment may have itsown special filter, as in the case of a frequency drive, in which case the filter is fitted adjacent to, or under theinverter.

Cable routing: The correct operation of a filter can be compromised by incorrect wiring. There should bemaximum separation between external input and output cables and also between internal filter and externalcables. Otherwise cross coupling will occur.

An example of bad filter position and cable routing A much better arrangement

Earth bonding: For correct operation filters rely on a good connection to earth. The earth conductor shouldbe as short as possible and have a very low inductance. The ideal method of earthing is to use a non-paintedchassis plate for the reference plane and to have metal to metal contact, at multiple points, between this andthe filter housing. It is also very important to avoid any earth conductor loops. A much better arrangement is to

connect earths in a star configuration, using a central earth stud welded into the chassis plate.

Earth leakage current: Earth leakage current is a current that is created by the Y capacitors in the filter thatare connected to earth, together with stray capacitance. This current is an important characteristic of filterswhen providing suppression. There are a number of situations where this leakage current must be limited e.g.for personal safety, circuits with earth leakage trips and IT networks.

Typical values are 0.75mA or 3.5mA. However; fixed equipment, permanently wired, may be allowed up to5% of phase current (providing appropriate warning labels are fitted). When a number of filters are usedtogether the earth leakages can accumulate to an unacceptably dangerous level. After filters are installedthey must be checked to ensure that the possible earth leakage current corresponds with local regulations.

It is also dangerous to use filters on supplies that have a frequency higher than that on the rating label.

Current Rating: Always check that the filter is correctly rated for the true current drawn by the load. This isnot always as straightforward as it appears. For example a capacitor and rectifier load; found typically inswitch mode power supplies; has a high peak current relative to RMS. The inductors in a filter will saturate ifthe current is too high and so will not function correctly.

Cooling: Generally filters are designed to operate at temperatures of up to 85OC when there is a high

ambient and with continuous current flow. Therefore it is important to allow sufficient clearance, of at least6cm, for air circulation. Filters with air vents should always be mounted in the correct orientation to allow agood through flow of air.


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EMC Fact Sheet 4


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EMC Filters

Filters work by providing an impedance mismatch between the power line and the

equipment, which reflects the generated noise back to its source. In order tomaximise the impedance mismatch the choice of filter circuit should take intoaccount the impedances of the source and load.

The main components inside the filter are chokes, capacitors and resistors.

The capacitors oppose the AC flow of electrons more at lower frequencies and lessat higher frequencies. Inductors on the other hand react against the rate of changeof current; they are more effective at opposing AC flow of electrons at higherfrequencies. Therefore, a combination of series connected inductors and shuntconnected capacitors is chosen to provide suppression over a wide frequencyspectrum. The resistors serve to discharge the capacitors when the supply isdisconnected and for damping resonances. The enclosure is normally producedfrom metal to provide good earth bonding. Above right A typical REO filter built into

metal enclosure (lid removed). The main components; capacitors and inductors canbe clearly seen.

The filter circuit is designed to contend with two types of noise. There isCOMMON MODE NOISE, which manifests itself as a current in phasein the live and neutral conductors and returns via the safety earth. Thisproduces a noise voltage between live/neutral and earth. It is oftencaused by capacitive coupling to the case earth. The other isDIFFERENTIAL MODE NOISE produced by current flowing along eitherthe live or neutral conductor and returning by the other. This produces anoise voltage between the live and neutral conductors.

The chokes fall into two groups; current compensated or common

mode and series or differential mode types. The current compensatedchoke has two or three windings on a toroidal core. The direction ofeach winding is chosen to give an opposing current flow, hencebalancing the flux. The result is that a much smaller choke can beused. Furthermore, the common mode currents, which are in phase inthe two or three conductors, have an additive effect, thus presentinghigher impedance against the common mode noise.

The differential mode chokes are larger due to the higher current handling requirement. Using a core made from ahighly permeable material will reduce its size.

Capacitors also fall into two groups; X Class and Y Class. The X Class capacitors are connected between live andneutral, or between phases, to reduce differential noise. They are tested to withstand mains voltage. Y Classcapacitors on the other hand are more critical because they are connected between live/neutral and earth to reducecommon mode noise. Because of this they have to be tested to ensure that they cannot fail to short circuit.Needless to say they are more expensive.

For higher levels of attenuation, several stages of chokes and capacitors can be added and this is known as amulti-stage filter.

Another important factor is the earth leakage current. The larger the Y Class capacitor the more the 50 Hz currentthat will leak to earth, raising the potential of the filter enclosure. The maximum permissible leakage currentdepends on the application but to give an indication; the maximum protective earthing conductor current for hand-held equipment it is 0.25 mA. Equipment that is permanently connected to the mains supply may have a leakagecurrent of up to 3.5mA. Industrial equipment normally has higher leakage current limits (up to 5%) but in each caseparticular care must be taken to ensure that local earthing regulations are observed.

Cx 1 Cx 2

Cy

Cy

L

SymmetricalInterferencecurrent

Asymmetrical Interference current

( asymmetrical interference )

( Symmetrical Interference )

Earth Current

Cp = parasitic capacitance

Common Mode

Differential Mode

Source Load


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EMC Fact Sheet 5


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What makes up the electromagnetic spectrum?

The electromagnetic spectrum is a family of waves that travel through space by way of theproduction of electric and magnetic fields. Changing electric fields are set up by the oscillation ofcharged particles and these changing electric fields induce changing magnetic fields in thesurrounding space. Changing magnetic fields then set up more changing electric fields and so on.The net result is that the wave energy travels across space.

All electromagnetic waves travel at the same speed through the same medium or substance butthey have a variety of frequencies which provide a corresponding variety of wavelengths. If theoriginal charged particle vibrates rapidly, the frequency of the wave is high. Because there aremany oscillations per second, the corresponding wavelength is short. Conversely, if the originalcharged particle vibrates slowly, the frequency of the wave is low and the correspondingwavelength is long.

The whole range of frequencies and wavelengths is called the electromagnetic spectrum anddifferent parts of the spectrum are given different names. These parts of the spectrum have differentproperties and, consequently, they have different uses.

What is the relationship between frequency and wavelength?

As stated above all electromagnetic waves travel at the same speed; the speed of light..

2.99792558 x 108 metres/sec

and wavelength () = the speed of light (c)/ frequency (Hz)

On the following page there is a diagram showing the whole magnetic spectrum and thetechnologies that use the different wavebands. The limits for conducted (150kHz 3MHz) andradiated emissions (3MHz 1GHz) have been transposed on the diagram and this shows theirsignificance in real life. They are very similar to the radio frequency bands. There is somediscussion on increasing the limits to 3 GHz the frequency used by 3G mobile phones.

Interestingly the medical equipment standard EN 60601 (CISPR 11) goes up to 18 GHZ for radiatedemissions and down to 9 kHz for conducted emissions demonstrating just how tough thesestandards are.

Sheet 1/2

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EMC Fact Sheet 5


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1AMe

Sheet 2/2eqe

The Electromagnetic Spectrum

10 pm

1 nm

10 nm

100 nm

1 m

1 mm

10 cm

1 m

10 m

100 m

1 km

10 km

100 km

1 Mm

10 Mm

100 Mm

1 pm

10 m

100 m

1 cm

100 EHz

10 EHz

1 EHz

100 PHz

10 PHz

1 PHz

100 THz

10 THz

1 THz

100 GHz

10 GHz

1 GHz

100 MHz

10 MHz

1 MHz

100 kHz

10 kHz

1 kHz

100 Hz

10 Hz

1 Hz

dical X-Rays

Visible Light

Violet 430 nm 700 Thz

Indigo

Blue 470 nm 640 Thz

520 nm 580 Thz

Yellow 570 nm 530 Thz

Orange 600 nm 500 Thz

Red 650 nm 460 Thz

Green

Fibre Optics

850 nm 1300 nm 1550 nm

Aircraft RadarPolice Radar

Satellite TV

Cellular

Microwave Oven 2.45 GHzCellular

UHF TV

Shortwave Radio

AM Radio 535-1605 kHz

Radio Beacons

Submarine Radio

Power Lines 50Hz

VHF TVFM Radio 88-108 MhzVHF TV

Audio

LF

MF

HF

VHF

UHF

Radio

Microwave

FarInfrared

X-Ray

Ultraviolet

Gamma-

ray

Near

Infrare

d

CircuitTheory(Electronics

RFTechniques Mic

rowaveTechniques

GeometricO

ptics

Photonics

Frequency

Wavelength

Radiate

d

Conducted

Emissions


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EMC Fact Sheet 6

To see lots more fact sheets like this one, or to register for ourseries of informative mini guides on related key topics go towww.reo.co.uk.

The small print: Every effort has been made to ensure the integrity of the information in this data sheet, which has been provided in good faithand the authors do not accept liability for any loss or damage caused by omissions, errors or the interpretation of the reader.

equipment

(and/orspark gap)

LR

CVesd Ce

The following information was kindly provided and compiled by:-Dr Jeremy Smallwood of Electrostatic Solutions Ltd www.static-sol.com

contact details [email protected] and phone number +44 23 8090 5600

Achieving Electrostatic Discharge (ESD) immunity by design

ESD sources

ESD sources have at least three basic elements; astorage capacitor C; series resistance R; seriesinductance L (R and L may be stray components of theinterconnects).

Example: Person using ATM Charge is stored on the body capacitance Discharge occurs to the ATM The ESD current returns to earth by the easiest

routeWhere does the current go??

How ESD affects a system

If ESD flows through the system thereis a risk of a problem.

Therefore the task is to:

Anticipate typical ESD sources Manage the ESD current flow Reduce conducted EMI to

insignificant level Reduce radiated EMI to

insignificant level

ESD

ESDRadiated

EMIuser

interface(frontpanel)

Enclosure

Conducted

EMI
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8/24

EMC Fact Sheet 6



ESD waveform characteristics

Human body ESD

A person gets charged upfrom walking, sitting ordoing anything ! ESDoccurs when they touchanother conductor suchas an equipment panel,chassis, trunking ormachine parts.

C=100 300 pFR= 0.1 100 k

L=stray

Rise time ~10 nsDuration ~100ns

Frequencies up to GHzPeak current ~0.3 A @ 500V

~7 A @ 10 kV

ESD from larger charged metallic parts

A tool or machine partgets charged e.g,

insulated metal objects ortrolleys.

ESD occurs when ittouches anotherconductor.

C=10s -100s pFR=strayL=stray

Rise time ~10 ns or lessDuration ~100ns or lessFrequencies up to GHz

Peak current ~8 A @ 500 V

EESSDD ffrroomm ssmmaallll cchhaarrggeedd mmeettaalllliicc ppaarrttss

A device gets chargedthrough packaging orhandling

ESD occurs whencharged part touches aconductor

C= a few pFR=strayL=stray

Rise time ~0.1 nsDuration ~1ns

Frequencies to > GHzPeak current ~14 A @ 500 V

Key ESD features

Feature Characteristics Typical values

High dV/dt Source collapses from kV to nearzero in nanoseconds

1 kV in 1 ns = 1012 V/s

High peak current Tens of Amps possible

High dI/dt Tens of amps in nanoseconds 10 A in 1 ns = 1010 A/s


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EMC Fact Sheet 6



How systems react to ESD

Components and wiring at high frequencies.

Small stray inductances have high impedance and small capacitances have low impedance. Chassis parts andground wires have inductance and cannot be considered to be at zero volts. Wires and tracks act as antennasand transmission lines.

Small capacitances at high frequencies.

All metalwork, wires and tracks have a capacitance Cto nearby conductors and components also have straycapacitances inherent in their construction. The impedance ZCof a capacitor reduces with increasing frequency f.

Even very small capacitances have low impedance abovea few hundred MHz. This means that the smallcapacitances between tracks can easily cause coupling across tracks at high frequencies.

Another way of seeing capacitance is to consider the effect of high dV/dt. A transient current Iflows in a capacitoras a result of a fast changing voltage V.

This shows that the high dV/dt will inject a charge pulse (current transient) through the small capacitance.

Capacitive coupling

A high impedance E-field couples most effectively with high impedance circuits and high impedance E-field

coupling can be considered to be capacitive coupling. Coupling is reduced by reducing the effective capacitancewhich can be achieved by reducing size of receptor (length of track, area), increasing separation and reducingdielectric constant of materials separating source and receptor.

Inductance at high frequency

All wires, component leads and tracks have some inductance L. The inductive impedance ZL increases withincreasing frequency fand often becomes significant.

Even a short track can look like a high impedance at high frequencies

Another way of seeing inductance effects is as a transient voltage Vdeveloped across an inductance as a result

of a fast changing current I.

All conductors have inductance so a high dI/dt transient can cause significant voltage impulses on any conductor even ground tracks or chassis parts.

Magnetic coupling

A low impedance H-field couples most effectively with low impedance circuits and low impedance H-field couplingcan be considered magnetic coupling. Coupling can be reduced by reducing the effective mutual inductancebetween the source and the receptor e.g. reducing the area of source and receptor circuit loops and increasingseparation.

fCZc

2

1=

Idt

dQ

dt

dVC ==

fLZL 2=

dt

dILV =


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EMC Fact Sheet 6



Designing for ESD immunity

EMC design needs a holistic approach and the best ESD immunity will involve many disciplines:- ESD ground paths Enclosure Circuit design PCB design and layout Software design System wiring and interconnects

A combined view of the options and interplay of these different aspects will be required to achieve good ESDimmunity.

ESD Entry points - ESD can inject fast transients into a panel control, keyboard, switch,connector, component or a PCB. Any wire, metal part or ground plane can take EMI into, or outof, a system. Wires leading to, or from, a PCB can conduct EMI into or out of a board. Wiresand tracks act as antennas to receive or radiate EMI.

Design ESD Ground paths - Where possible provide a direct ground path to keep the ESDcurrent outside the system. Ideally this should be as short as possible with a low inductance.Dont take the ESD through circuit boards or EMI sensitive regions.

ESD groundpaths user

interface(frontpanel)ESD

ESD

ESDRadiated

EMIuserinterface

(frontpanel)

Enclosure

ConductedEMI

Enclosure

Plastic enclosures -A plastic enclosure canprevent ESD occurring. The weak points aregaps for user interface controls andconnectors. ESD will jump through the airgaps to circuitry behind. Make separation

enough to prevent ESD or provide apreferred ESD ground path e.g. via metalback-plate. Prevent ESD/EMI entering viacables. Use transient suppressors and filtersat cable entry points.


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EMC Fact Sheet 6



Filters can be used to prevent ESD andconducted EMI entering or leaving via cables

or connectors

Some types of on-board filter:-

Capacitors - provide simple decoupling.High voltage capacitors may be required towithstand ESD transients

Transorb - Transient suppressors featurefast turn-on and clamping to protectsemiconductor devices against ESD

L-C filters - block transients and EMIentering or leaving

Series resistors - can attenuate transientsand high frequencies - can be used withclamp diodes or decoupling capacitors

Ferrite beads - attenuate EMI and ESD

Shielding,filtering and

interface design

Design interface circuits to prevent ESD and EMI crossing onto or out of, the system.Connection to ground may be made through a tightly bolted mounting bolt or metal pillar tochassis - make sure no paint, anodisation or coating can impair the ground connection.

Screened cableincorrectly

terminated onPCB

At first sight the screen on this cable mayappear reasonably well terminated directly toa ground track. However the ground track islong and thin and crosses the PCB to theground point. EMI currents have plenty ofchance to couple into other circuitry on thePCB. The long ground track presentsconsiderable impedance to the EMI currents.

R-C filter on I/Oline

Low frequency I/O lines may be filtered usinga simple R-C filter circuit. The capacitorshould be as close as possible to theconnector and connected directly to chassisor RF ground. A similar approach may betaken with a transient suppressor.

RF ground

EMIOther circuitsground plane

C

I/O Line

R

C

Othercircuits

I/Oground

I/Oconnector

I/O moduleapproach

Filter components may be placed separately on a multilayer I/O module at the connector entry.Connect the ground plane to chassis ground via short low inductance connections - Advantage:The filter may be optimised independently from the design of the remainder of the board.

Filter capacitortypes

Decoupling capacitors should have high self resonant frequency- Low inductance- Minimise leads and tracks- Chip capacitor packages can be excellentMultilayer ceramics are often quoted as being the best for HF decoupling. Polymer dielectriccapacitors can have good performance. The capacitor value is often not critical.

Coax cableLong thin ground

Coaxconnectoron PCB

Groundpoint

Othercircuits


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EMC Fact Sheet 6



Shieldedenclosure

Shields can effectively prevent ESD and EMI radiation entering or leaving a systemMake sure it cant get in at cable entry points.

Incorrectlyterminated

screened cable

An un-screened connector has been used.The cable screen has been incorrectlyterminated in a pigtail within the screenedenclosure. The pigtail carries EMI currentsthat will re-radiate within the screen.

Equivalent circuit

"Pigtails" have inductiveimpedance L at RF

"Pigtail" cancarry groundcurrent andradiate EMI

insideenclosure

Shield

L L

Correctlyterminated

cable screen

A screened connector been correctlyterminated outside the screened enclosure.No EMI currents will enter the screen.

Equivalent circuit

Conductive connector shellclamps to cable screen and

mates to shield formingcontinuous screen Shield

Outside screen Inside Screen

Correctly terminated cableforms an ubroken screen

Circuit designconsiderations

Hardware can be designed to help give ESD immunity. High impedance lines will be sensitiveto fast dV/dt therefore use lower impedances where possible, keep line lengths short and avoidedge triggered circuits. Low impedance loops may pick up fast dI/dt changes -minimise loopsizes.

Software andfirmware

considerations

Assume that any port or data line state may be corrupted - design-in ways of detecting faultstates. ESD transients are short, so sample states more than once over longer time frame.Design-in safe recovery from possible fault states.

Key points ESD immunity design requires a holistic view Remember ESD has high dI/dt and dV/dt and large peak current flow but is short duration Evaluate likely ESD source entry points Design enclosure and ground paths to take ESD direct to earth away from susceptible circuitry Block ESD/EMI entry at connectors and cables with appropriate filters

Use hardware/software design to reduce basic susceptibility


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EMC Fact Sheet 7

Cherry Clough Consultants, 26 Aug 2004 Page 1 of 12

To see lots more fact sheets like this one, or to register for ourseries of informative mini guides on related key topics go to

www.reo.co.uk.The small print: Every effort has been made to ensure the integrity of the information in this data sheet, which has been provided in good


Techniques for assessing an electromagnetic

environment,

plus guidelines for simple calculationsEur Ing Keith Armstrong C.Eng MIEE MIEEE

Partner, Cherry Clough Consultants,http://www.cherryclough.com

Phone: +44 (0)1457 871 605, fax: +44 (0)1457 820 145, email:[email protected]

Note: A rule of thumb is an expression that refers to a simple calculation or engineering guide or

estimate. Most rules of thumb can only give an estimation of the order of magnitude.

1. The process of assessing the EM environment .....................................................................................1

1.1 Assessing EM threats to the apparatus ............................................................ ..........................................................1

1.2 An example of a checklist of simple EMC questions................................................................................................21.3 Engineering analysis of EMC requirements..............................................................................................................4

1.4 Sources of information on the EM environment ................................................................... ....................................5

1.5 What if you can't predict the environment?...............................................................................................................6

1.6 Example of limitations to use emissions ........................................................ ........................................................7

1.7 Example of limitations to use immunity.................................................................................................................7

2. Estimating the low frequency radiated fields emitted by long conductors...........................................7

2.1 Estimating electric field emissions at low frequencies (DC-100 kHz).......................... ...........................................7

2.2 Estimating magnetic field emissions at low frequencies (DC-100 kHz)...................................................................8

2.3 Notes on running conductors close together:.............................................................................................................9

2.4 Notes on frequencies higher than 100 kHz:...............................................................................................................93. Estimating how radiated fields vary with distance ...............................................................................9

3.1 Electric field strength ........................................................... ............................................................ .......................10

3.2 Magnetic field strength............................................................................................................................................10

3.3 The relationship between electric and magnetic fields at higher frequencies..........................................................11

4. A list of the current standards in the IEC 61000-2-x series ................................................................11

1. The process of assessing the EM environment

What EM threats are present which could interfere with the apparatus?What EM threats are emitted by the apparatus and might interfere with sensitive equipment, even if

it is not nearby?

Always best to agree specifications for the above with the customer in a written contract, which

should include limitations to use, to ease design and manufacture without harming sales too much.

1.1 Assessing EM threats to the apparatus

First decide where the apparatus is to be installed (if it is fixed equipment) or the range of locations

where it could foreseeably be used (especially if it is portable).
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EMC Fact Sheet 7





Initial assessment of EM threats is relatively easy, for example (in household, commercial and

industrial situations) by using an initial checklist of simple questions and assessing its results (see

below) using the tables above, IEC 61000-2-5, the IEC 61000-2-x series (see below), EM emissions

data on other apparatus nearby and/or interconnected by cables (supplies, signal, data, control, etc.),plus researching numerous other relevant sources of information (see below).

This assessment should be supported by simple calculations using known currents, powers,

distances, etc. (see later) and (where practicable) by simulation on a computer using a calibrated

program.

Significant EM threats are then compared with proposed technology and construction of the

equipment concerned. Most of them will be found to be so negligible that further investigation is not

warranted. But there will often be a few threats that will need to be investigated in mode detail.

Instrumented site surveys should be done for the worrisome disturbances, but are only cost-effective

for frequent and continuous EM disturbances, or for transient disturbances that can be made to occur

(e.g. by switching large machines off and on, simulating earth-faults, opening circuit-breakers, etc.).Low probability disturbances that are uncontrollable (such as lightning) may need to be assessed

from literature (articles, books, standards, etc.) and/or calculations. Fault events need to be assessed

too. Consider fault currents, fuse-blowing transients, proximity of arcs and sparks.

1.2 An example of a checklist of simple EMC questions

This checklist should be completed by salespeople in conjunction with potential customers, and used

by EMC specialists working for the Technical or Engineering Department.

The purpose of this checklist is to help begin the process of assessing the electromagnetic

environments that equipment could be exposed to to assist with design and development that will

achieve reliable products with low warranty costs, that will also comply with legal regulations thatinclude EMC requirements (especially the EUs EMC, R&TTE and Medical Devices Directives,

etc., that include immunity requirements).

For custom engineering projects an EM environment assessment should be a contributory factor to

each tender submittal, or quotation of price or delivery. For volume-manufactured products, an EM

environment assessment should contribute to the initial technical specification process.

SAFETY NOTE: Where inaccuracy, errors or malfunctions in electrical, electronic and/orprogrammable electronic devices couldpossibly have safety implications checklists like this canalso be used as the start of the EM environment assessment process. In such cases, what matters is

not just the environments that the equipment is intended to be used in, the replies to this checklistsquestions should also consider all reasonably foreseeable environments, including incidental and

accidental uses of the equipment, and foreseeable misuse. Never assume that people could not bestupid enough to do something you would be wrong.
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EMC Fact Sheet 7





Questions a) to e) below are intended to use with EMC Directive compliance, to identify whether the

final product will be used in domestic, commercial, or light industrial environments, or in industrial

environments. Of course, some equipment might be used in all these environments.

Where a product-specific standard is relevant (e.g. EN 55014-1 and 2; EN 55013 and EN 55020;

EN 55024; etc.) it may be best to apply the most relevant generic standards as well, to helpovercome the well-known shortcomings in some product standards.

a) Will the final product be operated from a low-voltage AC mains supply where the supplyfrom the distribution transformer is shared by more than one organisation? YES NO

b) Will the final products low voltage AC mains supply be shared by heavy powerequipment, industrial manufacturing or processes and the like? YES NO

c) Will the final product be physically located


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l) Will the final product be 10milliseconds; plastic welders or sealers; induction heaters;industrial microwave heaters or dryers; diathermic processes; RF-assisted welders;

RF-assisted wood or card gluers; electromagnetic pulse devices; variable speed drivesfor AC or DC motors >100kW or their motors; etc.) YES NO

If the answer is Yes, please provide all details.

m) Will the final product be


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1.4 Sources of information on the EM environment

The IEC 61000-2-x series generally addresses the household, commercial or industrial

environments, but electronic equipment can find itself in other environments such as military,

marine, automotive, civil aviation, space, etc., and there are other standards and documents thatprovide information on the likely magnitudes of the EM threats in such situations.

Although telecomms are generally located in household, commercial or industrial environments, the

telecomms industry places great emphasis on reliability, especially for central office (telephone

exchange) equipment, so their immunity requirements can be a lot tougher. Also, some telecomms

equipment is located outdoors in situations very exposed to lightning (e.g. on a pole). So telecomms

resistibility and immunity standards can be very helpful when high reliability is required.

For International Telecommunication Union (ITU) EMC recommendations: visit www.itu.org, click

on ITU Publications Online then click on ITU-T Recommendations then click on K, to see the

list of their EMC standards. Note that they sometimes use the word resistibility to mean immunity.

They usually make a small charge for providing these standards.

Telcordia in the USA is the successor to Bell Telephone Labs and maintains a number of Telecomm

standards that are widely used in the USA to ensure high reliability of telecomm network equipment

despite earthquakes, lightning, and all other manifestations of the physical environment. Visit

http://telecom-info.telcordia.com and click on Technical Documents to visit their Information

Superstore. Then click on Document Centre and look for relevant documents. It may be necessary

to search the site for electromagnetic or lightning since few of the documents mention EMC in

their titles. A very important EMC document is GR-1089-CORE Electromagnetic Compatibility

and Electrical Safety - Generic Criteria for Network Telecommunication Equipment.

Military EM environments are often different, sometimes harsher than household, commercial or

industrial environments, and information on them is contained in the immunity tests and test levels

of Military standards. There are military EMC standards for ships, land vehicles, and aircraft, andthey are all different.

For British Defence EMC standards: visit http://www.dstan.mod.uk/home.htm and download

appropriate parts of DEF STAN 59-41. For the US Military EMC standards MIL-STD-461E and -

464: the official site is supposed to be www.astimage.daps.dla.mil but if this doesnt work the

Google search engine (www.google.com) will turn up locations you can download them from for

free, for example. (at the time of writing) http://www.multilek.ca/Specifications.htm. Simply typing

the MIL standards reference number into Google will usually find a number of sites where it can be

downloaded.

Civil aircraft EM environments are covered by RCTA/DO-160D, published by RTCA Inc. in 1997

(www.rcta.org), which is continually being improved. The civil working groups developing thisstandard (DO-160E is due in 2004) are SC135 (in USA) and EUROCAE WG 14 (in Europe), and

Google should find their websites. It is interesting to note that over the past 15 years there has been a

dramatic increase in immunity test levels required by DO-160, with the maximum test limits for

radiated immunity increasing from 1V/m to over 6kV/m.

Road (automotive) and roadside EM environments are again different from household, commercial

or industrial environments, and the UKs Motor Industry Research Association (MIRA

http://www.mira.co.uk) surveys the EM environment of the UKs roads every few years and

publishes a report that can be purchased.

Rail networks also have special EM threat characteristics. A great deal of information on railway

EM environments has been collected, but is probably in the hands of the rail network operators and
http://www.reo.co.uk/http://www.reo.co.uk/http://www.multilek.ca/Specifications.htmhttp://www.multilek.ca/Specifications.htmhttp://www.multilek.ca/Specifications.htmhttp://www.multilek.ca/Specifications.htmhttp://www.reo.co.uk/http://www.multilek.ca/Specifications.htm


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their prime contractors. It might be worthwhile looking in the EN 50121 series (railway EMC) but

(like medical EMC standards) they only describe the normal environment and do not give much

help when it comes to worst-case or low-probability threats. Some EMC consultancies specialise in

railway EMC (e.g. York EMC Services http://www.york-emc.co.uk and ERA Technology Ltd

http://www.era.co.uk) and they may be able to help.

The marine (non-military) EM environment is addressed in IEC 60533, which also includes some

procedures for ensuring compatibility when its minimum immunity requirements are not enough.

A sites electricity supply Authority should be able to provide data on the disturbances that can occur

in their supplies, and may be able to provide data on the actual disturbances present at a given

location (for a fee). However, they can be unwilling to admit how bad their services are with regard

to dips, short interruptions, overvoltages and waveform distortions. We would expect thick reports

citing many years of serious measurements, and appropriate data reduction in to a digestible form, so

if all you get is bland assurances that they comply with their statutory requirements it may be better

to rely on the relevant IEC 61000-2 series and/or on-site measurements (over a lengthy period) if

they can be arranged.

Military authorities have contour maps of field strengths from fixed transmitters over most of theworld, but it may be hard to obtain them unless you are a member of that countrys military or an

allied nation.

The UKs Civil Aviation Authority (www.caa.co.uk) keep a record of all the radars in use in the UK

(frequencies, power levels, and pulse characteristics), and are a good source of information on

mobile radars. They may also be able to help with field strength contour maps from fixed

transmitter and industrial RF processing sites. Presumably other countries equivalent authorities are

similar. Civilian pilots can also get contour maps of no-fly areas from appropriate civil authorities,

but they might not give actual field strength information.

Lightning protection standards (e.g. BS 6651, IEEE C62-41, IEC 61024-1 and IEC 61312-1) and

lightning incidence maps (isokeraunic maps) and knowledge of the site's lightning protectionsystem helps determine the threats from lightning. Isokeraunic maps are available from a number of

sources, including national weather forecasting organisations. But it is probably easiest to search for

them, for the area concerned, using the Google search engine (http://www.google.com).

IEEE International EMC Symposia are also a good source of information on real-world EM

environments (http://www.ieee.org).

Simulation of the EM environment is increasingly possible, and some software products are

available, e.g. for the fields created by overhead HV power lines. Some consulting companies have

written their own software and can offer bureau services (e.g. Qinetiq, BAE Systems, ERA

Technology).

For intentional interference, Metatech Corporation are experts in assessing EM environments andintentional interference and their website http://www.metatechcorp.com has a number of usefuldownloads.

1.5 What if you can't predict the environment?

This is usually only a problem for custom engineers where the customer is unhelpful.

Choose the most relevant harmonised standards and "apply" their informative annexes too. Put the

EM environments they cover in the contract, with their limitations to use written out in full, and get

the customer to agree to them.
http://www.reo.co.uk/http://www.reo.co.uk/http://www.era.co.uk/http://www.york-emc.co.uk/http://www.york-emc.co.uk/http://www.york-emc.co.uk/http://www.era.co.uk/http://www.era.co.uk/http://www.metatechcorp.com/http://www.metatechcorp.com/http://www.metatechcorp.com/http://www.reo.co.uk/http://www.metatechcorp.com/http://www.era.co.uk/http://www.york-emc.co.uk/


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Then if/when things go wrong due to EM problems with the environment, you will have some

leverage

1.6 Example of limitations to use emissions

Developed from Section 3 of the heavy industrial generic emission standard EN 61000-6-4:"This apparatus might interfere with radio and television reception when it is used closer than 30metres to the receiving antenna(e). And it might also interfere with highly susceptible apparatus

being used in proximity. In all such instances the user is responsible for employing special

mitigation measures, at his own expense."

1.7 Example of limitations to use immunity

Developed from Section 3 of the heavy industrial generic immunity standard EN 61000-6-2:

"Situations may arise where the electromagnetic disturbances in the environment may exceed the

immunity of this apparatus, e.g. where it is installed in proximity to ISM equipment (as defined by

EN 55011) or where a mobile transmitter is used in proximity. "In all such instances the user isresponsible for employing special mitigation measures, at his own expense."

2. Estimating the low frequency radiated fields emitted by longconductors

At frequencies from DC to 100 kHz it is possible to crudely estimate the strengths of the electric and

magnetic fields emitted by voltages and currents in conductors, using the simple formulae below.

Measurements of electric and magnetic fields at these low frequencies are easy to do, for fields of

magnitudes down to 0.1 V/m, or 0.1 A/m, using low-cost handheld instruments, so the main use for

these rules-of-thumb will be where the apparatus concerned does not yet exist.

These rules-of-thumb will mostly be used for estimating high levels of magnetic fields fromconductors carrying high levels of DC and AC power, such as motor drives, electromagnetic stirrers,

etc., especially to determine whether CRT type monitors in the vicinity will give stable images.

These rules assume free-space radiation, but actual fields strengths will be modified by the proximity

of cables, cable trays ducts and conduits, equipment and cabinets, structural steelwork, etc., and may

be higher or lower than those estimated from these simple formulae.

Where safety-related issues are concernedit will be important to perform more exact assessments

or by performing measurements as soon as possible, even on partially constructed apparatus or

apparatus of a similar type. If these rules are to be used in the initial stages of a project on safety-

related issues their results should be multiplied by at least 10 to provide a suitable margin for error

until a more accurate assessment or measurement can be made. The actual margins required willthen depend upon the safety integrity level (see IEC 61508) of the safety-related function(s)

concerned.

These rules-of-thumb all assume that the length of the conductors is much greater than the distance

(d) at which the field strength is to be estimated. When the cable length equals d, a rule of thumb

would be to divide the field by two, with further reductions as the cables become even shorter.

2.1 Estimating electric field emissions at low frequencies (DC-100 kHz)

Electric field strength is given the symbol E and measured in Volts/metre (V/m).


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EMC test equipment is usually calibrated in dBV/m, where 0 dBV/m = 1V/m, since EMC was

traditionally concerned with interference to radio receivers which were intended to pick up radio

signals with merely a few V/m field strength.

Personnel hazard measuring instruments for non-ionising radiation are usually calibrated in kV/m,

since it is long-term exposure to such magnitudes of electric fields that may cause health problems.Electric fields are difficult to calculate for real-life situations because free-space conditions are never

found and the proximity of other conductors, metalwork, and ground have a profound effect. A verycrude rule of thumb for the electric field between a long single conductor and anything else is to

divide their voltage difference (Vdiff) by their spacing (s) in metres: E = Vdiff s

E.g. A long cable carrying 1 kV is 1 metre from an opto-isolator device which may be

assumed to be at earth potential. The resulting electric field experienced by the opto-isolator is

1 kV/m. (At 2 metres distance the field would be reduced to 500 V/m.)

Where there are multiple long cables running in free space, the electric field at any point is the

vector sum of all their individual contributions. In most cases cables are run parallel to each

other, so the vector addition is merely a straight addition of the fields.E.g. for +1 kV on a long cable 1 metre away from an "earthy" optical sensor, with a second

long cable run in parallel with 100 mm spacing from the first cable and 1.1 metres from the

optical sensor: when the second cable carries an equal and opposite voltage of -1 kV the

resulting field strength at the optical sensor is very approximately (1,000) + (-909) = 91 V/m.

If instead of 100 mm the cable spacing was reduced to 10 mm, the resulting field strength at

the optical sensor would be roughly (1,000) + (-990) = 10 V/m.

The presence of large masses of earthed metalwork nearby is likely to reduce the size of

electric fields. If this mass of earthed metalwork is between the conductor with the high

voltage and the sensitive part, it may reduce the electric field dramatically by acting as a

shield. (If the mass of metalwork is not earthed its shielding effect could be much less.)

2.2 Estimating magnetic field emissions at low frequencies (DC-100 kHz)

Magnetic fields are measured in Amps/metre (A/m), Tesla (T), or Gauss (G).

Conversion factors between these three units in free airare: 1 A/m 1.25 T 12.5 mG

1 T = 10 kG 800 kA/m

1 G = 100 T 80 A/m

EMC test equipment is usually calibrated in dBA/m, where 0 dBA/m = 1 A/m, since EMC was

traditionally concerned with interference to radio receivers which were intended to pick up radio

signals with merely a few A/m field strength.

Personnel hazard measuring instruments for non-ionising radiation are usually calibrated inkAmps/metre, kGauss, or Tesla, since it is long-term exposure to these magnitudes of magnetic

fields that may cause health problems.

In the special case of a long single conductor in free space, the magnetic field strength it produces at

a nearby point may be calculated from Amps (2d), in A/m, where d is the perpendicular line-of-

site distance from the point concerned to the centre of the conductor (in metres).

E.g. For 100A in a long cable that is 1 metre away (the shortest distance at right angles to

cable run) the field strength according to this formula is 16 A/m (approx. 20 T).

Where there are two or more long cables similarly running in free space, the magnetic field at

a point is the vector sum of all their individual contributions. In most cases these cables will be


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running parallel to each other (e.g. send and return currents to a DC motor, three-phase or

three-phase-plus-neutral power) and the maximum resulting field strength is simply the

straight addition of their individual fields.

E.g. For +100 A in a long cable 1 metre away with its -100 A return current in a parallel cable

1.1 metres away (e.g. a cable spacing of 100 mm when the point of interest and the two cables

all lie in a plane): the field strength at the point of interest is (16) + (-14.5) = 1.5 A/m.

If instead the cable spacing is 10mm (i.e. the send/return cables are almost side-by-side, since

d is measured to the centre of the conductor) the resulting field strength is (16) + (-15.8) =

0.2 A/m.

2.3 Notes on running conductors close together:

The above examples show the great reduction in electric and magnetic fields which can be achieved

by running send and return conductors, carrying equal and opposite voltages and currents, as close

together as possible. Twisting send/return conductors together is even better (although easier forsmall-signal cables than for power).

For three-phase (or three-phase and neutral) power conductors the voltages and currents (and hence

their fields) are all at 120o to each other, and running them together in a single cable or bundle (with

a twist if possible) helps reduce electric and magnetic fields in exactly the same way.

Where very heavy currents are concerned, the mechanical stresses caused by running cables with

opposing currents close to each other may damage the insulation in the cables in a relatively short

period of time, leading to fire or shock hazards. Busbars which use solid insulation may be a better

solution in such cases.

As well as considerably reducing the emitted electric and magnetic fields, running send/return or

three-phase power conductors closely together also helps to reduce their pickup of interference fromtheir environment, so this technique is important for immunity as well as for emissions.

2.4 Notes on frequencies higher than 100 kHz:

At higher frequencies the wavelengths become comparable with typical cable lengths in industrial

situations, making the above rules-of-thumb useless.

Where intentional radio transmitters are involved Table 2 gives useful guidance on field strengths,

but for other high-frequency signals it is impossible to use the above rules-of-thumb and

measurements are the only option.

Crude measurements may be done with simple low-cost test gear, but if the apparatus concerned is

of recent manufacture, its manufacturer should already have emission test results.

3. Estimating how radiated fields vary with distance

Where the field strength at one distance from the emitter is known (e.g. from manufacturers test

results, or from a calculation) the rules-of-thumb below allow the field strength at other distances (d)

to be crudely estimated.

These simple rules work over a very wide frequency range, at least to 1GHz, providing the distances

concerned are not too near to the emitter (less than /6, see 2.3).

These rules assume free-space radiation, but actual field strengths will be modified by the proximity

of cables, cable trays ducts and conduits, equipment and cabinets, structural steelwork, etc.


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Consequently an "engineering margin" of at least 100% is recommended over and above the levels

calculated using these rules to allow for these real-world effects, but it should be realised that such

effects can sometimes cause field strengths to be 10 times (+1,000%) or reduced to negligible

values, especially at frequencies above 10 MHz.

Where safety-critical functions are concerned it will be important to initially either measure theactual field, or allow for the level to be at least 10 times higher than these calculations give and then

measure the actual field as soon as it becomes possible to do so.

3.1 Electric field strength

Electric field strength tends to be proportional to Volts d

E.g. An ISM apparatus is known to emit 135 dBV/m (= 5.6 V/m) at 84 MHz at 3 metres

radial distance from a part of its structure.

At 1 metre radially from the same part of its structure it may be expected to have a field

strength of the order of 145 dBV/m (= 17.8 V/m).

At 30 metres radial distance from that part it may be expected to have a field strength of theorder of 115 dBV/m (= 0.56 V/m).

3.2 Magnetic field strength

For single conductors, magnetic field strength tends to be proportional to Amps d

E.g. A long single cable is known to emit a magnetic field strength of 16 A/m at a distance of

1 metre (perpendicular to the run of the cable).

The field strength at 100 mm distance may be expected to be of the order of 160 A/m.

The field strength at 10 metres distance may be expected to be of the order of 1.6 A/m (which

is still too high for a normal CRT-type computer monitor to be sure of meeting the Health and

Safety "VDU Directive").

Where a number of conductors run very close together in parallel and carry currents that

balance out (e.g. send and return currents to a DC motor, three-phase or three-phase-plus-neutral

power), at distances (d) which are very much larger than the separation between the individual

conductors the resulting magnetic field strength tends to be proportional to {(Amps) (separation)}

d2

E.g. A pair of DC drive cables (send/return) have a spacing of 10 mm, and are known to

create a magnetic field of 0.2 A/m at a distance of 1 metre.

At a distance of 2 metres their magnetic field may be expected to be of the order of 0.05 A/m.

For transformers, solenoids, and the coils of induction heaters, the magnetic field strength tendsto be proportional to Amps d3.

E.g. An 800 kW 1.1 kHz steel billet induction heating coil is known to produce 100 A/m at 1

metre distance from the side of its coil.

At 100 mm distance it may be expected to create a field of the order of 100 kA/m, getting

close to the levels at which health hazards may occur.

At 10 metres distance it may be expected to create a field of roughly 0.1 A/m, quite low

enough to be confident about fitting a CRT type of monitor at this distance and achieving good

image stability.


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Mixtures: in the real world coils and transformers are connected to other devices and to cables, and

the rate of change of magnetic field strength with distance will be a mixture of all three of the above

approximations.

E.g. In the above example of the steel billet induction heater, although the 1.1 kHz magnetic

field emitted by the coil has diminished to roughly 0.1 A/m at a distance of 10 metres, the 11

kV 3 50 Hz power cables to its power electronics cabinet would be likely to be carrying

around 100 Amps each.

If these long cables had a spacing of 100 mm from each other in the same plane as the

computer monitor, and were 5 metres away from it on average, their magnetic field would be

of the order of 0.06 A/m, still a negligible amount.

However, if the power to the electronics cabinet was supplied at 1.1 kV 3 50Hz and their

three 1000 A supply cables were each spaced apart by 500 mm: at 5 metres distance their

resulting 50 Hz magnetic field would be of the order of 3 A/m, which could be expected to

have a significant effect on the image stability on a normal CRT-type VDU.

3.3 The relationship between electric and magnetic fields at higher frequencies

All fields are emitted as either electric or magnetic fields, but after travelling a distance equivalent to

roughly one-sixth of their wavelength they all turn into electromagnetic fields.

Electromagnetic fields consist of both electric and magnetic fields in a ratio that depends on the

characteristic impedance of the medium they are travelling in. For air, the characteristic impedance

is 377, so it is possible to measure either the electric or magnetic component and calculate the

other by dividing or multiplying by 377.

The wavelength () of a frequency (f) is given by = v/f, where v is the velocity of propagation (thespeed of light) in the medium the wave is travelling in.

In air, v = 3.108 metres/sec so the wavelength of a 30 MHz wave in air is 10 metres, so at more thanabout 1.5 metres from an emitter, whether it initially emit electric or magnetic fields, the result will

be an electromagnetic wave with its electrical (E) and magnetic (H) field components in the ratio

E/H = 377 (just like V/I=R, Ohms law).

Below 30MHz, most test methods measure the magnetic component of electromagnetic fields with a

loop antenna. Above 30 MHz most test methods use an electric-field antenna. However, the results

from each type of antenna can easily be converted into E or H fields as required.

In PVC cables the velocity of propagation is less than in air, and is often as low as 2.108 metres/sec

(depending on the cable type). This means that all frequencies have shorter wavelengths when they

are conducted in a cable, compared with being radiated through the air.

In section 1 we deliberately limited the frequency range of the simple formulae to 100 kHz, since thewavelength (in air) at this frequency is 3,000 metres. One-sixth of this is 500 metres, a large

enough distance to enable us to ignore the effects of wavelength even in a large building.

4. A list of the current standards in the IEC 61000-2-x series

These standards will be found to be very useful in helping to assess the electromagnetic environment

without using site surveys. IEC 61000-2-5 is particularly helpful.

(Note that all the IEC 1000 series are gradually being renumbered as the IEC 61000 series.

Consequently some of the numbers are still using the 1000 series.)


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