aemc-scientific industrial emc

7/31/2019 AEMC-Scientific Industrial EMC

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Scientific & industrial EMC

Alain CHAROY - [email protected]


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IEC 60617 - S00200 Symbol

Name: Earth, general symbol

Alternative names: Ground (US), grounding (US), general symbol

To be clear : The earth electrode and its connections

Regardless of its resistance, an earth electrode is not functional

Even for safety reasons, fortunately for medical implants, automotive

electronics, aircrafts, helicopters, satellites


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Name: Noiseless earth

Status level: Obsolete - for reference only

To be clear : Stupid !

1) To cancel earth currents, just remove all earth conductors!

2) In voltage, any earth electrode is noiseless regarding itself!

3) In voltage, any earth electrode is noisy regarding any separate one!


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Name: Functional earthing, functional grounding (US)

Alternative names: Functional earthing conductor, Functional

earthing terminal

To be clear : There is no functional earthing in industry !

Only used for long distance DC links (mainly ancient telegraphy)


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Name: Frame

Alternative names: Chassis

Status level: Obsolete - for reference only (Why to cancel it?)

To be clear : We need a symbol for local common bonding

network. Why not to use this one?


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Name: Functional equipotential bonding

Alternative names: Functional bonding conductor

Functional bonding terminalTo be clear : Exactly the same definition as S01410 ! ! !

No symbol any more for chassis or frame

So, this new symbol (2005) be may confusing.Can we use it for Common Bonding Network?


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Name: Protective equipotential bonding

Alternative names: Protective bonding terminal

To be clear : ? ? ?

The old meaning of this symbol was 0 V

Does this symbol mean frame thereafter?

If not, what is its new meaning?


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IEC Definition: Reference Earth

Reference Earth: Part of the Earth considered as conductive, the

electric potential of which is conventionally taken as zero, being

outside the zone of influence of any earthing arrangement.The concept "Earth" means the planet and all its physical matter.

(IEC 60050 - 195-01-01 - No symbol for this definition)

Comments : No Reference Earth exist in the real world:

it is just a fictive - purely imaginary - concept.

Fortunately for safety, it is a useless concept!

Fortunately for EMC, it is a useless concept!

Equipotentiality is a necessary virtue.


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IEC Definition: earth electrode

earth electrode (or ground electrode - US): conductive part, which

may be embedded in a specific conductive medium, e.g. concrete or

coke, in electric contact with the Earth (IEC 60050 - 195-02-01).

Comments : No trouble with this definition, but

What is the importance of an earth electrode?

What is a correct earth electrode resistance?

Both for safety and for EMC, equipotentiality

is the most desirable virtue.


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Maximum Earth Electrode Resistance

Minimum length l1 of each earth electrode according to the class of LPS ( 2 electrodes required)

490

125

78

IEC 62305-3: Protection against lightning - Physical damage to structures & life hazard

56


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Soil resistivity

Copper

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 100 103 104

Aluminium

Iron

Resistivity (in m)

1 billion

Excellent

soil

Average

soil

Bad

soil

Conclude by yourself

GoldSilver


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IEC Definition: Equipotentiality

Equipotentiality: state when conductive parts are at a substantially

equal electric potential (IEC 60050 - 195-01-09)

Comments : Equipotentiality is necessary for safety at 50 Hz.

This is obtained by CBN, including PE conductors.

A correct equipotentiality is necessary for EMC

at all frequencies, from DC to several GHz.

This can be obtained up to several MHz by a mesh

common bonding network.


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IEC Definition: Protective conductor

Protective conductor (PE): conductor provided for purposes of

safety, for example protection against electric shock. This is the

green/yellow cable (IEC 60050 - 195-02-09)

Comments : PE conductors are sufficient for safety at 50 Hz.

But in a noisy environment, a PE conductor is not

sufficient for EMC : a mesh-CBN is needed.

In a very noisy environment a metal plate is better.

In an extreme EMC environment, a shielded room

(or a shielded cabinet) may be necessary.


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Summary of useful symbols

Earth (both for safety & for lightning protection)

Common Bonding Network (& frame)

0 V (may be floating or not)

PE conductor (a part of the CBN)


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Correct use of symbols (example)


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34

15

Voltage

reference

The Theory

Star (single point) grounding principle

2


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4

15

The TheoryThe real world !

Single Point Grounding : reality

2

3

Actual conductors + pipes +


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Star grounding = common impedance

P. Supply

500 mA

Pulses

at 500 kHz

Variable

Frequency

Drive

Control

cabinet

50 V

Isolatedlink

cable

30 m long PEZ 100

at 500 kHz

500 mA

100

500 mA

Pulses

at 500 kHz

Variable

Frequency

Drive

Control

cabinet

< 1 V

250 mA 250 mA

< 1 m

Bond

Single point groundingcreates large common

impedances between

interconnected

equipments


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Star grounding = large ground loop

Equipment

#1

5 kV

Isolated

linkcable

50 m2

Ground

loop

Lightning magnetic

field (= 80 Am-1s-1)

Equipment

#2

< 50 V

< .5 m2loop

80 Am-1

s-1

Equipment

#1

Equipment

#2

Single point groundingcreates large area

ground loops between

interconnected

equipments


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Star grounding of added capacitors

Added capacitors (3 x 2,2 F) downstream side

Noise with star grounded caps

added at each side of the filter+ 12 dB (degradation)

Conducted noise with an EMC filter

(without added capacitors)

EMC Filter

Added capacitors (3 x 2,2 F) upstream side

Star point

Mutual inductance


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Star grounding = mutual inductance

Better cabling (still perfectible: too long wires)

Noise with star grounded caps

Noise with separately grounded

caps (Improvement = 31 dB)

M

EMI Filter

EMC Filter

EMI Filter


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Poor power distribution

RIGHT:

Single point groundingfor the neutral bus.

WRONG:

Single point grounding

for the ground bus (i.e.

the PE conductor): a

mesh-CBN is should

be specified.


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Poor grounding distribution

RIGHT: Modems (with galvanic isolation) installed

between unconnected earth terminals.

WRONG: Single point grounding for all PE wires.

Large

ground

loop


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Isolated grounding networks

RIGHT: Nothing !

WRONG: Simultaneously accessible unconnected networks,

so it is quite illegal (due to a safety issue).


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Isolated grounding electrodes

For safety: 2 isolated grounding networks are illegal.

For EMC: 2 isolated grounding networks are calamitous


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The Ground Loop problem

Small DM loop

A variable magnetic field (difficult to shield at LF) induces voltage across loops

a

Larger CM

ground loop

A

This ground loop area is unavoidable

It should be reduced by:

- Laying the cables down over the (metallic) ground plane

- Using cable trays (in contact from one end to the other)

- Using shielded cables with connection at both ends

A raised floor loop is not a ground loop

Multiple loops are favourable:

- They lower the common mode impedance

- They divide the common mode currents

- They reduce the external EM fields effects


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Tolerable noise between equipments

In case of trouble over the limit B , improve the equipotentiality of the installation.

In case of trouble below the limit A , improve the immunity of sensitive equipments.

1000 V

100 V

10 V

1 V

0,1 V

10 mV

DC 10Hz 100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz

Maximum permanent rms voltage between equipments

A

1 s

1 s B

IEC 61000-4-16 61000-4-6


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Impedance of a meshed network

For a conductor, the impedance between the ends increases as its length.

For a 2D grid, the impedance between 2 points does not depend on their distance.

For a 3D grid, the impedance between 2 points decreases with the size of the structure !

Inductance : L

(L 1 H/m)

LAB L

LCD L

A

B

C

D


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1000 A

100 A

10 A

1 A

100 mA

10 mA

100Hz 1kHz 10kHz 100kHz 1MHz 10MHz

Current for a 1 V maximum common mode noise

Maximum current in a meshed network

Current in a 1 m x 1 m grid



Zone of usual total

leakage currents for

a 10MW installation

Zone of usual CM

currents of a large

variable speed drive

(100kW inverter)


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Keeping EMC under control

All those cable trays are correctly cross-bonded together at everyopportunity, including bonding them to the cabinets at both ends, to

cost-effectively create the most closely-meshed three-dimensional CBN

possible from the existing metalwork (Thank you, Keith, for reviewing).

C l i


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Conclusions

- In scientific/industrial installations most EMC concerns appear over 1 MHz.

- A single point grounding creates large common impedances

between interconnected circuits or equipments.

- A single point grounding creates large area ground loops

between interconnected circuits or equipments.

- A single point grounding creates a large mutual inductance coupling

between parallel grounding wires of independent circuits.

- A single point grounding is only good for the neutral conductor

(to limit an homopolar current flow in the common bonding network).

- A meshed common bonding network is efficient for EMC up to severalmegahertz (possibly > 10 MHz). Such a cost effective mesh-CBN allows

grounding of shielded differential cables at both ends without any risk.

- Properly connected shielded cables are effective from 1 MHz to > 1 GHz.

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