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7/31/2019 AEMC-Scientific Industrial EMC
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Scientific & industrial EMC
Alain CHAROY - a.charoy@aemc.fr
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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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IEC 60617 - S00201 Symbol
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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IEC 60617 - S01408 Symbol
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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IEC 60617 - S00203 Symbol
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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IEC 60617 - S01409 Symbol
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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IEC 60617 - S00204 Symbol
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
Current in a 2 m x 2 m grid
Current in a 5 m x 5 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.
Recommended