Scientific & industrial EMC. Alain CHAROY -

Transcription

1 Scientific & industrial EMC Alain CHAROY - a.charoy@aemc.fr

2 IEC S00200 Symbol Name: Alternative names: To be clear : Earth, general symbol Ground (US), grounding (US), general symbol 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

3 IEC S00202 Symbol Name: Alternative names: To be clear : Protective earthing Protective grounding (US), protective earthing conductor, protective earthing terminal, protective grounding conductor (US), protective grounding terminal (US) The earthing conductor role is to protect people (for safety), so why 2 different symbols?

4 IEC 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!

5 IEC 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)

6 IEC S00203 Symbol Name: Alternative names: Status level: To be clear : Frame Chassis Obsolete - for reference only (Why to cancel it?) We need a symbol for local common bonding network. Why not to use this one?

7 IEC S01409 Symbol Name: Functional equipotential bonding Alternative names: Functional bonding conductor Functional bonding terminal To 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?

8 IEC S01410 Symbol Name: Alternative names: To be clear : Functional equipotential bonding Functional bonding conductor Functional bonding terminal Signal ground (usually called 0 V ). May be earthed (connected to frame), or floated (primary of SMPS for instance).

9 IEC 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?

10 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 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.

11 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 ). 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.

12 Maximum Earth Electrode Resistance IEC : Protection against lightning - Physical damage to structures & life hazard Minimum length l 1 of each earth electrode according to the class of LPS ( 2 electrodes required)

13 Soil resistivity Copper Aluminium Iron Excellent soil Average soil Bad soil Silver Gold Resistivity (in m) 1 billion Conclude by yourself

14 IEC Definition: Equipotentiality Equipotentiality: state when conductive parts are at a substantially equal electric potential (IEC ) 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.

15 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 ) 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.

16 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)

17 Correct use of symbols (example)

18 Star (single point) grounding principle Voltage reference The Theory

19 Single Point Grounding : reality Actual conductors + pipes + The real Theory world!

20 Star grounding = common impedance P. Supply 500 ma Pulses at 500 khz Variable Frequency Drive 500 ma 50 V Isolated link cable Control cabinet 500 ma Pulses at 500 khz Variable Frequency Drive < 1 V Control cabinet < 1 m Bond 250 ma 250 ma 30 m long PE Z 100 at 500 khz 100 Single point grounding creates large common impedances between interconnected equipments

21 Star grounding = large ground loop Equipment 5 kv Equipment Equipment < 50 V Equipment #1 Isolated link cable #2 #1 <.5 m 2 loop #2 50 m 2 Ground loop 80 Am -1 s -1 Lightning magnetic field (= 80 Am -1 s -1 ) Single point grounding creates large area ground loops between interconnected equipments

22 Star grounding of added capacitors Conducted noise with an EMC filter (without added capacitors) Added capacitors (3 x 2,2 F) upstream side EMC Filter Star point Noise with star grounded caps added at each side of the filter + 12 db (degradation) Mutual inductance Added capacitors (3 x 2,2 F) downstream side

23 Star grounding = mutual inductance Noise with star grounded caps EMC Filter EMI Filter M Noise with separately grounded caps (Improvement = 31 db) EMI Filter Better cabling (still perfectible: too long wires)

24 Poor power distribution RIGHT: Single point grounding for the neutral bus. WRONG: Single point grounding for the ground bus (i.e. the PE conductor): a mesh-cbn is should be specified.

25 Poor grounding distribution Large ground loop RIGHT: WRONG: Modems (with galvanic isolation) installed between unconnected earth terminals. Single point grounding for all PE wires.

26 Isolated grounding networks RIGHT: Nothing! WRONG: Simultaneously accessible unconnected networks, so it is quite illegal (due to a safety issue).

27 Isolated grounding electrodes For safety: 2 isolated grounding networks are illegal. For EMC: 2 isolated grounding networks are calamitous

28 The Ground Loop problem A variable magnetic field (difficult to shield at LF) induces voltage across loops Small DM loop a Larger CM ground loop A This ground loop area is unavoidable It should be reduced by: A raised floor loop is not a ground loop Multiple loops are favourable: - 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 - They lower the common mode impedance - They divide the common mode currents - They reduce the external EM fields effects

29 Tolerable noise between equipments 1000 V Maximum permanent rms voltage between equipments 100 V 1 s 10 V 1 s B 1 V A 0,1 V 10 mv IEC DC 10Hz 100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz 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.

30 Impedance of a meshed network Inductance : L (L 1 H/m) A B L AB L C D L CD L 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!

31 Maximum current in a meshed network 1000 A Current for a 1 V maximum common mode noise 100 A Zone of usual total leakage currents for 10 A a 10MW installation Zone of usual CM currents of a large variable speed drive (100kW inverter) 1 A 100 ma 10 ma 100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 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

32 Keeping EMC under control All those cable trays are correctly cross-bonded together at every opportunity, 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).

33 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 several megahertz (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.