EMC Compatible enclosure assembly 1
The definition of electromagnetic compatibility (EMC) is the ability of an electrical device to function satisfactorily in its electromagnetic environment without adversely affecting this environment, which may include other equipment. On the basis of this, the essential requirements of EMC are: To prevent/reduce interference emission and to offer defined resistance against interference. EMC is an indispensable element of quality, and the protection requirements regulated by law, along with the technical risks must be taken into account at the planning stage when developing equipment. The Rittal EMC concept With the enclosure, as a housing for electrical/electronic controls and systems, the following points must be observed: These days, the intelligence contained inside enclosures is becoming ever faster, i. e. shorter switching times and steeper pulse edges, leading to ever higher frequencies of voltages and currents. Ever lower energy consumption, i. e. lower voltage/ current levels, means that components are more readily influenced by interference. The sitting of controls in ever more confined spaces, i. e. smaller distances between components and cables, causing ever more frequent interference on different paths. Technical progress will exacerbate these risks even further. A standard enclosure made of coated sheet steel can make a significant contribution towards the EMC of controls for machinery and systems, provided some simple population rules are taken into account. In applications with high frequency field-bound interference, the use of an RF shielded enclosure with a superior shielding effect may be required. The only way to draw definite conclusions about the type of enclosure which is necessary or adequate to comply with certain standard limits is by conducting measurements. 2
Explanation of symbols 3
Comprehensive potential equalization grounding grid 1 Enclosure for power distribution 2 3 4 5 Enclosure for control and measurement technology Tray for power cable Tray for low-power cable Steel reinforcement in concrete ceilings, metallic bases in false ceilings Earth connections For EMC purposes, ideally there should be low and high-frequency potential equalization between all metallic masses, enclosures, machine and system components, which should be meshed as closely as possible. 4
Comprehensive potential equalization routing A Power supply B 1 2 3 4 5 6 7 8 Data/control connection Enclosure Cable Earth connection Reference potential Loop Enclosure Machine Measuring device Cable routing in machinery and equipment Basic principle: Avoid large cable loops; lay current-carrying cables as close as possible to the reference potential. 5
Comprehensive potential equalization connections L < About 2 A 0.015 in 2 In practice: Maximum possible cross section, large-area conductive mounting, low-inductive (therefore, a rectangle is better than a round conductor). 6
Comprehensive potential equalization connections 1 Plain washer and spring lock washer 2 3 Screw connection Spring lock washer 7
Comprehensive potential equalization connections 8
Comprehensive potential equalization connections 1 Green with yellow stripe conductor 9
Comprehensive potential equalization connections How to connect metallic cable tray to the enclosure 1 Conductive connection 2 Contact paint provides protection against corrosion 10
Comprehensive potential equalization connections Conductive connection of metallic cable tray 11
Comprehensive potential equalization connections L < About 2 A 0.015 in 2 Conductive connection of metallic cable ducts 12
Comprehensive potential equalization connections Conductive attachment of metallic cable trays 13
Comprehensive potential equalization connections L < About 2 A 0.015 in 2 Conductive connection between machine and system components 14
Functional enclosure layout / spatial division 1 High outputs 2 Low outputs 15
Functional enclosure layout / spatial division 1 High outputs 2 3 Low outputs Inputs 16
Measures within the enclosure 1 High outputs 2 3 4 5 6 7 8 Low outputs More powerful Components Mains Dividing area Actuators Transducers, probes, Detectors Shielded enclosures Protect sensitive assemblies by means of encapsulation, shielded case / sub rack within the enclosure 17
Measures within the enclosure for optimum shielding effect 1 Shielded viewing window, as small as possible 2 3 4 5 6 7 8 Conductive seal between the enclosure and removable enclosure panels Enclosure within an enclosure Potential equalization via suitable rails or conductive mounting plate Air-conditioning apertures with RF filters Actuators Mains filter/overvoltage protection at the point of entry, with large-area contact Unshielded control cables via filter openings conductively connected to the enclosure entry point Shielded cables via EMC cable glands 18
Optimum potential equalization of enclosure surfaces L < About 2 A 0.015 in 2 19
Enclosure back panel as a potential equalization surface 1 Conductive mounting plate 2 3 Long PE conductor Painted sheet metal All components with a conductive housing can be conductively mounted with a large contact area. L < About 2 A 0.015 in 2 20
Shielding interface with the enclosure 1 5 Ideal to have all-round EMC cable glands Conductive metal sheet 2 5 EMC gland plates Conductive metal sheet 3 5 EMC shield bus Conductive metal sheet 4 5 Potential equalization on the mounting plate Conductive metal sheet Solder ring terminal Tin plated braiding Soldered earth strap Cable shields should be contacted directly at the point of cable entry, where possible. 21
Filters Important: Spatially separate input and output cables from one another; use as shielded cables 1 2 3 4 5 Conductive metal sheet Control assembly Output: To actuator or machine Filter Power supply 22
Filters 1 Conductive metal sheet 2 3 4 5 Control assembly Output: To actuator or machine Filter Power supply 23
Filters 1 2 3 Conductive metal sheet Filter Power supply Important: Ensure there is large-area conductive connection between the filter housing and the mounting surface; avoid cable loops to the reference potential. 24
Transformers 1 Conductive metal sheet 2 Plastic material or painted Position transformers on gland plates with large-area conduction, conductive connection of the shield 25
Cabling rules, cable selection, and routing Immunity to interference of connected equipment Shielded, twisted cable pair Shielded, twisted cable pair with extra shielding Unused conductor Multi-conductor wire Unshielded, twisted cable pair 26
Cabling rules, cable selection, and routing Immunity to interference of connected equipment Unshielded multi-conductor cable Shielded multi-conductor cable Shielded multi-conductor cable, in metal conduit or in metallic cable tray 27
Cabling routing between panels and machine/system parts Steel conduit Steel cable tray Steel cable tray or Sheet steel supports Sheathings, open laying of bus cables etc. Surface cabling with cable clamps or other fastening components PVC plastic conduit, surface mounted 28
Cabling routing in tray selection / population Closed metallic cable tray Plastic cable tray Areas with effective shielding against electromagnetic fields Open metal cable tray Open metal mounting angle 29
Cables emission and interference sensitivity in cable trays Unshielded cable D Large as possible Immunity to Interference Interference Emission Unshielded cable Very sensitive High Shielded cable Sensitive Low 30
Potential equalization of shielded cable l= Lambda: Wavelength of the maximum frequency occurring in the vicinity 1 Reference potential (machine chassis or separate rail) 2 Reference potential or potential equalization rail with connection to the chassis Cable shield with potential equalization on one side Cable shield without potential equalization contact 31
Cable routing Connect unused conductor to reference potential 1 2 3 Output Control Signals Keep cables perpendicular wherever possible, and ensure adequate distance between interference-emitting and sensitive cables. 32
Information from Rittal: Practical assembly tips 33