In European law , manufacturers of electronic devices are advised to run EMC tests in order to comply with compulsory CE-labeling. More are given in the list of EMC directives. Electromagnetic noise is produced in the source due to rapid current and voltage changes, and spread via the coupling mechanisms described earlier.
Since breaking a coupling path is equally effective at either the start or the end of the path, many aspects of good EMC design practice apply equally to potential emitters and to potential victims.
Further, a circuit which easily couples energy to the outside world will equally easily couple energy in and will be susceptible. A single design improvement often reduces both emissions and susceptibility.
4.2 The importance of zoning in an installation
Grounding and shielding aim to reduce emissions or divert EMI away from the victim by providing an alternative, low-impedance path. Testing is required to confirm that a particular device meets the required standards. It divides broadly into emissions testing and susceptibility testing. Open-air test sites, or OATS, are the reference sites in most standards. They are especially useful for emissions testing of large equipment systems.
However RF testing of a physical prototype is most often carried out indoors, in a specialised EMC test chamber. Types of chamber include anechoic , reverberation and the gigahertz transverse electromagnetic cell GTEM cell. Sometimes computational electromagnetics simulations are used to test virtual models.
Like all compliance testing, it is important that the test equipment, including the test chamber or site and any software used, be properly calibrated and maintained. Typically, a given run of tests for a particular piece of equipment will require an EMC test plan and follow-up test report. The full test program may require the production of several such documents.
Emissions are typically measured for radiated field strength and where appropriate for conducted emissions along cables and wiring. Inductive magnetic and capacitive electric field strengths are near-field effects, and are only important if the device under test DUT is designed for location close to other electrical equipment. Typically a spectrum analyzer is used to measure the emission levels of the DUT across a wide band of frequencies frequency domain. These incorporate bandwidths and detectors as specified by international EMC standards.
EMC - Earthing principles and structures - Electrical Installation Guide
EMI receivers along with specified transducers can often be used for both conducted and radiated emissions. Pre-selector filters may also be used to reduce the effect of strong out-of-band signals on the front-end of the receiver. For radiated emission measurement, antennas are used as transducers. Typical antennas specified include dipole , biconical , log-periodic , double ridged guide and conical log-spiral designs.
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Radiated emissions must be measured in all directions around the DUT. Some pulse emissions are more usefully characterized using an oscilloscope to capture the pulse waveform in the time domain. Radiated field susceptibility testing typically involves a high-powered source of RF or EM pulse energy and a radiating antenna to direct the energy at the potential victim or device under test DUT. Conducted voltage and current susceptibility testing typically involves a high-powered signal or pulse generator, and a current clamp or other type of transformer to inject the test signal.
Transient immunity is used to test the immunity of the DUT against powerline disturbances including surges, lightning strikes and switching noise. Electrostatic discharge testing is typically performed with a piezo spark generator called an " ESD pistol ".
Higher energy pulses, such as lightning or nuclear EMP simulations, can require a large current clamp or a large antenna which completely surrounds the DUT. Some antennas are so large that they are located outdoors, and care must be taken not to cause an EMP hazard to the surrounding environment. Lightning rods or lightning conductors began to appear in the midth century. With the advent of widespread electricity generation and power supply lines from the late 19th century on, problems also arose with equipment short-circuit failure affecting the power supply, and with local fire and shock hazard when the power line was struck by lightning.
This approach is advised for general use, both in terms of safety and EMC. This recommendation does not exclude other special configurations that, when correctly maintained, are also suitable. Installation with multiple earth electrodes. In a typical installation for a multi-level building, each level should have its own earthing network generally a mesh and all the networks must be both interconnected and connected to the earth electrode. At least two connections are required built in redundancy to ensure that, if one conductor breaks, no section of the earthing network is isolated.
Practically speaking, more than two connections are made to obtain better symmetry in current flow, thus reducing differences in voltage and the overall impedance between the various levels in the building. The many parallel paths have different resonance frequencies.
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If one path has a high impedance, it is most probably shunted by another path with a different resonance frequency. On the whole, over a wide frequency spectrum dozens of Hz and MHz , a large number of paths results in a low-impedance system see Fig.
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Each room in the building should have earthing-network conductors for equipotential bonding of devices and systems, cableways, trunking systems and structures. This system can be reinforced by connecting metal pipes, gutters, supports, frames, etc. In certain special cases, such as control rooms or computers installed on false floors, ground reference plane or earthing strips in areas for electronic systems can be used to improve earthing of sensitive devices and protection interconnection cables.
Each level has a mesh and the meshes are interconnected at several points between levels. Certain ground-floor meshes are reinforced to meet the needs of certain areas. Personal tools Create account Log in.
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