Electromagnetic Interference (EMI)
Electromagnetic Interference (EMI) is the general term used for electrical signals that interfere with the normal operation of electronic equipment. All electric and electronic systems and equipment generate some kind of signal that could potentially interfere with the normal operation of another nearby piece of equipment.
Due to the number of electronic devices used within construction, EMI is on the increase and should be taken seriously, especially where safety critical instrumentation is concerned. All Geotechnical & Structural Instruments should be designed and certified to comply to international regulations such as CE, FCC and UKCA.
Electromagnetic interference (EMI) is an electromagnetic emission that causes a disturbance in another piece of electrical equipment. EMI can be attributed to a wide span of the electromagnetic spectrum including radio, DC and even microwave frequencies. Anything that carries rapidly changing electrical currents can produce electromagnetic emissions and it is possible for one object’s emissions to “interfere” with another’s. EMI compromises the performance of instrumentation by affecting data and sometimes leading to the loss of data.
Due to the number of electronic devices used within construction, EMI is on the increase and should be taken seriously, especially where safety critical instrumentation is concerned
Sources of EMI
EMI comes from two main sources.
- Conducted EMI caused by direct physical contact with a conductor
- Radiated EMI induction, without physical contact. While nearly every electrical device (electric motors, Bluetooth devices, cell phones) creates emissions that can cause EMI, electric power cords and high-speed cabling are the most common sources of radiated EMI.
How to protect against EMI – Design your instruments to be EMC Compliant
EMC stands for Electromagnetic Compatibility. This means that a product has been designed so as not to be interfered with by other products as a result of EMI, whether it is radiated or conducted. In other words, it is not affected by noise, in the form of electromagnetic radiation, coming from another source that can cause a product to malfunction. Creating a Faraday cage within your product can solve this problem. Where necessary, Geosense instruments are built as a Faraday cage.
Faraday cages are metallic enclosures that surround a product that is radiating energy in the EMI band. The Faraday cage reflects this energy back into the product but rarely absorbs it, and it is the ultimate method for containing EMI.
Testing depends on the type of product and environment
Different levels of EMC testing
In general, Emission limits are lower in the Residential, Commercial and Light Industrial Environment and the immunity requirements are higher in the Industrial Environment as outlined below.
EN 61000-6-1 – Immunity requirements for the Residential, Commercial and Light Industrial Environment
EN 61000-6-2 – Immunity requirements for the Industrial Environment
EN 61000-6-3 – Emission requirements for the Residential, Commercial and Light Industrial Environment
EN 61000-6-4 – Emission requirements for the Industrial Environment
All Geosense MEMS sensors are tested to these standards and also in accordance with London Underground Standards S1222 and G222 so you can be assured of their performance even in high EMI environments.
Cables are the biggest source of EMI as they can become an antenna
Geotechnical Instrumentation often requires long lengths of cabling and whilst the instrument itself may be EMC Compliant the addition of some types of cables may introduce EMI.
360 degree shielding
For the most effective shielding, cable shields need to be circumferentially bonded to connector back-shells in a 360-degree manner, and in turn, through the connectors to the equipment chassis at each connector.
All interfaces should be provided with connectors capable of bonding to double over-braid shielded cables. Connectors must provide electromagnetic shielding and allow 360-degree circumferential bonding from the cable connector body through to the equipment chassis. The maximum mated resistance between the cable connector body and the equipment chassis should be less than 2.5 milliohms. Additionally, high quality bonding of the connector to the interface is standard procedure as it is imperative for maximum performance of filters and filtered connectors.
All electronic instruments manufactured by Geosense, and in particular MEMS sensors, are all designed on the Faraday cage principle with fully shielded cables and 360-degree terminations and have been independently tested to prove EMC Compliance for CE marking to Industrial Test standards.
All electronic instruments manufactured by Geosense are designed on the Faraday cage principle with fully shielded cables and 360-degree terminations and have been independently tested to prove EMC compliance
Typical questions asked about EMI & shielding
Q. What can cause EMI in a cable?
A. Electromagnetic interference (EMI) is caused when the flux lines of a strong magnetic field produced by a power conductor cut other nearby conductors and cause induced voltages to appear across them. When signal cables are involved in the EMI process, this causes a noise in signal circuits.
Q. What is the purpose of shielded cable?
A. Shielding reduces electrical noise and reduces its impact on signals and also lowers electromagnetic radiation. Shielding prevents crosstalk between cables near each other.
Q. Which type of shielding is the best?
A. Braided shielding generally will provide between 70% to 95% protection from EMI, depending on how tightly the braid is woven. Braided shields are more effective than foil shields due to the fact the copper has a higher rate of conductivity than aluminium and because they are less prone to internal damage and are easier to make 360-degree terminations.
Another way to reduce magnetically induced interference is to use twisted pair wires. This applies both for shielded and unshielded cables and for interference caused by shield currents or from other sources. Twisting the wires forces them close together, reducing the loop area and therefore the induced voltage. Having braided shielding and twisted pairs provides the maximum protection against EMI.
Q. Should shielded cable be grounded at both ends?
A. The best way to wire shielded cables for screening is to ground the shield at both ends of the cable. Traditionally there existed a rule of thumb to ground the shield at one end only, to avoid ground loops. Best practice is to ground at both ends, but there is a possibility of ground loops.
Q. What is the difference between shielding & grounding
A. Grounding means to connect electrical equipment to a common reference ground or earth. Shielding is used both for immunity (protecting against external interference) and emission (preventing interference to be radiated).
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