EMC Testing and Standards in Transient Immunity Testing - Assignment Example

Introduction A control room in a manufacturing industry is susceptible to data loss due to power supply failures that might be accidental or malicious. It is therefore necessary to ensure that Electromagnetic compatibility (EMC) has been adhered to. EMC refers to the capability of an electronic component or system to work optimally without any fault in the electromagnetic sense. There are many techniques and good practice controls that should be used in the design and management of the control room since very high frequency emissions are expected. These emissions are not desirable since they will either ultimately interfere with the local reception or have an impact on the external reception (Keenan, 2003). Historical and legal perspective As early as 1979, the United States government started handling complaints from people that were complaining about their television receivers being interfered with other signals from computers and radio. The arm that was responsible for receiving and processing these complaints was the federal communications commission (FCC). The FCC decided to establish regulations to guide them and system designers in order to curb future EMIs. There are various digital devices that are subject to the codes established by the FCC where a digital device is any unintentional device or system which produces and uses timing pulses in the excess of nine thousand pulses per second. This definition covers anything from a computer and a typewriter to laser printers (Fassbinder, 2002). The regulations put a limit to acceptable emissions that can be generated by a digital device. A conducted emission is said to be the emission that exits a power cord of a device and the frequency is typically in the range of 150 kHz to 30 MHz the FCC compliance is required to be displayed on the device (usually a sticker on the back of the device) and devices that don’t meet this criteria cannot be sold in the United Sates. A violation of these regulations can lead the FCC to invoke their statutory powers and the violator can be jailed or fined heavily. The implication of this is that designing of a system that fails to keep in mind these standards might be cheap to produce but eventually expensive in the long run since it would be impossible to market a noncompliant device. It is therefore mandatory for the system designers to ensure that the electronic systems they develop are compliant (Keenan, 2003). It is however important to note that the statutory requirements for controlling electronic noise is not unique to the USA. Most other countries subscribe to the same ideology. In fact, some may even have more stringent requirements to limit digital device emissions. In short, electromagnetic compatibility is a global issue. For instance, the FCC can be seen as a development based on the foundation of the European limits of radiation emissions. The limits can be traced back to the CISR which is a French acronym that is translated to mean International Special Committee on Radio Interference. CISPR is one of the committees of the International Electromagnetic Commission (IEC), which is an international body mandated with the task of ensuring that standards are upheld by countries in order to facilitate cross border and overseas trade. The CISPR 22 is a standard that was published by CISPR to describe how information technology systems and other relevant digital devices ought to be designed. This is perhaps the most reused standard with most countries adopting the standard not only in the region of Europe but also in the entire world (Fassbinder, 2002) Elementary Coupling A simple system can be used to describe the principles that will be important in the design of the control room. The simple system will have three parts, the source, the cause of the interference, and some means of coupling. The figure below illustrates this:- The source maybe anything from an electrical wire to antennas of a wireless Local Area Network. Coupling will be realized by the current should common conductors from different circuits share the electromagnetic field. Drain of EM disturbance could be any type of apparatus or even any part of the electrical system. It is important to develop a matrix that has all the possible coupling paths, sources as well as the likely disturbed objects. The matrix will identify four types of EMI that should be of concern in our system namely impedance, inductive, capacitive, and radiative coupling (Kodali, 1996). A manufacturing factory is bound to have high electronic noisethanks to inductive coupling followed by capacitive coupling and impedance coupling. It is also expected that the heightened use of wireless media in the future will significantly cause an increase in the EMI from this source (Kodali, 1996). Impedance coupling When different circuits use common coupling impedances and common lines, it will result in galvanic coupling. An example of this case would be when different circuits use the same power source in the circuit. Should the current and coupling impedance be significantly large, then the voltage will be large enough as opposed to the disturbance Inductive coupling How strong inductive coupling is will largely depend on three parameters namely, how strong the disturbing current is, how far or near the source and drain are located and last but not least, the frequency of the disturbing field. There are many scenarios that could affect the size of the disturbing signal. For starters, if the currents of the external circuits are large, then the signal will be significantly be larger. An unbalanced go-and-return current will also enlarge the signal. Other factors include the surface area of the circuit as well as the variation of the signals. Ideally, a high variation in the signals of the external circuit will impact on the size of the disturbing signal. Should the coaxial cables be done properly, inductive coupling can be instrumental in preventing or controlling of disturbance (Rabindra, 1996). Capacitive coupling The factors that affect the disturbance in the inductive coupling remain true for capacitive coupling as well. An example would be cables of a LAN that are running in the same tray as the power cables. Assuming that the power cables have a power sine in the range of 50Hz to 230Hz, the disturbing signal will reach an average amplitude of 10v- which can be said to be a negligible voltage. The presence of high frequency components in the power cables will inflate the amplitude to well over 90V and that will lead to malfunctions in t he LAN. The implication of this is that proper planning can either reduce the EMIs to acceptable levels or even eliminate them altogether (Nelson, 2011). Radiation coupling Electromagnetic fields will usually travel in space with the same velocity as light. This means that they can impact on installations not only near the source but also in environments that are far from the source. Cell phones, radio and TV transmitters are good examples of electromagnetic fields. The fast signals have some high frequency parts and this is what leads to the radiation of electromagnetic fields by cables. There is also a possibility of some components of the system to act as antennae that eventually broadcast radiation. Using the hertz dipole will be instrumental in the detection or estimation of the magnitude of the radiation (Nelson, 2011). Preventive measures It is advisable to identify and alleviate possible EMI related disturbances in the control room. The first solution is using grounding. A good connection is needed to ensure that the currents are returned back to the circuit without causing unnecessary electronic noise. Using a flat braided cable will yield lower impedance as opposed to using a rounded one. A Variable Frequency Drive (VFD) and a motor can be used to achieve this. Cable routing can be used as a way of avoiding AC line wiring, motor wiring and parallel signal wiring. Cable rerouting should be done in way that avoids routing via free air. In cases where parallel routing cannot be avoided, it is advisable to try to separate the cables (e.g. to have a 6-8 inch space in between). Another way to achieve this would be by using grounded conductive partition (Sue, 2011; Rabindra, 1996). When laying of the cables in the control room, it is important to use the proper cabling. Twisted-pair cables as well as cable shielded twisted-pair cables that have especially been designed to counter any possibility of electromagnetic interference. It is advisable to avoid using the single conductor six hundred volt cables as they have been known to provide the least protection against electromagnetic interference. Caution should be taken when terminating the shielded cables as terminating it at the pigtail will ultimately increase the impendence thereby affecting the effectiveness of the shield (Nelson, 2011). Motor cable selection is another possible remedy. Motor conductors should be one of the most important considerations when handling EMI related noise. Using single conductor wires will provide one with very little protection from interference. Installation of shielded cable media is the surest way to guard against electromagnetic interference. The shield works by making the electronic noise to flow back before it contaminates the power network. One difference this approach has from the signal wiring is the fact that shielding on the motor cabling will be terminated on both of the ends. In the event that a shielded motor cable is not available, a three phase conductor with a ground connected in conduit will serve the same purpose. However, this technique may have some limitations owing to the contact the conduit has with various parts of the system (Nelson, 2011). Another practical solution for the control room is insulation. Using of a shielded room helps to avoid any possible elctronic contaminations to the original signal. For instance, the walls of the control room can be lined with carbon-impregnated foam in order to provide walls that will absorb most if not all of the electronic noise (Ott, 2008). On adition to this, the floor of the control room should be typically a grounded plane. This precautions will make sure that only the interference that emanate from the power cords of the system conducted. It is possible to measure this using line impendence stabilization network (LISN). LISN will help maintain constant impedance between the neutral and the ground as well as between the phase and ground. Another function of the LISN is to eliminate interference on the commercial power net from contaminating of the test (Electronics project design, 2011). Spread spectrum should also be used to reduce the emissions. Ideally, spread spectrum works by modulating the clock signal. The rationale is to provide a lesser value of measurement on the CISPR16 (as well as the FCC) scale(s) (Ott, 2008). The fact that this mechanism uses the bandwidths and derivatives of the time constant makes it a bit complex but nonetheless, it is a standard that is accepted by both Europe and America. In order to preserve the integrity of the smoothness of the clock, the rates of modulation take place in the audio band (Nelson, 2011). In order to attain optimal stability in the circuits used in the room, it is imperative to use capacitive loads that have been buffered up with some resistance. An integrator feedback will typically use a resistor in series with all the integrator capacitors that are bigger than 10pF. It is more advisable to use passive filters outside the feedback loops as opposed to using the RF (Electronics project design, 2011). It is also important to ensure that the connections have been secured by applying passive filters or any other relevant suppression techniques. It is expected that any digital circuitry in the control room will exhibit some level of noise on all of the internal interconnections. The appropriate filtering mechanisms can be used together with galvanic isolation to ensure there is protection from interference. However, while input and output filters are often good solutions in the external cables, they may not be optimal solutions where opamps are interconnected with other opamps via PCB traces. All wired connections in the unshielded enclosures will need filtering as a result of the antenna effect (Ott, 2008; Learn EMC, 2011). Also, comparators must have hysteresis (positive feedback) to prevent false output transitions due to noise and interference, also to prevent oscillation near to the trip point. Faster output-slewing comparators than are really necessary (i.e. keep their dV/dt low) should be avoided. Some analogue ICs themselves are particularly susceptible to radiated fields. They may benefit from being shielded by their own little metal box soldered to the PCB ground plane (take care to provide adequate heat dissipation too). It is also worth noting that most of the curricula of electrical engineering don’t address the electromagnetic compatibility issues. This means that most engineers graduate from college without enough knowledge on what electromagnetic compatibility really is and the implications it has on the design and distribution process of a system. The engineers will in most cases learn the importance of electromagnetic compatibility ad hoc while in the job market. It would therefore be useful if the present day universities and colleges revised their curriculum to include electromagnetic compatibility as one of the modules they teach the upcoming engineers as this will go a long way into ensuring the next generation engineers will comply with the electromagnetic compatibility regulations (Rabindra, 1996; Nelson, 2011). Conclusion It is only natural that a control room in a manufacturing factory will be highly susceptible to electronic noise. With this in mind, it is important to ensure that all principles of good practice as far as electromagenetic compatibility have been taken into consideration. In addition to this, various precautionary measures like insuklatign the control room as well as restricting access to it should be taken into account too. While it is possible to comp[ly with the elctronomagnetic compatibility after the completion of setting up the control roomIt is important to think of the electromagnetic compatibility principles during the design , it is also true that it will cost much less to have it desingend into the ssytem from the word go.It has been established that only a mere fifteen percent of the products that haven’t been designed with electromagnetic compatibility principles in mind pass the EMC testing (Paul, 2007; Keiser, 2007). Electrodynamics usually provides the basic foundation for all electromagnetic interference. When operating at low frequencies, the magnetic fields will often operate independently. However, when at high frequencies, the only significant field is the one propagating the electromagnetic fields References Electronics project design. 2011.EMC Testing and Standards in Transient Immunity Testing, RF Immunity. [online]. Available at: < http://www.electronics-project- design.com/EMCTesting.html> [accessed Jan. 8, 2012 Fassbinder, S., 2002. Disturbances of the Power Supply Network by Active and Passive Components (in German), VDE, Verlag Keenan K.,2003 Digital design for interference specifications. Vienna, Va: Keenan Corp. Keiser, B., 2007. Principles of EMC, New Jersey: Artech House, 1987 Kodali, V., 1996. Engineering Electromagnetic Compatibility, Washington: IEEE Press Learn EMC. 2011. Common impedence cpupling. [online] Availble at: [Accessed Jan. 8, 2012] Mardiguian, M. 1992.Controlling radiated emissions by design. New York : Van Nostrand Reinhold. Nelson, W. Ed., 2001. Interference handbook. New York: Radio Publications.  Ott H., 2008. Noise Reduction Techniques in Electronic Systems, New York: A Wiley Paul, C. 1992. Introduction to Electromagnetic Compatibility, New York: John Wiley Rabindra N., G., 1996. Interference mitigation: theory and application New York: IEEE Press. Sue, M.K. 2011.Radio frequency interference at the geostationary orbit. NASA. Jet Propulsion Laboratory. [Online] accessed on I5th December 2011. Read More