The designers allow a reduction of the angle as the protective device – ground wire, mast, or rod – height increases because those angles may be inadequate for tall structures. The resultant shielding might not protect the objects for strike currents I2 < I1 with a shorter striking distance, but will likely guard the items for strike currents I3 > I1 with a more considerable striking distance. Figure 7 shows the same sphere with a lower structure. In case of necessity for functional reasons, other grounding point may be installed through small ceramic capacitors, lower than 1 nF and to 1500V, as long as the total of the capacitances does not exceed 10 nF. When referring to shield and grounding, in practice there are other ways of handling this subject, one with much controversy, as for example, the shield grounding can be made in each station with the sub D 9-pin connector (figure 34), where the connector housing makes contact with the shield and is grounded when connecting with the station. While most manufacturers take due precaution on their projects to minimize this effect, the complete immunity is not attainable. This capacitance effect between two bodies with electric loads separated by a dielectric is called mutual capacitance effect. This is very common in industries and factories, where EMI is fairly frequent in function of the larger use of machines such as welding instruments, motors (MCCs) and in digital networks and computers in the vicinity of these areas. This year, we’re adding an article on simulating electromagnetic shielding for aeronautical applications. Another additional manner is to ground the equipment junction boxes and housings on a ground equipotential line on the non-safe side. 1 Multipoint grounding is permissible if connections are made to an equipotential ground plane dimensioned to help avoid cable shield damage in the event of power system short-circuit currents.. Use a shielded twisted cable or a triaxial cable on low frequencies. The device may be comprised of a plurality of individually mountable shell-like housings which are joined in mating contiguous relation to surround the lower terminus of a control rod drive. The angle α describes an inclined line that limits the protective zone. For those on the Scottish shielding list, the new Scottish tier system includes new guidance. The shield must cover entirely the electric circuits through the connectors, couplings, splices and junction and distribution boxes. The protection may not be complete if the equipment is beyond the device’s protective zone. Limit the cable length that runs in parallel, Increase the distance between the disturber cable and the victim cable, Ground one of the shield ends on both sides, Reduce the disturber signal on the dv/dt by increasing the signal peak whenever possible (lower the signal frequency). The currents are distributed in function of their frequencies, because they follow the path with least impedance. Make sure that the plates and the coupling joints are made with the same material as the cable ducts/boxes. See figure 33. and shielding diagramearly in the development phase of a space project and to continuously update it further on. Another arrangement employs self-sustaining masts inside and next to the substation. The protection of a substation against direct lightning strikes consists of providing secure conducting paths to carry the lightning currents to the ground without damaging equipment and jeopardizing personnel. When having TE and PE terminals, ground them separately at the closest panel point using a twisted loop. The most common purpose is to prevent electromagnetic interference (EMI) from affecting sensitive electronics. Figure 5 – Example of how important are grounding and equipotentialization and their influence on the signal. Individual standard conductors that meet the general type specifications concerning temperature, gauge and applicable codes are acceptable, provided they are distant from high voltage cables to minimize the coupling noise. Not only to assure proper operation, bu… Figure 15 shows an example of protection against transients. Jan 1968 R G Jaeger The rolling sphere method allows us to determine the protective cone and the equivalent protective angle of a mast or rod. The neutral conductor works basically as a means of conducting system return currents. It typically functions as the Faraday cage and protects against lightning. Ground connectors must be classified according to the maximum capacity of the inverter current. jan. 27, 1970 e. johnson 3,492,568 . Depending on the location of the stepped leaders, the shielding will capture some of these strikes. The “disturber cable” and the “victim cable” are followed by a magnetic field. All closet/cabinet metal parts should be electrically connected to the larger contact area. Within each system, the individual subsystems are single-point grounded. Therefore if the coating compound underneath the backing is damaged, and a holiday is created in the corrosion coating, the backing will not shield the passage of protective currents. These provide protection to peak currents and offer a deviation path of low impedance for the ground point. The mesh provides multiple ways for the lightning current to flow to the ground. This is critical in inverters with nominal power of 200 HP (150 KW) and keep the power source cables and the control cables separated. The lowest level is 0 and the highest level is 4. It uses a fictitious sphere of radius S to locate the lightning protection devices on structures. eur-lex.europa.eu. As high frequency inverters (up to 300MHz) generate harmonics through the commuting amplifiers on these frequency levels, the grounding system resembles more a series of inductors and capacitors rather than a low resistance channel. This may deteriorate the dielectric, whose width is limited by the capacitor operation voltage, which may produce a gradient of potential inferior to the dielectric rigidity of the material, causing malfunctioning and even the capacitor burning. A typical example of how the EMI may affect the work of an electronic component is a capacitor exposed to a voltage peak higher than its specified nominal voltage. The voltage difference between both ends should be a maximum of 1 V (rms) to minimize the ground loop effects. This will keep the noise in common mode away from the PLC backplane. When considering the question of shield and grounding on fied buses, take into account: According to IEC 61158-2, to ground means to be permanently connected to the ground by a sufficiently low impedance and with enough conductive capacity to prevent any voltage from causing damages to equipment or persons. Don't have an EEPower account? The one-pair wire is wound in spiral and through the cancellation effect reduce the noise and keep the medium electric properties constant through its whole length. The coexistence of equipment of different technologies and the inadequacy of the installations favors the emission of electro-magnetic energy and often causes problems of electro-magnetic compatibility. This condition is the method’s drawback because field observations on buildings show that most strikes finish on sharp points or projecting corners. Retest If any gas fails to meet its requirements, that test shall be repeated twice. EMI (Electromagnetic Interference) is a process by which disruptive electromagnetic energy is transmitted from one electronic device to another via radiated or conducted paths, or both. The coupling is a system problem involving the … To keep them apart, direct them through a separated duct or use box divisions. GPs demand clarity on 'trigger point' for return of COVID-19 shielding. Every circuit must have a conductor for protection all through its length. This is the effect of metal plates and screens on the incidence of electromagnetic waves; they generate their own fields that minimize or even nullify the field through them, therefore working as true shields against electromagnetic waves. EM shielding is conducted for several reasons. The Scottish government guidance continues to be that everyone who can work from home, should do so. Grounding systems must execute multiple simultaneous functions: provide personal safety and protect the equipment. All this may provoke alterations with the resulting overload, sub-voltage, peaks, voltage transients etc., which may cause high impact on a communication network. Never install a device that has been previously used without an intrinsically safe barrier on an intrinsically safe system, as the protection zener may be burned and will not work in instrinsically safe areas. One of the main goals of a project is to keep all common signal return points on the same potential. On high frequencies when the cable length is longer than 1/20 of the wave length of the noise fequency, the shield will present high sensibility to noise and the grounding on both shield ends is recommended. In these cases, it is very important that the differences in ground potential in both connection points are the minimum possible. Discrete I/O as the start and stop commands may be connected to the inverter with several cables. Special cables: another important detail to help minimize the effects of the electromagnetic noises generated in installations with inverters and AC motors is the use of special cables to avoid the corona effect of discharges that may deteriorate the dielectric rigidity of the insulation and allow the presence of stationary waves and noises on the ground loops. Shielding plays an essential role in maximizing the effectiveness of cable systems and is designed to minimize signal leakage and the reception of signals produced by external sources. Long and parallel cables work as a large capacitor. The cable gauge specs and the recommendations are normally based on 75º C. Do not reduce the wire gauge when using a wire for higher temperature. Never use the shield as a signal conductor. An intrinsically safe circuit must float or be connected to the equipotantial system associated with the classified area on only one point. A common error is the use of protection ground as signal ground. It uses the protective angle for the location of grounding wires, masts, and rods. The overlapping of the protective zones decreases the likelihood of direct impacts (Figure 4). In fact, shielding and grounding problems are frequently responsible for measurement errors, but many test system developers aren’t quite sure why. The length of the conductor extending outside the shield should be minimized and guarantee good connection between the ground and the shield. Braden Shielding Systems services include the manufacture, installation and independent 3rd-party testing of your Radio Frequency Shielded enclosure. Therefore, only use trained and qualified professionals on installation, operation and maintenance jobs. Whenever possible refer to the physical regulations, as well as the safety practices for each area. This article provides information and tips on grounding but in case of doubt the local regulations always prevail. In case of discrepancies, the norms, standards, profiles, technical guides and manufacturer manuals will prevail. The rolling sphere method applies equally to flat surfaces, sharp points, edges, and corners. All motors activated by inverters must be powered preferably by shielded cables grounded on both ends. Line reactors are a simple and inexpensive way to increase the impedance of an isolated load source (as a variable frequency command, in the case of inverters). Such substations should be protected from direct lightning strikes and from traveling waves arriving through the overhead lines. Common high frequency conductors have the disadvantage of having high impedance and current loops should be avoided. Radiation shielding may be described as an interaction between particles and matter through collisions and capture by atoms. Note that the protective cone and angle figures depend on the device’s height for a constant sphere radius. In this instance, the shield is insulated on all grounds, with exception to the negative ground point of the power source or the intrinsic safety barrier on the safe side. Get well nformed about them before starting work. Burning of electronic components without apparent motive, even on reliable equipment. This is known as the polarization principle. When installing the equipment, normally the housing is in contact with the structural part or piping and consequently is grounded. Wave Guide Air Vents. Avoid unnecessary wiring to reduce coupling capacitances and inductances. The impedance of the victim circuit input (circuits of high input impedance are more vulnerable). Do not bring the network cable close to the power source and output cables of the inverters, to avoid the common-mode current. Following is the use of metal ducts to minimize Foucault currents. Or, still, the transistor polarization currents may be altered and cause their saturation or cut, or burn its components by the joule effect, depending on the intensity. Figure 24. Notice the unprotected portions of equipment inside the sphere. In addition, EM shielding often is required to limit EM radiation according to various standards. Use RFI filter and always connect it the closest possible to the noise source (between the RFI filter and the drive). Each of the system ground points is then connected to the tree ground bus with a single insulated conductor. Most probably this commuting noise may produce intermittence in nearby equipment. All the housings must be connected to the protection conductor. The inverter and the automation equipment should not be connected directly on the same power source. The results of both retests shall meet the requirement of this specification. The victim cable insulation (cable insulation εr), mainly for strongly coupled cable pairs. The main effects of inadequate grounding are electric shocks to users through contact, low or intermittent response from the protection systems, such as fuses, circuit breakers etc.). To prevent radiator from a conductor, a grounded shield on both sides is generally used above the cut frequency, but some precautions must be taken. There is no protection against ground loops. However, this is true only at the levels of the shield transfer function, but not at the system level shielding effectiveness. The reactors are connected in series to the load that generates harmonics and, by increasing the source impedance, the magnitude of the harmonic distortion may be reduced to the load where the reactor is added. Electrical Grounding – Alexandre Capelli, Revista Saber Eletrônica, Edition 329, 2000, EMC for Systems and Installations - Part 2 – EMC techniques for installations, Eur Ing Keith Armstrong, The benefits of applying IEC 61000-5-2 to cable screen bonding and earthing, Eur Ing Keith Armstrong, Borges, Giovanni Hummel - Intrinsic Safety Manual, Sanches, Durval - Electromagnetic Interference, Noises and Interferences in PROFIBUS Installations. The implications of poor or even inadequate grounding are not limited to safety aspects. Most lightning currents go through the wires and grounding electrodes close to the impact point. If using a PE ground wire on the panel, connect it on the same side as the wire duct/housing connections. Figure 30 – Shield and Ground Ideal Combination. The ground wires of the incoming and outgoing overhead lines should terminate at the top of the substation’s steel structure — this might require steel columns extending above the main structure. 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