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Medical approaches, for example from Asia, make it clear to us how important it is not to look at individual symptoms, but the entire system. Often, it's the only way to find and reasonably treat causes instead of fiddling with symptoms. This also applies to automation technology with its increasingly complex systems. The post shows how Indu-Sol with an eye for the whole and one comprehensive diagnosiswhich improves the reliability and quality of communication and thus of a whole system.

Many years ago, the experts for bus communication specialized in the development of systems for fieldbus diagnostics and related services. The focus is on the permanent network monitoring with the aim of warning of failure. For this purpose, the bus was virtually missed a long-term ECG. While in the past it had to be said that the bus works or not, in the meantime one can find out in a very differentiated way which problems exist where or could occur in the foreseeable future. This is also the case today if the cause of communication problems does not arise in the bus itself. While the doctor asks the patient when evaluating a long-term ECG what he has done or eaten at times with unusual readings, it would also be interesting to analyze how other systems in the bus environment are behaving at this point in the analysis of communication problems on the bus to have. Often there are the causes of the problem.

Screen currents and their causes

Indusol10114Since time immemorial, one has to deal with screen currents in automation systems. That these can occur is not new knowledge. True to the motto: "What I do not know does not make me hot," umbrella currents are given little attention. Measurements in practice show, however, that today screen currents are not infrequently 500 mA or even in the single-digit A range and are classified in the kHz range. How does this happen?

Indu-Sol sees great discrepancies in machines or systems that have been upgraded in recent years or modernized in the field of automation. The I / O device is now decentralized in the last corner of the system and the drive technology has been switched to energy-saving frequency inverters. Both are absolutely important, but you have left the potential equalization completely untouched in the modernization idea.

In the DIN VDE 50310 it is explicitly pointed out that in buildings with information technology equipment an improved equipotential bonding (at least improved type A) has to be carried out. The improvement is that in addition to the typical star structure additional equipotential bonding connections between the locations of electrical and electronic devices are manufactured. But also with newly planned machines or plants problems can occur, if one does not observe some important principles. Within the supply line for high-frequency consumers, high-frequency currents are coupled into the PE conductors, which then want to return to the source of generation. Theoretically, this happens via the equipotential bonding.

However, it becomes problematic in practice if there is a Profibus subscriber in the vicinity of the drive whose feed line is at ground potential on both sides. This installation is absolutely correct, because only then the screen functionality is fully functional. But there is a downside to be considered here: Since the shield and protective conductor now lie on the same end point and in most cases run parallel to each other, according to the current divider rule, the 'unwanted' current also takes the path via the shield of the Profibus line as the return path and not just the potential equalization connection. Because high-frequency leakage currents do not take the path of least ohmic resistance but always the path of lowest impedance.

Symmetrical motor cables

Indusol20114According to DIN VDE 0100-540 / DIN EN 61140 protective earth currents (PE) of permanently closed equipment with a rated current of consumables above 20 A may reach a maximum of 10 mA. However, practical measurements show that PE currents of up to 10% of the phase current are not uncommon. Since such high shield currents can be dangerous for machine or plant and bus communication, there is an urgent need for action. Instead of dissipating the shield currents via secondary solutions such as shield clamps, it would make more sense to prevent their formation at such a height from the outset.

In conventional motor cables, the protective conductor runs parallel to the phases L1, L2 and L3, In an alternative design of the motor cables offered by the bus experts, the PE was divided into three lines. The capacitive and inductive coupling is thus distributed over three lines whose phases are shifted by 120 ° to each other. As a result, the injected currents almost cancel each other out, namely more than 80%.

Indusol40114Such a design is not a novelty, but on the contrary actually recommended by all frequency converter manufacturers. This proven solution has only been forgotten in recent years. However, it is becoming increasingly interesting again today because frequency converters near bus modules quickly lead to the problems described above. In addition to the motor cables, the Schmöllner experts also offer an EMC service box containing all the necessary components for standard-compliant system wiring.

Improve concepts for equipotential bonding

Indusol50114The bus experts today dare to suggest that the reliability of the plant of the future will depend to a large extent on the quality of the low-voltage switchgear. Anyone who can not replace all motor cables or wants to consistently proceed correctly when planning a plant should think about its equipotential bonding. In old factories, the question must be allowed, as it has been ordered after all the years of operation around the foundation earth. At the same time, concepts for equipotential bonding are required, which ensure that the impedance of the equipotential bonding system is lower than the impedance of the shield. As a guideline, shield loop resistances of data lines such as bus cables should be at a maximum of approx. 0,6 ohms (impedance value at 2,2 kHz) and loop resistances of the equipotential bonding system (CBN) within a range of approx. 0,3 ohms (impedance value at 2.2 kHz). With a meshed equipotential bonding MESH-BN, electromagnetic interference can be significantly reduced. The smaller the mesh size, the better the function.

By using the "Emcheck MWMZ I" mesh resistance measuring pliers, for example, these values ​​can be determined and a statement about the equipotential bonding quality can easily be made. In order to spare the user the time-consuming search for the respective supplier, a catalog offers a selection of low-impedance compensating cables that comply with the applicable standards and guidelines.

Causes of EMC problems uncover

Indusol6a0114Those who want to avoid or remedy problems must know their causes. The long-term ECG of the Profibus mentioned at the beginning only helps a little if the cause of the communication disturbance can be identified elsewhere. Therefore, further measurements over a longer measurement period may be necessary, the results of which are then compared to reveal relationships. Automated long-term tests, for example with the "EMC Inspector V2", can show, among other things, the temporal behavior of the Profibus screen current, the current in the PE / PA system, the PE current in the motor cable and the course of the 24 VDC supply.

In addition to the necessary measurement and analysis tools that can be used to achieve stable bus communication by minimizing the effects of EMC, the experts also offer their broad know-how as services and in training courses. They know the standards and guidelines that specify useful limit values ​​and, incidentally, are not derived directly from plant engineering but, for example, from building services engineering.
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