Electronics & Scientific Guide, Measuring & Testing

Guideline on Earthing and Shielding of Automation Networks and Meteorological Verification

For a long time, there was no right and no wrong with the topic of earthing and shielding in industrial automation. A PI guideline should provide clarity. But what does the corresponding measurement look like?

Companies in almost all manufacturing industries are steadily increasing the level of automation in their value chain. As a result, the use of higher-frequency power electronics such as frequency converters is also increasing. Increased dynamics (control frequency) of the drives and communication networking in distributed I / Os are two further factors that increasingly place the Fieldbus-related electromagnetic compatibility (EMC) in the spotlight of industrial automation.

This is also shown in a recent study by Indu-Sol: In the so-called Vortex Report 2018, the Thuringian technology company is preparing the findings from more than 500 missions of its measurement engineers to evaluate the communication quality in industrial networks from the 2017 calendar year. One finding: In almost every fourth case, the main reasons for reduced quality of data transmission in the areas of EMC and equipotential bonding were. In the run-up to the trade fair, however, this was only suspected by the customer in 7% of the cases.

Policy released specifically for automation

Now there has been a pent-up demand from the proper side: The World Association Profibus & Profinet International (PI) published a guideline on functional equipotential bonding and shielding in Profibus and Profinet networks in the run-up to this year’s Hannover Messe. This is the first time that a Fieldbus user organization has made an attempt to comprehensively put the current state of technology on the subject of protection and functional grounding as well as protection and functional equipotential bonding in the area of mechanical engineering – and not without many “old braids “Have to cut off.

The document has succeeded in aligning the latest findings in the planning, construction and operation of automation systems with the current state of standardization to increase their EMC resistance and thus their availability. On this basis, the directive makes appropriate recommendations. In many cases, slight modifications to existing solutions and systems can already save material and installation costs, which can increase immunity to interference and maintain competitiveness.

Plan equipotential bonding

So it is worthwhile for companies not to let the equipotential bonding “live” on the construction site, but to think in advance of its construction or to get technical advice in this regard. The metrological practice shows very clearly that a strong meshing in the equipotential bonding system reduces its load and that of the devices and components. This is achieved by as many short connections between the devices and conductive elements as possible – the current splits and the load decreases. If bare conductors are used, contact with metal objects along the connection will even increase this effect.

To facilitate the effort involved in the construction of such a so-called Common Bonding Network (CBN), Indu-Sol has developed the fastening elements of the EmClots series in addition to a selection of conductor cables for a wide variety of purposes. This conductor connection and connection terminals are fastened to tracks or other electrically conductive equipment using their M6 × 9 screw connection to allow the connection, attachment and branching of bare conductors, depending on the design. The company attached importance to a quick and easy installation as well as a redetachability of the components in case of modifications or modifications.

Measure the success of the measures

If the equipotential bonding system is installed correctly, it must pass the functional test during operation. It is important that it not only fulfills the function of protection but also of the function potential equalization. For metrological verification, it is recommended to use a so-called mesh resistance clamp such as the check MWMZ II. With the tool designed especially for this purpose, the uninterruptible proof of a sufficient equipotential bonding by DIN EN 50310 can be performed. The following are guideline values: For a frequency range of 2.3 kHz, screen loop resistances of data cables such as bus cables should have an impedance value of approx. 0.6 ohms maximum, loop resistances of the equipotential bonding system (CBN) max. 0.3 ohms. This would ensure that the equipotential bonding is performed with lower impedance than the shield of the data lines. This should be the goal, in any case. Otherwise, there is a risk that higher-frequency leakage currents will flow back through the bus line instead of using the equipotential bonding system provided for this purpose. In this case, users should expect interference with data communication in the network.

In addition to a metrological check of the impedances, the height of the screen mentioned above currents should also be monitored during operation. If anything at all, this is often still done by a punctual measurement with the aid of a leakage current clamp. However, the load on the equipotential bonding system is not always constant; It changes, for example, during certain operations or the temporary connection of participants. For this reason, a permanent monitoring option should be provided.

The benefit of permanent monitoring during operation

Continuous monitoring of shield currents on data lines during operation has several advantages: Instead of a point-by-point measurement, which shows only a snapshot of the load on the equipotential bonding system, an analysis of the current profile also records temporary load peaks that would otherwise remain hidden. Especially in the case of troubleshooting, these historical data are valuable starting points to isolate the cause of possible impairments quickly. Care must be taken to ensure that the measuring instrument used does not only record the so-called root mean square (RMS) value since it represents only a quadratic mean calculated over time and is therefore inaccurate as a measurement result. Just a recording and representation of the exact current waveform provides accurate diagnostic and analysis data.

Anyone using Profinet or Industrial Ethernet as the automation network can gain an advantage: As the only switch so far, the mesh P9 not only fulfills the actual switch function of data distribution but also provides the service of leakage current monitoring in addition to numerous other diagnostic options. The continuous measurement over the entire frequency spectrum (20 kHz), as well as the recording of average values (RMS measurement) and peak values (peaks), make the causes and relationships understandable for EMC interference. If this leakage current exceeds a value of 200 mA (practical) and 400 mA (peak), this represents a risk for trouble-free communication and a targeted warning is issued.

As an alternative or for all other communication protocols, the measuring and diagnostic device EMV-INspektor V2 also has the mentioned analysis function as well as the option to monitor up to four current-carrying channels in parallel and permanently.

Trust is good; control is better

With the guideline “Functional Potential Equalization and Shielding of Profibus and Profinet” of PI there are written notes and experimental values for the structure of an equipotential bonding system specially tailored to the needs of automation.

Conscientious planning allows time and material to be kept within manageable limits, while at the same time achieving high EMC resistance of machines and systems. Nevertheless, there should be possibilities right from the start to be able to test the verification of the function of the equipotential bonding system as well as the prevailing shield current level. According to the numbers of Indu-Sol alone, more than 100 cases of process-relevant reductions in data communication quality due to electromagnetic interference could be avoided.

SPECIALIST IN INDUSTRIAL NETWORKS

Indu-Sol has set itself the task of objective evaluation of the quality of industrial data communication. To ensure the long-term, trouble-free operation of machines and plants for industrial automation, a complete system solution for permanent network monitoring (PNÜ) was developed with the aim of warning against failure. The more than 16 years of experience of the Thuringian technology company shows that not only their physics and logic must be analyzed for a stable and safe functioning and high availability of field buses and networks, but also a professionally executed equipotential bonding and electromagnetic compatibility (EMC) are indispensable.

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