Notes regarding analog equipment - shielding and earth

Common signal potential, basic measures and notices

In some applications the reference potentials of different devices have to be linked, e.g. in order to be able to perform a measurement.

• Usually, no equalising currents should flow via such connections – for remedial measures see the following section.
• Buffer amplifiers may have to be used in some cases
• Potential-free connections on the device side may be suitable in some cases – note the permitted potential difference!

FE/shielding, basic measures and information

A list of exemplary measures, taking into account the information provided above, which may be considered in order to reduce interference, is provided below.

• Temporal effectiveness: the effectiveness of implemented measures may reduce over time and should therefore be checked regularly, particularly in the event of anomalies. Negative influencing factors include wire break, oxidation at contact points, mechanical damage, change in earthing characteristics, change in environment (new interference sources?), etc.
• Selected reference potential
• The reference potential used for discharge/earthing may itself be subject to interference, so that a connection to it may introduce more interference in the system than it discharges. In this case, a different, low-interference reference potential should be used.
• To ensure good discharge, it may be helpful to install a separate FE earthing point in the building and use it for sensitive signals/shielding.
  Caution in the event of lightning strike: a lightning strike in the vicinity can result in large potential differences between buildings and earth, which can affect locally separated earthing equipment. Spark gaps may be able to prevent equipment damage. VDE guidelines must be followed!
• Wiring
• The cable connections should be as short as possible
• The denser the cables can be laid over a metallic, grounded area/potential equalization, the less interference can be introduced, and the more interference is discharged capacitively via earth.
• Installation instructions for analog interference-sensitive signal lines next to highly interfering load lines:
• - avoid parallel installation, if unavoidable then separate with a large distance (> 20 cm) from each other, shield from each other with metal separators
• - unshielded cables should be twisted, if possible
• Use wire end sleeves or cable lugs to connect flexible cables/strands. Tinning is no longer permitted.
• Unused wires/cables should be earthed on one side as a minimum.
• All cables in a cable channel must comply with the insulation class of the cable with the highest voltage
• Metal cable channels are preferable for shielding reasons
• Shielding
• Shields must not assume the function of an N or PE conductor.
  Functional earth intended for improving EMC (electromagnetic compatibility) must not be used as protective earth according to VDE 0100.
• The shielding should not be used to carry discharge/fault currents (DC). These can significantly interfere with the useful signals and possibly damage devices!
  If in doubt, use suitable measuring devices (current measuring clamp) to check whether the shield carries current. Possible remedy:
  - only one-sided shield connection, or one-sided RC connection (resistance/capacitance)
  - Network cable: use fiber optic instead of copper
• Some connector technologies such as Coax (2-wire connection) require that signal ground and shield are connected to the same conductor. This can be disadvantageous in special environments. Then check whether a connection technology can be used that guides the shield separately, e.g. Triax.
• Shield configuration
• If braided screen is used, it should consist of tin-coated/nickel-plated copper. Aluminum braid may be suitable, provided the specific properties are taken into account.
• For cables shielded with braid, the cover should be 60% to 95%
• In special cases magnetic shielding using magnetically conductive, highly permeable material may be required.
• Cable shielding may consist of braid and/or electrically conductive foil. The use of foil on its own is not recommended, since it can easily be interrupted.
• Contacting of the electrically conductive foil alone for the purpose of shield coupling is not permitted; the braiding must be contacted. The contacting of the electrically conductive foil creates, among other things, too little galvanic coupling and is also mechanically less resilient.
• Earthed metal tubes used for shrouding cable can offer additional shielding



• Shield integration
  For discharge purposes, a “good connection” of the shield should be aimed for, i.e.
• low impedance/ low resistance → cross-section as large as possible, finely stranded, earth strap if necessary
• short cables
• large-area contact, EMC seal if necessary
• as 360° comprehensive as possible
• metallic conductive components without dirt, paint, grease, oxide layer

• Beckhoff offers the ZB8500 shield connection system.



• If there is more than 1 conductive connection between 2 devices, hum interference ("earth loop") can occur. Alternating magnetic fields can induce circular currents. This can be done simply by connecting cables/shield connections at both ends between two devices that are located on a common electrically conductive foundation, e.g. the machine structure.
  In such cases
  However, unilateral separation of the shield can significantly reduce the effectiveness of the shield. A better solution is coupling of the respective shield on one side via a coupling capacitor (C = 10... 100 nF, bipolar). This provides separation for direct current, while currents from HF influence can still be discharged.

• If the shielded cable continues after the shield contact, the further free cable length under shielding should be no longer than 10 cm. This also applies to cables within control cabinets.



• Manufacturers sometimes equip devices with an RC combination between earth and PE. On the one hand this can achieve good discharge capacity for HF interference, on the other hand the device may not be damaged inadvertently through high leakage currents. Such an RC combination on the device side as connection between earth and PE counts as earth-free coupling.



• A high-resistance resistor prevents excessive leakage currents. The capacitor short-circuits high-frequency peaks with low impedance. A specified dielectric strength applies to the combination.
  Good interference protection can be achieved if a shielded cable is laid completely earth-free (only RC connection on both sides).
• If a shielded cable has a drain wire, connection of this conductor to the shield coupling only is inadequate. At the cable end the shield and the drain wire should be connected together at the designated shield point.
• Connection of the shielding with sources of interference - only expected outside the control cabinet
• Apply the shielding at the control cabinet entry
• Continuation of the shielding within the control cabinet may not be necessary



• Connection of the shielding with sources of interference - also expected inside the control cabinet
• see: notices on control cabinet design
• The shielding should be opened after the entry into the control cabinet then connected and continued up to the terminal. At the device it should be contacted again (terminal contact or separate shield coupling).

Equalization of potential difference

• If signal or communication lines are run over longer distances, the installation should be checked for potential differences. Example: ribbon conductors in a wind turbine tower. To prevent equalizing currents in the shielding:
• suitable equipotential bonding conductors can be provided
• fiber optic can be used
• buffer amplifiers can be used



• The equipotential bonding conductor should be fine-wired, so that it has a large surface area to ensure effectiveness for high-frequency interference currents. In addition, compliance with the minimum diameters according to IEC 60364-5-54 is required
• copper 6 mm²
• aluminum 16 mm²
• steel 50 mm²
• The earthing system should have a star configuration.
• The PE connection replaces neither HF earthing nor the shielding, but is required for safety reasons.
• Lightning protection may have to be provided.
• Atmospheric influences can lead to significant potential shifts.

Additional safety measures

• For connected inductors and capacitors protective circuits/extinguishing equipment must be provided on the device side, in order to prevent voltage peaks, unless the connected device already has such a function.



• Filter components against interference emission should be used if necessary, e.g. in the form of current-compensating chokes or toroidal core ferrites
• Pick up the power supply for the measuring equipment in a star shape from the central supply source.
  Lay the forward and return conductors next to each other



• Thermoelectric effects in the mV range can interfere with analog signals, therefore
• avoid potential differences between different materials
• check the temperature or material, if necessary

Practice-oriented diagnostic methods for system examination

The following section lists some options for checking the effectiveness of shielding measures:

• Visual inspection
• Acoustic inspection (listen for voltage flash-overs)
• Voltage measurement with voltmeter between suspect system points
• Monitoring of voltages on shielding conductors with an oscilloscope that is suitable for high frequencies
• Current measurement of equalizing currents on the shielding cable with a clamp-on ammeter.
  The current on the shielding should not exceed a few milliamps (True RMS).
• Continuity measurement of the shielding and checking for unacceptable shunt
• Temperature measurement of surge arresters; the contact point will warm up in the presence of high current passage and high contact resistance
• Device-integrated diagnostics such as bus error counters etc.