Introduction to Industrial Automation

Figure 6.45  Grounding configuration in an industrial enclosure with a PLC and additional 

groups of I/O modules.

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Basic Operating Principles of PLCs 

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presented from a ground electrode to bus-bar, from bus-bar to mounting metallic base, from 

mounting base to PS module of the PLC, from mounting base to braided shielding, etc.

  5. The grounding and its quality of implementation is a particularly important issue when the 

system is equipped with lightning protection.

6.10.4   Electromagnetic Shielding of Cables

Shielding generally has different forms and may involve different things, such as shielding of 

three-dimensional housing, a single location, individual devices, and simple wires. In many cases 

the shielding is created by itself, for example, a small or large metallic enclosure in which a piece of 

electrical equipment could be placed; this is in itself a kind of shielding from the electromagnetic 

environment.

In the section on electromagnetic interference, the need to protect the cables from electromag-

netic interference has already been mentioned. Figure 6.44 in particular has been presented as the 

screening of a RS485 two-pin interface. In this section, all the basic rules of correct shielding will 

be listed only for cases of industrial automation signals (low voltage signals like analog measure-

ments and data).

The purpose of the shielding is to stop the electromagnetic lines of the interfering field and, 

as a result, block the electromagnetic interaction between adjacent cables. The protective metallic 

mesh absorbs the interference by playing the role of the escape line. In other words, the shield sur-

rounding the inner signal carrying conductor acts on EMI in two ways, which either can reflect 

the energy or pick up the radiated EMI and conduct it to the ground. In either case, the EMI does 

not reach the signal conductors. There are two basic types of shielding typically used for cables, the 

foil type and the braided type. For each type, there are also different kinds of shielding construc-

tion, the most important ones being:

  1. Foil shield (using a thin layer of aluminum attached to a polyester carrier)

  2. Braid shield (a woven mesh of tinned copper wires which provides coverage between 70% 

and 95%, depending on the tightness)

  3. Double foil and braid shield (better coverage and protection)

  4. Triple shield (foil-braid-foil shielding combination for very noisy environments)

In each of these items, the shielding mesh has a different impedance that is frequency depen-

dent. In general, the following rules can be established:

  1. The less impedance the mesh presents, the better the shielding becomes. In this case, the 

mesh may receive large discharge currents.

  2. For low-frequency magnetic fields, the twisting of the conductors is a fairly effective method.

  3. Data cables (network) and cables for analog measurements must always be shielded.

  4. In the analog signal cables, the protective mesh must be grounded at only one end 

(obviously on the side of the PLC) as shown in Figure 6.46. This is required in order to 

avoid creating ground loops with corresponding currents that may pose a problem to the 

PLC’s processor.

  5. In all other types of cables for data transmission, networks, etc., the shielding mesh must be 

earthed at both ends, as shown in Figure 6.44. Only the double ground at both ends of the shield-

ing mesh ensures that the conductors are shielded from inductive and high frequency interference.

  6. The shielding mesh should never be used as equipotential bonding conductor.

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  Introduction to Industrial Automation

Figure 6.47 shows the connection-contacting of the shielding mesh of four cables for analog 

signals in detail, with the grounded mounting rack in a Siemens PLC.