What are the two equipotential bonding surfaces
So that the shield can effectively shield high-frequency interference, it must be grounded at both ends.
One solution is to install an additional equipotential bonding cable between the individual potentials. The equipotential bonding line should also be able to discharge large currents (a cross-section of ≥16 mm2 is not uncommon). Litz cables with a good surface should be used so that high-frequency currents can also be diverted efficiently.
Fig. 61: Installation of an equipotential bonding line
The bus cable should be laid as close as possible to the compensating line. This makes the area between the two cables as small as possible and reduces inductive coupling.
IMPORTANT: The shield of a bus cable must NEVER be used for equipotential bonding!
With modern PE cabling (5-wire = TN-S) as shown in Figure 62, the neutral (N) and protective earth (PE) are consistently separated. Even with an unbalanced load, no current flows on the earth and thus the shield of the PROFIBUS cable remains de-energized.
Figure 62: PE cabling (5-wire = TN-S)
With old, no longer recommended PEN cabling (4-wire = TN-C) as shown in Figure 63, the equalizing current I1 flows on the common PEN wire if the load is asymmetrical. He seeks the path of least resistance. Part of the current I3 can therefore also be diverted through the grounding. It is possible that a not insignificant part of I2 is also derived via the shield of the PROFIBUS.
Figure 63: PEN cabling (4-wire = TN-C)
This can be easily checked with a clamp meter. The current on the screen should not exceed a few milliamperes. In practice, a current on the screen of more than 40 mA is considered problematic. A current of more than 150 mA can heat the cable impermissibly and cause fire damage.
Fig. 64: Measurements with a clamp meter
Fig. 65: A current of 350 mA caused by insufficient grounding
In special installations, potential differences can occur between different locations in an installation and thus lead to potential equalization currents along a cable shield. Such equalizing currents on a cable shield must be absolutely avoided because they can lead to interference. Grounding problems occur when:
i.the bus cable covers a large area or bridges a large distance
ii. the electrical energy comes from different sources (e.g. several substations)
iii. large amounts of electrical power (welding robots, large drives, etc.) are consumed
As a remedy, it is recommended to carry out part of this connection with a fiber optic cable or to implement galvanic isolation of the shield with a repeater.
As an alternative, the standards also suggest capacitive grounding, particularly for explosion-proof installations. The screen is connected with an RC combination. The small capacitance (<10 nF) dissipates the high-frequency interference, but has a high-resistance effect for the mains frequency (50 or 60Hz) and thus prevents equalizing currents through the screen. A high-resistance resistor connected in parallel prevents the capacitance from being charged with a direct voltage. This capacitive grounding can be carried out at one or all ends of a PROFIBUS cable. The correct function can also be checked here with the clamp meter.
Fig. 66: Capacitive grounding of the screen
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