This section describes three possible grounding and shielding schemes for a PROFIBUS PA network.
• Full isolation
• Installation with multiple grounding
• Capacitive isolation
The difference between the schemes lies in the grounding of the bus cable shield. In one case it is connected to a separate ground and in the other integrated into the general plant grounding system.
Plant grounding schemes also differ according to national and or local practice. For example, British practice is neutral star earth bonding, German practice is the use of a potential equalisation line and Americans often run cables in grounded steel conduits and enclose control equipment in steel cabinets.
Fig. 6-1: Device grounding schemes:
In neutral star earth bonding it is assumed that the device is electrically connected to the tank via the process connection.
The Tanks are grounded at a central grounding point.
In German practice, each device is connected directly to a thick potential equalisation line which runs throughout the plant and is connected to a single grounding point.
The purpose of grounding the shielding is to protect the digital signals on the fieldbus from high-frequency electromagnetic interference caused, e.g., by cellular telephones or harmonics from frequency converters and other plant equipment. It can be seen, therefore, that local installation practice may also have some influence on the suitability of a particular scheme. The conditions for transmission may also be poorer in extensive networks or in networks with many branches. Finally, Ex considerations may rule out some schemes in some countries.
Applicability The grounding schemes shown overleaf assume that the T-boxes/junction boxes are serving plant sections that are some distance from each other and the control room, making it likely that each is connected to a different earth potential. They address the problem of preventing current loops developing within the cable shield, should a multiple grounding scheme be chosen, see Fig. 6-2. For simplicity, the wiring is shown with screw terminals. If a M12 connector is used, the device connection compartment should be opened to check whether the shield wire is already connected to the internal device grounding terminal. If this is the case, it must be detached and isolated, if a isolated scheme is required. For T-boxes and junction boxes, the manufacturer’s instructions must be followed.
Central grounding Potential matching line
Full isolation Full isolation is the scheme described in IEC 61158-2, and is the favoured method in Britain and the USA. In this case, the cable shield ground is fully isolated from the device grounds. The shield is grounded at the power supply or safety barrier only.
The disadvantage of this method is that the bus signal is not optimally protected from high frequency interference. Just how much this disturbs communication, depends upon the length of the bus, its topology and the sources of interference. As pointed out at the start of this chapter, if the bus runs in grounded metal conduits, as e.g. in American installations, then this scheme may work quite well.
Fig. 6-2: Full isolation with optional grounding of devices:
1 Power supply/segment coupler, 2 T-Box or junction box, 3 Bus terminator, 4 Grounding point for bus cable shield, 5 Optional grounding of devices, e.g. by neutral star bonding or conduit
Fig. 6-3: Full isolation with optional potential equalisation of devices:
1 Power supply/segment coupler, 2 T-Box or junction box, 3 Bus terminator
4 Grounding point for bus cable shield, 5 Local device grounds, 6 Optional potential equalisation line
Multiple grounding Multiple grounding provides enhanced protection against electromagnetic interference in noisy environments. It is the favoured method in Germany. All devices and cable shields are grounded locally. Each local ground is connected to a thick potential equalisation line, which itself is grounded in a safe area.
According to IEC 79-13, Paragraph 12.2.2.3, this method can be used provided that the installation is effected and maintained in such a manner that there is a high level of assurance that potential equalisation exists between each end of the circuit. Under these circumstances it fulfils intrinsic safety requirements.
Fig. 6-4: Multible grounding with neutral star earth bonding:
1 Power supply/segment coupler, 2 T-Box or junction box, 3 Bus terminator, 4 Potential equalisation through star bonding
Fig. 6-5: Multible grounding with potential equalisation line:
1 Power supply/segment coupler, 2 T-Box or junction box, 3 Bus terminator, 4 Local ground, 5 Potential equalisation line
Capacitive isolation In the case of capacitive grounding, the shield is connected to the grounding system (here shown as potential equalisation line) via a capacitor at all points except the power supply or safety barrier, which is grounded in the normal way. Small capacitors (e.g. 1 nF, 1500 V, dielectric strength, ceramic) are used and the total capacitance connected to the shielding may not exceed 10 nF. The capacitors are normally built into the T-box and junction box connectors, and offered as such by the component manufacturers. This method is not permitted in Ex applications.
Fig. 6-6: Capacitive grounding in a safe area:
1 Power supply/segment coupler, 2 T-Box or junction box with capacitive shield grounding, 3 Bus terminator with capacitive shield grounding, 4 Local grounds, 5 Potential equalisation line (optional)
If the bus runs into a hazardous area the T-boxes and junction boxes must be wired in the normal way and the power supply or barrier grounded via a capacitor.
Fig. 6-7: Capacitive grounding in a explosion hazardous area:
1 Power supply/segment coupler with capacitive shield grounding, 2 T-Box or junction box, 3 Bus terminator, 4 Local grounds, 5 Potential equalisation line (optional)