Power supply, potential groups

The electronics of a fieldbus-connected I/O device generally consists of two potential groups (exceptions possible, see respective device documentation):

Both potential groups are usually electrically isolated. The "load capacity" of the isolation must then be observed in detail, i.e. the voltage difference/potential difference in continuous operation or for a short time between the two areas.

The internal electronics can be supplied via the bus side, field side or both, depending on the device. See the relevant notes about this in the respective device specification.

The plug used can also have an influence on the potential groups; if necessary, its housing is conductively connected to the housing of the box.

The external system GND (DIN rail, SGND, FE) is always present and represents the reference ground.

In the following the permissible potential difference is referred to only as "Insulation"; the exact specification (value, type and, if applicable, insulating strength) can be found in the respective specifications of the device.


Isolation between the potential groups in practice

The potential groups are theoretically electrically isolated, i.e. there are only parasitic ohmic connections in the range of MΩ and higher that are unavoidable due to the electronics.
The load capacity of the isolation with regard to voltage level and duration is specified. It results among other things from internal isolation distances and the group-spanning components used, e.g. data transmitters or transformers, and is formulated in view of the underlying standards, which describe application aspects such as aging, contamination or defined overvoltage events.
From this it can be seen that, in practice, potential groups cannot to be operated arbitrarily isolated from the environment. In particular, if EMC disturbances penetrate the potential group, conducted by the external cables or radiated, then this energy seeks its way to SGND and finds it in every case undefined in the group-spanning elements mentioned above. Therefore, practice has shown that potential groups of all kinds should be purposefully and intentionally connected to each other and to SGND with small capacitances in the nF range for interference dissipation, so that the HF interference (and this already starts at 50 Hz) finds a defined path and does not impair the operability.
The ohmic effect of the capacitors in relation to the parasitic ohmic effects is negligible.

The following potential scheme can be specified:

Power supply, potential groups 1: