Autors: Franco Bua, Francesco Buratti and Antoni Klajn
Source: Handbook of Power Quality Edited by Angelo Baggin, John Wiley & Sons, Ltd
The following case study refers to a steel factory in the north of Italy where the entire electronic equipment in the plant’s laboratories was operating apparently in a crazy way: all device functionality was seriously affected by some unidentified problem which forced the factory managers to ask for expert help.
The causes of all the problems experienced can be found in the constructive aspects of the earthing system.
In the case of a large current flowing into the ground system of a big plant (Figure C10.1, the voltage drop between two different points of the ground system, due to that current, could be not negligible (Figure C10.2), causing problems related, for example, to reference ground, as the whole net no longer has the same potential all over.
Therefore, the first solution proposed to this problem was simply to split the ground system of the sensible electronic appliances (Figure C10.3) from that of machinery causing disturbances (in this case some VSDs).
The datasheets of the sensible electronic appliances were then checked: the manufacturer had prescribed connection of its appliances only to a so-called clean earth.
This solution was then adopted: a new ground system was installed, far from the existing one near the boundary of the plant, and used only for the connection of sensible electronic appliances. As a result, every problem disappeared.
A further analysis of the problem was to determine if the implemented solution was correct or not: the answer is that it depends on the point of view.
If there is a possibility of someone touching simultaneously two exposed conductive parts connected to the two different systems, the solution cannot be considered correct: the person would be subjected to the voltage difference between the two ground systems.
As much as the two systems are independent, the situation is dangerous because the voltage disturbance becomes a touching voltage.
If it is guaranteed that nobody can touch two exposed conductive parts connected to the two different systems at the same time, the solution is obviously correct.
The real issue is to have a situation in which nobody can ever do this. So the eventuality should be considered that it is not possible to divide the two earthing systems: how can the problem be solved in this situation?
In principle two different solutions exist and can be adopted:
- To operate at the level of the appliances, choosing them with a higher immunity level.
- To work at the level of the installation.
The first one is of course the easiest solution, even if it cannot be considered a solution but just circumventing the problem. For this reason it is better to concentrate on the second solution, and to add just a small philosophical comment on the first one.
A system engineer could probably argue that it his duty to feed appliances, not to select them on the basis of some kinds of issues. This is partially true, but if we consider that engineers usually have to select equipment on the basis of International Protection (IP) grade against pollution, why not to choose that equipment on the basis of electromagnetic IP grade against electromagnetic pollution? The concept is exactly the same.
The international standard does not contain any prescription or suggestion about lownoise equipotential bonding except for a special symbol (Figure 10.31).
The methods used to deal with the problem at installation level come from good practice, which every system engineer should know:
- Requirements related to protective measures must prevail.
- Independent earthing systems are allowed only in case exposed conductive parts are not contemporaneously accessible.
- Separate sensitive and disturbing circuits.
- Avoid parallelisms.
- Adopt line balance.
- Use shields and double shields.
- Adopt differentiated conductors.