038

a)

Figure 70: example of separation into zones in a building

Generally speaking, from the EMC viewpoint, it is necessary to eliminate or reduce the ground loops, i.e. those surfaces which form between the earth conductor and a functional cable (figure 71 a). This can be done by connecting the metallic enclosures of the apparatus by means of equipotential bonding that follows the route of the cable or, vice versa, have the cable follow the route of the existing equipotential bonding (figure 71 c). where Rs and Ls are the resistance and inductance of the connection element, Zc is the contact impedance and Cp are the parassitic capacitances. Note how contact resistance plays an important part and how, obviously, connections on painted surfaces are to be avoided unless the paint is removed. Once the paint has been removed, it is advisable to remember to protect that point with paint or protective grease. Different connection techniques can be used, including soldering, use of nuts and bolts, rivets, etc. The typical values of the connection substantially depend on its length and shape. Generally speaking, if high frequencies are present (> 10 MHz) it is advisable to avoid circular section conductors and to prefer flat ribbon cable or equipotential bondings leads. The typical value of the ratio between length and width of the lead must be less than 5. In any case, these connections must be as short as possible to avoid the previously mentioned ground loops.

A single connection is normally sufficient for switchgear compartments (fig. 73).

b)

c)

Figure 71: various ground loop configurations The equipotential bonding mentioned in the previous paragraph is represented by the simplified circuit in figure 72:

Figure 72: simplified equipotential bonding circuit

Figure 73: connection to the busbar in an MV panel