Page 246 - Electrical Safety of Low Voltage Systems

Testing the Electrical Safety
229
FIGURE 14.7 Current and potential electrodes coincide with an EXCP of
negligible resistance-to-ground.
placing the auxiliary current electrode at a distance of five times the
grid’s maximum diagonal (e.g., distance of the order of kilometers)
may be challenging, especially in urban areas.
In such cases, the grounding grid of the power station feeding the
facility (Fig. 14.8) might conveniently constitute the current electrode.
The test current (order of tens of amperes) is injected through a
de-energized power conductor running between the station and the
facility.
In overhead distribution lines, an overhead ground wire is usu-
ally present as a protection against lightning strikes. As previously
seen, users can conveniently use this overhead wire, or suitable metal
armor/sheath of the incoming medium voltage cables, as a conduc-
tor to connect their earthing grid to the utility’s grid. This connection
FIGURE 14.8 Grounding grid of the power station feeding the facility as the
current electrode.