In TN systems, ZLoop exclusively depends on parameters known
to the user, such as the impedances of circuits and transformers. In
PME, instead, the fault-loop impedance includes the impedance Ze of
the utility low-voltage distribution system, which is usually unknown
to the customer. Ze, which increases with the distance of the fault’s
location from the supply source, may also change in time without the
user knowing it because of modifications in the utility distribution
system. If the total fault-loop impedance ZLoop = Ze + Zuser is excessive,
the ground fault current might be so low that Eq. (7.4) cannot be
fulfilled, and there would be no effective protection of persons against
indirect contact.