In extended
installations, a bus cable, for example, can often stretch many hundreds of metres
between buildings. In particular with older installations, it can arise that one part
of the earth-termination system is no longer in operation, or that no meshed
equipotential bonding exists. In such cases, interference can occur as a result of
multiple shield earthing. Potential differences of the different building earthing
systems can allow low-frequency equalizing currents (n 50 Hz), and the transients
superimposed thereon, to flow.