A different but related problem is seen when the Neutral to Earth Voltage (NEV) is substantially higher, in the multiple volt range.
When POCO's metallic primary neutral is compromised, the result can be a significant amount of primary current flowing through the earth electrode at each pole or other grounding point in the Multiply Grounded Neutral (MGN) network. Because the source of the primary current is at high voltage, the resistance of each earth electrode will not significantly change the current, but will result in a significant voltage offset between the earth electrode and "remote earth" where the current density is low.
The result for each customer is that the POCO neutral, connected at the transformer secondary to the same earth electrode is also offset from remote earth by as much as tens of volts. This can cause unpleasant or even hazardous voltages between EGC-connected appliance surfaces and direct earth grounds such as metallic water or drain pipes.
Only POCO can fully correct this situation by restoring the competence of their metallic neutral. But, especially in rural areas with long primary runs, they may be unwilling to immediately go to the necessary expense (since from their point of view the system still "works".)
From the customer side, one possibility is to install a whole house isolation transformer with no bond between the two neutrals (a separately derived system). If the input neutral to the transformer is bonded to the local earth grounding system but the secondary neutral is, the result will be that no stray POCO primary current will flow through the local earth grounding system.
A more affordable solution, often used at farms, and in particular dairy farms where stray voltages can drastically cut milk production, is the
neutral isolator. This is installed by POCO at their pole and breaks the connection between the primary neutral (still connected to the pole earth electrode) and the secondary neutral. The result is that the only earth connection for the secondary neutral is the bond at the customer service. At this point you may be asking why you need a substantial device just to NOT make a connection. So there is more to the neutral isolator than just that. In particular a fault on the POCO side which applies primary voltage to the secondary side will result in that primary voltage trying to return through the customer's earth ground system (GES). This could be extremely dangerous because a high voltage can develop between the customer EGC and remote earth. The neutral isolator deals with this event by sensing the higher than usual offset between primary and secondary neutral and restoring the metallic connection between two neutrals, carrying the fault current. The threshold of the neutral isolator is set by POCO, and usually ranges between a maximum of about 40V and a minimum of about 10 volts. Neutral isolators are not generally used for residential consumers, where the only recognized solution for high Netural Earth Voltage (NEV) is for POCO to fix their problem.
