How the earth terminal on a TN-C-S supply can be at a different voltage to the true Earth.

As the neutral and earth conductor are combined outside the installation, current in the neutral causes a voltage drop, which results in the earth terminal having a different potential (voltage) to the true Earth outside.

AC substation grounding for safety
https://www.youtube.com/watch?v=NXtA5rxJXGA

UPE = 2¼fIMNET (7)
with MNET as the net mutual inductance derived from the superposition of the individual mutual
inductances. According to this relation a mutual inductance M0N
ET = 70 nH per meter length
results when the induced voltage UPE is considered as derived above. The mutual inductance
is expected to depend on several cable parameters.

It shall be mentioned that this phenomenon takes place for cables with twisted PE conductors
only. It does not exist in the case of cables with concentric PE conductors which therefore should
preferably be used when low magnetic stray fields are required.

Source:
Inductive Coupling between Wires in Cables with a Grounded Conductor
https://www.researchgate.net/publication/266065334_Inductive_Coupling_between_Wires_in_Cables_with_a_Grounded_Conductor

Robustness and reliability have made RS-485 the industrial workhorse over the past 40 years. Its large
differential signal swing of 1.5V minimum and reliable operation over a wide common-mode voltage range of -7V to +12V have catapulted the RS-485’s widespread deployment. Initially used as a communication network in laboratory instrumentation, RS-485 has spread to control networks in industrial and building automation, PLC networks on the factory floor, process control, commercial heating, ventilation and air-conditioning systems, seismic networks, traffic monitoring systems, and alarm indication systems in oil rigs, coal mines and the petro-chemical industry.

this white paper focuses on: RS-485 transceiver protection against large over-voltages.

The 24V and 48V DC supplies in industrial and telecom systems are commonly distributed through the same conduits as the data lines of an RS-485 network.

If a DC supply shares the same connector or screw terminal block with the data lines of a
n adjacent bus node circuit, miss-wiring faults can occur that connect one or more supply conductors with the transceiver bus terminals.

Another failure cause is the layout of the conduit. Sharp bends often violate the minimum cable radius
specified for data and supply cables. Over time, the increased mechanical pressure on the cable will cause a break in the insulation, causing shorts between power and data lines.

Engineers new to over-voltage protection often assume that adding external transient voltage suppressors (TVS) to a non-fault protected, standard transceiver ensures protection against short-and long-term over-voltages.

to protect your bus nodes against the wide range of over-voltages, you need fault-protected transceivers, such as Intersil’s ISL3245xE family. These transceivers provide protection against DC over-voltages of up to ±60V and transient over -voltages of up to ±80V.

Occasionally the question arises: Why not use a non-fault protected, standard transceiver and a few discrete low-cost transistors with sufficient high voltage breakdown for over-voltage protection?
The answer is simple: A discrete solution adds more cost and development time, and it consumes more space than a fault-protected transceiver.

Fault-protected transceivers with common-mode ranges wider than specified in the RS-485 standard require double fold-back current limiting within the driver stage.

current limiting scheme ensures that the output current never exceeds the RS-485 specification, even at the common modeand fault condition voltage range extremes