The touch voltage display will be 0.0V for TN-S networks,
because the nominal residual current of 30 mA together
with the low loop resistance gives a very small voltage.
Example: UIdN = RE x IdN = 1 ohm x 30mA = 30mV = 0.03V
The measurement resolution is 100 mV, so the value is rounded off
and 0.0V is displayed.