The problem arises when the house earth is taken to a metal object for an extended time, it seems annually there is around 500 times when the PEN is lost, and 10% of those causes an electric shock, this is from a video on the methods used to keep electric car charging safe.
We have considered that 50 volt is the safe limit to touch AC supply to earth, but for car charging this has been raised to 70 volt, to detect this there are two basic methods, one is a reference earth rod, the second is the voltage of the supply. With a three phase supply it is very unlikely you will loose a PEN without the supply voltage on one phase going over 254 or under 207 volt, so with a three phase supply measuring the voltage is a good way to detect loss of the PEN.
What is however a problem is with the single phase supply, the 254 volt limit still works, but the 207 volt limit does not, tests show you can get over 70 volt to earth when the supply voltage is still over 207 volt, so the earthed frame of a car can have over 70 volt to earth without the system detecting a loss of the PEN and so present a real danger.
For cars being charged in a gated enclosure the time this is likely to happen means it is unlikely to cause a lethal shock, where the problem lies is where the car is being charged on the street or where people or other animals can easy touch the car. We have for many years banned the supply of boats, petrol stations, and caravans with a TN-C-S supply for good reasons, but the pressure to role out electric cars has resulted in the problem of TN-C-S being redressed in unacceptable ways, with salt with snow and ice clearance vehicles I have had a good belt from 24 volt, under normal conditions 24 volt is not even felt, but with wet gritters I have had a nasty belt with just 24 volt. Although on a dry summers day 70 volt may not be a problem, on a winters day when gritters have been salting the street 70 volt could be lethal. The gritters do not grit the drives, so off street not so much a problem, but with on street charging of electric cars it is just a matter of time before some one is killed. Unless better protection is employed.