A very dangerous condition can occur on TN-C-S electrical systems, which are very common in the UK. A PEN connection, combined earth/neutral, is passed along with live in the supply cable. PEN is split into neutral and earth at the main entry fuse. Most UK homes do not have additional earthing (PME)
The risk is that if PEN fails due to cable damage, corrosion, poor maintenance etc then, although power to the building will be lost, the PEN can effectively float towards Live as any appliance that conducts current in the "off state" (think fridge compressors, heating elements, even some SMPSes) will pull that PEN node up via a low impedance node. This will then mean if someone touches one of their now-floating appliances and an earthed device (such as water piping, or walks on wet ground outside while touching their PEN-earthed electric car) then they could receive a fatal electric shock.
This is a particular pain for EV charging installations, as far as I am aware there are only two ways to solve the problem:
- Earth rods until the impedance is below some nominal figure, which can often require 5+ earth rods to be drilled in to the driveway and fitted (expensive, time consuming)
- An EV charging station that isolates PE as well as L/N when power fails and only connects PE when the vehicle has been detected
It's for this reason PME is now standard at newer builds ... but again, one of those cost saving decisions made 50 years ago that bites us in the ass now.