Broken PEN in TN-C-S

[KeenPensioner]

I'm learning so be gentle.

I'm trying to understand the consequences of a broken PEN conductor downstream of the DNO service head. If I'm correct, this will cause all exposed conductive parts to potentially rise to mains voltage but this voltage will be reduced by any parallel paths created by supplementary bonding to metallic water, gas supplies etc. Also any RCD in the circuit will not trip.

My questions are:

(1) Will the parallel paths have a low enough resistance to generate enough current to trip any MCB connected to the circuit? Effectively a TT system?

(2) What happens if the gas, water supplies coming into the property are plastic? Surely this will mean no supplementary bonding is required and no path to Earth. All exposed conductive parts will remain live?

(3) Supply to outbuilding. If "extraneous conductive parts" in the building then bonding required and the bonding conductor would normally be 10mm2. Why is supplying outbuildings, caravans, marquees etc additionally problematic if supplied by TN-C-S?

As I've said, I'm not qualified, just interested so be gentle!!

 

[westward10]

An open circuit PEN conductor will mean the neutral flow is diverted by way of the earth, assuming as you say it has somewhere to go. This is why the sizing of main protective bonding conductors is important on TN-C-S systems, it would be easy for any protective conductor to overheat in this situation.
Just as hazardous if not more so is a dwelling connected from a three phase supply along with several other dwellings because if the neutral is lost at source you may have all sorts of voltage winging through your appliances due to a certain imbalance across the phases.