Cut Outs and Floods

[Lectrician]

in your scenario then, if we disconnected all the neutral / star points of the transformers from earth, then no one would ever get a shock unless they touched live and neutral since the fault current could not run through the general mass of earth to the star point..

True ish.

Not sure how that relates to the original question/point? It doesnt make any sence.

Apart from transformer losses giving a rouge voltage to earth, which they will......which is why we tie neutral down to from the 'earth' & which is why it is generalied as an earth, and not a "protective conductor".

An earth metal lump (the sub) sat on 'earth' with the neutral floating will still give volts from N or a phase to mother earth due to internal losses.

 

[Spark123]

If you disconnected the star point from earth then as you say, there isn't in theory an immediate risk. However if any phase was to become in contact with earth (undetected single fault), the other two phases would pose a higher risk.

 

[plugwash]

in your scenario then, if we disconnected all the neutral / star points of the transformers from earth, then no one would ever get a shock unless they touched live and neutral since the fault current could not run through the general mass of earth to the star point..

What you are describing is an IT system, for small systems like a portable generator running a single appliance or the output of the isolation transformer on a maintinace technicians workbench it works very well.

For larger systems there are two big problems with IT

Firstly any system of nontrivial size WILL have faults and leakages. If one phase has more leakage to earth through faults than others then the other phases could rise up to near 415V relative to earth with enough current to cause a shock. The neutral again through such faults or leakages could rise up to near 240V with enough current to cause a shock.

Secondly detecting faults locally is virtually impossible. You can detect that there is a fault somewhere by having high value balancing resistors (sufficiantly large value that they won't carry leathal current) from each phase to earth but there is no real way to detect where in the system that fault is.

 

[Spark123]

Thought isolation transformers and portable generators were electrically separate as opposed to IT (eebads)?

 

[plugwash]

Well a portable genny isn't strictly speaking IT because there isn't generally any earth connection at all. 240-240 isolation transformers have a floating output but the incoming earth is generally availible on the output socket making thier output IT.