What you describe is an IT system (insulated from Terra-firma) It certainly can be done, and in some cases it has advantages, and indeed is used for UK shaver sockets in bathrooms, and some types of specialist medical equipment.

However the shaver socket only supplies one shaver, and the medical kit is very carefully maintained and monitored when in use

So why do we not do it more commonly ? Well the answer is all to do with automatic disconnection of supply (ADS).

If you do cannot detect  a fault perhaps from one phase to the case of  an appliance, . then nothing is disconnected, and the presence of a second fault between another phase and earth makes things suddenly lethal with no warning.

When your substation feeds many houses each with some tens to up to perhaps a  hundred or so items connected for years, the chances of at least one  fault being present at some time become quite high.

With the N-E connection, you can detect and automatically isolate the faulty equipment with fuses if the fault loop is low impedance, and with an RCD if it is not. Neither form of ADS will detect faults on an IT system.

Note also that there will always be a rather indeterminate path between every conductor in the system and earth, as there is capacitance and insulation resistance to be considered, so in effect things float about, unless they are tied down and there is a voltage but not a well defined one. And when the installation is large then the current available via these unavoidable leakage paths becomes enough to make touching the conductors  dangerous anyway.

However it is only convention that we earth the central star point of a 3 phase transformer - you could earth one corner, or as sometimes seen in the states, the centre of one side. The 3 phases still have the same voltages between them, and motors etc work just fine but the voltages and phase angles relative  to earth are all over the place..

Mike.

What you describe is an IT system (insulated from Terra-firma) It certainly can be done, and in some cases it has advantages, and indeed is used for UK shaver sockets in bathrooms, and some types of specialist medical equipment.

However the shaver socket only supplies one shaver, and the medical kit is very carefully maintained and monitored when in use

So why do we not do it more commonly ? Well the answer is all to do with automatic disconnection of supply (ADS).

If you do cannot detect  a fault perhaps from one phase to the case of  an appliance, . then nothing is disconnected, and the presence of a second fault between another phase and earth makes things suddenly lethal with no warning.

When your substation feeds many houses each with some tens to up to perhaps a  hundred or so items connected for years, the chances of at least one  fault being present at some time become quite high.

With the N-E connection, you can detect and automatically isolate the faulty equipment with fuses if the fault loop is low impedance, and with an RCD if it is not. Neither form of ADS will detect faults on an IT system.

Note also that there will always be a rather indeterminate path between every conductor in the system and earth, as there is capacitance and insulation resistance to be considered, so in effect things float about, unless they are tied down and there is a voltage but not a well defined one. And when the installation is large then the current available via these unavoidable leakage paths becomes enough to make touching the conductors  dangerous anyway.

However it is only convention that we earth the central star point of a 3 phase transformer - you could earth one corner, or as sometimes seen in the states, the centre of one side. The 3 phases still have the same voltages between them, and motors etc work just fine but the voltages and phase angles relative  to earth are all over the place..

Mike.