If I poke my finger into the live terminal of a 230v socket, the line current travels through me, through the floor (earth), back through the earth to the supply transformer (because the transformer star point is also tied down to the general mass of earth) the fault currents travel back into the transformer star point and hopefully the completed path is of low enough impedance to be able to blow or trip the protective device serving the socket I touched.

Almost, but not quite. The human body itself has considerable resistance (varies between individuals, but typically in the region of 1000 Ohms) - so the 'path of low enough resistance' only works for faults between line conductors and deliberately earthed metalwork (exposed-conductive-parts) - people are then given some protection if touching that metalwork while in contact with the general mass of the earth (or other earthed metalwork). What used to be called 'protection against indirect contact'. Putting your finger on a line terminal (or other hazardous live part) is much more difficult - traditionally the risks were avoided by making it difficult to touch - e.g. by enclosures of insulating material or earthed metal - nowadays we can provide some additional protection as it's called by 30mA RCDs - which spot the small currents leaving the normal circuit.

After all, this is exactly why we use SELV transformers in bathrooms no?

Indeed. Usually comes under the heading of "separated" rather than IT if there's nothing earthed on the load/consumer side. As mentioned above it's actually quite hard to achieve sufficient separation - cable insulation is never going to be totally perfect (especially buried ones) and will have considerable capacitance to Earth (as we use a.c.) The regs recognise separated systems, but put a limit of 500m and 100,000Vm (so around 400m for 240V) on the wiring system - which probably counts out even a normal dwelling.

   - Andy.

If I poke my finger into the live terminal of a 230v socket, the line current travels through me, through the floor (earth), back through the earth to the supply transformer (because the transformer star point is also tied down to the general mass of earth) the fault currents travel back into the transformer star point and hopefully the completed path is of low enough impedance to be able to blow or trip the protective device serving the socket I touched.

Almost, but not quite. The human body itself has considerable resistance (varies between individuals, but typically in the region of 1000 Ohms) - so the 'path of low enough resistance' only works for faults between line conductors and deliberately earthed metalwork (exposed-conductive-parts) - people are then given some protection if touching that metalwork while in contact with the general mass of the earth (or other earthed metalwork). What used to be called 'protection against indirect contact'. Putting your finger on a line terminal (or other hazardous live part) is much more difficult - traditionally the risks were avoided by making it difficult to touch - e.g. by enclosures of insulating material or earthed metal - nowadays we can provide some additional protection as it's called by 30mA RCDs - which spot the small currents leaving the normal circuit.

After all, this is exactly why we use SELV transformers in bathrooms no?

Indeed. Usually comes under the heading of "separated" rather than IT if there's nothing earthed on the load/consumer side. As mentioned above it's actually quite hard to achieve sufficient separation - cable insulation is never going to be totally perfect (especially buried ones) and will have considerable capacitance to Earth (as we use a.c.) The regs recognise separated systems, but put a limit of 500m and 100,000Vm (so around 400m for 240V) on the wiring system - which probably counts out even a normal dwelling.

   - Andy.

That is before you have the several hundred metres of thinly insulated wire in the average motor or transformer wrapped around metal bits that may or may not be connected to the chassis, where the winding to core capacitance in hand held things are of order of a few nanofards unless extra thick insulation is used between windings and frame.

Ordinary twin and earth manages 50-100picoFarads per metre between adjacent cores in the common sizes, and more like 10-20pF per meter between the cores and metal tray or trunking, less to things like basket that are mostly holes and form a minimal second plate to the capacitor more when pressed to solid metal.

Mike.