What voltage would you expect on an IT system with an undistributed neutral between live and earth?

Thinking out loud: When you say 'undistributed neutral', do you mean that you have L1, L2, L3 wires, without any neutral or earth wires being distributed to you (i.e 3 phase). Or, that you have an L and a Neutral (single phase), but the neutral isn't connected into follow-on circuits (not distributed any further)?

  and then where/how is the local earth (the T in IT) connected? I'm guessing [academically] this is a 3phase system and a local earth rod, with no loads on the three line conductors.

There can be sufficient upstream line capacitances to the mass of earth such that the 3-ph system is 'balanced' , or there could be sneak path(s) that bring one line close to earth potential such that the other lines show as line to line voltage. Using a load may tame the confusions.

If a line is 'floating' then it will have capacitance to whatever it is floating 'from' .

Thinking out loud: When you say 'undistributed neutral', do you mean that you have L1, L2, L3 wires, without any neutral or earth wires being distributed to you (i.e 3 phase). Or, that you have an L and a Neutral (single phase), but the neutral isn't connected into follow-on circuits (not distributed any further)?

  and then where/how is the local earth (the T in IT) connected? I'm guessing [academically] this is a 3phase system and a local earth rod, with no loads on the three line conductors.

There can be sufficient upstream line capacitances to the mass of earth such that the 3-ph system is 'balanced' , or there could be sneak path(s) that bring one line close to earth potential such that the other lines show as line to line voltage. Using a load may tame the confusions.

If a line is 'floating' then it will have capacitance to whatever it is floating 'from' .

then where/how is the local earth (the T in IT) connected?

If it's the isolated version of IT, T is just connected to the exposed-conductive-parts of the loads - the supply end is separated from Earth. (In the impedance earthed version, the star point of the supply is connected to Earth via an impedance - but that star point may or may not be brought out as a N conductor (typically not, as that opens the possibility of N-PE faults which can be hard to detect).

  - Andy.

Thanks, That's useful clarification.

I was reading recently of the supply upgrade issues in Cornwall area where they had a neutral-Earth impedance (ASC?) reactor that allowed them the time to fault find line to earth faults without having to take the lines off supply, and how they were upgrading because of 'issues'. Unfortunately I can't find it in my browsing history .

I do hope that was HV !! Where we distribute neither neutral nor earth in the UK at least on overheads, so rural fault currents are actually quite low, considering the voltages.
In some countries is quite common to have neutral earthing resistors of single ohm sort of value at the source end on HV systems that have metallic earth, to limit the destructive power and impressed additional earth voltage of any phase to earth fault.

Cornwall, and a few other places in the UK, suffer from very variable ground conditions, all the way from wet mud to solid rock, and the latter can be awkward as step voltages don't  decay over distance as fast as they do over a more uniform soil.
Mike.

Edit According to P27 of this Western power https://www.nationalgrid.co.uk/downloads/3238

they use Peterson coil (a parallel resonant coil) earthing to limit the HV fault currents to a very low value to allow fault tracing, especially  in Cornwall and on the Gower Peninsula in Wales.

Yes (your Edit) the report I mentioned was a WPD (Western Power Distribution). It included discussions about the replacement of the resonant coil system to meet various requirements, and how that was proceeding.

There is rather more here on that here..
https://www.nationalgrid.co.uk/downloads/4102/5-arc-suppression-coils.pdf

 The decisions are essentially driven by the problems of a tricky soil type, but buried cables are more like giant TV co-ax and have a far higher capacitance per unit length, making the tuned circuit method less safe, as the impedances  get lower and the circulating resonance currents get correspondingly higher - segmenting the network into shorter lengths is only a partial solution.

The path finder fault detector basically sees the region of faulted cable as carrying an extra current, like a rather wide single wire with earth return, while beyond fault, balance is restored - the fields under the line are quite different, so fault finding from a safe distance is often possible.

Those interesting in some related  history may find the tests of the Alabama Power Company in 1923 ( page 31 of the PDF onwards ) an interesting read, as they tuned their reactor earthing coil up for the first time. The problems of capacitance in an IT system are similar, but much less severe ;-)
Mike

Hi Mike, Yes it was that '5-Arc-suppression-coils' file! It was back in January that I downloaded it - time flies.

I'll have a look at that AIEE journal paper. Always interesting how things were presented back then.

The comment about capacitance reminds me of an anecdote of a substation switch that was always burning out. The solution (probably temporary..) was simply to wire in as drum of cable (open ended!) - the problem was the resonance of the [original] low Capacitance connection  and local load transformer created an HF arc that never quenched. Adding the extra cable lowered the frequency allowing enough time at zero crossing to quench the arc. Funny stuff this electricity thing