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

Between where and where are you seeing line voltage, and at what impedance - the whole point is that it can 'float' however a dead short to earth anywhere is bad.
if this is a 3 phase system I can well believe that the centre of the triangle will be near zero, so the line to earth voltage will seem close to line, due to all 3 line to ground capacitances being equal and having more or less equal voltages being the equilibrium condition/

Mike.

Sorry Mike, can you correct my simple thinking..

High resistance to earth meaning voltage dropped and a lower voltage to earth than line

How many phases have you got, and what voltages do you see on each of them ?

In the simple view, just measuring voltage draws no current, so no voltage drop even if loop impedance is infinite. A real volt meter is tens of megohms, but the real earth loop impedance is a bit uncertain but could easily be a few hundred k ohms, so ~ 2 orders of magnitude lower than your volt meter.

But add an impedance between 1 line and ground, that is a lot lower than that loop impedance combined with the reactance of the capacitance of the wiring (you can estimate that -  how much wire have you got on each phase ?  - allow 50-100pF per metre for small diameter stuff.) and if the system is IT, the loaded phase voltage to ground will drop away, and the others will rise to compensate and the loads wont notice as the line to line voltage is unaffected.

Mike.

Just to add a couple more questions - which version of an IT system do you have - isolated from Earth or impedance earthed? If the latter, what sort of value of impedance?

Are there any appliances or anything else with filters, that could reference the system to Earth? (e.g. via a capacitor or three.)

   - Andy.

or even an insulation monitor...

  - Andy.