about IT system

Voltmeter reads zero

It should read zero or fairly close to it theoretically. In actuality you might read something, especially if you have wires up against a junction box or conduit. This can have a capacitive effect which will cause a path for current to flow... however you won't have a very large voltage, most likely less than one volt. In a perfectly ungrounded system you would have no path for current to flow and you could grab a live wire and not get shocked. However, there are no perfectly ungrounded systems. There is always some capacitive or inductive path for current flow. The only reason (besides plain laziness) that some systems are built ungrounded is for fault tolerance. If you have an ungrounded three phase AC system that somehow has one phase ground out, you still have a system that will work just fine... it will just have one leg grounded. Once a second leg gets grounded however you have a short circuit and bad things happen, anything from a blown fuse to an arc flash explosion. I hope you can understand my meandering

Are you reading the question correctly? your answer seems to apply to a reading from neutral to ground, but the question is from one phase to a neutral and ground tied together.

"an isolated neutral point and groundş"

The way I'm reading this is that the neutral is isolated from the ground. which would be an ungrounded system.

If your meter is across one phase of 3 phase transformer and that transformer is energized, you should read the rated voltage. Doesn't matter if the neutral point is ungrounded or not.

one phase to ground will read approximately phase / root3. If system is corner grounded you may read unbalance between all lines to earth (one being zero for the grounded leg)

How long is a piece of string?

I had said "grounded",so tell me why do we have 55 volt (half of phase-phase) when we have a voltmeter connected across phase-ground.it it the point that in an isolated system we dont get shocked by 55 volt if we touch the live wire. pls tell me why? in reality i guess the answer but i wanna know the real reason..

That's a horse of a different color. The answer is that you have a wye connected transformer with a center point ground. With this set up you will read half of your phase to phase voltage. This is assuming that all of your other phases are 55 Volts Phase to ground. If not you have some other arrangement. As to why you don't get shocked when you touch a live wire, you either had enough resistance in your body that you couldn't feel the current flow, or you weren't touching something else to create a path for current flow. If you know what you are doing, you can touch live wires all day long without getting shocked. However, I wouldn't reccomend doing this even if you do know what you are doing. All it takes is to brush up against something that will make a path for current flow and you get shocked.

Is that a single phase transformer?

What is the L-L rated voltage?

Can you give us a drawing of your setup?

it is hospital instensive care IT System, and it has a l-l value of 220 volt.İ hold my voltmeter across the phase (line )-ground and i read 110 volt but when i hold the line i do not get shocked. Can you explain why?? if you read 110 volt and u dont get shocked so it is paradoxial isnt it? As you know my transformer is single phase and has no any connection to the ground.İs the reason for my reading 110 volt between phase and ground the fact that we have a capacitive impedance throughout the lines? pls give me a clear answer.it is a weird situation to explain simply, pls dont forget that i have an amazing earthing system at the room and it is so simple that i do get shocked via the path the current is supposed to follow...

Although the secondary of the transformer is magnetically isolated from the primary, it is still capacitively and resistively connected to all surrounding conducting parts. If the transformer has a shield between the primary and secondary then the secondary is "shielded" from the capacitive coupling to the primary. However, you still have the secondary capacitively coupled to the ground referenced shield. The resistance of the windings and insulation to ground will be several or even tens or hundreds of megohms. Many digital DVMS have impedances of many tens of megohms, so will read a voltage. If you use an old fashioned meter movement with 10k or 100k impedance you may find you read practically nothing.

In summary, you are simply balancing very high impedance (low capacitance) systems. Very high impedance meters will read higher than low impedance meters.

As an experiment you could check the voltage with you in the circuit. (However if it is a low impedance to ground we may not be hearing back from you )

You don't get shocked because the system implemented is IT earthing

psl tell me if we bond our IT System in healthcare facilities like intensive care operation rooms etc.. to the hospital's common earthing bus system?? or we just do a reference grounding bus and earth it directly on the isolated power panel? so what? pls tell me the way we must do...