Floating Source Voltage Difference to Earth

I was reading about floating voltage on web but there is something that I couldn't understand. I attached photo of circuit. As you see it shows 60v difference between neutral of flating source and earth but how is that possible since there is no physical connection between them ? What is the reason of this voltage difference.

• Because it's floating, it can be any voltage with respect to ground. 60 was just something they chose as an example. – Hearth Mar 9 '19 at 21:57

Floating means that there is a high impedance with respect to ground. However 'high', in practice, does not mean infinite.

If you attach a light bulb between the neutral and ground it will be unlikely to light and will most likely reduce the measured voltage to close to zero. However with a typical voltmeter that may have a 10M or higher input impedance, the measured voltage may be pretty much anything, depending in part on where that 120VAC comes from.

That's because there will be capacitive and possibly resistive coupling to both sides of the 120VAC and possibly to other voltages, depending on the circuit. There could also, in theory, be a static buildup of relatively high voltage. Usually the measured AC voltage on a real meter will be less than half the mains supply voltage.

Think of something like this:

simulate this circuit – Schematic created using CircuitLab

If C1 == C2 and the meters are ideal (and the transformer is ideal) each meter will read half the transformer secondary voltage.

• So even if the main transformer neutral isn't grounded someone can be shocked by touching live cable ? But why the current flows(if voltage difference exists there may be current flow) from live to ground though we didn't wire neutral transformer to earth – d.alex Mar 9 '19 at 22:57

As you see it shows 60v difference between neutral of flating source and earth but how is that possible since there is no physical connection between them ? What is the reason of this voltage difference.

Strictly speaking the circuit is not-sensical. Conductors are neutralized by connecting them to ground. If the supply is truly floating then neither line can be "neutral". One possible scenario is that the lower line is supposed to be neutral but due to a fault is now at 60 V AC.

The other point to notice is that the circuit is incomplete. We have no idea what the supply is or how it is configured. The fact that one line is labelled "Neutral" should lead us to expect a connection to ground at the supply end of that conductor.

It's either a measurement error ( stray capacitance) from a floating transformer ) or a line fault condition as Neutral is low Resistance to the earth-ground by design in North American power systems. e.g. Line to ground FAULT, thus raising local earth ground voltage relative to neutral which is earth grounded at one or more places or a another ground fault is a dry arid zone with inadequate earth bonding> 100 OHms and ground currents. One cannot say exactly where yet, but it is a fault condition.

If that were the residential voltage, you should expect Neutral may reach 5% of line voltage by design. The Neutral-earth bonding at the transformer and distribution wire R = V=IR , at maximum Neutral current. That means the panel rating of say 200A and wire resistance installed to present day codes ought to never exceed 5% of line. (Using only a single line and Neutral being 100A. ) If current for L1=L2 then neutral current would be near 0.

If that were simply a small transformer secondary, it would never be called L1 , N , but regardless when floating and measuring with a high impedance meter say 1Meg or 10Meg then the voltage you might see depends on the impedance division from stray capacitance as it floats according to these parameters..

• Depends on the earthing system, which is off-topic. Some words missing in the formula? Doesn't address the question? Seems confusing. – Heath Raftery Mar 9 '19 at 22:51
• @HeathRaftery North American standards (120V) regulate grid to 5% and distribution is another 5% = 10% total regardless of ground system . – Tony Stewart EE75 Mar 9 '19 at 23:05
• Interesting, but not helpful in explaining the 60V in the question. – Heath Raftery Mar 9 '19 at 23:12
• OK @HeathRaftery I added clarity – Tony Stewart EE75 Mar 9 '19 at 23:14

The reason is that we define ground to be zero volts compared to the V+ or in alternating current compared to the L1, L2 and L3. Volts is the potential difference between two points and makes no meaning without a reference. So any other grounding in another system or even the actual ground can have a different potential to the grounding in your system.

In other words there will be a voltage difference between them, for example 60 volts. Your schematic shows that the transformer of your system is not grounded to the earth and the voltage between the earth and your transformers neutral is 60 volt.

simulate this circuit – Schematic created using CircuitLab

So say you wanna connect two systems together that has their own power source you should usually do what we call equalize ground which is basicly to connect them together so they have the same defined ground at zero compared to their power source.

• You say "doesn't mean the ground is actually zero", and that's true, but I think you should emphasize that it makes no sense to say the ground is "actually" anything; there is no such thing as an absolute voltage magnitude. – Hearth Mar 9 '19 at 22:42
• Well ground is only the return path for the current defined at zero compared to the source path. It can certainly have an "absolute" potential difference to another system or is your point that by itself it's randomly floating and thus not anything? – C. K. Mar 9 '19 at 22:57
• My point is that by itself it has no voltage, as voltage is defined as a difference. – Hearth Mar 10 '19 at 0:56
• @Hearth I clarified my answer – C. K. Mar 10 '19 at 1:11