Floating Source Voltage Difference to Earth

Thread Starter

alexfrey

Joined Feb 23, 2019
22
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 when our circuit has no path to ground ?

floating_volt.jpg
 

dendad

Joined Feb 20, 2016
3,084
There will be capacitive coupling and a high resistance in practice.
The connection as shown will be high impedance, and a low impeadance meter may load it down sufficiently to read nothing, but a high impeadance meter will read some voltage.
For a real world test, get a plug pack that does not have a ground pin, plug it in, turn it on and measure the AC voltage from its output to a handy ground point.
 

Yaakov

Joined Jan 27, 2019
1,604
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 when our circuit has no path to ground ?
Not enough context for that diagram but remember, the neutral is grounded somewhere back along the path if this is normal household service.
 

Thread Starter

alexfrey

Joined Feb 23, 2019
22
Not enough context for that diagram but remember, the neutral is grounded somewhere back along the path if this is normal household service.
Yes you are right but please consider this circuit is after an isolation transformer I am not sure if it changes your answer
 

Thread Starter

alexfrey

Joined Feb 23, 2019
22
There will be capacitive coupling and a high resistance in practice.
The connection as shown will be high impedance, and a low impeadance meter may load it down sufficiently to read nothing, but a high impeadance meter will read some voltage.
For a real world test, get a plug pack that does not have a ground pin, plug it in, turn it on and measure the AC voltage from its output to a handy ground point.
Thanks for respond. Is that normal to see such a big voltage because of capacitive coupling ? (please consider this circuit is after isolation transformer) . So let’s think we didn’t wire the neutral of transformer to earth but we still see such a big
Voltage difference so someone can be shocked even by touching neutral cable although no physical connection to earth in transformer side ?
 

dendad

Joined Feb 20, 2016
3,084
It is possible you could get a shock, but if it is correctly designed, probably not. The capacitance between the primary and secondary is pretty low and so the impedance is high, limiting the current to a very low value. For example, a Neon lamp may light but I doubt a pair of back to back LEDs (for AC) would.
Have you seen the Neon test lights built into screwdrivers? The allow you to test for main bu touching a terminal with the blade, and the return path is the person's capacitance to ground (along with a very high series resistor in the handle for just in case...).
Very little current is needed to ionize the neon and light the lamp.
 
I had this fault exist at work once. Take the schematic in this https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=MEDIUM_P_SERIES&DocType=Customer+Drawing&DocLang=English datasheet,

Look at the two capacitors that is grounded in the center.

If you leave the ground disconnected and only apply 120 v between L and N, you will end up with about 60V at the ground terminal, The parasitics/leakage current are about the same, so 120 VAC gets divided by 2 and you get 60.

You cannot draw appreciable current at 60 V. 25 to 50 mA from the datasheet.

So, picture an outlet strip and something like a computer which has a 3-prong plug and an RFI filter in it. Now suppose that ground is not connected to Earth because of a wiring defect. That one device and/or others will drive the unhooked ground to the 60 V potential. now suppose that a few of the outlets are grounded. Now, some of the outlets will have ground raised by 60 v and the others won't.

If something like a parallel port printer and computer were plugged in and one of the outlets were defective, it would kill one of the devices since a parallel port printer does use ground as a TTL level reference.
 

Tonyr1084

Joined Sep 24, 2015
3,680
[edited] Had a MAJOR flaw with my drawing.

You can have an isolated voltage doing its own thing without having to reference it to earth ground. In the illustrations below you see an isolated 3 VAC sine wave and a 3 VAC sine wave offset by 6 volts. The offset voltage is referenced to ground - BUT you can still have it entirely isolated from ground if you choose.

In the illustration you provided there is a "Potential" difference between the isolated 120 VAC and ground. Honestly, it could be ANY voltage. As to why they site 60 VAC - that I can't answer. But in the hopes to help you understand how a circuit can be above ground - this illustration just might shed some light on it. Incidentally, without the battery being grounded at its negative terminal you can have an offset voltage as shown and STILL be isolated from ground.

With a "Potential" difference there isn't necessarily any current flowing.

Z Gnd Isolation.jpg
 
Last edited:
Top