Floating Voltage

When you float, do you touch ground?

I am not an electrical engineer (just a technician) but I am going to attempt to answer this "dumb question" because this same question was asked by my boss when I tried to explain why a DC power supply that I was installing needed to be grounded. And this is the way I understand it

Voltage is a measure of electrical potential between any two points in an electrical circuit. Earth Ground is just a reference point considered to be zero potential relative to any other point in a circuit. If a circuit is not connected to Earth Ground at some point, then any point in the circuit can have any potential relative to ground, therefor be said to be floating.

A floating voltage source is not connected to ground, but maintains the potential difference between the pos and neg, but the actual voltage can float to well above the stated voltage....A floating voltage source is more versatile in certain situations such as DC charging circuits...

A floating voltage is simply one that is not in anyway, shape or form, earthed, or related to earth.

It simply "floats".

If you placed a sensitive, low resistance voltmeter between one of the outputs and anything truly earthed, you would read no voltage, as they are not in anyway, "related" to each other....

While in the RN, when we had our ships in the dry dock, the on-board generators/alternators could not be run, as there was no cooling water available!!

So we used 115V AC lighting, powered from a shore alternator, that was simply un-earthed. There would be a fuse-board mounted temporarily near to the gangway, that we would attach long strings of 115V lamps to.

The outputs of the fuses would be made via large brass thumbscrews to the open ends of the cable strings.

Occasionally the thumbscrews would loosen, someone might have tripped over a cable or something, and the duty electrical rating would be asked to come and take a look and fix it.

Normally they would remove power to that board completely, darkening a large part of the ship, where possibly many were working, making it quite dangerous for all concerned!

Obviously warnings were made over the loudspeaker (Tannoy) system.....BUT, still dangerous.

When I found out that the supply was completely unearthed, the first time I test/touched each terminal (still live of course!) and felt nothing, so I simply replaced the wire back and screwed down the thumbscrew, while it emitted flashes and sparks due to the current being taken by the long string of (old fashioned) lamps!!

I was careful that the sparks were not touching me of course. That might have given me some small burns.

This caused amazement and even "dread" in the non electrical servicemen. They would take a very wide berth around me at all times after that. If I made a sudden movement in their direction (enjoying myself) they would step away in panic!

Telling them it was quite safe and that they could touch it too and feel nothing, did not persuade a single one to believe me.......

It turned into a bit of a party trick, especially at night when the sparks were even more impressive........

I had forgotten all about that until I read this blog. Thanks for the reminder.

Oh, by the way, I take NO responsibility for anyone trying this and ending up being electrocuted or getting burned!!!

It seems as if it depends on context. I have worked on DC powered communication consoles and boats. In this context both systems are/were grounded. Once the batteries are completely charged, the charger provides sufficient current to operate the load(s) which change in change depending on what accessories are operating. This is called "float".

For different reasons......

Yea, it depends on the context, and what he is trying to accomplish. Same general field, different applications yield the same term with vastly different scenarios. That is why I like this site.

In a non-grounded circuit, such as supplied on the secondary of a transformer or by many DC power supplies or by a generator, the voltage between the circuit conductors is well known, but the voltage between any one of them and ground is not controlled. In practice this voltage is determined by the inductive and capacitive coupling between the circuit conductors and their nearby energized conductors and grounded surfaces. If you were to measure the voltage to ground with a low-impedance meter (an older analog meter) and then with a high-impedance meter (a modern multimeter) you would get different readings because the low-impedance meter would significantly change the coupling between the conductor being measured and ground. With a non-floating circuit, either type of meter would give the same value (other than internal errors in the calibration of the meter).

Circuits with a floating (non-grounded) output are required for safety in certain circumstances, such as the control circuit on a bridge crane or in certain critical care areas of hospitals. They are preferred in many industrial plants where they are used along with ground detection devices, so the first instance of a fault to ground does not interrupt the process but is given a warning; then there is enough time to locate and repair the ground fault before a second one occurs to shut down the process. In the USA, codes generally require circuits that have a neutral point (such as 3-phase wye or 120/240V single phase) to have the neutral point grounded if this will result in no conductor having a voltage greater than 300 volts to ground. At voltages above this there is no requirement to ground them; if grounding is desired there are various methods for doing it, but that is beyond the scope of your question.

--JMM

http://cr4.globalspec.com/thread/100449

Two years later and still the same question.

Well you can bring a horse to water. You cannot make them drink.

Undefined term: <...we...>. The forum cannot possibly know what the <...we...> does unless the <...we...> is defined and the <...we...> tells the forum.

A torch/flashlight battery might be described as having a <...floating voltage...>, as it is not inherently connected to or referenced to earth/ground voltage.