Ungrounded Systems & Fault?

<.....a danger of unbalanced voltages....>

The distribution system cabling insulation properties must be selected so as to render danger from unbalanced voltages extremely unlikely. See the lastest issue of the Wiring Regulations, published and maintained by the Institution of Electrical Engineers.

http://en.wikipedia.org/wiki/Earthing_system

Consider 'IT' earthing systems and their implications.

Hello Guest,

Current will flow whether your neutral is gounded or not, because if a person touches a live wire and is grounded the current has a path.

Cheers.

In a properly installed system, the ground and neutral are connected at the source, in your case at the generator, but usually at the transformer substation where the HV is transformed down to normal 3 phase voltage....this is normal and conventional for various safety reasons.

This also allows safety equipment such as ELCBs to function and to usually save the lives of such idiots that go around touching phases while standing on the ground!!

Check the link for more details of this:-

Ground to neutral connection

Therefore that person would be dead (you did not mention an ELCB) as the path would be from phase via the person to earth to neutral....= dead meat!!!

Dear Guest,

I don't mean to be insulting, but the generator set-up you describe is certainly unusual. It seems that you are describing a 3 phase machine, however the rotor typically generates the field, and receives its excitation from a DC source in the case of a synchronous machine, which is typical for the size of machine you stated. Synchronous or asynchronous, it has no direct connection to the load.

But enough of semantics. If in the scenario you described neither the source or the load has a grounded neutral (the neutral has no path to ground), then it is possible for someone to come into contact with a single energized part or phase and receive no shock, even with bear feet, as there is no completed path for the current.

Also, though a "floating neutral" is as you described (one not being connected to ground at some point), the danger of unbalanced voltages arises from an "open neutral", (the neutral of the load in not connected to the neutral of the source) not a "floating neutral".

The voltage required to breakdown the insulation present in the generator to create a possibility of shock in your scenario would also likely destroy the generator as well.

You wrote:-

even with bear feet

I love it!!! But what do the Bears have to say with you stealing their feet!!!!

d'oh! Well, you know, better resistance... I just gotta laugh at myself sometimes...

lendog, you had the best answer, there is a 3 phase or maybe a 5 phase rotor in this generator feeding 6 or 10 diodes in a full wave bridge for rotor control

brushless excitation... three machines in one. The first is an alternator to create the control voltage used by the DVR, which send a DC voltage to the "exciter stator" (for lack of a better name), then the "exciter rotor" creates a 3 phase voltage, which is sent through a 6 diode bridge to the main field (main rotor).

Please forgive my nomenclature (and spelling too). I don't know the common names for some of the parts, but hope you can still understand my ramblings.

I have not seen a 10 diode bridge yet.

Although you think your system is "ungrounded", the reality is that your system is "grounded" by the capacitive reactances of the system insulations to the ground.

The capacitive reactances are spread along the conductor length and the winding insulations of the generator, hence it is not measurable at a single point, unless you intentionally ground a point on a line and insert an ammeter to measure the ground current. An approximation of the available ground current (also called "charging current) provided by a cable manufacturer is 0.2 X Line kV X line length in km.

On the question of a person touching a live line (415V LL or 240V L-G) with bare feet, the amount of ground current depends on the person's body resistance + contact resistance to the wire + the capacitive reactance of the system. The ground current will pass through the person's body, to the ground, to the insulation of the system and finally, back to the point which we call the "neutral". The degree of the person feeling the shock will depend on the ground current passing through his/her body. The mA limit for a person to be able to extricate himself from accidental contact of live wires is 10 mA (IEC 479) or 15mA -where the victim cannot let go of the conductive material (OSHA).

GA, I rated it so too.....

Even with capacitive reactances present, the person is not going to get any shock if he is touching live conductor from ground. The contact voltage developed across him will be too small to pass any current of the order of 15mA and above.

So with bear feet or without bear feet, you can touch the live conductors without any doubt... But if at some place the system has earthed by accident, you are finished....