Ungrounded Systems: Explaining Natural Capacitance & Fault Current

[metsfan1395]

Can somebody please explain what is meant by an ungrounded system, while not having any intentionally applied grounding, is grounded by the natural capacitance of the system to ground?

What is meant by the natural capacitance of the system to ground? Is there actually capacitance between an overhead conductor and the earth?

How does capacitance provide a ground in the system?

How does this keep fault current low?

Thank you for all of your previous replies, and I look forward to a discussion on this as well.

 

[mtworkowski@o]

Have a question. I noticed that all the outlets in one room have no ground. None in the boxes. This was a system that was used way back. I installed a gfci and used a test light which showed an open ground. When i connect a jumper between the ground screw on the gfci and the line neutral the tester reads normal. I have my own thoughts on the safety of this but would like to here some others.

 

[dlgoff]

Can somebody please explain what is meant by an ungrounded system, while not having any intentionally applied grounding, is grounded by the natural capacitance of the system to ground?

What is meant by the natural capacitance of the system to ground? Is there actually capacitance between an overhead conductor and the earth?

How does capacitance provide a ground in the system?

How does this keep fault current low?

Thank you for all of your previous replies, and I look forward to a discussion on this as well.

This wiki page on http://en.wikipedia.org/wiki/Ground_(electricity)" might be a good read.

 

[Naty1]

Can somebody please explain what is meant by an ungrounded system, while not having any intentionally applied grounding, is grounded by the natural capacitance of the system to ground?

I'll assume you are referring to a house system. Hot (ungrounded, black) and neutral, (grounded, white) are run to all fixtures and appliances. The incoming electrical box (where all the circuit breakers or fuses are located) is considered the generating source and so the neutral wire is connected to a rod buried in the ground at this point...usually by a bare cable from the box inside to the rod outside. In old systems that's it. If a fault develops, and a human touches say a "hot" appliance case, shock results...there is no protection via circuit breaker or fuse. This is an ungrounded system and I can't imagine it is legal anywhere in the USA.

A third grounding wire has been required for many years and connects external cases of appliances and fixtures via green wire back the the grounded neutral wire at the electrical box. In normal situations this wire carries no power, but will do so if a fault develops. Further, if its a serious "short" enough current can flow to trip a circuit breaker or blow open a fuse. So all homes should be wired this way.

An ungrounded system may be utilized aboard boats if an isolation transformer is used between shore power and the boat electrical system, but in yachts up to 50 or 60 feet, where I have knolwedge, I don't believe it is used any more. Most modern boats are grounded but in a special way much different than a house. This is because stray current and galvanic corrosion can be a serious problem in the wet marine environment.

Any natural capacitance provides no protection whatsoever.

And none of the above comments relate to lightning grounding which is an entirely separate subject and almost always a separate grounding system.

 

[Naty1]

I noticed that all the outlets in one room have no ground. None in the boxes. This was a system that was used way back. I installed a gfci and used a test light which showed an open ground.

A gfci constantly monitors milliamp current differences between the hot and neutral wires. In a properly grounded system if some current leaks the current between hot and neutral is different. The GFCI should shut off power before a fire occurs, before power is wasted, before a lethal dose of electricity is received by a person... USA uses about 5ma, Europe about 30ma trip (activation) currents. In multiphase systems, requirements may be different.

What does your GFCI instruction booklet say about an open ground? There may be a type GFCI for two wire circuits, I am not sure...

Try Wikipedia: There is a lot of helpful information, even a little on requirements in a few different countries.
http://en.wikipedia.org/wiki/GFCI

In some countries,[1] two-wire (ungrounded) outlets may be replaced with three-wire GFCIs to protect against electrocution, and a grounding wire does not need to be supplied to that GFCI, but the outlet must be tagged as such. The GFCI manufacturers provide tags for the appropriate installation description

.

 

[mtworkowski@o]

Naty1,
Very good. What are your thoughts on the jumper between the ground terminal on the gfi and the neutral terminal on same? I am thinking in the event of a separation of the upstream neutral that the jumper will feed the returning energy from the appliance neutral right back up to the appliance metal work. The gfi should trip on imbalance, but what do think? Anybody? Thanks