Grounding, basic but tough topic

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unsaint34

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There are many questions I have about grounding. But I am willig to solve them one at a time. I am intentionally avoiding the term "ground" since it has such many usages.

Question 1) Among many reasons, connecting an electrical system to earth provides a reference point for input voltage, therefore achieving steady voltage to loads. But, since we connect all neutral conductors together at the main service panel, why can't the neutral conductor be used as the reference point, instead of connecting the circuit to earth? If I have a neutral conductor bonded at the service panel and no part of my system (not even the service transfromers) is connected to the earth, wouldn't I still get stable voltages to the loads in the circuit?

Question 2) Along the same question ... I also read that connecting circuit to earth stabilizes the "voltage to earth" or "voltage to ground" under normal operation. But, why do I care about the voltage to earth as long as I have voltage to loads?


I'll appreciate your answer.
 
Re: Grounding, basic but tough topic

Both of your points make sense. The supposed reason we "earth" electrical systems is spelled out in section 250.4(A)(1), yet I have never seen nor read any verifiable evidence that these things occur because of grounding.

I also don't know why it is important to have a stable voltage to earth reference. Perhaps one of the theory junkies here can explain the necessity of that.

[ February 23, 2005, 05:47 PM: Message edited by: bphgravity ]
 
Re: Grounding, basic but tough topic

I try to give a simple answer that was taught to me.

If we allowed all electrical conductors to sort of say "Float" above Earth then there would be no way to guarantee that these conductors will remain at a "0" potential in reference to Earth. any long run of a conductor has the potential to pick up stray eddies or magnetic current fields that can induce a current into these conductors. Most of us has had this experience with a un connected conductor in a conduit or cable while using a tic-tracer or a DVM type meter. Now there are fields around Earth and even from the Sun that can and has produced very large fields that can put a un connected conductor at many thousand's of volts above Earth potential. Look at what happens even when they are bonded to Earth after a solar flair! so this is the main reason to bond all conductors to Earth is to keep them at the same potential as Earth to lower the risk of a fatal shock. And it is easy to prove just roll out your next 500' roll of any size THHN wire and support it about 3' above the ground and use a DVM and measure the voltage between this single wire and Earth.

[ February 23, 2005, 07:03 PM: Message edited by: hurk27 ]
 
Re: Grounding, basic but tough topic

On the early ungrounded systems, a person in contact with the earth, or anything connected to the earth, would not get a shock by touching any one conductor.
The only voltage to ground would be due to distributed capacitance, which would be very low, with few metal raceways in use.
The early electrical customer would get used to that situation, and become careless about touching one wire at a time. (Sketch A)

Sounds great, but how do we ensure that a system remains ungrounded? We can?t!

Keep in mind the fact that it is difficult to isolate equipment and conductor enclosures from the earth, especially in masonry buildings that have steel frames.

Any insulation breakdown that results in a conductor coming in contact with the earth, or any object connected to the earth, will have the effect of creating a grounded system, and now a person touching the other conductor receives an unexpected shock. (Sketch B)

I don?t think it would have taken very long for the early engineers to realize that, because they couldn?t prevent the system from eventually becoming grounded, they might as well ground one of the conductors in the first place, and by grounding the neutral conductor, the maximum voltage to ground would be half the transformer voltage (in single-phase systems), or the phase voltage (in wye connected three phase systems).

I firmly believe that this was the main reason for introducing the use of grounded systems.

Of course, there are also the standard reasons -

a. lightning, and
b. inadvertant contact with a conductor of a higher voltage system.

Ed

UnGround.gif
 
Re: Grounding, basic but tough topic

Originally posted by hurk27: I try to give a simple answer that was taught to me.
Click to expand...
That?s a good answer, Wayne. Let me add another detail to this scenario.

The neutral and equipment grounds are connected together at the main panel. Suppose they were not connection to planet Earth.

You would not normally want to touch the neutral wire of a live system anyway. But you should think that it would be safe to touch the front cover of the main panel.

The voltage between the ?hot leg? and the ?neutral leg? will be held constant by the power source. Now consider what happens when the voltage from either of them to planet Earth is allowed to ?float.? There could be, for examples, 50 volts or more from the neutral leg to Earth. But because of the existence of the main bonding jumper, this same voltage is present on the Equipment Ground Bus. A bond wire from the Equipment Ground Bus caused this same voltage to be present on the enclosure of the main panel. So when you touch the cover of the main panel, you would receive a shock.
 
Re: Grounding, basic but tough topic

Re: Ed MacLaren

The pictures are clasic.
 
Re: Grounding, basic but tough topic

Maybe this will help.

The unplanned system (not referenced to ground) has distributed leakage capacitance connection to earth whereas the planned system (an intentionally ground reference) has a known connection to earth thereby shorting out the capacitance. The leakage capacitance can cause havoc by:

In large ac ungrounded systems, the line-to-ground short should occur, up to 25-amps or steady-state ac leakage could flow through the leakage capacitance without causing any protection devices to operate. The 25-amps represent a lethal hazard if personnel get into this path.

Part of the steady-state leakage current described above flows through conductor insulation, speeding up deterioration and thus decreasing useful life.

Under transient conditions, the leakage capacitance can resonate with an inductance in the circuit causing very high voltage across insulation and loads.

Not wanting to labor or hog the subject, but grounding the system makes it very easy and economical to employ over current protection systems. If it were not grounded or referenced you would have to install elaborate ground fault detections/protection systems. With a grounded system throw in a breaker or fuse in the ungrounded circuit conductor and call it done.
 
Re: Grounding, basic but tough topic

I was told that when they were first designing the telegraph system back in the 1830's the wood poles kept the leakage high and also kept the voltage down. But when non-alkaloids glass insulators was invented, many telegraph line men was hurt or killed by this phenomenon. And at first they didn't have a clue as voltage and amp meters were in the infancies and many times they would just find the linemen laying on the ground and thought that he just fell. They never expected that the wire was it. But it was when the doctors noticed that they had heart failure type injuries they started looking deeper as to why so many were being killed. and the ones that wasn't were saying that they experiencing some kind of grabbing sensation and a flashing before they fell. :)
 
Re: Grounding, basic but tough topic

Hurk, I think the old telegraph systems consisted of a string of electromagnets, one at each station, in series. Could be quite an inductive kick when the loop opened.
 
Re: Grounding, basic but tough topic

Terry,

With the system being ungrounded, there is no return path to the source through earth. However, if a conductor faults to a non-current carrying enclousre or raceway that is grounded, and you are touching one of the other phase conductors while being grounded, the current has a circuit to flow back to its source.
 
Re: Grounding, basic but tough topic

Another referance for a ground is data. Ones and Zeros need to be reconized by computers in order for them to operate. The 1's measure between 3.5 and 5 volts and 0 is any thing below that voltage. When the ground starts to float around data can become corrupted as 0's can start to measure as 1's.
 
Re: Grounding, basic but tough topic

rjfrankn, your wording gives me the impression you are saying a signal ground would be sepearte and isolated from the AC electrical ground. I hope that is not your understanding as it violates NEC code and all telco data practices.
 
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