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#129405 - 02/23/05 02:16 PM grounding (basic yet tough theory)
unsaint32 Offline
Member
Registered: 08/24/04
Posts: 15
Loc: eden prairie, MN, USA
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.
Sung.
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#129406 - 02/23/05 03:11 PM Re: grounding (basic yet tough theory)
BPiersel Offline
Member
Registered: 01/06/05
Posts: 18
Loc: Minnesota
Think about this:

If you don't have a controlled conductor-to-ground voltage, and you are running the conductor in metallic conduit, what voltage should the insulation be rated at? Assume the conduit is connected to earth.
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#129407 - 02/23/05 09:55 PM Re: grounding (basic yet tough theory)
trollog Offline
Member
Registered: 10/02/04
Posts: 276
Loc: San Diego California USA
>>why can't the neutral conductor be used as the reference point, instead of connecting the circuit to earth? <<

One purpose of grounding is safety. When a loose conductor hits a grounded metallic surface, it causes breakers to trip, thus (in theory at least) preventing dangerous current where it doesn't belong. Much grounding is done for reasons of safety, rather than to make things run right. If there seems to be a lot of superfluous grounding, its purpose is to make electrical circuits and systems safer. Yes, AC in 50 year old houses runs pretty good without the strict grounding practices used in newer homes, but they are far less safe from the dangers of fire and electrocution. From an EE or physics standpoint, yes, grounding panels isn't strictly necessary for things to work, but it is far less safe.

>>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?<<

All service transformers *are* grounded. Utility poles have ground wires. If it appears you are able to *magically* run your house with no ground and yet it somehow still works it is because you are just piggybacking off the utility's grounding at the transformer.. so there is no mystery here.. just an unsafe condition.


>>But, why do I care about the voltage to earth as long as I have voltage to loads?<<

"loads" are nothing but controlled paths to ground that perform some useful work on the way. Think of water flowing downhill. It doesn't flow uphill, only downhill. The consistency of gravity and of the path the water flows in determine whether water flows fast or slow, in a trickle or in a torrent. If the water in your house randomly flowed from the faucet at 1 psi, then suddenly rose to 10,000 psi in 1/60 of a second, then settled down to something like 80 psi then taking a shower or washing your hands or even using the water at all would be a disaster all of the time. Having a stable voltage to earth in this case is much the same as having reliable stable water pressure in your pipes- without stable voltage in your wires (voltage being nothing more than "electron pressure" like water pressure in a pipe) you can "cook" a wire's insulation (much like high water pressure "bursting a pipe") or damage electrical equipment you are using in your home, which is engineered to operate at a set range of voltage/hz. Grounding provides a reference point for loads- without a ground there is no "voltage" for the loads to use. If you put both probes of a voltmeter on the hot wire you will see that it gives no reading because voltmeters measure the difference between "ground" and what voltage is in a wire, but it is only when you look at what difference there is between a load and the ground that you can even detect voltage. In an indirect way, voltage to ground IS voltage to load, channelled though a lightbulb or motor or whatever.
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#129408 - 02/24/05 08:21 AM Re: grounding (basic yet tough theory)
BPiersel Offline
Member
Registered: 01/06/05
Posts: 18
Loc: Minnesota
Quote:
But, why do I care about the voltage to earth as long as I have voltage to loads?


For normal operation, it doesn't really matter. A 240V load will operate with one conductor at earth and the other at 240V above earth, or each at 120V from earth but 180 degrees out of phase, or even one at 480V and one at 240V. So why are systems grounded? For safety. Since almost all loads and wiring are in close proximity to objects at or close to earth ground potential, safety reqirements (including insulation type, air gap requirements, etc.) are determined in part by the maximum voltage to earth.
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#129409 - 02/24/05 07:20 PM Re: grounding (basic yet tough theory)
peterpang Offline
Member
Registered: 01/08/05
Posts: 19
Loc: toronto
The questions below is helpful for u to understand what is ground!
1)What is your voltage to ground
(earth,your machine,car,hourse....)?Is it stable?
2)Provided that the Primary voltage is given,will the RMS voltage of the transformer secondery coil variable?

Now let me answer your questions one by one!
Question 1)why can't the neutral conductor be used as the reference point, instead of connecting the circuit to earth?
A:My answer is very simple:The voltage of the neutral to you(ground) is not stable unless the neutral is grounded!!!!BUT the voltage between hot line and neutral is stable even the neutral is not grounded!!So surelly you can get a stable voltages to the loads in the circuit!!!why? If you can answer my second question then you will understand it,and finally you will understand what is a transformer!
Question 2)But, why do I care about the voltage to earth as long as I have voltage to loads?
A:The answer is the same as above!!!!!!That is:The voltage of the neutral to you(ground) is not stable unless the neutral is grounded!!!!Sometimes this voltage will goes to very ver high to ground(you).Do you think the neutral have a transient 1000v to you(ground) is good things?Will this voltage damage the electrical insulation of your machine(equipment etc.)?


[This message has been edited by peterpang (edited 02-24-2005).]
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#129410 - 02/25/05 01:46 AM Re: grounding (basic yet tough theory)
pdh Offline
Member
Registered: 01/20/05
Posts: 354
Why is it necessary (theory rather than code) to have one of the conductors be a grounded conductor? Already some circuit configurations do not have a neutral as such: 2-wire 240 volt single phase, and 3-wire delta (208, 240, 480, or 600 volts as the case may be). Yet they can be derived from a system that has a neutral point at the source transformer which can be bonded to earth ground there, giving the same lower ground potential voltages as if there was a neutral (e.g. 120, 139, 277, or 347 volts). Some of the reasons I do see include: Standard Edison screw base light sockets expose one of the conductors (so code requires it be on a L-N connection at 120 volts), though some newer base designs avoid this issue. Switches would have to be 2-pole, increasing some costs. Any others?
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#129411 - 02/25/05 04:18 AM Re: grounding (basic yet tough theory)
IanR Offline
Member
Registered: 12/06/04
Posts: 328
Loc: Palm Bay FL USA
Here's another quesion concerning ungrounded neutrals not having a stable voltage to ground.When working on old TV's with neutral bonded chassis, technitians will use an isolation transformer for protection from shocks. Well, this creates an ungrounded neutral. So, if an ungrounded neutral can cause high transients to ground, is the isolation XFMR actually improving safety or does it merely cause a false sense of security? Maybe OT, if so I apologize for the thread jack.
Ian

[This message has been edited by IanR (edited 02-25-2005).]
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#129412 - 02/25/05 05:39 AM Re: grounding (basic yet tough theory)
pdh Offline
Member
Registered: 01/20/05
Posts: 354
A low voltage lighting system in compliance with 411.5(A) and (B) would have the same kind of issues any ungrounded and isolated system would have, right? At least the TV repairman could intentionally ground the old TV chassis that's plugged into an isolated secondary, and have an effective zero reference right there.
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#129413 - 02/25/05 08:01 AM Re: grounding (basic yet tough theory)
BPiersel Offline
Member
Registered: 01/06/05
Posts: 18
Loc: Minnesota
Quote:
Why is it necessary (theory rather than code) to have one of the conductors be a grounded conductor?

In addition to the safety issues you mentioned (Edison base sockets, for example), I suspect it is also a matter of convenience in some cases. Take the standard 120/240 residential service. Since it needs to be referenced to earth, what is the easiest way to do it? I can't think of anything easier than bonding one of the 3 conductors to earth.
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#129414 - 02/25/05 08:13 AM Re: grounding (basic yet tough theory)
BPiersel Offline
Member
Registered: 01/06/05
Posts: 18
Loc: Minnesota
The TV repairman probably does end up grounding the set through the test equipment. It is very common for line-powered oscilloscopes to have the signal ground connected to earth ground, for example.

Isolated systems can have high voltages with respect to earth from static charges or unintended AC coupling. The larger the system is, the more likely these will occur. For a small isolated system such as low-voltage lighting, this is typically not a problem.
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