Explaining Why we use Neutral (Grounded) Systems

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I was talking with someone and I was trying to explain why we use 'grounded' systems vs non grounded systems because he heard that we use ungrounded isolation panels in Hospitals to help protect patients.

Other than it being a NEC requirment to run the neutral (grounded) conductor to the service per NEC 250.21 (C), I could only come up with very few reasons. I could use some Help.

1) Its safer to deal with only one hot (ungrounded) conductor in most cases especially in residential homes
2) Grounded systems are to connected to the earth, which helps to limit overvoltages and lighting surges in the system.


What other reasons can I give?? Thanks.
 
Visit 250.4 and read it in it's entirety. Be a good spot to warm up 250.20 and 250.21 are other code specific areas to further enforce thoughts on grounding. in regard to grounded, and ungrounded systems.
 
I say common sence is if you ground the centertap of a xformer you keep the touch voltage to ground to an absolute minimun. I know that not everyone will agree with this common sence aproach so let the clubing begin.
 
Shooting for THE main reasons:

  1. Consistent voltage - that does not change voltage at varied points in the circuit path depending on load. i.e. a simular type of effect seen in a lost neutral condition. (AKA High/low voltage event) The grounded conductor helps 'anchor' that neutral point at one place in the circuit, as well as it's voltage in relation to other legs or phases in the system. Other loads on different legs or phases may heavily change under varying load conditions.
  2. Touch potential - an ungrounded system voltage could be in excess of the nominal system voltage when compared to the grounded surfaces areas around electrical equipment. Making these potentials less predictable.
  3. So guys like Mike Holt can sell book about how, where and when to bond the grounded conductor.
I too will now duck.....
 
Shooting for THE main reasons:

  1. Consistent voltage - that does not change voltage at varied points in the circuit path depending on load. i.e. a simular type of effect seen in a lost neutral condition. (AKA High/low voltage event) The grounded conductor helps 'anchor' that neutral point at one place in the circuit, as well as it's voltage in relation to other legs or phases in the system. Other loads on different legs or phases may heavily change under varying load conditions.
  2. Touch potential - an ungrounded system voltage could be in excess of the nominal system voltage when compared to the grounded surfaces areas around electrical equipment. Making these potentials less predictable.
  3. So guys like Mike Holt can sell book about how, where and when to bond the grounded conductor.
I too will now duck.....


Ok, so mike holt can sell more books. sounds good enough for me. lol
 
Most of the time a solidly grounded electrical system enhances safety over a non-grounded system. Sometimes the reverse is true. Hospital ORs and cow milking parlours are just two places when lack of solid grounding (with alternative measures) are less dangerous.
 
If you get a chance to read 'Soares Grounding' , an appendix at the back gives a snapshot of the tremendous controversy regarding grounding back when the industry was first getting off the ground. Which installation is actually safer is not totally clear, and depends both upon the 'ideal' qualities of the functioning system, and the safety of the system when it suffers from expected failures and degradation.

IMHO A "perfect" ungrounded system will be safer than a grounded system...but no system is perfect. Insulation breaks down over time, and even new insulation leaks a bit of current. If you don't have a path for dissipation electrostatic discharge, then static zaps will help break down the insulation, etc. Net result from a practical safety standpoint is that a grounded system will be safer, unless you are talking small systems that are regularly and professionally monitored for very small defects (eg OR systems).

-Jon
 
---I was trying to explain why we use 'grounded' systems vs non grounded systems ---

Other than it being a NEC requirment to run the neutral (grounded) conductor to the service per NEC 250.21 (C), I could only come up with very few reasons. ---

1) Its safer to deal with only one hot (ungrounded) conductor in most cases especially in residential homes
2) Grounded systems are to connected to the earth, which helps to limit overvoltages and lighting surges in the system. ---

30 years ago I understood clearly what the NEC was saying about why we ground electrical systems. About 15 years ago I figured out I didn't have a clue what they were talking about and I finally came to the conclusion the code panel likely didn't either. (I still don't know what "line surge is".) But they have been saying this for so long that they are stuck.

Here is an example: I just recently had a nationally recognized code expert (that I shall leave un-named) tell me that I should definitely drive ground rods until the grounding resistance to earth was 25 ohms or less and that most knowledgable AHJs would agree. :-?:-? I asked for some science or physics on why that was true and the conversation deteriorated quickly.

Are grounded systems safer? - maybe not. The Norwegians use an ungrounded (or high impedance grounded) system called IT. It looks pretty safe to me.

We (under the NEC and NESC) use completely different systems depending on which side of the service disconnect we are on. We insist on tying the local house transformer neutral to the MV neutral and we insist on grounding both neutrals multiple times. Leaves me baffled why this is considered good practice.

Perhaps you saw the thread on this:
http://forums.mikeholt.com/showthread.php?t=116383

Here is a paper on grounding systems that is pretty good. MH had a reference on his website for a while. The first couple of pages on the history is pretty interesting, then it's pretty boring until page 15, section 3. Sections 3 and 4 may be what you are most interested in.
http://www.schneider-electric.com/d...ility-safety/low-voltage-minus-1kv/ect173.pdf

cf
 
I just recently had a nationally recognized code expert (that I shall leave un-named) tell me that I should definitely drive ground rods until the grounding resistance to earth was 25 ohms or less and that most knowledgable AHJs would agree. :-?:-? I asked for some science or physics on why that was true and the conversation deteriorated quickly.
See if you can ask him whether you have to go back and re-check installs you did after wet weather once they have dried out. :cool:
 
My two cents...or pence
Assume a domestic single phase 230V supply. The neutral is connected to ground at the POCO supply transformer LV star point. So there are three conductors, live neutral and ground. The exposed conductive parts of appliances are connected to ground. It's a typical system for UK and elsewhere in the world.

If a live conductor has a fault and contacts a grounded part, fault current shoud take out the over current protection. If the neutral does, a residual earth current device should.

For an isolated supply you could possibly have a live to ground fault on one bit of kit and a neutral to ground on another. So possibly 230V for two adjacent bits of kit.
A grounded system takes that risk out.
 
My two cents...or pence---
---For an isolated supply you could possibly have a live to ground fault on one bit of kit and a neutral to ground on another. So possibly 230V for two adjacent bits of kit.
A grounded system takes that risk out.
Got a half pence change for you ;)

I don't think it is the grounding that matters, it is having all of the adjacent "bits of kit" bonded together. And of course, one needs ground detectors.

cf
 
See if you can ask him whether you have to go back and re-check installs you did after wet weather once they have dried out. :cool:

I do my darnest to not be beligerent. Whether I agree or not, he is the nationally recognized expert - not me.:confused:

Heck, I can't even figure out why running the neutral current over the grounding/bonding conductors is dangerous.

cf
 
My two cents...or pence
Assume a domestic single phase 230V supply. The neutral is connected to ground at the POCO supply transformer LV star point. So there are three conductors, live neutral and ground. The exposed conductive parts of appliances are connected to ground. It's a typical system for UK and elsewhere in the world.

If a live conductor has a fault and contacts a grounded part, fault current should take out the over current protection. If the neutral does, a residual earth current device should.

For an isolated supply you could possibly have a live to ground fault on one bit of kit and a neutral to ground on another. So possibly 230V for two adjacent bits of kit.
A grounded system takes that risk out.


This would be true in a grounded system where the POCO Earth the XO, but what if the XO wasn't grounded by the POCO, here in America we allow a un-grounded delta for industrial if a phase to earth monitor alarm is used, I would say if kept isolated, and a monitor it would be safer on the load end.

All most all electrocutions accrue between a ungrounded conductor and Earth or something connected to earth, with a reference of the XO to Earth this completes the path through your body, now if after the last transformer to your service was kept isolated, and a monitor with a bell or siren/red lights on it, and since there is no reference to Earth accidentally touching a current conductor and something that is referenced to earth such as concrete, there would not be a complete path for current.

In understanding why Earthing was brought into existence, we have to travel back to the days of telegraph, at first the telegraph wire was strung along wooden poles for miles connecting to wooden insulators or stand offs, the problem with this arrangement was it provided a resistance through the insulator and down the pole to earth, this placed a higher load on the batteries used to supply the power for the telegraph and limited the distance to each station.
Well someone invented a glass insulator and it was a success, it had such a high resistance to Earth the batteries lasted longer and distance could be now greater, but another problem arose, it seemed that line men and operators working on and repairing the lines was being electrocuted, at first they weren't sure what it was, but it was discovered, that winds, and later soler winds would cause a voltage to build up on the lines to a very high potential, while the wooden insulators would keep this voltage drained down, the glass ones would allow it to build till something came into contact with it and Earth.

so they found that by Earthing one of the conductors would allow the voltage to drain off without increasing the load on the batteries.

this was carried over to developing electrical transmission lines, and some how over to even secondaries of local transformers feeding loads less then 1,000 feet from it.

In turn it developed to a fault path to automatically open the line for voltages above 500 volts and by miss information over to lower voltages, when it was discovered that the Earth couldn't reliably open a OCPD it turned into voltage stabilization, and protection from lightning.

and like the energizer bunny it keeps on going from there.

the rest is in the PDF file Cold Fusion posted at the bottom of post 8
 
Got a half pence change for you ;)

I don't think it is the grounding that matters, it is having all of the adjacent "bits of kit" bonded together. And of course, one needs ground detectors.

cf
I was using the concept of "ground" to cover exposed conductive (rather than conducting) parts. The notion of what's live an what's neutral also become a bit tenuous. Sloppy on my part and for that I apologise.
 
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I believe the OP was trying to explain why we have such a system, not if it was safer than than the alternative. As the pressures on businessmen never seem to change, I believe the answer is related to what was cheaper to install back when large scale electrical systems were being developed. :)
 
This would be true in a grounded system where the POCO Earth the XO, but what if the XO wasn't grounded by the POCO, here in America we allow a un-grounded delta for industrial if a phase to earth monitor alarm is used, I would say if kept isolated, and a monitor it would be safer on the load end.

All most all electrocutions accrue between a ungrounded conductor and Earth or something connected to earth, with a reference of the XO to Earth this completes the path through your body, now if after the last transformer to your service was kept isolated, and a monitor with a bell or siren/red lights on it, and since there is no reference to Earth accidentally touching a current conductor and something that is referenced to earth such as concrete, there would not be a complete path for current.

In understanding why Earthing was brought into existence, we have to travel back to the days of telegraph, at first the telegraph wire was strung along wooden poles for miles connecting to wooden insulators or stand offs, the problem with this arrangement was it provided a resistance through the insulator and down the pole to earth, this placed a higher load on the batteries used to supply the power for the telegraph and limited the distance to each station.
Well someone invented a glass insulator and it was a success, it had such a high resistance to Earth the batteries lasted longer and distance could be now greater, but another problem arose, it seemed that line men and operators working on and repairing the lines was being electrocuted, at first they weren't sure what it was, but it was discovered, that winds, and later soler winds would cause a voltage to build up on the lines to a very high potential, while the wooden insulators would keep this voltage drained down, the glass ones would allow it to build till something came into contact with it and Earth.

so they found that by Earthing one of the conductors would allow the voltage to drain off without increasing the load on the batteries.

this was carried over to developing electrical transmission lines, and some how over to even secondaries of local transformers feeding loads less then 1,000 feet from it.

In turn it developed to a fault path to automatically open the line for voltages above 500 volts and by miss information over to lower voltages, when it was discovered that the Earth couldn't reliably open a OCPD it turned into voltage stabilization, and protection from lightning.

and like the energizer bunny it keeps on going from there.

the rest is in the PDF file Cold Fusion posted at the bottom of post 8

Interesting story, but not quite true.

Telegraph never had anything to do with the intentional grounding of a current carrying conductor.

Telegraph lines use one wire conductor and an earth return. That's where we get the 25 ohm resistance figure for grounding electrodes. The sounders operated typically at 3-6 volts. Two to four batteries was the norm. Even on long lines voltages as high as 100 were seldom used as the amount of batteries needed approaches the unmanageable. The higher voltage systems were called main line systems and were for long runs only.

The story about arc over has it's roots from a severe solar storm in 1859 and another in 1882 that caused massive surges on telegraph conductors and destroyed a whole bunch of equipment. Solar storms also have damaged modern electrical systems. We will see the most severe storms of the next cycle at it's peak in 2012 or 2013.
 
All arguments beaten to death here my personal preference is a grounded system. I know exactly when I am in jeopardy and when I am not.
I think that's a reasonable opinion. (both the arguments are beaten to death and you like grounded systems)

My preference is for:

Distribution at impedance grounded 13.8kv, trip on first fault.

Main equipment source is impedance grounded 480Y - limits the first fault current to 5A - 10A. Ground detected so one knows when to look for the first fault. No trip on first fault

For the low end: If I were electrical emperor-for-a-day, and the Europeans did not already have a standard, I'd go with high impedance grounded 240Y - all residential appliances/lighting decreed to be 240V.

But I'm not EEFAD, so my low end preference is 208Y, with a Ronk Blocker to get rid of the neural shift.

I think my industrial bias is showing:D

cf
 
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