Why Ground Split Phase Residential?

Status
Not open for further replies.
G

GBMorris

Member
Location
Regina, SK, Canada
Hi Folks,

Why is it common practice to ground (earth) the ground terminal in split-phase residential service? I fully understand the reason for the ground wire connection to the enclosures of appliances, etc., however, it is not clear why this ground wire must actually be "grounded" or "earthed". In what way would it not be feasible for the entire 120/240V circuit to be floating with respect to earth?

I do not know the answer, but I do not believe arguments around safety or GFCI operation hold water:

It does not seem that a grounded system is more safe, since in a grounded system there is a path for the current to return to the source through an earthed person. This is not the case with a floating circuit?if the person is the only part of the system that contacts earth, no current will flow through them.

The GFCI system can operate regardless of ground potential, since it only detects a discrepancy in the currents on the hot and neutral conductors and has nothing to do with voltage. Whether the faulty current path is through ground or elsewhere is irrelevant.

A colleague and I have been struggling with this question. Any insight you can provide would be appreciated! :D

For background, I am a transmission P&C engineer with a masters degree in electrical engineering, and about four years of experience.

Thanks!
Greg
 
I think it is important to consider that there is no way to make sure it remained isolated.


Consider the hoops that need to be jumped through to install isolated systems in hospital ORs. The circuits have to be short or they start to connect to ground through capacitance coupling / insulation leakage.

We do use ungrounded delta systems in industry but you can still get a fatal shock from hot to ground.
 
Another thing to remember is that with a grounded neutral you know what voltage the insulation in cords, motors, lamp sockets, breakers and other equipment needs to withstand. With an ungrounded system that voltage can get pumped up by non linear leakage to many times the nominal voltage.
Most important though is what iwire stated, namely that the first fault on the isolated system will cause a reference and current path to ground. That fault will NOT trip a GFCI since no current will flow. Now you have an "ungrounded" system that is no longer as safe as people assume. :(
 
Seems to me that the OP may be confusing terms. Are we talking about the Service of a residence or the System that provides the power?

The System is grounded by the PoCo; the Service is grounded by the installing electrician. The only safety then for grounding the service would be to alleviate (somewhat) surges from the PoCo or lightning strike.
 
Thanks for the replies, folks!

Sparky, to clarify, I am referring to the entire 120/240V circuit from the low side of the distribution transformer, including any connected residences.

I can buy the non-linear leakage current and maybe the capacitive/inductive coupling current arguments. That said, these effects would probably result in quite a small overall leakage current (a few hundreds of uA maybe?)?would it be practical to keep the neutral reference to ground through a high-impedance ground? This might be high impedance enough to prevent fatal shocks through a grounded person who might touch an energized part of the circuit, but low enough to keep the neutral from floating above some set voltage (say <50 V) given a large-margin assumption of total expected leakage and coupling currents.

Greg
 
I believe most utility transformers have the secondary neutral bonded to the supply side grounded conductors at the power pole. Most utility poles with a transformer have a ground electrode. So the distribution system is already grounded before the service drop. Since they (the utility) grounded it, the earth is now part of the system. To change this would be nearly impossible and a huge undertaking.

I'm not sure what difference it makes to put an electrode at the residence, other than an extra margin of safety from downed HV lines or lightning closer to the house than the distribution poles. Certainly, the house equipment grounding system needs to be bonded to the neutral, but anything needing an earth path (e.g. getting rid of static build up or RF systems which need an earth reference) could get it from the grounded wire coming from the utility.

If you wanted to start from a clean slate and design your own power distribution system, you could pick up the IEEE green book. It goes through many of the different distribution schemes and give some pro's and con's for each.
 
Last edited:
GBMorris said:
Thanks for the replies, folks!
Click to expand...

It is an interesting topic and for myself if I could be sure the system would stay floating and safe I would be all for it. :)

Lets put capacitance coupling aside for the time being and just look at the problems with trying to maintain an ungrounded system in a typical home.

Lets assume the power company will play along and provide us with a reliable isolated ungrounded supply. My own home is supplied by a pole mounted transformer down the street, there are likely 5 or 6 buildings supplied from it. A ground fault at anyone of these homes or the wiring on the poles between them would result in a grounded system and it might not be the neutral that goes to ground so suddenly the potential for a 240 to ground shock would exist at all the homes suplied by that transformer.

Instead of me going on I will ask you a question.

How would you set up the system so it would remain isolated and if it does fault to ground what would happen? How would that be indicated to the residents? How would safety be maintained?
 
iwire said:
...

Lets assume the power company will play along and provide us with a reliable isolated ungrounded supply. My own home is supplied by a pole mounted transformer down the street, there are likely 5 or 6 buildings supplied from it. A ground fault at anyone of these homes or the wiring on the poles between them would result in a grounded system and it might not be the neutral that goes to ground so suddenly the potential for a 240 to ground shock would exist at all the homes suplied by that transformer.

...
Click to expand...

So instead we intentionally build in the same thing, just at 120V instead of 240V? :huh:
 
Last edited:
iwire said:
Its not the same thing when the system is designed around it being so.
Click to expand...

I made my comment because I didn't think your explanation was very complete. I'd say it is the same thing, electrically, as far as I'm concerned. I'd only agree that the intentionally grounded system is safer to work on because the different sides are marked as such. It's the knowledge factor that makes the difference, not the electrical schematic.

When it is a grounded system ground faults trip breakers and open fuses stopping the hazard.
Click to expand...

Well, you hope they do, and in most cases they do. And when they don't (e.g. fault on the grounded conductor), you count on it not being quite so dangerous. (And then there's also the cases, like the PV systems I work on, where for a random reason you can't actually use a fuse or breaker to interrupt hazards.)

There's no reason that ground-faults can't be detected and hazards stopped on ungrounded systems when the right devices are employed. The various types of GFP devices are arguably a better way to handle this than relying on OCPDs; they are just more expensive, that's all. I think that grounding systems actually represents something of a tradeoff between a higher level of safety and a satisfactory level of safety at a lower cost. Or to put it another way, back when society lacked the technology for cheap compact GFP devices, grounding systems was the most practical way to meaningfully increase safety. As someone alluded to above, if you were building a system from scratch, nowadays you have other choices.

FWIW, I think these pages probably offer some better explanations than any of us probably have time to type in this thread...
http://www.allaboutcircuits.com/vol_1/chpt_3/3.html
http://en.wikipedia.org/wiki/Earthing_system
 
I believe that an unintentional ground detector, unlike a GF detector, will have a hard time indicating the location of the fault, even if attached to branch circuits, since very little current will flow through the fault. Can someone support or refute that?
 
jaggedben said:
I made my comment because I didn't think your explanation was very complete. I'd say it is the same thing, electrically, as far as I'm concerned. I'd only agree that the intentionally grounded system is safer to work on because the different sides are marked as such. It's the knowledge factor that makes the difference, not the electrical schematic.



Well, you hope they do, and in most cases they do. And when they don't (e.g. fault on the grounded conductor), you count on it not being quite so dangerous. (And then there's also the cases, like the PV systems I work on, where for a random reason you can't actually use a fuse or breaker to interrupt hazards.)

There's no reason that ground-faults can't be detected and hazards stopped on ungrounded systems when the right devices are employed. The various types of GFP devices are arguably a better way to handle this than relying on OCPDs; they are just more expensive, that's all. I think that grounding systems actually represents something of a tradeoff between a higher level of safety and a satisfactory level of safety at a lower cost. Or to put it another way, back when society lacked the technology for cheap compact GFP devices, grounding systems was the most practical way to meaningfully increase safety. As someone alluded to above, if you were building a system from scratch, nowadays you have other choices.

FWIW, I think these pages probably offer some better explanations than any of us probably have time to type in this thread...
http://www.allaboutcircuits.com/vol_1/chpt_3/3.html
http://en.wikipedia.org/wiki/Earthing_system
Click to expand...

You have some good points in there. Modern technologies make some differences as well. There is still a lot of old stuff out there, and the grounded systems have been around a long time.

NEC does have provisions for using ungrounded systems though, and there are times it is beneficial to use them - if done properly. Most ungrounded systems are going to be separately derived with customer equipment and will not be delivered by a POCO though. Then there is also the cousin of the ungrounded system - the impedance grounded system.
 
What is it with this fascination of always thinking what we do not have is better?

We use a solidly grounded system because after near one hundred years of using electricity it has proved to be the best among all possible systems.

Okay..... sure, there are one or two cases where something different can be called out; but for most of the world most of the time, actually almost all of the time, grounded wins.
 
jaggedben said:
There's no reason that ground-faults can't be detected and hazards stopped on ungrounded systems when the right devices are employed. The various types of GFP devices are arguably a better way to handle this than relying on OCPDs; they are just more expensive, that's all.
Click to expand...

No that is not all. Those devices are full of electronics that die. Fuses and for the most part small molded case breakers are extremely reliable.

There was a study done in Florida on standard GFCI devices that found a very large percentage to be faulty.
 
iwire said:
No that is not all. Those devices are full of electronics that die. Fuses and for the most part small molded case breakers are extremely reliable.

There was a study done in Florida on standard GFCI devices that found a very large percentage to be faulty.
Click to expand...
And we all know the instructions that say to test monthly - usually right on the face of the device are followed religiously:happyno:

Even myself, one that maybe better understands how important this test could be, just never thinks of testing them. I usually test them when I install them or when there are troubles, but almost never to just see if it is still functioning with no trigger to make me suspicious.

Same for smoke alarms.
 
iwire said:
No that is not all. Those devices are full of electronics that die. Fuses and for the most part small molded case breakers are extremely reliable.

There was a study done in Florida on standard GFCI devices that found a very large percentage to be faulty.
Click to expand...

Yes, maintenance adds to expense. So does quality control at the factory. So I think you are just repeating part of what I said in a roundabout way. As for OCPDs, there have been some threads here about many of them not being reliable. And I've seen my share of installations, including my own apartment, where the fuses clearly are not blowing at their rating or they would have done so long ago. And of course, the whole reason that GFCIs were mandated is because OCPDs don't afford very complete personnel protection. To repeat, it's a tradeoff.

It occurs to me at this point that perhaps the simplest answer to the OP's question could have been "to guard against an open neutral".
 
kwired said:
And we all know the instructions that say to test monthly - usually right on the face of the device are followed religiously:happyno:

Even myself, one that maybe better understands how important this test could be, just never thinks of testing them. I usually test them when I install them or when there are troubles, but almost never to just see if it is still functioning with no trigger to make me suspicious.

Same for smoke alarms.
Click to expand...

When I'm in a public bathroom or a motel room I usually punch them just for grins. I've reported a few to management (who probably vows never to book an inspectors' convention ever again.)
 
Status
Not open for further replies.
Top