ground questions


  #1  
Old 06-17-04, 03:55 AM
T
Member
Thread Starter
Join Date: May 2003
Location: USA
Posts: 102
Upvotes: 0
Received 0 Upvotes on 0 Posts
ground questions

subpanel in detached garage. 4 wires (Hot, Hot, Neutral and Ground) come in underground from house off a 100 amp breaker. The grounds and neutrals are separated, but I haven't yet installed the ground rod at the garage. So, the incoming ground wire (from the house) is connected to the ground bus, as are all the equip. grounds. Is this panel still providing a path to ground through the house's ground rod? I think it is, but just want to be sure. I know the ground rod has to be installed at the garage, but it's taking some time to get it done (work, etc). The power is on at the garage, but nothing much is being used out there yet. I'm just wondering if everything is safe for the time being, or whether I should temporarily bond the grounds and neutrals until I get the ground rod at the garage installed. Is there any kinds of current (amps) tests that could be done to make sure that all metal appliances are not conducting? I have a multitester with the clamp type ammeter.

thanks for your help
 
  #2  
Old 06-17-04, 04:30 AM
R
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,245
Upvotes: 0
Received 0 Upvotes on 0 Posts
Do not bond the neutral and the ground bars at the sub-panel in the garage. This would provide a second path for return current back to the panel and would provide problems with your ground in the garage.
 
  #3  
Old 06-17-04, 06:44 AM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
Your equipment grounding needs are being served by the grounding wire back to the house. This is what protects you and your family from electrocution in the event of an electrical fault. Neither the grounding rods at the house, nor the yet-to-be-installed grounding rods at the garage play a role in this. But install the grounding rods anyway for lightning protection. Don't wait too long. June is prime storm season in many parts of the country.

As Bob says, do not bond the neutral and grounding in the garage, even temporarily. It won't help and it does hurt.
 
  #4  
Old 06-18-04, 02:25 PM
T
Member
Thread Starter
Join Date: May 2003
Location: USA
Posts: 102
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by John Nelson
Your equipment grounding needs are being served by the grounding wire back to the house. This is what protects you and your family from electrocution in the event of an electrical fault. Neither the grounding rods at the house, nor the yet-to-be-installed grounding rods at the garage play a role in this. But install the grounding rods anyway for lightning protection. Don't wait too long. June is prime storm season in many parts of the country.
John, this is confusing to me. On one point, you say that "This is what protects you and your family from electrocution in the event of an electrical fault". Then, you say "Neither the grounding rods at the house, nor the yet-to-be-installed grounding rods at the garage play a role in this".


What are the purposes, theoretically, in ground rods (EG system)? This is somewhat confusing, because some say that the rod provides no protection in cases of fault current, that it provides only for Lightning and surges. Other materials I have read, says that its purpose is to activate overcurrent devices (breakers, fuses). But a grounding system of less than 25 ohms will not provide enough current to trip a 15 amp breaker. On the other hand, it seems that this would be a fault, which would cause a surge and would therefore cause the breaker to trip. I don't know. What's up?

OK, hypo: Hot wire in a service entrance panel comes loose and touches the metal chassis of the panel. Different scenario: The hot wire comes loose in an appliance and touches the chassis of that appliance:

1) what's the effect on the circuit breaker in an ungrounded system?
2) what's the effect on the circuit breaker in a grounded system?

thanks
 
  #5  
Old 06-18-04, 02:37 PM
R
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,245
Upvotes: 0
Received 0 Upvotes on 0 Posts
The ground wire that runs back to the main panel provides a path for fault current which will cause the breaker to trip without putting the high current on the neutral wire. If a fault occurs in a grounded appliance (say a washing machine) and the hot wire comes in contact with the metal body of the appliance, the current will have a path back to the source. Thiss means that high current will flow, causing the 15 or 20 amp breaker to trip.

The ground rod provides a means of equalizing the potential of the neutral and ground wires to the potential of the ground. This helps to deter lightning strikes as it puts the potential of the wires at the same potential as the ground.
 
  #6  
Old 06-18-04, 03:06 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
some say that the rod provides no protection in cases of fault current, that it provides only for Lightning and surges
This is correct.

I cannot answer your hypothetical questions unless you define the terms "grounded system" and "ungrounded system". Those terms mean different things to different people.
 
  #7  
Old 06-18-04, 03:31 PM
R
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,245
Upvotes: 0
Received 0 Upvotes on 0 Posts
The path for any fault current is not into the earth and/or through the arth. The path for a fault is through the grounded wires, back the ground/neutral bond and then back to the source of the power, power company.
 
  #8  
Old 06-18-04, 03:49 PM
T
Member
Thread Starter
Join Date: May 2003
Location: USA
Posts: 102
Upvotes: 0
Received 0 Upvotes on 0 Posts
John,

I define a grounded system as one which has a point of contact with a grounding electrode, i.e. a driven ground rod.
 
  #9  
Old 06-18-04, 04:44 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
In the case of a fault from hot to ground, several tens of thousands of amps will flow through the power company neutral for a few milliseconds until the breaker trips. In the appliance scenario, the equipment grounding wire from the appliance outlet to the panel will also carry those tens of thousands of amps to the panel. If there is a grounding electrode conductor to a grounding rod, a couple of amps will also flow to the earth during those few milliseconds. Those couple of amps won't make any difference to anything.
 
  #10  
Old 06-18-04, 05:10 PM
T
Member
Thread Starter
Join Date: May 2003
Location: USA
Posts: 102
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by John Nelson
.... Those couple of amps won't make any difference to anything.
Meaning that the breaker won't trip? Sounds like the gec is useless for a fault situation, then?
 
  #11  
Old 06-18-04, 05:13 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
Yes. Yes. Yes. Yes. Yes.

Clearing a fault is not the purpose of the GEC.

Repeat the following as often as necessary:

"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
"The GEC is useless for a fault situation."
 
  #12  
Old 06-18-04, 05:16 PM
T
Member
Thread Starter
Join Date: May 2003
Location: USA
Posts: 102
Upvotes: 0
Received 0 Upvotes on 0 Posts
sounds like a "yes" Tell me, would there be a copyright infringement if I were to copy/paste parts of an article from another website here? What about parts of a book? I did earlier, then I deleted it cause I didn't want to get in any troubles
 
  #13  
Old 06-20-04, 04:05 AM
T
Member
Thread Starter
Join Date: May 2003
Location: USA
Posts: 102
Upvotes: 0
Received 0 Upvotes on 0 Posts
So, is it correct to say that, in a ground fault (hot to ground short) situation, it is the neutral grounded conductor which takes the fault current back to the transformer? And if so, how would this trip the breaker or blow the fuse? Does the neutral become like the EGC?

Just trying to learn and understand all this, John. I don't mean to irritate you.
 
  #14  
Old 06-20-04, 05:00 AM
R
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,245
Upvotes: 0
Received 0 Upvotes on 0 Posts
There are only three wires back to the transformer. The two hot wires and the neutral. There is not other wire to the transformer.

So yes, the neutral carries the fault current back to the transformer.
 
  #15  
Old 06-20-04, 05:28 AM
W
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,104
Upvotes: 0
Received 0 Upvotes on 0 Posts
Exactly, fault current flows back to the utility transformer; ideally through the neutral only, and _not_ through the grounding electrodes. In some environments (notably urban installations where homes are supplied with metal piping systems, and several homes share a single transformer) a large fraction of the fault current will return through the grounding electrode system, but this is only because of the _metal_ interconnects in the ground (the underground pipes), and in many ways this is _not_ desirable.

The way to understand this is to draw a picture of the paths that current could follow. Remember that current can only flow through a closed circuit (complete circular path). Another way of saying this is that electricity always tries to travel back to its source.

In the scenarios that you described:
1) proper single point grounded system. Main to panel chassis fault: current flows from the point of contact to the ground/neutral bar, back to the neutral feed and back to the transformer. There are _no_ circuit breakers in this path. Current flow continues until the wire 'burns free' or a fuse opens on the transformer. The lack of OCPD is the main reason that service entrance conductors are required to be as short as possible and kept on the outside of the house. Branch circuit fault to 'grounded' appliance frame: current flows from the point of contact to the EGC, back to the main service entrance ground/neutral bar, back to the neutral feed, and back to the transformer.
2) 'normal' grounded system, where there is a ground to neutral bond both in the main service panel, and also back at the transformer, and at the service panels of every other customer connected to the same transformer. As above, except that a small amount of current flows from the ground/neutral bar to the GEC and then through the ground back to the source, in parallel with the main current flowing back on the neutral.
3) true 'ungrounded' system: In a true _ungrounded_ system, there is _no_ intentional connection between the neutral and ground anywhere. No bond at the transformer, no bond in the service entrance. This is an archaic system used in industrial situations, never in a home. Note that an 'ungrounded' system has grounding electrodes and GEC and EGCs, and all exposed metal is carefully bonded to earth potential through the ground rods. But the current carrying conductors are not connected to this ground system. Main to panel chassis fault: the panel gets 'energized' to the voltage of the main....but because of the careful Earth bonding, everything around gets 'energized' to this potential, so there is no shock hazard. No significant fault current flows. In the event of a _second_ fault between an energized conductor and the ground system, lots of current will flow. Branch circuit fault to 'grounded' appliance frame: same; everything gets energized, but no current flows. True ungrounded systems require careful monitoring in order to detect the first fault before a second occurs, and are subject to wild voltage swings in the event of a fault. Equipment and insulation can be destroyed by extremely low current high voltage leakage, since there is no path to dissipate these high 'static' voltages.

-Jon
 
  #16  
Old 06-21-04, 06:01 AM
hornetd's Avatar
Member
Join Date: Aug 2001
Location: Maryland
Posts: 646
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by tacticaltal
sounds like a "yes" Tell me, would there be a copyright infringement if I were to copy/paste parts of an article from another website here? What about parts of a book? I did earlier, then I deleted it cause I didn't want to get in any troubles
I'm not a lawyer but I believe that posting a small part of copyrighted material is considered "fair use" but that quoting all or a major portion is copyright infringement. It would be fair use to post one code section but not an entire article or chapter.
--
Tom H
 
  #17  
Old 06-21-04, 06:35 AM
hornetd's Avatar
Member
Join Date: Aug 2001
Location: Maryland
Posts: 646
Upvotes: 0
Received 0 Upvotes on 0 Posts
Purpose of Grounding / Earthing

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
(2) Grounding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.
(3) Bonding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path. (copyright 2002 National Fire Protection Association)
The name Equipment Grounding Conductor is misleading because as subsection 1 & 2 above tell us Grounding is done to limit or stabilize the voltage to ground. This limiting function does not limit the voltage to ground to a level that is safe for human contact but only to a level that is less likely to cause a catastrophic failure or fire. The Code Making Panel that is responsible for this article is presently considering a name change from Equipment Grounding conductor to Equipment Bonding Conductor in order to clear up the confusion. Subsections 3, 4, & 5 make it clear that an effective fault clearing path is what will provide safety from electric shock. That effective fault clearing path is provided by bonding the non current carrying conductive parts of the electrical system, and any other conductive object that is likely to become energized, back to the source of supply.

This can best be illustrated by a passenger on an airplane who uses the convenience plug at their seat to power a device while in flight. There is no path to ground / earth but the entire frame of the seat is bonded to the frame of the aircraft and so is the source transformer. Any fault between the circuit's current carrying conductors and the seat frame causes an immediate overcurrent condition which opens the breaker or fuse that protects the circuit. The fact that the seat is bonded keeps the voltage between the seat frame and the other conductive surfaces of the aircraft to such a low potential that there is no shock hazard to the person in that seat or anyone touching the seat. Effective bonding keeps the touch potential below a level that could cause a dangerous shock.
--
Tom Horne
 
  #18  
Old 06-21-04, 06:37 AM
M
Member
Join Date: Dec 2000
Posts: 475
Upvotes: 0
Received 0 Upvotes on 0 Posts
So, is it correct to say that, in a ground fault (hot to ground short) situation, it is the neutral grounded conductor which takes the fault current back to the transformer? And if so, how would this trip the breaker or blow the fuse?
The breaker will trip (or the fuse will blow) because the current in the hot conductor exceeds the breaker trip current. The breaker doesn't care how the fault current flows back to the source, only that the trip current through the breaker is exceeded.
 
  #19  
Old 06-21-04, 07:17 AM
hornetd's Avatar
Member
Join Date: Aug 2001
Location: Maryland
Posts: 646
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by mikewu99
The breaker will trip (or the fuse will blow) because the current in the hot conductor exceeds the breaker trip current. The breaker doesn't care how the fault current flows back to the source, only that the trip current through the breaker is exceeded.
Your reply is misleading. On a fifteen ampere circuit at 120 volts the impedance of the two grounding electrodes would have to be less than eight (8.0) ohms total, or an average of four ohms apiece, in order to carry enough current to trip the breaker via the earth alone. Such a low impedance on utility transformer or premises wiring grounding electrode systems is very seldom achieved. A very common measured impedance on utility transformer grounding electrodes is seventy ohms or more. On all but municipal water system grounding electrodes premises wiring system grounding electrode impedances are often fifty or more ohms even when concientiously installed.
--
Tom Horne
 
  #20  
Old 06-21-04, 07:24 AM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
I believe Mike's point was no more than he stated, that the breaker doesn't care how the current gets back. Which is true.
 
  #21  
Old 06-21-04, 10:14 AM
hornetd's Avatar
Member
Join Date: Aug 2001
Location: Maryland
Posts: 646
Upvotes: 0
Received 0 Upvotes on 0 Posts
That may be true John but Iwas anxious to avoid any misunderstanding about the role of circuit and system grounding in clearing faults which is negligable. The important point is that for almost all homes the only reliable fault clearing path is via the main bonding jumper and the service grounded conductor (neutral). At 100 ohms total grounding electrode system impedance; and in truth it is often far higher; the home and utility transformer grounding electrode conductors will carry only 1.2 amperes at 120 volts. Since the available fault current will often exceed 1000 amps that is not a significant factor in clearing the fault.
--
Tom H
 
  #22  
Old 06-21-04, 01:19 PM
T
Member
Thread Starter
Join Date: May 2003
Location: USA
Posts: 102
Upvotes: 0
Received 0 Upvotes on 0 Posts
My whole idea of the system ground was wrong, as I'm sure it is for many untrained persons. I thank everyone for clearing this up. I suppose my posts go beyond the general idea for this forum, which is to assist in helping the diy perform actual tasks. I tend to ask the questions that are more suitable to the classroom, I suppose, but if you'd indulge me just one more time:

With the idea that a fault, indeed even a normal, current attempts to reach its source, suppose that an open neutral, in a 120 V circuit occurred somewhere in a branch circuit. I guess the circuit would become open? Or would the EGC, which is bonded to the neutral in the SE panel, become the return to the source? If this is so, then might there be an appliance touch hazard since current is now flowing on the EGC?

Thanks to hornetd for supplying the code article. That helped alot. And winnie, your posts are always very detailed and helpful - ever think of writing a book?

Thanks to everyone for their replies.
 
  #23  
Old 06-21-04, 01:39 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
At the high level, we often lie to keep things simple. When we get in deeper, the earlier lie is often exposed.

The current doesn't actually try to get back to its source. It really has no idea where it came from. It is just seeking a point of lower potential than where it is now. Of course, this paragraph is also a lie, but it's a lie closer to the true than the prior lie.

Since there is no bonding outside the panel between the neutral and the grounding connection, once the neutral is open, the circuit is broken and no current flows at all on that branch circuit. There is just no path for the current to get to that grounding conductor outside the panel. Moreover, once the current (from this or other circuits) has reached the panel, it has much more attractive places to go than the branch grounding wires, so it snubs its nose at all the branch grounding wires in the panel. Of course, a few rogue electrons may flow back out that EGC, but it's only a million-trillionth of the electrons that choose to go down the power company neutral, so there is no touch hazard at the appliance.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: