grounding electrode

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domnic

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Electrical Contractor
Question from student? The purpose of the grounding electrode (as i understand it to be )is to direct lightning to earth if the building or structure is struck by lightning and also helps to stablize the voltage from the transformer. What I dont understand is how the grounding electrode helps stablize the voltage from the transformer and what affect the grounding electrode has on the lines of flux in the transformer,please explane?
 
Re: grounding electrode

If you had a 600 VDC and 480 VAC supply, you could have the 480 VAC riding on top of the 600 VDC. I is possible for one voltage to ride another voltage. A grounded system will attenuate that problem. Most likely, the 600 VDC will actually be a static buildup that doesn't have a path to ground and will add electrical stresses to the insulation.

I did not check this information before posting since I am at home today. I trust someone will verify my facts. :D
 
Re: grounding electrode

Another function of the grounding electrode is for safety reasons. Since the neutral and ground conductors are bonded together, and bonded to the grounding electrode, there is no voltage between them. Now add a concrete basement (the concrete is a pretty good conductor of electricity). Since the concrete is in contact with the same dirt as the electrode, the voltage between the concrete and the neutral is zero. Therefore, the neutral wire cannot shock you unless you are somehow connected to the hot wire.

In addition, if a hot wire were to come in contact with the concrete slab, the grounding electrode provides a path for current to get back to the transformer, and hopefully this will trip a circuit breaker. On the other hand, take away the grounding electrode, and a short could cause the entire slab to be at a dangerous voltage.

The grounding electrode has no significant effect on the lines of flux in the transformer. (Under normal operation, no current flows through the grounding electrode).

STeve
 
Re: grounding electrode

a transformer is a magnetically coupled device and therefore the primary is isolated from the secondary.

i will use an example of an actual event that occured at a plant i worked at.

a plant had to install a new transformer at their facility when they expanded. the installation crew incorrectly wired the grounding point of the secondary to the wrong tap point (not x0). the following monday morning, some of the equipment would not operate. no one knew why. thank goodness that this was the first week of the shutdown period in july and production was building up reserve stock.

we also installed several pieces of equipment on that day, and of course modifications had to be made. it was hot and we were sweaty, when one emlpoyee got under the machine and tried to use a drill. as soon as he pulled the trigger, he yelled. he then showed me the sparks from the drill bit to the machinery when he pulled the trigger. after doing some investigation, we found a 68 volt potential between the ground and neutral of all plant receptacles. this was caused by the incorrect grounding point. we now had 2 paths for return current - the neutral and earth ground. earth ground was also connected to the plant structure.

we then connected a ground wire from x0 of our control transformer to the machine ground. this then established a 0 voltage point for the 120 volt controls of the secondary and eliminated the dual paths and the shock potential was eliminated. this had to be done to a lot of equipment that week. the transformer has since been rewired and the dual path has been eliminated. we also ran across incorrect ground wiring at 2 other plants also.

what charlie said is also true. i am not allowed to discuss the details of this plant, it was a good educational experience.
this occurred 16 years ago. i hope my information is helpful.
 
Re: grounding electrode

Originally posted by steve66:
In addition, if a hot wire were to come in contact with the concrete slab, the grounding electrode provides a path for current to get back to the transformer, and hopefully this will trip a circuit breaker
Click to expand...
Steve, that is a incorrect statement. Concrete would not exhibit a low enough impedance to even trip a 15-amp breaker IMO. Earth alone cannot be used as a fault clearing path per NEC 250.4(5).

The reason we use a ground is strictly for safety. They are:
1. Provide a fault path for lightning.
2. Provide a fault path in case of accidental contact with HV or MV systems.
3. Provide a zero voltage reference with respect too earth to minimize touch potential of metallic objects.
4. Stabilize system voltages and prevent stress to cable and circuit insulation.
5. Provide a discharge path for static electricity.

[ February 17, 2004, 10:23 AM: Message edited by: dereckbc ]
 
Re: grounding electrode

Quote:
_________________________________________________
Provide a zero voltage reference with respect too earth to minimize touch potential of metallic objects.
_________________________________________
Dereck
Grounding is a subject I have been dealing with a lot in the last few weeks. I have been researching the subject and keep getting mixed signals in regards to ground rods reducing touch potential. I had always agreed with you on ground rods reducing touch potential until I was reading Grounding Myths (by Mike Holt).

Quote from Mike Holt:
__________________________________________________
Ground rod reduces Touch Potential
Myth: A ground rod can be used to reduce touch voltage from a ground-fault to a safe value by creating an equipotential plane.
False: Because the earth is a resistor, not a conductor, touch voltage will not be reduced to a safe value.
__________________________________________________
I understand that because of the resistance to earth of most ground rods the OCPD would never trip. If you were standing next to the ground rod the earth you would be standing on would be at almost the same potential as the metal object with the ground-fault, so in most cases you would not be shocked if you came in contact with the metal object. I infer from this that if you are standing a distance from the ground rod and touch an energized metal object the resistance of the earth would negate the effect of the grounding rod to a large degree and you would be more likely to receive a shock if you came in contact with the metal object. I know there are a lot of you who have a better understanding of this subject than I do. If you have a difference read on this could you explain your view?

Don
 
Re: grounding electrode

Don McClain you are correct. The reference I used comes from many sources, and IMO it is intended as normal operating conditions rather than fault conditions.

If there is a fault or load current flowing through the earth, there is going to be step potential differences. Earth cannot be used as a fault clearing path. It would take a grid or some sort of ground plane (equipotential) to minimize step potential differences.

Perhaps I should of said "Provide a zero reference point from which voltages differences can be taken" or something to that effect.

[ February 17, 2004, 01:44 PM: Message edited by: dereckbc ]
 
Re: grounding electrode

Maybe the text from a message I posted in the Ground Rod thread may be of help:

Ahhh, the Grounding Electrode System's usefulness topic has arrised once again! It's last
mega hot file discussion here at MHE was (I think) in the "Old Forum" just before Y2K rollover.

I'll make a quick and dirty description for the effectiveness of a local GES at a 1 Family
Dwelling - using any type(s) of Electrodes (C.W. pipe w/ >10' direct buried and metallic,
Driven Rod(s) as supplimental or sole GES, Encased Electrodes - AKA "Ufer", or whatever else
can be made).

Here's the "Geek-Dom" info:
Electrical Power is derived from a common use Power Transformer, which has a Medium
Voltage Primary feeder (4KV to 34.5KV). 1? Isolated Transformer with 2 to 5 %Z range, 25 to
37.5 KVA Apparent Power rating, center tapped Secondary winding, oil filled, Silicon Steel
laminated core.
120/240VAC 1? 3Wire Grounded AC system.

<end of Geek'o info string>

Surge supression and first point of contact for grounded secondary circuit is via #8 conductor,
attached to side of wood pole; and it extends into dirt (earth) with the pole. Connection to
center tapped terminal + bond to metallic Transformer enclosure.

At this location, the Center tapped point of the Secondary winding, along with the now
"Grounded Conductor" are at close to zero potential to the "Grounding Plane" surrounding
the Utility pole. This also results in each of the Ungrounded Conductors having a potential of
120V maximum to the local Ground Plane.

Simple so far?, let's toss some variables in.

100' from this point, the potential between the Grounded Center Tap Conductor (AKA
"Neutral") to the grounded plane will not be zero - but more like 10 or 15 volts. Also, the
ungrounded conductors have a changed potential to the grounding plane.
Why? Because the distance has introduced an Impedance within the soil, and now you have
the classic "Higher Voltages Across The Higher Impedances" deal, for Series Circuits.

Using this data, what happens if the Secondary is not grounded at all?

Via Reactive Coupling, there will be a potential difference (and a corresponding path for
current to flow) between the Primary and Secondary winding circuits.
Now each of the Secondary circuit conductors have a Series additive connection to the
Primary feeders - and for whatever the level of Impedance is, there will be a resulting
Voltage - at a level required to push a corresponding level of current through this Reactively
Coupled circuit.
If the Impedance is relatively low, so will be the potential difference between two points (the
Voltage). If Z is very high, the voltage will also be high.
At a point, the secondary circuit may easilly have potentials to earth ground as high as
15,000 VAC.

So how does this relate to a GES at a house?

Think of each house's "Ground Plane"! We are re-establishing the "Zero Point Of Potential"
for the Grounded Conductor, and as an extra added bonus, the Ungrounded Conductors will
be "Set" back to 120VAC potential to the local grounding plane.

In other words, the AC system has become bonded to a new local "Ground" at the house,
because we have bonded the Grounded Center Tapped Conductor to the Earth surrounding
the house.
To keep the Bonded metallic enclosures + EGCs to the same level (zero, or close to zero volts
to ground) - along with establishing a solid low Z connection to the AC system, the fault
clearing bonding conductors + main bonding jumper for enclosures are also connected to the
system's Grounded Center Tapped Conductor at the same location of the local ground
plane's connection - the GEC for the GES.

The GES does not need to be a current carrying structure - nor does it require a very low
Impedance (Z) to the Power Transformer's Grounding Plane (it should not have low Z,
because this will create parallel paths for large current levels to flow!). It also should not be
a very high Impedance, which will result in high potentials. A happy medium needs to be
found.

With this local grounding plane established, the chances of high potentials being created
across a semi-high/semi-low Impedance (a person) are nullified.

As with the Service feeders 100' from the pole and Xformer, when circuit length increases the
distance from the Service point + local GES bonding of the AC system, the potentials to
ground will also increase. This occurs on all related conductors - Grounded, Ungrounded and
Grounding.

To wrap it up, the Grounding Electrode System (GES) does not assist in conduction of fault
currents for L-G fault situations. This is done via the Bonded Metallic Enclosures + Raceways,
and supplimented with the Equipment Grounding Conductors (EGC). These items carry
Ground Fault currents, to the point in the local system where the AC system is physically
bonded (connected) to them. This is, of course, the point where the "Noodle" is connected to
both the grounded enclosures and EGCs, plus the GES - via the Grounding Electrode
Conductor (GEC).
What the GES does do during an L-G fault is keep the potential to ground on all items
bonded to it, as low as possible.

During a Line-to-Ground (L-G) fault, there will be a very low level of current flowing through
the earth back to the Transformer. Lucky to see anything as high as 5 amps at the time of
L-G fault.
Remember one simple thing here: there are many paths for ground currrents to flow back to
the Transformer besides the earth its self. The next-door neighbor's house has the same
bonding of the AC system to it's GES, so currents flow through their stuff, and the earth
ground.

(BTW: no significally large current flows in the grounded circuitry or structures when a
Line-to-Line fault occurs. Only circuit charging and Reactive coupled stuff will be 'a-flow'in on
grounded parts!).

As mentioned about driving rods and connecting them to a 15 amp 120VAC circuit, without an
overcurrent resulting (trip the breaker), this is very true - but is also very basic.
Drive a single 10' rod in semi damp, low Acidic, sandy soil. Drive it >30' from any buried
metallic things which are bonded to the AC system (directly or indirectly), then connect the 15
amp circuit to it.
Lucky to get 3 amps flowing.

Do the same in very damp, medium Acidic, unsandy soil - still >30' from buried treasures .
Now you might be drawing 8 amps!

Drive the rod about 5' from the Cold Water line coming from the street (which is bonded to
the AC system in some way), with the more conductive soil types described in the "8 amp"
scenario above. Chances are that breaker is gonna trip - and trip fast!

OK, now it's Audience Participation time!

Does this shed light on the whole GES thingee?

Let me know whazzup.

Send all flames to:

Mr. Spam.
1122 Green-Eggs and Ham Drive.
Sam I Am, CA. 90u812

They will read them on a boat; they will feed them to a Goat.

<end sillyness>
Click to expand...
Additional information posted by others may be found in this thread too!

Scott

Link to thread = http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=4;t=000151
 
Re: grounding electrode

Scott:
I found your post very informative. Most of it doesn?t apply to my location. For the last two months I have been fighting with co-workers and safety personnel who think the earth is an electric sponge and a ground rod will suck up any and all voltage and make everything safe. It was suggested at one point that we drive 6 ground rods to make everything safer. I will outline our electrical service and my view on what the ground rod is for. All comments are welcome.

Power is generate with three 1050 KW 600 volt generators. 600 volt and 480 volts systems are ungrounded delta systems with a ground detection system. 120/208 lighting system is a delta/wye (112.5 kva) with the start point bonded to the ground buss in the panel box. A ground rod is driven at the generator skid and a 12? pipe than is set in the earth appx 15? is used as the supplemental electrode. The GEC is connected to the ground buss at the panel box and a bonding jumper is connected to the metal skid. There are no under ground water pipes to bond to. All electrical cable contains an EGC. All equipment is mounded on metal skids. All skids are bonded to each other with a 3/0 cable in a loop configuration so if one bonding cable is cut a bond will still exist through the other bonding cable. Both ends of the loop bond back to the generator skid. We have five sites like this. All are portable and move every two to seven days. We try to conform to the NEC but there is no requirement that we have to.

As I see it the grounding electrodes we have are sufficient. In my view they are used to discharge static electricity and in the case of lighting strikes. There are no overhead power lines around the sites.

Don

[ February 18, 2004, 01:23 AM: Message edited by: mclain ]
 
Re: grounding electrode

I will concede that the resistance of concrete may be too high to trip a breaker. I was trying to keep it simple for domnic, and probably made some incorrect simplifications.

Steve
 
Re: grounding electrode

Pierre I live in Texas but work in Chad, Africa. There isn?t a village within one hundred miles of our location that even has electricity. Due to satellites and the internet I am able to post to the forum from Chad.

Don
 
Re: grounding electrode

It is a long commute. I work a 28 day rotation. I am in Chad for 28 days then fly home for 28 days.

Don
 
Re: grounding electrode

Don
An excellent reference on this material is the Soares Book on Grounding put out by the
International Association of Electrical Inspectors
972 235-1455 or fax 972 235-3855. You can pick one up next time you are home. They are at
901 Waterfall Way, Suite 602
Richardson Texas 75080-7702
 
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