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#128821 - 02/01/04 09:49 AM Members- please review this document 4 me
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
I posted the message (text) below at Mike Holt's site - in a thread regarding Ground Rods (it started out as a simple question, then became a 40 hit hotfile - so I of course had to add more hits...).

Would like to have some input / comments from you, as to what you think about of the following text.

Reason being is that I'm looking for inaccuracies and unclear areas of the message, plus trying to be sure the point(s) got across as planned.

Thanks in advance!

Scott35



 Quote:


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). 1Ø 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 1Ø 3Wire Grounded AC system.



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.



Scott35

_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128822 - 02/03/04 08:27 PM Re: Members- please review this document 4 me
PCBelarge Offline
Member

Registered: 06/08/03
Posts: 657
Loc: Dobbs Ferry, NY, USA
Scott
For the average person in our trade this post is too long without some visual aid to help describe what you are saying. That is a very detailed statement and can be hard to follow. You asked

Pierre
_________________________
Pierre Belarge

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#128823 - 02/04/04 06:44 PM Re: Members- please review this document 4 me
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Pierre;

Thanks for the reply.

Believe me, I take no offense to what you have mentioned - my messages are far from concise! (mini-series messages).
I was hoping to get 100% truthful responses for this.

Visiual aids would make these types of messages more interesting and comprehendable.

I'll try a prototype using the data in the clipped message.

Thanks.

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128824 - 02/04/04 08:15 PM Re: Members- please review this document 4 me
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Scott — I get bogged down on the term "reactive coupling" as it applies to utility power and distribution transformers. Should it be "leakage reactance"?

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#128825 - 02/04/04 11:18 PM Re: Members- please review this document 4 me
nesparky Offline
Member

Registered: 06/21/01
Posts: 650
Loc: omaha,ne
Scott
I would question your 8 amp with the ground rod witin 5 ft of the water pipe in wet soil.

I saw this type of thing happen by mistake once. A worker got in a hurry to terminate wire in a hot panel and shorted a #4 ground wire to a 20 amp breaker The # 6 bare melted but no breaker tripped. A HVAC guy told me that he saw the bare #6 turn red then melt. time was less than 30 sec.
( the existing panel was federal pacific)


You may have a ground loop there with most of the current not traveling to the source, thus the breaker may not see enough currrent to trip.
Just a question based on a scary experience.
_________________________
ed

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#128826 - 02/05/04 06:34 AM Re: Members- please review this document 4 me
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Thanks for the additional replies!

Bjarney;

 Quote:

Scott — I get bogged down on the term "reactive coupling" as it applies to utility power and distribution transformers. Should it be "leakage reactance"?


Sorry about the "Clear as Mud" descriptions used!

I was referring to Capacitive Coupling, Inductive Coupling and Leakage Reactances / Line Charging Reactances (Relative Permittivity) per the Coefficient of Coupling technobabble.

This data will be very, very, very-very-very difficult for readers to grasp, if that person does not understand the basics.
Even more difficult if this is the first time the reader has heard of such stuff!
What I need to do is make things much easier to understand, for those readers - while keeping a proper level for the more advanced reader.
Is this possible with a topic such as the one referenced here?

nesparky;

 Quote:

I would question your 8 amp with the ground rod witin 5 ft of the water pipe in wet soil.


This example scenario is to describe how distance and soil characteristics effect the conductive properties of an Earth-Ground type circuit.
In most normal conditions, the rod would need to be driven within 18" of the water pipe, to achieve a current of 8 amps flowing under a pressure of 120 Volts.

This example scenario is figured to be "Isolated" from all bonding for the rods, with the water pipe bonded to the power system in some direct method (such as N-G bonding at a service). The only conductive path between the rods and the water pipe is the Earth (dirt).

 Quote:

I saw this type of thing happen by mistake once.
A worker got in a hurry to terminate wire in a hot panel and shorted a #4 ground wire to a 20 amp breaker.
The # 6 bare melted but no breaker tripped.
An HVAC guy told me that he saw the bare #6 turn red, then melt.
time was less than 30 sec.
( the existing panel was federal pacific)


If this Fault resulted in a fused #6 cu conductor, then the fault level must have exceeded 300 amps for 30 seconds, or 500 amps for 10 seconds (don't have current density data at hand, just shootin' from the hip!).

That level of fault would exceed the typical "No-Trip" characteristics of FPE MCCBs, but also might exceed most other MCCBs too!
Hence, no trip of the device would occur.

Likely the conductor which faulted to the 20A Breaker, was solidly bonded to the equipment + system, or was connected to solidly bonded water lines, gas lines, etc.

If this happened, and the conductor was only connected to driven rods, it would be the best ground rod application ever!
The Resistance for the rod's part of the GES, would be less than 1 Ohm! Somewhere in the area of 0.5 Ohm would be likely here!

 Quote:

You may have a ground loop there with most of the current not traveling to the source, thus the breaker may not see enough currrent to trip.
Just a question based on a scary experience.


This is very true! The GES does not have the ability to conduct a sufficient level of current needed to trip an OCPD - even one of 10 Amp rating.

By its self (no bonding to the AC system involved), the GES will not clear a Ground Fault - due to the high Impedance between it and the Power Source.

Proper and solid bonding techniques are important to follow, to avoid the above situations.

The currents in a closed circuit will flow from the Power Supply (source), thru one conduction path (wire), thru a given contact point / load (for instance, a metallic enclosure), thru another conduction path (wire), and finally back to the Power Supply (source).
What flows out will eventually return again.
If a heavy flow originates from a given point - like an OCPD, the device will respond accordingly.
If flow is below rated capacity for Maximum load current, the device remains closed (unless it sucks!).
If flow exceeds the device's rated capacity for an extended period of time, then a trip function will open the device and flow will cease (again, unless it sucks!).

Thanks to everyone for the replies!

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128827 - 02/09/04 10:55 PM Re: Members- please review this document 4 me
wye-delta Offline
Junior Member

Registered: 01/18/04
Posts: 2
Scott35:

With regards to the subject matter, what we need is effective grounding system and that is proper understanding of grounding and its method.

In your post it only shows that a very bad grounding is present.In your reply to NESPARKY, if we can attain less 1 ohms resistance of grounding rod it would be better.Did American electrician follow the NEC when they installed grounding rod or they just push it to ground(earth) and didn't measure the resistance itself.For me I kept it as low as 2 ohms( 2 ohms is my max. allowable) so as to have low impedance grounding.

In my view, water pipes and gas pipes are suplemental grounding only and must not rely on it alone.Whenever there is neutral/grounding conductor a grounding rod outside the perimeter is a must and must conform with min. resistance, on how to make it low resistance is up to the knowledge and method of electrician making it.Although , we know that there is no perfect grounding.

Second, I am wondering why we have to put ocpd on grounded/neutral conductor.Whenever I wired I see to it that those conductor are continous and no ocpd this will help the ground fault current to return to its source without interuption of high impedance grounding rod.
_________________________
DON'T MEDDLE WITH ELECTRICITY...
THINK SAFETY FIRST...

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#128828 - 02/12/04 08:32 AM Re: Members- please review this document 4 me
wocolt Offline
Member

Registered: 12/06/02
Posts: 117
Wye-Delta:
Interesting post.
First I would like to know how you measure the resistance to ground and how you achieve a maximum of 2 ohms ground resistance.
Since there are so many variables involved in soil conditions ie, 'Average Resistivity' which is measured in (Ohm-CM) and these values are highly dependant on moisture content, temparature and soil conductivity itself. In other words the resistivity is that of the electrolyte in the soil.
You asked about putting an OCPD in the neutral. When a system has only one source of power(generator or transformer), grounding may be accomplished by connecting the source neutral to earth directly or througth a neutral impedance. Providing a switch or circuit breaker to open the neutral circuit is NOT recommended. It is not desirable to operate the system ungrounded by having the ground connection open while the generator or transformer is in service.

WOC

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#128829 - 02/13/04 09:00 AM Re: Members- please review this document 4 me
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Wye-delta,

there is a point in having a switch or OCPD in the neutral under certain circumstances. These conditions typically pertain to European systems, not systems designed in accordance with the NEC.

If you have fitted a sensitive ground fault interrupter that protects several circuits, contact between neutral and ground can mean you loose power to all the protected circuits. To find such a fault, you have to lift the neutral wires from the bus until you find the offending circuit. As the norm here is a single ground fault interrupter to procted an entire single family home, an neutral-ground fault will leave the house without power until an electrician arrives.

In addition, there are a couple of special conditions where a switch or OCPD in the neutral will in fact enhance safety somewhat.

Note that you are not normally allowed to switch a grounding conductor, nor a conductor which serves as both ground and neutral (grounded).

Could you please elaborate on the reasons for the 2 ohms and how you achieve this value?

[This message has been edited by C-H (edited 02-13-2004).]

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