Re: ground rod
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>
Scott35
