What exactly is 'voltage gradient' all about? Are there some exacting parameters in the physics dept that would lend definition to this?
Is this an established bond or connection made to equalize potential from internal or external influence?
Or should i ask is it to isolate from, or gain a common denominator to all?
Can an area or group of conductables that are connected in some manner have a different 'gradient' than a similar scenario next door?
Does this 'gradient' need to be established by man made means, or can it occur naturally?

Voltage gradient is the rate of change of voltage with distance.It has the units of volts per meter and is generally studied in the transmission of electro-magnetic energy in free space or some other dielectric/transmission media.
A VOLTAGE GRADIENT can be man made and depends upon the physical dimensions and frequency and amplitude of the applied voltage waveform.
I would also assume that there are some voltage gradients that occur naturally like lightning strikes .

Chris

Sparky,

Many years ago an electrical inspector told me proper bonding of a swimming pool eliminates voltage gradients. I didn't want to tell him I didn't know what voltage gradients were. However, somehow equal potential applies when bonding is done correctly. I know I didn't give you the electrical engineers answer you were looking for. It's because I'm just an average electrician. The other application in the NEC is for livestock. Other than that I'm not aware of additional applications.

Frank

Frank,
It is a term i have always simply taken for granted, as many of us average electricians probably do.
Is this something that can be metered?, or has tangible parameters in relation to some constant? (earths' core??)

Chris,
i have a poor understanding of the physics here, are you applying RF as a reference? If so, please expand....

~Steve

n the electrical trade, a voltage gradient is most often a difference in potential between places on the earth within a few feet. Also known as the 'step potential". This occurs when there is current flowing through the earth. The difference in potential is due to the voltage drop across the resistance of the earth. The higher the resistance, the higher the "step potential".
Grounding grids just below the earth like those used in substations or ag facilities will reduce the voltage gradient by reducing the resistance and the voltage drop.
Don(resqcapt19)

Sparky,
A voltage gradient can be at any frequency,not necessarily at RF,but the value of the gradient given in volts per meter is a function of a lot of things like physical dimensions,applied voltage impedance/resistance,etc.Don and Frank have explained it much better than I in terms of a practical problem that is the voltage difference between two or more grounding points.It can be measured with some speciol metering that I saw advertised in the ECN magazine.

Chris

Interesting reply, thanks.
Assuming ( key word..) that a grid or other such installation were to achieve the minimal impedance, and given a current flow that would follow suit in like readings from end to end would then constitute a 'gradient'?
In other words, 'Voltage Gradient' would not necessarily be a zero deal? If in fact , say a 15ma constant where to be maintained ?
So the step potential, (i take this as literally from one step to another, man or beast) would yield no potential?
Please unfuzz me if i've gone off to the ozone...

Sparky,
Apply 120 volt,60 Hz,to a wire 100 feet long in free space that has a resistance of 10 ohms per 100 feet or 0.1ohm per foot.The current flow is 120 divided by 10 or 12 amps.The voltage gradient is 12amps times 0.1 ohm per foot or 1.2 volts per foot.
If we bury that wire in the dirt/ground,no longer free space,then we pick up capactive reactance and that will enter into the impedance and change the value of the voltage gradient.Remember that capacitive and inductive reactances/impedances are a function of frequency.60 cycle suff is so low in freq that it can be ignored for the example that I have given.
I hope this helps.It sure got me "juiced up".

Chris

Steve,

Have a look here , where I described the method we normally use in England to measure the impedance of a ground rod.

Following on from that description, when we measure the voltage between electrodes X and Z, then move Z closer and farther away, we're making sure that electrode Z is outside the voltage gradient of the rod being tested (X).

If Z remains outside the voltage gradients of X and Y, then the reading will stay the same on all three tests. If the readings differ, then Z must be within the voltage gradient.

I once heard of power distribution to some very remote parts of Australia being a single HV line with a return path by just a local ground rod. With enough current flowing, there could be enough of a gradient to get a p.d. of several hundred volts over a space of a couple of feet. I wouldn't want to be standing barefoot near that rod!



[This message has been edited by pauluk (edited 05-20-2002).]

Chris, Paul...good examples here, thanks

"...single HV line with a return path by just a local ground rod." 'Earth return' has been done in southern Alberta at 14.4kV to rural homes. It's quite odd to see the first time.

Bjarney;
would'nt this be a somewhat inefficent circuit, given earth impedance

sparky, it's for real. I found a few comments about it...a bit unusal. Don't know if it's used in the US.

“The Single Wire Earth Return (SWER) system was developed to achieve minimum cost in supplying widely dispersed rural customers, and is used in most States [of Australia.]”
http://www.worldaware.org.uk/awards/awards1997/gibb.html http://www.ctech.ac.za/conf/due/2001/abstracts/ab13.html

Regarding hobby flying in Australia: “The SWERs do not run along roads but cut through farm land and are not easy to see from the air. They consist of a single wire strung from the tops of pencil poles. The wires are high tensile strength steel. The poles are needed only every 500 feet, not the 200 feet we see in the US.”

Gives a new meaning to giving someone a hot foot!

It would be interesting to know what precautions they will be taking in Africa to keep people and animals away from any potential (excuse the pun) voltage gradient area around the rod.

Barrier? Fences?

In southern Alberta, Canada, SWER was observed at probably a dozen or two farmhouses. At each transformer the one visible “safety feature” was two driven rods—one at the transformer pole and one at the second pole out. For each transformer there was one lower neutral span between the first and second poles for interconnecting the two rods. They utility was required to conduct periodic ground-resistance testing by governmental mandate.

Each 14.4kV-primary transformer was limited to something like 15kVA, making the hi-side ground current a bit above 1 ampere at full secondary load. I’m sure earthworms considered it their version of The Vulcan Death Ray. In maybe 50 miles of road, I noticed only one ‘open-wye-primary’ transformer bank, to serve what appeared to be a small-town water pump. {This was couple of decades ago.}


[This message has been edited by Bjarney (edited 05-22-2002).]

Vulcan Death Ray Hehe... Conjures up memories of watching the old 1930s "Flash Gordon" serial on children's TV during the school holidays!

Leaving aside SWER, just how common are single-phase xfmrs fed line-to-neutral? I've never been on the HV distribution side of things here (except a local site system), but I've never seen anything other than 3w delta with single-ph xfmrs run phase-to-phase.

The majority of the utility transformers that serve single family houses in this area are single pahse line to neutral, with a common primary/secondary neutral.
Don(resqcapt19)

Bjarney,
this SWER is giving me a real short circuit, what say we move it a tad mainsteam???

One more way-off-topic post and then I'll quit. pauluk— phase-to-phase versus phase-to-neutral primary transformer connections in the US are a regional thing, and often passionately defended on local turf. Overhead or underground, the choices are the same. Within a mile of my house is ~ten 1ø ø-ø, [12,470V…and two primary fused cutouts] one 1ø ø-n, [7200V—probably a 50-year-old installation…one primary fused cutout] 3ø delta-primary dairy and <150Hp irrigation pumps, but also a new ~25-unit housing tract fed full 3ø underground from 3ø4W overhead […all with 3 primary fused cutouts.] One older ungrounded-wye-primary bank to a ~40Hp pump was recently retired. But also, I don’t think a homeowner could get 3ø residential service in the same development at any price. The subsurface 1ø transformers there are served ‘staggered’ ø-ø; probably 3-4 homes per transformer. In most cases, phase-to-phase powered 1ø transformers have better voltage regulation than otherwise equal ø-n versions, but are probably somewhat more expensive to install and maintain.

There are a lot of reasons for either, and in 3ø arrangements they manifest as delta-primary and wye-primary connections. Your favorite open-delta service can even be fed as open-delta or open-wye on bank primaries.

I understand that 3ø servce [208Y/120] in larger homes is common—as larger houses go, but usually billed at commercial power rates. (If you’ve got enough the money for a home, power cost isn’t a make-or-break decision.) There are cases of homes having indoor atriums cooled by 60-ton water chillers…i.e., with ~60Hp motors.

"America -- Land of contrasts." Also applies to power distribution systems!

Many thanks for putting up with my questions on this, but I thought it was about time I investigated the HV distribution a little more deeply.

Steve,
Sorry for unintentionally hijacking your thread. Didn't mean to, but one thing just seems to lead to another.