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#128012 - 05/19/02 02:33 AM Voltage Gradient
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
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?

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#128013 - 05/19/02 01:01 PM Re: Voltage Gradient
Chris Rudolph Offline
Member

Registered: 05/01/01
Posts: 160
Loc: Winter Park,Fl USA
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

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#128014 - 05/19/02 03:42 PM Re: Voltage Gradient
Frank Cinker Offline
Member

Registered: 07/29/01
Posts: 317
Loc: Pennsylvania
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

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#128015 - 05/19/02 03:59 PM Re: Voltage Gradient
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
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

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#128016 - 05/19/02 04:26 PM Re: Voltage Gradient
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
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)
_________________________
Don(resqcapt19)

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#128017 - 05/19/02 06:21 PM Re: Voltage Gradient
Chris Rudolph Offline
Member

Registered: 05/01/01
Posts: 160
Loc: Winter Park,Fl USA
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

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#128018 - 05/20/02 03:48 AM Re: Voltage Gradient
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
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...

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#128019 - 05/20/02 08:25 AM Re: Voltage Gradient
Chris Rudolph Offline
Member

Registered: 05/01/01
Posts: 160
Loc: Winter Park,Fl USA
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

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#128020 - 05/20/02 11:17 AM Re: Voltage Gradient
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
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).]

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#128021 - 05/21/02 03:21 AM Re: Voltage Gradient
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Chris, Paul...good examples here, thanks

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