Earth as only neutral

[karl riley]

Re: Earth as only neutral

Bennie, if no one (electricians) believed one of my electrical assumptions I would do some deep thinking about that assumption. Either I have a misunderstanding or else it would be important for me to explain to everyone exactly why they are wrong.

If the 90% of the resistance of a rod is within a foot (I have heard 6 inches) of the rod, then anyone standing a foot away would get the 500V touch potential.

Bennie, can you explain your reasoning?

If there is someone on the forum who has worked on these systems, perhaps they can explain how the grounding system is made safe for personnel.
Karl

 

[bennie]

Re: Earth as only neutral

You must not be grounded when working live lines barehanded.
You must not touch the active line when working on the ground system bare handed.

The only precautions for the earth connection on a SWER system is to have the lowest possible resistance.

The conductor from the transformer is not protected from touch.
There would be a lot of dead kangaroos if this conductor is lethal.

I am still having problems with the earth shell theory. This voltage drop is common mode and is dropped in the earth. I will do some more research in my old documents.

 

[bennie]

Re: Earth as only neutral

The 230 KV iron maiden transmission towers, owned by PGE, have a minimum of 3 amperes flowing in the MGN system to ground rods at the legs, at all times.

Children play around these towers. If the rod has 25 ohms resistance this would be 75 volts. There is no special precautions with these ground connections.

 

[bennie]

Re: Earth as only neutral

Karl: I am not saying everyone is wrong. I am saying something is being left out.

The touch potential calculations do not factor in the resistance of the surface area in contact with the human.

I do retract my statement concerning charging the area to equal potential. That statement is in error.

The 16 KVA transformer, shown on the SWER site, will draw 1.26 amps, full load, at 12.7 KV.
The voltage drop, with the surface resistance, makes this a safe installation method provided resistance at rod is not over 24 Ohms.
24 Ohms will create 30 volts drop.

 

[dereckbc]

Re: Earth as only neutral

Is it possible a ground grid is being used? This is what utilities use to eliminate ground radiant or step potential problems.

 

[bennie]

Re: Earth as only neutral

Dereck: My information indicates driven rods, up to three.

 

[bennie]

Re: Earth as only neutral

To clarify the statement about bare handed live line maintenance, the surface resistance, in most cases is of a high resistance. Standing on this surface, and touching the ground wire, will have limited potential.

The body is charged to the system voltage, but without an opposite polarity present there is no difference in potential.

This is similiar to the live line scenario.

 

[hurk27]

Re: Earth as only neutral

think that what being missed here is the load current on this primary. As most SWER system's I have read about only serve a 60 amp or less services and are fed by a 19.5k or higher line. if voltage drop is a combination of resistance and current then it would be very low. 60 ampsx 240 = 14400 volts / 19500 volts (pri.)=.74 amps on the swer neutral .74 amp.s x 25 ohm's = 18 volts across this ground rod. I don't think that will bring many worms out of the ground. LOL

Correct me if I'm wrong


Here is a calcaulation that they did on 200kva system.

Voltage = 12.7 kV line to ground
Rating = 200 kVA
R = 1.6%
X = 3.8%
Earthing Resistance RE = 1 ohm
Therefore Ibase = 15.7A
Voltage drop over RE = 15.7A x 1 ohm = 15.7V
v% = 0.0012 = RE (pu)

[ March 17, 2003, 09:12 PM: Message edited by: hurk27 ]

 

[bennie]

Re: Earth as only neutral

hurk: Looks good to me. There is no way the ground conductor should be a problem, other than disconnect it and your dead.
I suspect a pole butt ground plate is also used with a cover over the conductor to prevent damage.

A line to ground fault is the same as a line to line fault. The recloser should activate in a short time to attempt clearing.

 

[hurk27]

Re: Earth as only neutral

Bennie I have thought about that very subject what would happen if there was a transformer fault. this would have to include some kind of very fast acting ocp. to keep the ground from going high, and or ground potential monitoring that will shut down the primary if this condition were to occure. Like in a case of the ground rods burning off due to galvantic action

trying to follow the thread I was trying to figure how they were getting those high voltage drops across to ground rod?

P.S. Glad to see you back. Missed your valuble insite's

[ March 17, 2003, 09:41 PM: Message edited by: hurk27 ]

 

[bennie]

Re: Earth as only neutral

Thanks hurk: I know who to ask for math calcs. You did good.

The worst case scenario would not create a hazard.

 

[hurk27]

Re: Earth as only neutral

Thanks to you but it was simple ohm's law.

I think that we as humans we try to read to much into the equations and often confuse ourselves when simple math is all that is needed. I too am guilty of this and when something doesn't sound right I have to slow down and go back and readdress the problem until I get it.
I am by far no mathematician even with the electronics courses that I took and all the math I still struggle to get it right. most of it I have forgot because of nonuse. but the teachers don't know what field we might go into and have to cram all of it in. But these computer's do help.

 

[don_resqcapt19]

Re: Earth as only neutral

A online document that I found suggests some potential safety problems with the grounding of SWER systems at the load end.
"Generally SWER lines within villages would have an under-strung earth wire for practical and safety reasons. The under-strung earth wire would be required within the boundaries of the village and where community gatherings take place, e.g. children ply areas, vicinity of sports facilities, churches and trading areas.
You can read Desk Research Project On Innovative Technologies for Rural Electrification Southern Africa Perspective here. It did not go into any details on the safety of SWER and is mostly concerned with the economics of large area rural distribution.
Don

 

[hurk27]

Re: Earth as only neutral

At 400v,1.1kv, 1.9kv and even 3.3kv there would be good reason to be concerned. as the load on the earth return would be higher. but it did say that 5kva would be what is supplyed to a hut and would be 12.5a @400v, 4.5@1.1kv, 2.6@1.9kv, 1.5@3.3kv The VD across the grounding would be at 25 ohms:312v@400v,112.5@1.1kv,65@1.9kv and 37.5@3.3kv

As we can see the Voltage drop across these lower voltages can result in a higher touch potential.
Now if the ground resistance could be kept at 1 ohm then the VD would be quite less as it would be the same as the current.

Just a note the voltage drop on the 400v would be too high to be of use if the grounding could not be maintained below the 2 ohm level.

And I'm not sure if they even meant that this voltage (400v) would be a SWER or a two wire from a transformer to the dewlling.(as they did say the final voltage to the dewlling would be 220v)

But with the lower voltages there would be some concerns if the resistance could not be maintained, and is probley the reasone for the grid grounding under the village. as with higher primary voltages this would not happen

[ March 18, 2003, 01:50 AM: Message edited by: hurk27 ]

 

[karl riley]

Re: Earth as only neutral

I searched for SWER on Google and finally found a good article in Transmission & Distribution World, April 1, 2001 by Neil Chapman. The lines start with an isolation transformer. They limit the earth return to this transformer to 8A, and the electrode impedance to 2 ohms "to avoid potential step and touch problems for livestock and humans". 8A X 2 Ohms = 16V. So OK. Typical voltages are 12.7kV and 19kV.

Distribution transformer grounding impedance is limited to from 10 ohms to 20 ohms depending on kVA (5 - 20 kVA).

Power losses are up to 100% more than standard 3-phase lines from a combination of the single conductor (can be 1000 ohms) plus the grounding electrode impedances. They are used in many countries in the world due to low cost of construction. Problems of voltage drop with loads are countered with voltage regulators.

Australia has 10s of thousands of miles of SWER.

Searching on the internet is great if you have a whole afternoon to give to it!
Karl

 

[hurk27]

Re: Earth as only neutral

Karl if you want a good little program that will use all the search engines at one time go to:

http://download.com.com/3000-2379-10156085.html

This is one of the best little programs I have ever used for searching as it will use all the search engings at one time and allow you to slect 4 differant mode to search.
It is a must have as I even bought the full version web ferret pro

 

[fallssparky]

Re: Earth as only neutral

Hurk

posted March 17, 2003 09:04 PM
--------------------------------------------------------------------------------
think that what being missed here is the load current on this primary. As most SWER system's I have read about only serve a 60 amp or less services and are fed by a 19.5k or higher line. if voltage drop is a combination of resistance and current then it would be very low. 60 ampsx 240 = 14400 volts / 19500 volts (pri.)=.74 amps on the swer neutral .74 amp.s x 25 ohm's = 18 volts across this ground rod. I don't think that will bring many worms out of the ground. LOL

Correct me if I'm wrong


Here is a calcaulation that they did on 200kva system.

Voltage = 12.7 kV line to ground
Rating = 200 kVA
R = 1.6%
X = 3.8%
Earthing Resistance RE = 1 ohm
Therefore Ibase = 15.7A
Voltage drop over RE = 15.7A x 1 ohm = 15.7V
v% = 0.0012 = RE (pu)

[ March 17, 2003, 09:12 PM: Message edited by: hurk27 ]

My question is where on this planet is the ground resistance of 1 ohm and how long does that last ?
Just in the United States soil resistance is on the average 40 ohms.
If this were the case then we would need only one ground rod.
Someone also suggested 10 ohms for these systems who goes around and checks this to ensure that the resistance hasnt changed.
Bennie as far as the shells around a ground rod I have checked this I have applied 120 volts to an 8 foot driven ground rod, and measured from the top of the rod to different places around the rod the farther away from the rod the voltage increases. The earth is a huge resistor. You never see/read 120 volts but the T.V. is certainly there.

FallsSparky