Why would an underground conductor experience a voltage drop?

Thread Starter

robertkang

Joined Jul 5, 2020
12
Hello I'm a student and I have conducted an experiment where I buried a wire underground. Since there are many underground conductors in Seoul(where I live), I've always thought whether metal substances from the earth would effect the conductor's transmission. If the magnetic fields of the substances could interfere the underground conductor's current, there must be a loss of electricity.
Therefore, I actually buried 10 meters of a wire nearly 40~60 centimetres under the ground and observed if there was any difference in voltage or ampere. As a result, the voltage actually dropped a little(about 0.475V). How could this be? and if I'm right, how could we prevent this from happening? Please share your opinions.
 

Thread Starter

robertkang

Joined Jul 5, 2020
12
I used electricity from my house-the standard one provided by seoul-and it was 220V, 60Hz (forgot to measure current). And when experimented underground, there was voltage drop for 0.475V
 

Papabravo

Joined Feb 24, 2006
13,939
Of course you can get any voltage drop that you want by setting the current through the wire. If you have 10 meters of wire how much current is going through it to produce the indicated voltage drop?
 

Thread Starter

robertkang

Joined Jul 5, 2020
12
Of course you can get any voltage drop that you want by setting the current through the wire. If you have 10 meters of wire how much current is going through it to produce the indicated voltage drop?
Well, I couldn't check the current then, but what made me think there was surely a voltage drop was
when I kept the wires above ground and turned on the power, averagely 220V was measured. However, when i buried the wires under ground, averagely 219V was measured. I had little electronics knowledge(still same..) so I wished to know what kind of reasons could be possible...
 

Thread Starter

robertkang

Joined Jul 5, 2020
12
If this is common parallel twin cable, ungrounded sheathing, it can be subject to all kinds of external influences also.
Max.
Oh then, can different temperatures of underground and above ground be the reason to this phenomenon? Underground was 6~7 degrees lower than above ground.. What could be the 'external influences' you said? Can you tell me more specifically, please?
 

Alec_t

Joined Sep 17, 2013
11,414
Are you sure the voltage change wasn't simply due to the usual mains voltage variations you get, particularly in an urban environment when your neighbours are randomly drawing power from the communal supply?
 

Thread Starter

robertkang

Joined Jul 5, 2020
12
Are you sure the voltage change wasn't simply due to the usual mains voltage variations you get, particularly in an urban environment when your neighbours are randomly drawing power from the communal supply?
Yes, I experimented this 10 times and results were the same each time. So I think urban environment could not be a problem.
 

MaxHeadRoom

Joined Jul 18, 2013
20,913
Oh then, can different temperatures of underground and above ground be the reason to this phenomenon? Underground was 6~7 degrees lower than above ground.. What could be the 'external influences' you said? Can you tell me more specifically, please?
Job to say specifically, a lot of dependent conditions, Untwisted power cables radiate magnetically, make up of ground conditions, etc, IOW could be a collection of any or all of a number of situations.
Without in-depth empirical testing, it is sheer speculation!
Max.
 

Papabravo

Joined Feb 24, 2006
13,939
All conductors have some resistance to the flow of charged particles. Conductors like copper and silver have less of it than insulators like glass and plastic. As an example there are tables that tell you the resistance of a given material given the length and the cross sectional area. In the table for AWG (American Wire Guage) #18 wire we find 6.51 Ω/1000'. This would be the same as 0.021358 Ω/m, and so your 10 m of wire(single conductor) would have a total resistance of 0.21358 Ω. If a current of 2.22 Amperes flows through the 10 m of wire the expected voltage drop, according to Ohms law would be:
0.21358 Ω * 2.22 Amperes = 0.474 Volts. This would be the voltage drop measured from one end of the 10 m wire to the other end.

Temperature will have an effect on the resistance of the wire, and the current flowing through the wire will generate some amount of heat and actually warm the wire up. How much heat you might ask. To start with, the power dissipated by the wire will be the current squared times the resistance. In our case
(2.22 Amperes)² * 0.21358 Ω = 1.0337 watts. I would expect the temperature rise in still air to be greater than that in the cooler ground.
 

Thread Starter

robertkang

Joined Jul 5, 2020
12
Job to say specifically, a lot of dependent conditions, Untwisted power cables radiate magnetically, make up of ground conditions, etc, IOW could be a collection of any or all of a number of situations.
Without in-depth empirical testing, it is sheer speculation!
Max.
I see. Thanks for replying!
 

Thread Starter

robertkang

Joined Jul 5, 2020
12
All conductors have some resistance to the flow of charged particles. Conductors like copper and silver have less of it than insulators like glass and plastic. As an example there are tables that tell you the resistance of a given material given the length and the cross sectional area. In the table for AWG (American Wire Guage) #18 wire we find 6.51 Ω/1000'. This would be the same as 0.021358 Ω/m, and so your 10 m of wire(single conductor) would have a total resistance of 0.21358 Ω. If a current of 2.22 Amperes flows through the 10 m of wire the expected voltage drop, according to Ohms law would be:
0.21358 Ω * 2.22 Amperes = 0.474 Volts. This would be the voltage drop measured from one end of the 10 m wire to the other end.

Temperature will have an effect on the resistance of the wire, and the current flowing through the wire will generate some amount of heat and actually warm the wire up. How much heat you might ask. To start with, the power dissipated by the wire will be the current squared times the resistance. In our case
(2.22 Amperes)² * 0.21358 Ω = 1.0337 watts. I would expect the temperature rise in still air to be greater than that in the cooler ground.
Oh I see. But if there was a metal substance underground, is there any possibility of its magnetic field interrupting the current and cause voltage drop??
 

Papabravo

Joined Feb 24, 2006
13,939
Oh I see. But if there was a metal substance underground, is there any possibility of its magnetic field interrupting the current and cause voltage drop??
I don't think the effect would even be the same order of magnitude as the intrinsic resistance of the wire. The metal external to the wire might change the shape of the magnetic field, but would not have much effect on the magnitude. You're welcome to try to calculate it though.
 
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