100 watt light
- Thread starter domnic
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- Placerville, CA, USA
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- Retired PV System Designer
Yes, but not at full brightness. And I doubt that an 8" rod would go as low as 25 ohms. An 8' rod would be a lot better.
In your example there would be at least a 25 volt drop across the electrode relative to remote ground since the current would be an amp or more.
Tapatalk!
In your example there would be at least a 25 volt drop across the electrode relative to remote ground since the current would be an amp or more.
Tapatalk!
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
120 - 25 = 95. Close enough for most purposes. If the bulb filament is really cool, the current may be even higher and the drop across the bulb even lower.
- Location
- Lockport, IL
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- Retired Electrical Engineer
I make it 89 volts. But then, I am taking into account that in addition to the 8' ground rod described in the first post, the current cannot return to the source without first passing through the other ground rod, the one associated with the building's service. That makes the total resisance (assuming both ground rods have the same resistance to ground) of 50 ohms. Here's the math:
- P = VxV/R
- Thus R = VxV/P
- R = 120 x 120/100 = 144.
- So the resistance of the light bulb is 144 ohms.
- You have a series circuit with the light bulb and two ground rods in series with the breaker.
- Total resistance is 144 + 50, or 194 ohms.
- Current is therefore 120 volts divided by 194 ohms, or .619 amps.
- Voltage across the light bulb is .619 amps times 144 ohms, or 89 volts.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Which would be correct if the resistance of the filament were not a function of temperature, among other things.
Tapatalk!
Tapatalk!
kwired
Electron manager
- Location
- NE Nebraska
Only one problem, the current has to travel through the rod the lamp is attached to, but once current enters the earth which path(s) (which electrodes) will it flow through to return to the source? Not very likely 100% of that current returns via a single rod only, which makes measuring the resistance beyond the first rod a difficult task to achieve any accuracy on.
Electrodes have higher resistance to earth but once you do get connected to earth, earth itself is considered to be low resistance.
K8MHZ
Senior Member
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- Electrician
A 100 watt bulb has a resistance of 144 ohms when fully lit, but as little as 9 or 10 ohms cold.
Crucial to the OPs question is how hot the filament needs to be in order to be considered lit. Also, a temp. co-efficient chart to show the resistance at that temp. Then Ohm's Law can take over from there.
Crucial to the OPs question is how hot the filament needs to be in order to be considered lit. Also, a temp. co-efficient chart to show the resistance at that temp. Then Ohm's Law can take over from there.
Its not that dangerous unless you're close to the rod where the light/power is attached. My dad made a similar device to collect worms for fishing. The current in the earth pushed them to the surface near the rod. Stupid kid me had to poke my finger into the ground and I could feel the buzz -- my knees were well away from the rod and I poked my finger into the ground probably 2' from it. I then understood why the worms came to the surface.
mbrooke
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I wonder if the same holds true for worms coming out after a rain storm from utility ground rods?
I've read newspaper articles about kids who have been killed by those.
And close is relative - if there happens to be a lower resistance path running nearby, close could be someone in a neighbors basement or swimming pool.
jumper
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- 3 Hr 2 Min from Winged Horses
Nice post, could actually save a life if someone reads this post.:thumbsup:
mbrooke
Batteries Included
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- United States
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- Technician
I guess this is relevant but its one of the best videos on the myths of grounding I have ever seen by far when dealing with the NEC :thumbsup::
http://www.youtube.com/watch?v=qNZC782SzAQ
Well worth the 30 minutes no matter where you are in the electrical industry!
: http://www.youtube.com/watch?v=qNZC782SzAQ
Well worth the 30 minutes no matter where you are in the electrical industry!
Though I have seen it done, and years ago did something similar, it is still dangerous and has way to many risk.
With that said it is a good theory and gets a very good point across in a classroom to get it across to students earth is not to be used as a conductor and why NEC forbids earth to be used as a conductor. For example let's say we run a 12 AWG conductor from a 20 amp breaker in a main panel, run it outside 20 feet away from the structure and bond it to a 8 foot rod pounded into the ground. Assume the rod has a earth impedance of 10 ohms.
What happens when you turn the breaker on?
Does the breaker trip?
If not, why?
Now ask the student this. Turn off the breaker and cut the wire mid span somewhere. Take one wire in one hand, and the other end in the other hand and have someone turn on the breaker. Remind the student he is grounded and safe. What happens? :angel:
Then take the lesson one step further and ask why POCO can use earth. So now say we have a 13.2 KV distribution feeder running overhead. Your home has a 17KV transformer, and the pole ground is 100 ohms, and the transformer is connected with a 2 amp fuse on the high side. For whatever reason the transformer fails and the primary shorts to the transformer case.
What happens besides Kaboom?
Will 132 amps flowing through a 2 amp fuse open circuit the fuse?
It is simple for us ole timers and experienced sparkies, but will open a students eyes and mind.
With that said it is a good theory and gets a very good point across in a classroom to get it across to students earth is not to be used as a conductor and why NEC forbids earth to be used as a conductor. For example let's say we run a 12 AWG conductor from a 20 amp breaker in a main panel, run it outside 20 feet away from the structure and bond it to a 8 foot rod pounded into the ground. Assume the rod has a earth impedance of 10 ohms.
What happens when you turn the breaker on?
Does the breaker trip?
If not, why?
Now ask the student this. Turn off the breaker and cut the wire mid span somewhere. Take one wire in one hand, and the other end in the other hand and have someone turn on the breaker. Remind the student he is grounded and safe. What happens? :angel:
Then take the lesson one step further and ask why POCO can use earth. So now say we have a 13.2 KV distribution feeder running overhead. Your home has a 17KV transformer, and the pole ground is 100 ohms, and the transformer is connected with a 2 amp fuse on the high side. For whatever reason the transformer fails and the primary shorts to the transformer case.
What happens besides Kaboom?
Will 132 amps flowing through a 2 amp fuse open circuit the fuse?
It is simple for us ole timers and experienced sparkies, but will open a students eyes and mind.
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kwired
Electron manager
- Location
- NE Nebraska
Remind the student he is grounded, but don't say he is safe, this will add confusion and is inaccurate. You could possibly tell him he is "allegedly" safe and then have him disprove that theory.
It was humor.
The overhead power distribution system has its neutral grounded every alternate poles. Result? Stray leakage current throughout the land of this country.Compared to it, 100 W bulb ground current is infinitesimal, But it is allowed without seemingly any objection here!
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