# Confused by earth grounded circuits

Discussion in 'General Electronics Chat' started by Kevin Shaughnessy, Nov 10, 2013.

1. ### Kevin Shaughnessy Thread Starter New Member

Jan 2, 2013
10
0
Hi,

I'm having a bit of difficulty wrapping my head around why this bird isn't getting shocked:

There's no voltage because the bird is connected in parallel with two points of zero resistance and therefore zero voltage, but isn't current still flowing through that point in the wire, and so through the bird? Isn't it that movement of electrons that causes a shock?

Thanks!

Kevin

2. ### tcmtech Well-Known Member

Nov 4, 2013
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1,752
The current flow is going past the bird not through it.

As far as the electrons are concerned the bird is nothing more than a high resistance resistor connected by only one end to the circuit.

3. ### #12 Expert

Nov 30, 2010
16,704
7,350
Suppose the wire has so much current that it is losing a volt per hundred feet. Consider also that the bird has 2 legs and they are 2 inches apart.
What is the voltage from one foot to the other?

.001666 volts.

Assume the bird is standing in the rain and its feet are wet so they only have 50 ohms of resistance each. How much current is going through its private parts?

16.7 microamps.

What is that compared to the 1 milliamp needed to notify a human that something is wrong?

1.7% of the minimum detectable current for a human.

If the bird is uncomfortable, it can still move its wings.
Proof: How many birds have you seen hanging dead on the power lines after a rainy night?

Jul 18, 2013
10,851
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He has to experience a potential difference, there is none as far as he is concerned.
Just curious, why the title?, there is no earth ground reference?
Max.

Last edited: Nov 10, 2013
5. ### Kevin Shaughnessy Thread Starter New Member

Jan 2, 2013
10
0
I chose the title due to the fact that the conceptual problem relates to why earth ground works.

Thanks for the responses, guys! #12, you're saying that the bird does experience a potential difference due to the resistance of the wire, but that its far too small to be relevant in any serious way?

I thought over the issue while I took my dog for a walk and I was wondering if someone could tell me if my reasoning is correct. Voltage = Joules / Coulomb. When the bird lands on the wire, it contacts, in parallel, at two points that have (an assumed) 0 resistance between them, so there are 0 potential joules acting at that point. The end result is that there is no force available to push current through the bird?

Last edited: Nov 10, 2013
6. ### #12 Expert

Nov 30, 2010
16,704
7,350
There is no assumed zero resistance in the main cable after I just did the math for you. There is an estimated .001 to .002 volts from one leg to the other.
How would you feel if somebody connected you to .002 volts?

7. ### GopherT AAC Fanatic!

Nov 23, 2012
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The swimming pool story in another post turns out to be just the reverse situation. In that case, the human body is a better conductor than the water in the pool so current will take the path of least resistance and flow through the body instead.

In the bird-on-a-wire case, the copper is a much better conductor and, therefore, the bird doesn't need any basting.

#12 likes this.
8. ### GopherT AAC Fanatic!

Nov 23, 2012
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2000 microVolts! OUCH!

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Jul 18, 2013
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Still not quite sure of the relationship as it pertains to the question?
Max.

10. ### jfwfmt New Member

Nov 10, 2013
4
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Actually the bird will not land on a wire when the AC RMS voltage-to-ground is about 22KV. The current flow into the air is enough to make the bird uncomfortable. This is corona.

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11. ### killivolt Active Member

Jan 10, 2010
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351
So, at what voltage might it be ok to land on a wire? 3k 10k ? Is that assumed?

Although I don't recall birds on wires of that voltage? But, the truth is I'm not sure of any Voltage they might sit comfortably.

Jul 18, 2013
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The problems occur during the migratory season, they can be seen here where flocks stop to rest and occupy the whole length of the conductor, it is usually the unlucky ones on the end that are close to metallic objects and get fried usually causing power outage.
The main distribution towers here carry 45kv to 500Kv conductors.
Max.

Nov 23, 2012
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