Hi. Is this line to ground capacitance?
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Power Lines
- Thread starter Cerkit
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- Moderator
- #2
Working in a 500 kV capacitor station I have been zapped by capacitive current more times than I care to remember.
However the description of current flowing does not match a capacitive discharge.
If the ground is so dry that the swing uprights are not effectively grounded then the swing chains and supports will act as a capacitor plate. They will form part of a capacitive voltage divider. One capacitor is formed by the swing, the air gap and the overhead lines.
This capacitor is in series with the capacitor formed by the swing, the effective ground and the gap between the swing and the effective ground.
Very little current will flow, but when you touch it the charge on the swing will be transferred to your body.
The shock is similar to a static shock but if you let go and then touch again you will generally get a second shock
If you hold on to the charged object, in this case the swing, there may not be enough current flow to be detectable.
Hard to say for sure in this case without measurements or an evaluation by someone with experience with capacitive shock effects.
I suspect that any effect other than a capacitive effect may be fatal, but this is not a guess that I would bet someones life on.
Bill
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"Why not the best?"
Jimmy Carter
However the description of current flowing does not match a capacitive discharge.
If the ground is so dry that the swing uprights are not effectively grounded then the swing chains and supports will act as a capacitor plate. They will form part of a capacitive voltage divider. One capacitor is formed by the swing, the air gap and the overhead lines.
This capacitor is in series with the capacitor formed by the swing, the effective ground and the gap between the swing and the effective ground.
Very little current will flow, but when you touch it the charge on the swing will be transferred to your body.
The shock is similar to a static shock but if you let go and then touch again you will generally get a second shock
If you hold on to the charged object, in this case the swing, there may not be enough current flow to be detectable.
Hard to say for sure in this case without measurements or an evaluation by someone with experience with capacitive shock effects.
I suspect that any effect other than a capacitive effect may be fatal, but this is not a guess that I would bet someones life on.
Bill
--------------------
"Why not the best?"
Jimmy Carter
- Moderator
- #4
I just looked at the Facebook video. I couldn't hear the sound but the swings are supported on a wooden framework.
In the hot dry weather that most likely is capacitive coupling.
It will take a voltmeter with an extremely high impedance to measure anything.
Bill
--------------------
"Why not the best?"
Jimmy Carter
In the hot dry weather that most likely is capacitive coupling.
It will take a voltmeter with an extremely high impedance to measure anything.
Bill
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"Why not the best?"
Jimmy Carter
The actual swing set is all wood and the chains are perpendicular to the overheads. How are those chains being excited?
Keith Cress
kcress -
Keith Cress
kcress -
HamburgerHelper
Electrical
What you are seeing is the voltage gradient from the power lines. The chains are maybe 6 ft or so. So, the voltage that is developing between the swinger and ground is whatever voltage gradient exist between the top of the swing and the ground. I don't know think this is a grounding issue but more of a bonding issue. If mats or conductive concrete or something more conductive was placed under the swings and bonded to the chains, there wouldn't be any shocks. I don't care about electrical ground. I care about the potentional difference between the chains and the dirt or whatever you stand on. Just connecting the chains to a ground rod might not get you the results you want.
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If you can't explain it to a six year old, you don't understand it yourself.
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If you can't explain it to a six year old, you don't understand it yourself.
So you're saying you'd measure a voltage from one end of a chain to the other?
Keith Cress
kcress -
Keith Cress
kcress -
HamburgerHelper
Electrical
No, I believe this is due to the voltage gradient through the air. You measure from the top of the swingset to the dirt you stand on, and that should give you the potential you are getting from the gradient. If you bond underneath the swing set to the chains, it will be hard for a potential voltage develop. If you put crushed rock under the swings set to increase the impedance, you reduce the energy that flows from the potential difference to electrical earth. I believe this is probably measurable with a regular voltmeter if people are lighting up fluorescent bulbs under transmission lines.
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If you can't explain it to a six year old, you don't understand it yourself.
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If you can't explain it to a six year old, you don't understand it yourself.
- Moderator
- #9
No Keith. Visualize a pair of capacitors in series formed by a conductor-the power line, and air gap, another conductor-the chains, another air gap and a third conductor-the effective ground.
The first capacitor is formed by the power lines and the chains,
The second conductor is formed by the chains and the effective ground.
With very dry conditions, the effective ground may be some distance below the surface of the actual ground.
If paint or corrosion is insulating the chain links from each other it gets more complicated.
Working under the 500 kV lines, we found that if you could take the shock, similar to a static discharge, you could hold on without feeling any more current.
If you touched the charged component with a wrench, a spark would jump between the wrench and the component, but you would not feel the shock.
If you first touched the object with a wrench you culd then touch and hold it with the other hand and thrn work with the wrench.
However it was almost impossible to remember to hold on with one hand. Every time that you forgot and released and then touched the object with your free hand, you got another poke.
The charging current to charge the chain as one plate of a capacitor was too small to feel.
However when you first touched the object you would discharge the stored charge. That you could feel.
We ended up wrapping one end of a bare #14 AWG stranded wire around one wrist and attaching the other end of the wire to the object that we were working on.
The voltage gradient mentioned by HH is there and his bonding method will work to avoid shocks by effectively shorting out the lower capacitor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
The first capacitor is formed by the power lines and the chains,
The second conductor is formed by the chains and the effective ground.
With very dry conditions, the effective ground may be some distance below the surface of the actual ground.
If paint or corrosion is insulating the chain links from each other it gets more complicated.
Working under the 500 kV lines, we found that if you could take the shock, similar to a static discharge, you could hold on without feeling any more current.
If you touched the charged component with a wrench, a spark would jump between the wrench and the component, but you would not feel the shock.
If you first touched the object with a wrench you culd then touch and hold it with the other hand and thrn work with the wrench.
However it was almost impossible to remember to hold on with one hand. Every time that you forgot and released and then touched the object with your free hand, you got another poke.
The charging current to charge the chain as one plate of a capacitor was too small to feel.
However when you first touched the object you would discharge the stored charge. That you could feel.
We ended up wrapping one end of a bare #14 AWG stranded wire around one wrist and attaching the other end of the wire to the object that we were working on.
The voltage gradient mentioned by HH is there and his bonding method will work to avoid shocks by effectively shorting out the lower capacitor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
I think I have that down Bill, thanks. My problem with the capacitor theory is that the plate area of those chains -almost perpendicular to the overheads- doesn't seem to me to provide diddly-squat for the (parallel plate) capacitor situation. I think HH's field gradient explanation makes more sense in this situation as area doesn't come into it directly.
Thanks HH I can see the gradient from chain to earth. That makes the most sense to me.
When I was a kid I built lots and lots and lots of diode radio receivers and of course found the higher the antenna the better. Except, I discovered the gradient of the earth's atmosphere could generate a tiny continuous arc to earth and would even light a neon bulb. I was amazed.
Keith Cress
kcress -
Thanks HH I can see the gradient from chain to earth. That makes the most sense to me.
When I was a kid I built lots and lots and lots of diode radio receivers and of course found the higher the antenna the better. Except, I discovered the gradient of the earth's atmosphere could generate a tiny continuous arc to earth and would even light a neon bulb. I was amazed.
Keith Cress
kcress -
- Moderator
- #11
As I understand it, the voltage gradient charges the capacitor. The stored charge is what gives the shock.
Yes, given the large separation and the small horizontal area, the capacitance is quite low.
However small, the discharge current is much greater than the charging current.
Effective ground:
Years ago a friend of mine was a dedicated Amateur Radio Operator. He found a job manning a weather station in the arctic.
Most of the operators were ham operators.
He claimed that because of the permafrost (frozen water and soil are insulators) A wire laid on the ground was the equivalent of an antenna over 100 feet in the air down south.
The ham operators were able to spend most of their time DXing. (Making distance contacts.)
The point is: If the ground is dry enough to become non-conductive the chains may be moved higher up the voltage gradient.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Yes, given the large separation and the small horizontal area, the capacitance is quite low.
However small, the discharge current is much greater than the charging current.
Effective ground:
Years ago a friend of mine was a dedicated Amateur Radio Operator. He found a job manning a weather station in the arctic.
Most of the operators were ham operators.
He claimed that because of the permafrost (frozen water and soil are insulators) A wire laid on the ground was the equivalent of an antenna over 100 feet in the air down south.
The ham operators were able to spend most of their time DXing. (Making distance contacts.)
The point is: If the ground is dry enough to become non-conductive the chains may be moved higher up the voltage gradient.
Bill
--------------------
"Why not the best?"
Jimmy Carter
HamburgerHelper
Electrical
I think that there may be something like 50-70 volts developing, enough to feel it but not enough to hurt you but it will be disconcerning. The current will by choked by the impedance from the transmission line to the swing. I believe that you can look at it with gradients since you never draw enough current to affect gradient. I agree with everything Waross is saying. The voltage gradient is what you have after the initial discharge, which will depend on the swings capacitance to ground compared to the impedance of the person touching the swing to ground.
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If you can't explain it to a six year old, you don't understand it yourself.
------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.
bacon4life
Electrical
The other capacitor in play is the human standing on rubber shoes on dry ground. When working under an EHV substation bus, I could touch the grounded tower and receive a shock because my body had built up a charge.
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