Surge Current

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"A typical lightning bolt contains 1 billion volts and contains between 10000 to 200000 amperes of current."

I wouldn't want to be near an MOV when it's taking this kind of punishment.
 
WasGSOHM said:
"A typical lightning bolt contains 1 billion volts and contains between 10000 to 200000 amperes of current."

I wouldn't want to be near an MOV when it's taking this kind of punishment.
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When someone says “contains current” I am convinced they don’t know what they’re talking about.
 
retirede said:
When someone says “contains current” I am convinced they don’t know what they’re talking about.
Click to expand...
Yeah, this text has been passed through several non-techies, but the numbers may be accurate.

Some guy got impaled on a rebar and a reporter described the rod perfectly but had no idea what it was called.
OK, not everybody knows but the guy could have asked someone onsite.
 
See the IEEE lightning standard. 200 kA is not unreasonable. The thing with lightning is that it effectively dumps a bunch of electrons on your system. It induces an opposite charge in the Earth, and the two charges are trying to do whatever is necessary to combine with each other and neutralize.

So we don’t look at the common resistance and inductance normally used but the surge impedance of the wire affected. This determines both the voltage and current as seen by the electrical system. At some point we exceed the BIL rating where it just bypasses the insulation and burns right through it.

In practice not all lightning strikes are severe. The voltage/current of any given strike is quite random (long tail distribution) but the probabilities are consistent anywhere on the Earth.

So in practice we can estimate insulation coordination which estimates the maximum surge a system is designed for. Then we can calculate what percentage of lightning strikes it will survive. Then knowing the amount of lightning activity in an area we can make estimates of how often we can expect to see damage. There is no upper end...probabilities can be low but never zero.

By way of example Duke Power is currently the largest US utility. Their transmission system is 230 kV. I don’t have access to the design data but BIL is probably up around 1500 kV. That’s based on typical specs their transformer suppliers use (another customer). Effectively most transmission lines are almost immune to lightning. During hurricane Irene a few years ago a massive lightning strike went well beyond damaging surge arresters. It blew apart one of their transmission line pole structures. They did quite a lot of investigation looking for any structural defects because lightning of that magnitude is very rare, and they had never seen the pole itself destroyed. This part of Duke includes a lot of area in Florida so they are very knowledgeable about the effects of lightning. As the largest customer on their system at the time and affected directly by that pole they kept us in the loop.

By way of example as the customer at the time our distribution system was 23 kV for historical reasons but built to 35 kV design with 120 MPH wind load design. We ran static lines across the top of the poles connected to the pole ground with the phase conductors well below them. Surge arresters were installed every fifth pole. When I ran the calculations the probability of a direct strike was very low. However there was a good probability of a strike hitting a static line then jumping across the cross arm through a phase conductor, ionizing the air and causing a trip, or of an indirect strike hitting the ground nearby and again jumping the cross arm and causing a trip. The biggest problem area was an area that was elevated on an artificial hill and the soil was pure washed sand so all the poles were almost ungrounded so it made the indirect strikes almost guaranteed whenever a storm discharged in that area.
 
paulengr said:
See the IEEE lightning standard. 200 kA is not unreasonable. The thing with lightning is that it effectively dumps a bunch of electrons on your system. It induces an opposite charge in the Earth, and the two charges are trying to do whatever is necessary to combine with each other and neutralize.

So we don’t look at the common resistance and inductance normally used but the surge impedance of the wire affected. This determines both the voltage and current as seen by the electrical system. At some point we exceed the BIL rating where it just bypasses the insulation and burns right through it.

In practice not all lightning strikes are severe. The voltage/current of any given strike is quite random (long tail distribution) but the probabilities are consistent anywhere on the Earth.

So in practice we can estimate insulation coordination which estimates the maximum surge a system is designed for. Then we can calculate what percentage of lightning strikes it will survive. Then knowing the amount of lightning activity in an area we can make estimates of how often we can expect to see damage. There is no upper end...probabilities can be low but never zero.

By way of example Duke Power is currently the largest US utility. Their transmission system is 230 kV. I don’t have access to the design data but BIL is probably up around 1500 kV. That’s based on typical specs their transformer suppliers use (another customer). Effectively most transmission lines are almost immune to lightning. During hurricane Irene a few years ago a massive lightning strike went well beyond damaging surge arresters. It blew apart one of their transmission line pole structures. They did quite a lot of investigation looking for any structural defects because lightning of that magnitude is very rare, and they had never seen the pole itself destroyed. This part of Duke includes a lot of area in Florida so they are very knowledgeable about the effects of lightning. As the largest customer on their system at the time and affected directly by that pole they kept us in the loop.

By way of example as the customer at the time our distribution system was 23 kV for historical reasons but built to 35 kV design with 120 MPH wind load design. We ran static lines across the top of the poles connected to the pole ground with the phase conductors well below them. Surge arresters were installed every fifth pole. When I ran the calculations the probability of a direct strike was very low. However there was a good probability of a strike hitting a static line then jumping across the cross arm through a phase conductor, ionizing the air and causing a trip, or of an indirect strike hitting the ground nearby and again jumping the cross arm and causing a trip. The biggest problem area was an area that was elevated on an artificial hill and the soil was pure washed sand so all the poles were almost ungrounded so it made the indirect strikes almost guaranteed whenever a storm discharged in that area.
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I don't have access to that IEEE standard, but I have no reason to doubt you. I knew lightning could be severe, but had no idea that it could be that bad. Good to know.

BTW, I saw a Mike Holt video saying that surge arrestors don't need a ground, but Eaton is showing the ground doing its job. Any idea regarding this discrepancy?
 
Cannotlogin said:
... BTW, I saw a Mike Holt video saying that surge arrestors don't need a ground, but Eaton is showing the ground doing its job. Any idea regarding this discrepancy?
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You can limit the line to line and line to neutral surges without a connection to earth. Where the system is a grounded system one of the system conductors is connected to earth. The equipment cares mostly about the line to line or the line to neutral voltages, so a surge protector can provide reasonable protection without an additional connection to the earth.
 
don_resqcapt19 said:
You can limit the line to line and line to neutral surges without a connection to earth. Where the system is a grounded system one of the system conductors is connected to earth. The equipment cares mostly about the line to line or the line to neutral voltages, so a surge protector can provide reasonable protection without an additional connection to the earth.
Click to expand...


Like shunting voltage away from the device?
 
Cannotlogin said:
Like shunting voltage away from the device?
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You need to stop thinking that somehow the surge protector shunts voltage away from the device. What it does is change its resistance enough that the voltage drop across the surge protector drops from a level that is high enough to damage the equipment to one that doesn't.
 
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