What is the electrical current of streamers?

[Gymnast]

I live in Denmark, and here the lightning strike probability is quite low and about 0,3 strike each square km each year. Most buildings have no lightning protection. However i find that installations on the roof or connected to the roof more often are damaged than due to real lightning. So I suspect that earth upwards streamers may cause the damage.

In this description: http://en.wikipedia.org/wiki/Lightning it says that the initial parts of the top down streamer involves tens or hundreds of amps.

I imagine that streamer currents are much more frequent that actual strikes, but they do also happen durring thunderstorms. Further I have read, that longer streamers develop from tall buildings that from low buildings. I do also imagine, that protection measures against possible streamer currents are quite inexpensive. So the questions are:

1) What is the probability of a upwards streamer current?
2) What is the current wave shape?
3) What is the current amplitude?

Do you know of any research dealing with this issue?
Do you know of any building codes (not lightning) preventing damage in this case?

 

[bphgravity]

Try not to think of streamers as an actual entity or physical property. Imagine for a second a geyser. Pressure from the earth pushes water and steam out the surface of the planet in a channel. The pressure allows the water to "climb" itself up the channel giving the impression of a single flowing mass of water. Streamers are identical. They are a channel of ions with a great density which "streams" itself though a gas by a continuous establishment of an electron "avalanche" just ahead of its advancing tip.

While an upward streamer may have a great potential to a downward stepped or dart leader, the streamer itself has mo real current flow. The ionized path that is created by the streamer provides a path for the return stroke when the charge separation is connected at a point just above the earth based object.

Humans can and do release streamers with no ill-effects. Corners of buildings, light poles, towers, ect. all exhibit streamers almost continuously as electrons are stripped from the earth via the tall object.

So to answer your questions,

1. While current flow is theoretically present; it is not current forced through a resistance due to an electromotive-force.

2. Don't know.

3. Don't know.

I would suggest you look into the research performed here: http://www.lightning.ece.ufl.edu/

There is nothing in the building codes, other than requirements for lightning protection for certain occupancy types, for this issue. (Because there is no issue)

 

[bphgravity]

Here is a comment on the subject from an expert with much more knowledge of the subject than I:

According to Berger (1967, 1968, 1977), a lightning strike to ground can produce secondary upward streamers from pointed ground objects within a relatively large distance. Of course, the intensity of the streamers and the radius from the strike point within which they occur will increase with increase in magnitude of the downward stroke.

Upward streamers can occur on "short" objects. For example, Berger (1967, 1968) observed such discharges on the steeple of a small church which was only about 10 m high. However, the intensity of the streamers increases with altitude of the peak of the object. For example, the above church was located on a hill about 250 m above Lake Lugano.

The damage from secondary upward streamers will be limited unless they are intense enough to reach into the clouds and draw subsequent downward strokes. Based on available field observations, Mousa (1986) estimated that subsequent stroke only occur when altitude of the peak of the ground object is about 300 m (1000 ft) or larger.

The damage apparently results from being a pointed object, i.e. their shape. Quality of the grounding should have minimal effect, if any, on the observed damage. The best solution appears to be to use more robust antennas, especially at locations where altitude of the site is high.



References:

[1] Berger, K. (June 1967). "Novel Observations on Lightning Discharges:
Results of Research on Mount San Salvatore", Journal of the Franklin Institute,
Vol. 283, No. 6, pp. 478-525.

[2] Berger, K. (1968). Discussion of Group 33 on "Lightning and Surges", CIGRE
Proceedings, Vol. II, pp. 2, 10-11.

[3] Berger, K. (1977). "The Earth Flash", Chapter 5 of Golde, R.H. (Editor),
Lightning, Vol. 1, Academic Press, London.

[4] Mousa, A.M. (1968). A Study of the Engineering Model of Lightning Strokes
and its Application to Unshielded Transmission Lines", Ph.D. Thesis, University
of British Columbia, Vancouver, BC, Canada.


Abdul M. Mousa, Ph.D., P. Eng., Fellow IEEE
abdul_mousa@...

 

[Gymnast]

No issue?

No issue?

There is nothing in the building codes, other than requirements for lightning protection for certain occupancy types, for this issue. (Because there is no issue)

I have seen antenna on buildings not grounded at all and the receiver is not grounded. The same goes for masts supporting small wheather station on top of buildings used for internal energy control. In this case even small streamer current or corona may cause a charge up and small spark-over which may damage some electronics. So some awareness of this nature may prevent bad maners on buildings. So a simple measure is to connect the mast to protective ground.

I have read that corona apears with a current below/about 1 microamp. You cannot feel such a low current. But streamers may cause a short and higher current level - but still much lower levels that real lightning strikes.

 

[bphgravity]

Other than violations of the NEC for required grounding and bonding of equipment and/or the NFPA 780 for installed LP systems, any additional grounding or bonding is a design consideration.

I suspect there is more going on than just transient streamer currents based on your description. However, I agree that specialized knowledge of these systems and the hazards associated with lightning must be had to install and maintain these systems.

 

[Gymnast]

Other than violations of the NEC for required grounding and bonding of equipment and/or the NFPA 780 for installed LP systems, any additional grounding or bonding is a design consideration.

I suspect there is more going on than just transient streamer currents based on your description. However, I agree that specialized knowledge of these systems and the hazards associated with lightning must be had to install and maintain these systems.

Yes, I agree. You can have SELV systems not grounded on the roof. From a safety point of view it shall be all right. The task is more like finding a good installation practice for the many different installations exposed to open air.

 

[ghostbuster]

I have read that corona apears with a current below/about 1 microamp. You cannot feel such a low current. But streamers may cause a short and higher current level - but still much lower levels that real lightning strikes.

Here is a picture of both positive and negative corona.The picture is of a high voltage insulator string supporting the transmission line conductors.The voltage was approx.400,000 volts and voltage gradient of 20 KV./cm.
Hope this of interest.:smile: