Lightning Strikes

[Sean Moran]

Was it something about lightning taking the path of least resistance? They say it actually flows from the ground upwards too???

There is lightning outside in the storm right now here and I'm one of the only two ten-storey buildings around here. I think it might be time to get some practice into that "nervousness" thing. Anyone know any good lightbulb jokes?


---o0o---

AAAAAARRRRRRRGGGGH!

[sulasno]

How many Seans do you need to change a light bulb?

[Ian]

Seems to be a lot of misunderstandings about lightning So Lets look at a ground strike rather than cloud to cloud. Firstly air and pure water are good insulators, so the first task is to create a conducting path from cloud to ground.
Most people are familiar with normal fluorescent tubes, these have a starter switch the purpose of which is to evaporate mercury to form a conducting parth between the two ends, once this path exists then the electron discharge will start and the heat of this discharge maintains the conducting path.

With lightning the conducting path has to be created before a strike can occur. If I may quote, to save me a lot of typing,
An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. Discharge ionized channels are known as leaders. The negatively charged leaders, generally a "stepped leader", proceed downward in a number of quick jumps (steps). Each step is on the order of 50 to 100 ft (15 to 30 metres) long but may be up to 165 ft (50 m). As it continues to descend, the stepped leader may branch into a number of paths. The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible when compared with the subsequent lightning channel."

It is the proximity of this 'leader" which we can detect on our skin and in our hair and clothes. As this stepped leader approaches the ground a conductive discharge (called a positive streamer) can develop. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. This is the return stroke and it is the most luminous and noticeable part of the lightning discharge.

The superheated air (hotter than the sun), disrupts the ionised path but it immediately reforms, lighting is often up to 6 superimposed discharges, this is why it flickers.

A lightning rod or arrester, is designed to prevent a lightning strike by allowing the ground charge to dissipate (corona discharge) fast enough for the ground potential voltage not to reach a value where ground leaders can form.

Most deaths by lightning are caused by damage from struck objects rather than electrocution.

When lighting hit a metallic object fireballs often form, these can last for up to a minute and follow random paths, they often chase cars because the car body is electrically charged, some people sensitive to this fit hanging straps beneath their cars.

[prommee_NE]

I was thinking more about reporting the dangers of lightning and some simple steps to help prevent being injured during an electrical storm. Stating the obvious is a pretty good start:

1. Despite Ian's remark about pure water being a good insulator...rain water and paddy fields are seldom 'pure'. Get out of the paddy fields for a start and seek some shelter...the car is a safe bet (as long as you don't touch any metal surface) or the 'field bahn'.

2. Do not shelter under isolated trees.

3. Outside, do not use the mobile and at home, disconnect the mobile if fitted to an external aerial.

4. Unplug the TV and any major electrical appliances.

5. Don't take a shower...and get out of the swiiming pool

[Ian]

Seems to be a lot of misunderstandings about lightning

I agree Ian. The next quote is a good example.

A lightning rod or arrester, is designed to prevent a lightning strike by allowing the ground charge to dissipate (corona discharge) fast enough for the ground potential voltage not to reach a value where ground leaders can form.

Lightning rods and lightning arrestors are two different things with similar jobs: Protection.

Lightning Rods are devices that divert the high electrical current from lightning strikes to ground. They are made of conductive materials (often copper) that are at ground potential. These are what you see sticking up from buildings. The lightning strikes the top of the rod and flows down to earth.

Lightning arrestors are used on High and Medium Voltage Power systems (and some other systems) as protection against excessive surges in voltage caused by lightning strikes. When there is a voltage surge caused by a lightning strike, the excess voltage jumps a gap to earth, (a bit like a water overflow in a tank or basin) reducing potential damage.

Neither of the above prevent a lightning strike. In fact the lightning rod is an attempt to get the lightning to strike it rather than the building it’s protecting. The lightning arrestor simply diverts excess voltage to earth after the power line has been struck.

Cheers

Well far be it for me to argue with an expert, perhaps he would explain to me how the cable to a lighting rod, usually a copper strip about 3cm wide by 0.5 cm thick, is going to survive the passage of on average 300,000 amps. As at these current densities skin effect is a major factor, he would also care to explain why they use a solid conductor rather than a multistranded or plaited one.
Perhaps you can explain why charge dissipation is not acceptable to you [Early Streamer Emission]? Does St. Elmo's fire also get ignored?
I was amiss at using the term arrestor, but I was try to be be non technical, yes the full term is spark gap arrestors, I did briefly mention their modern equivalent in my previous comment, VDRs (voltage dependent resistors), spark gaps are so outmoded now, even gas discharge tubes are dated, although you might find small one in older house phones.
I assume you know that the EMP from lightning is in some ways akin to the EMP from a nuclear strike. Further that military software has to be "hardened" to make it immune to EMP. I used to enjoy my trips to the NPL high voltage facility playing with artificial lightning strikes and seeing what it did to my electronics.

[Ian]

Ian, I see your computer has that virus again: nonsensicus H1aN. Either that or you’ve been drinking Sean’s home brewed elixir.

If you don’t want to be too technical, don’t copy and paste technical stuff. If you don’t want to be caught out writing rubbish, don’t try to look clever by writing about something you have little or no knowledge of.

What is an expert? I wouldn’t claim to be one but I worked for 17 years in the electrical power industry and I have an HNC (Higher National Certificate) in electrical and electronic engineering. So I know quite a bit more about electrical issues than the average punter and I can spot a +++++++++ a mile away.

Lightning rods are connected to solid copper strips that run all the way down to ground. I have never seen any lightning rods connected to cables of any kind.

What on earth has skin effect got to do with lightning conductors? Lightning charge is not alternating current so there will be no skin effect. But even if there were a skin effect, so what? The conductor is solid metal and it should be large enough to take the current.

I assume you know that the EMP from lightning is in some ways akin to the EMP from a nuclear strike.

I didn't. Thank you for telling me. I'll sleep better knowing that.

As for the following, why don’t you explain them yourself? You introduced them into the thread.
… explain why they use a solid conductor rather than a multistranded or plaited one
…explain charge dissipation
…explain Early Streamer Emission
…explain the relevance of St. Elmo's fire to lightning protection

Go ahead and explain those and show off your 'expertise', but please keep it simple for those of us who may have limited or no knowledge; unlike you, of course.

Cheers

So you worked for 17 years in the Electrical power industry and have an HNC, good for you
I also have an HNC in Electrical Engineering, later converted to a degree in Electronics. I cannot beat your 17 years, I only spent 6 years with GEC Coventry, But does having my own company installing factory electrical equipment count?

You actually display your ignorance by claiming to show mine, The skin effect you are referring to is due to eddy currents set up by the AC current, and is frequency dependent. I am referring to the skin effect caused by high electron packing densities, 300,000amps creates pretty high electron densities.

You call me ++++++++++++++ You seem to make a habit of challenging me, I'm sure the members find it fun

Now streamer emission is responsible for charge dissipation. Perhaps you are aware that in very strong winds the half a dozen or so flashes of a typical lightning strike are not all superimposed but actually appear as a series of parallel strikes.
Perhaps you would accept that the total energy of the strike has been dissipated in say 6 places, not just one.

Perhaps you can remember from student days The Wimshurst machine and Van de Graaff generator, these machines do not hold their charge for long, it dissipates in a few minutes, dissipates where you ask, why into the air. Perhaps your teacher also showed you that by affixing points to the electrodes the charge dissipated faster, it is called a point discharge. http://amsglossary.allenpress.com/gloss ... discharge1

The more points the faster the discharge, if the rate of discharge is high enough the ionised air becomes visible as a corona discharge. When this happens on ship's masts it is called St Elmos fire by sailors.

Now if the earth charge beneath a storm cloud can be dissipated fast enough, the earth charge potential cannot rise to a level where lightning feeder formation will start, this is why lightning rods are often tipped with a circle of spikes. The corona discharge from the lightning rod spikes dissipates the earth potential build up and so prevents a strike in that vicinity. There is a formula for the spacing of such rods but I forget what it is, perhaps you can remind me.

Basically if a lightning rod becomes a lightning conductor it has really failed in it primary role.

Note that I am not accusing you of drunkenness, nor any viral disease, such comments are used by people who attempt to ridicule rather than have a serious discussion, what were you a high voltage rigger?

[stilljustbrowsing]

What is an expert?
Cheers

Ex = has been.
spert (spurt) = drip under pressure.
I think it is more important to dissconect the aerial from the TV rather than the Electrical mains. But then again, have you noticed that here in Thailand, it is a "two wire" system? (no earth wire)

[prommee_NE]

On the ball as always SJB...disconnect TV aerial..and power in our case.

Incidentally, I made my own 'earth' using copper rods in a 15m long trench down the side of our house and connected it to the Junction box with 10mm cable. Should have stuck coiled copper wire under a few of the main house posts, when building...but just plain forgot!

[Ian]

On the ball as always SJB...disconnect TV aerial..and power in our case.

Incidentally, I made my own 'earth' using copper rods in a 15m long trench down the side of our house and connected it to the Junction box with 10mm cable. Should have stuck coiled copper wire under a few of the main house posts, when building...but just plain forgot!

Is your earth designed for electrical equipment protection or lightning protection? You might find this discussion of interest
http://www.eng-tips.com/viewthread.cfm? ... 621&page=1

If you have an earth resistance of say 4 ohms this would imply a maximum current capability of 55 amps. Here in Thailand my electricity meter is rated at 15 amps and my supply is two single 4mm cables. I guess your earth would be overkill for me
My earth system is a single 4mm wire connected to a buried metal plate, it is adequate for me as I do not use ELCB protection but prefer RCCB. As far as I know all new installations in the UK have to be RCCB as the electricity suppliers no longer supply a company earth. Perhaps Noaksy can confirm, I am about 25 years out of the business.

[prommee_NE]

The earth is for equipment/personal protection rather than lightning protection. I use a combination of RCD's and ELCB's. The house was originally wired up by the local builders...I was horrified when I came to inspect (The incoming power was joined to the house in the roof using good old insulating tape!...Actually all joins were insulation tape or those horrible wire nuts).

Anyway, I wired the house up myself after seeing their attempt. I couldn't find any decent junction boxes for lighting etc. so I shipped over a lot of stuff from England/Germany. It's not pretty...all cables are in conduit vertical down the walls rather than inside the plaster...but it works for me. Consumer boxes are RCD protected and the showers and out-house have additional ELCB protection. Kitchen equipment (hob, oven etc.) are metallic so earthed to prevent build up of static.