NZ Weather Forum

Mod Note: Moved this discussion about lightning effects from the 'Thunderstorm Potential Early December, 2009' thread to here (for continuity, spwill's link added below)...

spwill wrote:Lightning kills 12 cows on a Northland farm, TV3.
http://www.3news.co.nz/Lightning-kills- ... fault.aspx

that must have been a very powerfull lightning strike to kill those cows (cows are very sensitive to electricity though)

Yes. Poor moo-moos

JohnGaul
NZThS

That is a classic case of Earth Potential Rise allowing a small amount of the lightnings current to travel up one leg and out another I expect.

In my opinion that's our biggest risk as it doesn't need a direct strike to get you.

How else could you explain 1 lightning strike and 12 dead cows?

tgsnoopy wrote:That is a classic case of Earth Potential Rise allowing a small amount of the lightnings current to travel up one leg and out another I expect.

In my opinion that's our biggest risk as it doesn't need a direct strike to get you.

How else could you explain 1 lightning strike and 12 dead cows?

Yeah, in the trade, we call that 'step voltage'. A similar effect occurs around downed high voltage cables. The voltage dissipates to earth in concentric circles around the point of contact, which means the difference in voltage between legs can literally be thousands of volts.

That's how I got whacked in Arthur's Pass in January!

so maybe then stand on one leg during a thunderstorm?

Andrew Massie wrote:That's how I got whacked in Arthur's Pass in January!

Yes, I recall that discussion

Manukau heads observer wrote:so maybe then stand on one leg during a thunderstorm?

Not generally necessary, the current path up one leg and down the other doesn't go near your vitals. Add an arm into the equation and it's a different story.

tgsnoopy wrote:

Manukau heads observer wrote:so maybe then stand on one leg during a thunderstorm?

Not generally necessary, the current path up one leg and down the other doesn't go near your vitals. Add an arm into the equation and it's a different story.

Depends what you call vitals!

Andrew Massie wrote:

tgsnoopy wrote:Not generally necessary, the current path up one leg and down the other doesn't go near your vitals. Add an arm into the equation and it's a different story.

Depends what you call vitals!

LOL, if you are close enough to a CG strike, that the step voltage around the EPR is that great, that the current up one leg, down the other, across your vital organs is high enough to damage them, then I suggest the electric shock might be the least of your worries.

However, that exact scenario has killed quite a few, not from lightning generally, more mistakes and failures with HV Electrical Equipment.

Still... Ouch, just thinking about it.

I'd post some resource photos... but no!

Andrew, in the late 80's and early 90's I did a lot of work with Earthing PCM Cabinets. We found the typical 2m Galv earth rod up here in our volcanic soil was about 180 ohms to earth. Obtaining the required 5 ohm Telecom and power earth at the cabinets was hard.

For Telecom Earth the copper plated steel rods with tapered ends that slip into sleeves to join one another were hammered in until either 5 ohms was obtained or you couldn't hammer them in any further. Hard work for the lines staff with a 10 pound sledgehammer. Often 3 such long stakes were required.

For the power earth we (An Electrician and myself the Transmission (Bearers) Technician (Technical Officer)) used standard 2m Galv Steel rods with lugs welded onto them. We used a ditch digger to dig a number of trench's away from the cabinet on different angles. The trench was typically about 800mm deep. We used an uninsulated 16mm2 copper conductor between the rods, lugs crimped on and bolted with 316 stainless. Often we needed 15+ such rods to obtain 5 ohms. The copper link cable was buried with blue stone (crude copper sulphate) to improve long term conductivity.

Of course the two earths were bonded in the cabinet anyway. Long term with electrolysis in the equation I shudder to think how they lasted.

A couple of cabinets were supplied their mains by their own small transformer with no other loading. Their earths were tested. Lets just say they didn't meet the required standard by a considerable amount.

Early protection was just MOV's. That evolved to MOV's across the equipment, gaseous on the line and longitudinal chokes between. Later an Australian firm produced fantastic electronic arrestors that looked at the slew rate of the incoming rising voltage, they clamped extremely effectively and essentially let us get by without the elaborate earths.

Essentially be aware of how bad the conductivity of our volcanic soils is, add rocks and it gets a lot worse.

Some of the remote radio sites have had very substantial earths added to overcome problems due to these soil/rock issues. I imagine Arthurs Pass could be very similar to the Kaimai's with earthing issues.

Andrew Massie wrote:I'd post some resource photos... but no!

Probably not the best thing for a public forum.

Still, it would be interesting to see some of those... tgsnoopy at gmail dot com

Re the volcanic rock, I remember a lightning strike on mt eden to the Telecom Radio site receive building. Being a scoria cone the main current path was across the tie cables to the transmitter building and then down the mains earths to the generator room at the base of the mount... The signal cable was ruptured every few metres and a switchboard was blown off the wall in the lower room. I watched for hours as loads of equipment was changed out and the cable was dug up and dug up again each time they TDR'd the next fault..

tgsnoopy wrote:
For the power earth we (An Electrician and myself the Transmission (Bearers) Technician (Technical Officer)) used standard 2m Galv Steel rods with lugs welded onto them. We used a ditch digger to dig a number of trench's away from the cabinet on different angles. The trench was typically about 800mm deep. We used an uninsulated 16mm2 copper conductor between the rods, lugs crimped on and bolted with 316 stainless. Often we needed 15+ such rods to obtain 5 ohms. The copper link cable was buried with blue stone (crude copper sulphate) to improve long term conductivity.


Essentially be aware of how bad the conductivity of our volcanic soils is, add rocks and it gets a lot worse.

Some of the remote radio sites have had very substantial earths added to overcome problems due to these soil/rock issues. I imagine Arthurs Pass could be very similar to the Kaimai's with earthing issues.

OH MY GOD!! So much for Multiple Earthed Neutral!
If you don't mind, I'm going to pass your EXTREMELY INTERESTING comment onto my colleagues... Incredible reading! Thanks for that! I'll do your pics when I'm back at work next week, remind me!

Andrew Massie wrote:

tgsnoopy wrote:
For the power earth we (An Electrician and myself the Transmission (Bearers) Technician (Technical Officer)) used standard 2m Galv Steel rods with lugs welded onto them. We used a ditch digger to dig a number of trench's away from the cabinet on different angles. The trench was typically about 800mm deep. We used an uninsulated 16mm2 copper conductor between the rods, lugs crimped on and bolted with 316 stainless. Often we needed 15+ such rods to obtain 5 ohms. The copper link cable was buried with blue stone (crude copper sulphate) to improve long term conductivity.


Essentially be aware of how bad the conductivity of our volcanic soils is, add rocks and it gets a lot worse.

Some of the remote radio sites have had very substantial earths added to overcome problems due to these soil/rock issues. I imagine Arthurs Pass could be very similar to the Kaimai's with earthing issues.

OH MY GOD!! So much for Multiple Earthed Neutral!
If you don't mind, I'm going to pass your EXTREMELY INTERESTING comment onto my colleagues... Incredible reading! Thanks for that! I'll do your pics when I'm back at work next week, remind me!

[EDIT:] p.s Arthur's Pass is all gravel, really, which would explain the shocking earth continuity!

The guy from the power board was pretty sympathetic, but I got the impression he had his hands tied.

They did improve the earths on the transformers we did test though.

His name from memory was Rex Hencock, but I expect he retired a number of years ago.

Do some tests yourself. hammer two earth pins in about 5m apart and measure the resistance between them with a reputable multimeter.

Then Try passing some current through them and calculate the resistance with different currents and the voltage between them. I used a variac, mains isolating transformer and a 150W lamp as a resistor. Very interesting results.

Of course Telecom (NZPO previously) had the proper Earth meters, but we still had fun doing some practical experiments of our own to prove the meters weren't lying.

Try as I might, I don't remember the name of the Telecom Electrician from Hamilton I did the Earthing work with. Hopefully it'll return in time.

We DO have proper earth stake testers, it's pretty good down here, Chch is built on a big swamp! But I will try, sounds intriguing!

Oh ok, field trip time It's a good excuse.

Wow this topic has gone way off course.... can a mod make a new thread thats more appropriate?

Well, I dunno. The latest comments are from people who are sparkies, especially one who is a bright spark himself, and as there are no more like thunderstorm events for the North Island in the meanwhilst, with a lingering ridge pointing towards the north of the North Island and we are moving into mid December and the period towards Christmas, I think that that was a good way to finish off the topic.
I have my Thunder Likely topic or Thunder Unlikely but as we had the thunderstorm yesterday, was a vertual saver as I have threatoning to delete the topic.
I will remedy the situation by starting up a new topic shortly, which should cover both topics..

JohnGaul
NZThS

I agree Andrew & My comments should be split off to another topic.

Done.

my friends parents recently had some cows shelter under a tree during a lightning storm, all got struck by a lightning strike and died. just putting that out there