Lightning Strike Protection

[Darkness]

I was at a house the other day and they showed me a newspaper which had a story about a Dairy farmer who went out to milk his cows one morning and found the cows in his shed all dead. It appears the previous nights lighning hit somewhere and the strike went down the automatic manure scrapper and electocuted the cows standing in the wet dung. The CU was melted. Farmers are now being advised to fit earth rods to the scrapper systems.

Would fitting an earth rod make much difference? These large steel constructed sheds are a target for lightning strikes so if a shed was hit, what sort of protection can a farmer get to prevent lightning accidents from happening?

 

[Aragorn84]

Presumably lightning protection on the building itself is what would be required.

Its a fairly specialised field, but essentially involves bolting a large conductive rod to the roof, and using large copper cables to carry the lightning current safely to earth via an earth rod.

Many large building have these systems installed.

 

[Spark123]

Yep, however a rod may not be a great idea where livestock are concerned as you can end up with a voltage gradient around it, with 4 legged animals you can end up with current passing up one pair of legs and down the other effecting their heart.
A grid buried in the floor of the dairy may be a better idea but will cost a small fortune.

 

[AdrianUK]

As Spark123 says, the problem with quadrupeds is that they are very venerable to getting a shock between there front & rear feet.

There have been several documented cases where an earth fault in a farm building has lead to all the cows in a nearby field falling over dead. The problem is that when a current flows down an earth rod it creates a voltage gradient in the soil around it. Depending upon the conditions in the soil this gradient can spread for several tens if not hundreds of metres. If the conditions are right & a cow is standing in the right place it will experience a potential different between its front legs & is rear legs. Tis may be enough to kill it.

The IET wiring regulations are quiet specific about electrical installations in milking sheds & the like, even specifying conductive metal grids in the floors. These aren't to protect the farmer so much as to protect the livestock.

Going back the original post, I think this probably happened due to a nearby lightning strike setting up a voltage gradient across the ground, nothing to do with the manure scraper. I don't think fitting an earth rod would make the slightest difference, the only real answer would be a metallic grid inside the concrete floor to prevent the voltage gradient.

 

[Darkness]

From from I read in the story, two sheds of cattle were affected and both contained scrappers, the scrappers that have a steel track down the passage which the scrapper travels along so it must have something to do with it when the other nearby sheds were ok.

Basically, its a freak accident and most likely it will never happen again.