More dodgy tat

[RF Lighting]

I once attended a post lightning strike to a TV aerial on a house four streets away from me. The bang was so loud it rattled the windows in our house.

It vaporised the coax down his outside wall. All that remained was a scorch mark where it had previously been.

It blew up his TV with such ferocity it set fire to his living room carpet. Every piece of electronic equipment was destroyed, and also the phones in several neighbours houses. The RCD in his CU physically exploded trying to clear the fault.

 

[ban-all-sheds]

So basically they should not really be called "lightning conductors", since they are not installed with either the hope or expectation that they will actually carry the current of a lightning strike.

 

[JohnW2]

So basically they should not really be called "lightning conductors", since they are not installed with either the hope or expectation that they will actually carry the current of a lightning strike.

Is that necessarily true of the massive 'copper bars' one sees going down the sides of some buildings?

Kind Regards, John

 

[SimonH2]

So basically they should not really be called "lightning conductors", since they are not installed with either the hope or expectation that they will actually carry the current of a lightning strike.

Well they are conductors, and they are there for lightning protection.
Even if they fail during a direct strike, it'll still divert most of the current away from the fabric of the building. Without them, there's nothing to channel the strike down the outside of the building - so it'll go down the inside taking an unpredictable path.

But as mentioned, the primary function is to try and discharge the static without a strike - hence the sharp points. If they succeed in doing that then they've worked - and I suppose it's virtually impossible to determine how well they do that since you can't tell whether there would have been a strike without this. It could be like pink custard and tigers *

BTW - the thick copper bars probably aren't as good as many think - skin effect comes into play, reducing the effective cross section.


* As in the joke ...
A bloke sees his neighbour painting the front gate with pink custard, and asks why. "It's to keep the tigers away" comes the reply. "But there aren't any tigers around here" says the bloke. Only to get the reply "See, it works !".

Yeah, it wasn't that funny the first time I heard it

 

[JohnW2]

BTW - the thick copper bars probably aren't as good as many think - skin effect comes into play, reducing the effective cross section.

That makes sense (maybe they are sometimes hollow?) - but one does sometimes see multiple ones, which is perhaps an attempt to partially address that issue?

Kind Regards, John

 

[bernardgreen]

The approximate theory is to create a wide mesh Faraday cage around the building. In theory the discharge starts down one or more of the external conductors which may be vaporised but once the discharge is going down that route it won't be changing direction. The near straight line of plasma and / or copper vapour is far lower impedance than the building.

The rate of increase in current is rapid ( i.e. one half cycle at extremely high frequency ) and any bend or curve in the conductor is high impedance and the current coming to a bend will jump to anything providing a straight ( low impedance ) path to ground.

The damage to radio equipment can be reduced by a U bend in the aerial feeder close to an earthed metal plate. The impedance of the U bend encourages the discharge current to jump to the earthed plate.