Did Lightning Rods do any Good?

[homeowner]

A lot of older houses had lightning rods on their roofs. Particularly farm houses. They would have a thick braided cable that went to ground rods. Many of them had a glass ball in the center of the copper rod.

I'm wondering of they really did any good. Sure, you want to ground a strike, but on some ways, it would seem to me that they also could attract lightning. Anything that is a high point in an area attracts it, such as large trees. Of course metal attracts, so putting a metal rod on a roof is an attractant. In a direct strike, I doubt they would do anything, in fact they might get hot enough to start a fire. But a discharge nearby could be sent to ground via the lightning rods.

One other thing, was there any usefulness to the glass balls in the middle, or were they strictly decorations?

[Robert]

They do not "attract" lightning unless the charged area is within a few feet of them already, solely because they provide a slightly shorter path than the surrounding protrusions. Electric fields appear randomly, and move across the terrain. When the jump path is less than required for the discharge voltage, it flashes over. If the field moves over a house, the lightning rod defines the jump path, and discharge thru the house is avoided.

The glass ball is decoration. Makes it look like the customer is paying for more than a thick piece of metal. The metal needs to be set above the roof with an insulator, until it terminates in the ground.

Telephone poles usually have a piece of copper wire running down them to the ground, for the same reason.

This technique is also used in some types of aircraft and in missles, which are often struck by lightning. The metal strips act as a guide to channel the bolt in a particular path, and prevent it from jumping willy-nilly into anything that may be around, which could cause damage. A metal fuselage will take care of itself, but a composite fuselage , or long radome, needs "channels" for this.

Lightning is very dangerous for flagpole sitters.

[Tomsic]

Because the flow of the electric fields during a thunderstorm is so unpredictable, the best that lightning rods can do is reduce the chances of a damaging strike. That they do. What lightning rods also do, and not mentioned above, is to bleed the ground charge into the air harmlessly through the needle-sharp tip of the rod. That reduces the voltage between the ground and the cloud overhead and so further reduces the chances of a lightning strike.

My own experience is that I've never had damage to a barn or house that had lightning rods. On houses without rods, I've had several damage incidents. I can also say that the "cone-of-protection" provided by higher structures and trees does work. I've lived in a couple of houses surrounded by tall trees and had those trees stripped of bark by lightning with no damage to the houses.

Tomsic

[trader4]

My recollection is that the above has been disproven. For one thing, given the huge distance between the cloud and the air terminal, the fact that air is a decent insulator, and the huge amount of energy involved, the amount of bleed-off that could occur is negligible.

[harry]

Lightning conductors are necessary particularly on buildings that are a high local point and are hence more likely to be struck. They need to be checked regularly that the ground point is good ie low resistance. The lightning conductor provides a low resistance path to earth. The energy released by a strike is simple (I squared R) So the low resistance path provided by the lightning conductor means most of the energy is dissipated in the ground, not on the building. High commercial buildings are frequently struck and no-one even notices.

The problem is the copper gets stolen theses days.

[bud--]

A properly installed lightning rod (now called "air terminal") system provides very good protection to a building. They provide a safe "point of attachment" for lightning that will hit the building anyway. The do not attract lighting and "metal" does not attract.

As electrical charge descends in a "stepped leader" lightning will hit what is nearest to the leader when the leader is close enough to the ground. The rod system is designed so the rods are nearer than the building.

They do not "bleed the charge", and points are not necessarily needle-sharp. The rather limited research that has been done shows a rounded tip is more likely to be hit than a sharp one at the same location. A sharp one will be hit if there is no rounded one.

The rod system is not insulated from the building. It may have to be bonded to metal within 6 feet of the rods and down conductors. If not bonded, the rod system can be at high enough potential with respect to other building parts there can be a side flash during a lightning strike.

A properly installed system also includes other protection, including surge protection at the power service panel.

[HeyBub]

Early on, there was a fierce debate among various religions. Since churches in small towns, with their elevated bell towers and steeples were commonly hit, lightning rods seemed a good idea. There were those, however, who said even if they work, lightning rods would be interfering with God's will.

The debate eventually subsided as this group continued to get burnt out while the other denominations flourished.

[clare]

The house and barn on the farm my mother grew up on were on top of a hill - with a huge oak tree. Both had lightning rods. Bothe were struch MANY times - and no damage except when lightong struck the cistern pump, jumped to the house, through the back door to the aluminum strip on the edge of the counter - across the woodstove, through the water dipper, to the well pump at the wash-sink. It blew the enamel off the dipper about the size of a silver dollar. The oak was struck several times as well.

[nestork]

I think that if there was good evidence that lightening rods DIDN'T work, we would have heard about the controversy by now. If tall building are all equipped with lightening rods, that's good enough evidence for me to believe they do work.

Obviously, you want to ensure that you have a pretty massive cable to transfer all of that current around the house and into the ground. But, I'd rather fry a $300 cable than have lightening set my house on fire.

I'd put one in if it wuz my house.

[Robert]

The current may be many thousands of amps, but it only lasts for a few thousandths of a second. Consequently, the conductor doesn't have to be massive since the short conduction time means shorter heating time. Something like #6 wire is usually enough, even tho it may carry an instantaneous 50,000 amps or so. This can be confirmed by checking the size of the grounding wire at your service entrance. Local codes and the NEC call out the required size. As far as a previous poster commenting on "bonding to the structure", keep in mind that most houses have wood, non-conducting construction, and that doesn't apply..... If the structure has steel girders, it is another matter.

[Tim Watts]

On Thursday 24 January 2013 08:05 nestork wrote in alt.home.repair:

Cable?? The rods I've seen all use 1" x 1/4" (or roundabouts) solid copper bar.

[trader4]

That sure must be an expensive and very labor intensive installation. Here in the USA I've never seen such a thing. All the ones I've seen use cable, it's accepted practice and it works.

[Robert Macy]

Thickness of the conductor is NOT to survive heating, but to maintain LOW impedance during a strike. The sharp impulse of current is limited by the self inductance [relating to diameter and shape of the conductor] and the impedance [related to the cross sectional area]

Although a bar shape is probably the only available shape one can easily get for the conductor, a round cross sectional area is the best.

[Tim Watts]

On Thursday 24 January 2013 13:00 [e-mail] wrote in alt.home.repair:

Seems to be the standard here (and the whole thing of lightning rods has a complete set of standards all to itself).

OTOH, apart from pikeys nicking the copper, it doesn;t need re-doing very often (ever??).

Before Fred Dibnah took to knocking down loads of victorian chimneys, one of his more called upon jobs was installing lightning rods on them.

[Tim Watts]

On Thursday 24 January 2013 14:23 Robert Macy wrote in alt.home.repair:

Good point - hadn't thought of that.

Just thinking about it, our flat roof at work in central London has not only rods, but a load of the same copper bar running all over the roof cross bonding bits of metal handrail (those roofs are a fire escape route sometimes) and other misc bits of metal and ventilation ducts.

They are not leaving anything to chance - and this building is 4 floors high!

[trader4]

The too are kind of one and the same....

The sharp impulse of current is limited

Say what?

[bud--]

Your comments on short duration are entirely right. But the NEC applies to power systems, not lightning rod systems. (NFPA780 is the installation standard for lighting rods.)

The average lighting strike is about 20,000A. About 5% are over

100,000A. Some are over 200,000A.

The numbers I have seen for down conductors are a little over #3 for class 1 systems and over 1/0 for class 2. (I have no idea what the difference between class 1 and 2 is.)

More than just structural steel. For commercial/industrial a rooftop HVAC unit may have to be bonded if it is within 6 feet of the system conductors. Cast iron plumbing stack? Service entrance riser? The point was you can't effectively insulate the rod system from the building.

[Harry K]

Personal experience says they do work. Back in the 30s and 40s lightning rod salesmen were common out in the farming country. Lots of people bought them. I never heard of a house with them being hit...but then I never heard of a house without them being hit either :).

We had horrendous thunderstorms every summer back on the Camas Prarie in Central Idaho.

Harry K

[clare]

The cables on the old farm house were braided copper as thick as my thumb -but loosely braded of roughly 16 guage wire with an open core to allow good air cooling. After a heavy strike there was obvious signs of heating (the oxide coating changed colour, and in some places cracked)

[hrhofmann]

Mr/Ms nestork,

Lightening is what Michael Jackson did to his skin, What comes outof the sky with a flash of light and loud noises is lightning.