Lightning Protection System

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sandsnow

sandsnow

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Southern California
For all you guys who see these systems:

The building is a 20 story office building. There are air terminals on the roof all connected together. No problem so far.

The air terminals then use building steel as a down conductor. Is this standard practice?

At the column pads, a conductor is cadwelded to the base of the steel column and taken out to a bunch of ground rods.

I see maybe a conflict with 250.60, since we are using the building steel as a down conductor. You can't use down conductors for lieu of an electrode. So then I could not ground a XFMR to building steel.

The plans are stamped by a master lightning guy (official title escapes me) whose company provides a UL certification on the system.

Any thoughts?? Seemed strange to me, but then I work in my attic in the summer.
 
sandsnow
look at 250.60 again. It says that this prvision shall not prohibit the required bonding together of grounding electrodes of other systems.
 
sandsnow said:
The air terminals then use building steel as a down conductor. Is this standard practice?
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Yes it is. In fact I cannot think of a better down conductor with lower high frequency impedance. The surface area makes for very low inductance, the major component of high frequency impedance. The DC resistance of steel may be higher than copper, but it is not even part of the equation compared to inductance and can be ignored for all practical purposes.
 
I would suspect that the entire building becomes a faraday cage and offers a very low impedance ground to lighting and good protection to the interior wiring
 
dereckbc said:
Yes it is. In fact I cannot think of a better down conductor with lower high frequency impedance. The surface area makes for very low inductance, the major component of high frequency impedance. The DC resistance of steel may be higher than copper, but it is not even part of the equation compared to inductance and can be ignored for all practical purposes.
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I don't dispute the performance of the building steel.

Using building steel as a down conductor would prevent it from being used as an electrode per 250.66.??????
 
sandsnow said:
I don't dispute the performance of the building steel.

Using building steel as a down conductor would prevent it from being used as an electrode per 250.66.??????
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I think you meant 250.60, but IMHO and experience it would not prevent it from being used.
 
dereckbc said:
I think you meant 250.60, but IMHO and experience it would not prevent it from being used.
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Thanks for the reply.

I got input from UL since this will be a UL Master Label System. They are ok with it, so I was worried over nothing. Learned a little bit more.
 
Utilizing structural steel in lieu of a full conductor is allowed under UL, NFPA and LPI. Grounding steel at the BASE is every other column not to exceed avg of 60 feet. Grounding to the steel at the ROOF is not to exceed every 100 feet. Be advised that UL does not allow exothermic welding at the roof level - only the ground level.
 
Lightning Protection System

I am not wild about this practice since many steel to steel connections are bolted. The steel being bolted often has mill scale and rust and in some cases a coat of paint at the bolted surfaces. This can only increase the resistance. In addition the bolts are not fully tensioned so that area of steel in contact may not be very tight. Connecting directly to columns would be better but even the column to column connections are often bolted and not welded.

It is my understanding that there are high currents when there is a lightening strike. Thus I would be concerned about damage to concrete walls and foundations when the steel columns are partiall embedded in concrete. Also can anybody venture what sort of temperature rise in the steel can be expected as a rsult of a lightening strike.

I would like to know why exothermic connections are not allowed at the roof but at the base.

When you intend to weld anything to the steel frame you should consult with the Structural Engineer on the project. These welds if performed on members of the system used to resist earthquake forces could create stress risers and crack initiators when the members undergo inelastic deformation during an earthquake.


Mark Gilligan
I design steel buildings for a living.
 
Mark Gilligan said:
In addition the bolts are not fully tensioned so that area of steel in contact may not be very tight.
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Thanks. Now I have a fear of walking into big buildings. :D

Thus I would be concerned about damage to concrete walls and foundations when the steel columns are partiall embedded in concrete. Also can anybody venture what sort of temperature rise in the steel can be expected as a result of a lightening strike.
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From what I've heard, stories of lightning blasting apart concrete from use as a grounding electrode are mostly urban legend. But this is hearsay on my part, to be honest.
 
Lightning Protection System

My reference to tensioning of bolts connecting steel members was not intended to imply anything wrong structurally. In many situations the building codes only require that bolts be snug tight But what is good structurally is not necessarily good electrically.
 
Mark welcome to the forum.

I doubt there is much difference to a lightning strike between cleaned and torqued connections vs loose and rusty connections.

The lightning made it thousands of feet through the air, it will make the jump across the rust at each connection.
 
One thing that made me puzzled about using the building steel is a requirement in NEC 780 (lightning protection). It states that down conductors must always run a downward or horizontal angle. Use of building steel just didn't make sense in relation to that requirement.
 
Mark Gilligan said:
I would like to know why exothermic connections are not allowed at the roof but at the base.

When you intend to weld anything to the steel frame you should consult with the Structural Engineer on the project. These welds if performed on members of the system used to resist earthquake forces could create stress risers and crack initiators when the members undergo inelastic deformation during an earthquake.


Mark Gilligan
I design steel buildings for a living.
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I'm curious about exothermic connections at the base only, also.

Mark, what do you mean by welding. Does that include exothermic welding?? I see a lot of things welded to the frame such as strut for pipe or conduits and never gave it a second thought.
 
Mark Gilligan said:
This can only increase the resistance.
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Resistance is of no real concern when compared to the inductance. I will give you a great example. Let’s take a look at a 10 foot section of 750 MCM copper cable, and a steel I-Beam. The 750 cable has a resistance of about .000171 ohms at 10 feet, and a steel I-beam of roughly.0002-ohms. That is not enough difference to even consider. However I could care less about the resistance, I am concerned with the inductive impedance at lightning frequencies.

A 750 MCM cable will produce about 3K-ohms of impedance for every 10 feet of cable length, whereas an I-Beam only produces about 100-ohms. Take that one step further and add the resistances back in series, you still come up with 3K for copper cable and 100 for the I-Beam.

Now there is still one more thing to consider. A steel frame building is made of multiple loops formed by the grid of interconnecting steel I-Beams, which makes parallel circuits. The multiple loops greatly reduce the impedance and resistance further. You do not get that effect with a cable.

So in my professional opinion, there is no comparison to be made, even if you had very poor connections of even 1-K, which is impossible, more like less than 1-ohm under the poorest conditions.
 
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Larry

The only difference between exothermic welding and other types is the source of the energy that is used to melt the metal.

Dereck

Assuming you are right from an electrical point of view has there been any testing to assure that there will not be damage to bolts due to local arcing or that exothermic welding will not create stress risers.

I have asked the American Institute of Steel Construction about this practice and the response is that they are not aware of this practice nor are they aware of any research that would justify using steel frames to ground lighting strikes.

Considering the potential damage to the steel frame and the potential loss of life I believe that it is prudent not to use the steel frame for lighting protection unless we have better data. In any case there should be no welding to the structural frame without consulting with the structural engineer.

Mark Gilligan SE
 
Used as a down conductor or not, the building steel must still be bonded to the LPS. The hazard of not bonding the steel would certainly exceed that of the hazard you are concerned about.
 
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