lightning

[smarkv]

I have been working on a job that has been shut down over and over due to lightning.The building is only a metal structure at this time 5 floors.
One of the G.C. Employees is saying all the electrical contractor needs to do is install the lightning protection and the building will be safe ,I think he is wrong I know the lightning pro. system will provide a direct path to ground but I do not believe that this will be the only path it may take. drawings show us to install lightning rod at top and ground rod at bottom of every column both will be cad weld connections to the column, all along the outside edge of the building.the columns sit on concrete peers drilled about 80 feet into the ground does the 80 feet deep help with this issue and how will the concrete effect it? how would the lightning rod and the ground rod improve this?
thanks for any comments,
mark

 

[hbiss]

Re: lightning

Why has it been shut down? Has it been actually hit?

I'm not a lightning system guy but I would think that bonding the columns to the rebar in the piers (Ufer ground) would be much more effective than ground rods.

I also think that if the area is that prone to lightning they need to have someone who specializes in lightning protection review what's been spec'd.

-Hal

 

[hurk27]

Re: lightning

Go here and be prepared to do some reading.
National Lightning Safety Institute

Lightning protection is a very technical field and even with the best system installed there are no guarantees that it will protect anyone that is exposed to open air that can flow through these structures. If lightning were to strike a beam that is a few feet from a worker, even if this worker does not receive a direct voltage from the strike he/she could still receive severe burns from the microwave effect. Lightning is a high frequency AC event that there is still allot to learn about. Impedances of grounding systems can allow a lightning strike to go right through a building and be unaffected. The site I posted above has many pages on this and there is much to explore.

 

[charlie tuna]

Re: lightning

the one thing we "do" know about lightning is it is unpredictable! a "listed" lightning protection system is supposed to protect a building from damage. but, it's not 100 per cent protected. i have seen a building with a system get hit and blow a 200 pound chunk of concrete off the corner! and i have been told that a system that is not ul listed or engineered can actually draw lightning instead of protecting a facility???
i was installing a lightning protection system on a 30 story building --- we had not tied the roof loop into the ground risers and over a weekend lighning hit the loop on the roof and ruined a spool of 4/0 bare cable that was close to the cooling tower!!! it gets spooky when you are installing the roof spikes and you can hear the static snapping at the point of the spike!!!

 

[mc5w]

Re: lightning

hurk27,

Thanks for the link to the National Lightning Safety Institute.

Over at www.eng-tips.com I posted a comment that lightning protection is only as good as the ground. Someone from a Colorado electrical utility replied that on the portions of their system that use 17 foot ground rods (using rod couplings) they have ZERO lightning damage. Maybe in 10 years they might have a stroke that is big enough to toast a 17 foot rod.

An 8 foot rod that is underneath an at grade slab like will do zilch. John Labriola who is one of the top electrical inspectors in northeast Ohio says that when you drive an 8 foot rod from the surface only the bottom 2 feet acts as a grounding electrode. When he was a contractor he liked to go into a house before the basement slab was poured and drive a ground rod underneath the basement floor.

Also, a cable plant technician for Ohio Bell told me that 97% of the lightning damage in a telephone cable plant comes from silent lightning and invisible lightning. He then said that when a Big One does hit you have about 100 side strikes over a 100 yard radius. What he was trying to say is that most lightning damage comes from just a little lightning.

 

[petersonra]

Re: lightning

Originally posted by charlie tuna:
have been told that a system that is not ul listed or engineered can actually draw lightning instead of protecting a facility???
!!

How would the lightning know if the system was UL listed or not?

 

[hurk27]

Re: lightning

It checks the tag

 

[catchtwentytwo]

Re: lightning

I believe the installer, not the system gets the UL Listing.

 

[hurk27]

Re: lightning

Actually lightning protection system installations are UL field certified as well as the installer.

 

[catchtwentytwo]

Re: lightning

An excellent resource for lightning protection information is at the LightningProtection ? Lightning & Power Quality Issues newsgroup
http://groups.yahoo.com/group/LightningProtection/

 

[oceanaradio]

Re: lightning

Couple of comments on some widely spread rumours about grounding for lightning protection:

>Maybe in 10 years they might have a stroke that is big enough to toast a 17 foot rod.

Grounding electrodes are not "toasted", but repeated strikes to any grounding system will render it's conection to conductive soil ineffective. Silicas in most soil can "glassify "and become insulators around the rod, reducing it's effectiveness with each heating.

>An 8 foot rod that is underneath an at grade slab like will do zilch. John Labriola who is one of the top electrical inspectors in northeast Ohio says that when you drive an 8 foot rod from the surface only the bottom 2 feet acts as a grounding electrode.

Well, poor John hopefully does not impose his mistaken understanding of this feature of a lightning protection system to his inspection clients. In fact the top 2 feet are just as likely to be the only conductive portion, based entirely on the soil strata conductivity and little to do with the rod depth through it. Going deeper just increases the likelihood of reaching more conductive soil. But driving couplers is very controversial, as it leaves a wide hole around a narrow rod and little connection to earth along the way.

>When he was a contractor he liked to go into a house before the basement slab was poured and drive a ground rod underneath the basement floor.

Yes this does establish quite a baseline - and one that is a nightmare for all other utilities such as telephone and cable. Their service entrances up at grade may be as high as 10,000v potential above that service panel ground in the basement. Leaving poor telco installers little choice but to ground to the cold water pipe or other outdoor feature - and it can meet code, proving that NEC/NFPA do not always protect us from ourselves, or over-exuberant inspectors.

Another comment in the session was about the so called "microwaving" hazard of lightning when standing near bonded structural steel. Sorry, no such thing. Lightning has very little energy as high as 100 megahertz, and most energy at less than 5 MHz. Microwave is above 1 Gigahertz. There is however, always a step-and-touch hazard to people who come in contact with any lightning protection system. We work with this hazard in VHF/HF radio communications, and must apply standard electrical safety practices to avoid making separate paths through human touch to such equipments. Single point ground and proper bonding are the lifesavers to both equipment and personnel using them in our field.

Best regards,

Jack Painter
USCG Oceana Radio
Virginia Beach, Virginia

 

[don_resqcapt19]

Re: lightning

Jack,

Yes this does establish quite a baseline - and one that is a nightmare for all other utilities such as telephone and cable. Their service entrances up at grade may be as high as 10,000v potential above that service panel ground in the basement.

The code requires that the grounding systems for the phone and cable be bonded to the building grounding system to prevent this type of problem. There can be large voltage differences anytime there are multiple grounding electrodes that are not bonded together.
Don

 

[oceanaradio]

Re: lightning

Originally posted by don_resqcapt19:
Jack,

Yes this does establish quite a baseline - and one that is a nightmare for all other utilities such as telephone and cable. Their service entrances up at grade may be as high as 10,000v potential above that service panel ground in the basement.

The code requires that the grounding systems for the phone and cable be bonded to the building grounding system to prevent this type of problem. There can be large voltage differences anytime there are multiple grounding electrodes that are not bonded together.
Don

Hi Don, that is exactly the example of meeting code but allowing an unsafe condition that I am talking about. Where lightning is concerned, there will be large differences anyway in that case. NEC bonding jumper requirements cover only DC impedances, and do little to impress lightning surge voltages, whether from ground potential rise, magnetically induced or capacitively coupled surges.

If the utilities are not grounded together in the first place (the only recommended practice), bonding together across such a large difference as grade to 10 feet below grade (plus any lateral distance)creates an inductance that allows thousands of volts potential between the two. And there is little an owner can do once that situation is created for him. Some contractors building new homes with large basements create this very situation. When direct TV satellite, cable modem or telco technicians arrive later, many never bond at all, and if they did, do they care what impedance/inductance they allowed between separately grounded systems? I doubt it.

Cheers,

Jack

 

[hurk27]

Re: lightning

Hi Jack This is a very good thread on this problem and I have to agree with you on these impedances that will not block a voltage differently between different points when runs are beyond 20' Or even less in some cases. Derick has made this statement many time here that the impedance of a #6 10' long can be several thousand ohms and will do very little to reduce differently voltage.

One thing that I do have to say is about your statement:

Another comment in the session was about the so called "microwaving" hazard of lightning when standing near bonded structural steel. Sorry, no such thing. Lightning has very little energy as high as 100 megahertz, and most energy at less than 5 MHz. Microwave is above 1 Gigahertz.

This is just one of the most common ways some use to describe the effect of "RF" heating which can have high energy heating that can happen at much lower frequency's and with enough energy (which I think most lightning strikes have) this effect can happen at frequency's as low as 100khz. I have been burned many times by getting to close to a high powered antenna system for ham radio. And I have worked on a industrial RF amp that had a output of over 200,000 watts at 27mhz's which was used to seal a fiber product to a car door liner. You didn't get very close to it before it started to warm your hand. It has been known that even in a small shelter that a close by lightning strike can cause burns within a person's body without even striking the person. I have seen a few cases of this in Florida and I have been close enough to feel the heating effect of a close by lightning strike that felt like I just drank a hot cup of coffee. But the research into this is very vague and there is not much data that supports this as most strikes are miss diagnosed or referred to as a direct strike even when there is no entry or exit wounds. The link I posted in my first post has a little on this but very little as without the data most in the science communities wont discuss much about it openly.

But I do believe there is still much to learn in this field and still much to discover.

 

[dereckbc]

Re: lightning

smarkv, the only thing it does is make the site as safe as one can reasonable make it. However, that does not imply it makes it safe to work on a exposed steel structure.

Three possible problems can still occur.

One is you or someone else could be directly struck by lightning.

Two If lightning were to strike the structure, than as the fault current flowed through the steel, step potential voltage diffences could be high enough to cause injury of death.

Three, same as two except from arc blast if someone was located close to the strike area.

 

[mc5w]

Re: lightning

You forgot that you have to provide a point where the telephone and CATV people can hook up to the electrical service ground, such as a piece of #4 solid copper wire sticking outside.

A lot of AHJs around here require that aluminum siding be bonded both at the service and at the opposit end of the house.

The only ways that the bottom 4 feet of a ground rod could be ineffective is if it is driven into hardpan clay or hits a rock and turns 90 degrees. Ohio Edison's electricians tell me that they sometimes dig up an old ground rod when removing a pole it is bent 90 degrees somewhere. Someone else told me that a ground rod did a 190 degree turn and almost boned him up his rear end.

If you have hardpan clay you can only drive a ground rod 4 or 5 feet into the ground and you would need to dig a deep trench and lay the rod in the trench.

 

[mc5w]

Re: lightning

Forgot to add that NEC also requires and extra ground rod if a telephone or CATV protector block is more than 20 feet from the electrical service grounding electrode. What that means is that typically the telephone or CATV would need a dedicated ground rod that is bonded to the service GEC unless the telephone or CATV protector block is right by the service panel.

Also, if a meter socket is too far from a service switch the surge protection for an electronic meter is no good.

 

[iwire]

Re: lightning



[ May 05, 2005, 05:26 AM: Message edited by: iwire ]

 

[iwire]

Re: lightning

Never mind.

 

[mc5w]

Re: lightning

Hey Jack,

The idea behind ground rod couplings is that unless driven into a swamp ( and we are no longer allowed to build in one ) the bottom half of a ground rod will always be more effective than the tope half. Also, in new construction a ground rod that is driven into the backfill is no good becuase the soil is never compacted enough until about 20 years after backfilling. Foundation drains and gravel also dictate that a ground rod would have to be driven 6 to 10 feet horizontally from the foundation to have any more than the bottom 2 feet in permanent soil moisture. You would be better off to drive the rod well into the bottom of the foundation excavation inside or outside of the basement wall.

Also, lightning rods manage to work effectively even though they are tens of feet or even hundreds of feet above the grounding electrodes. Army Manual 5-690 specifies a 10 foot deep 1/0 copper ground ring as part of the grounding electrode system for lightning rods. For each down conductor there has a to be a ground rods driven below the bottom of the trench and bonded to both the down conductor and the ground ring.

There are also a lot of small lot homes where the sp[acing between houses is just enough for a 1 lane driveway. That means that the only way to install a ground rod that is close to the service is to jackhammer the basement floor and drive a rod underneath. I can run the ground wire from the service panel, loop wire through the ground rod clamp, and then go back outside and bond to the service conduit or to the outside of the meter socket. This would provide and excellent place for the telephone and CATV people to hook up their grounds.

Theoretically, I need to jackhammer the floor underneath a service panel and install grounded wire mesh to prevent a tingle voltage on the service when somebody's old underground wiring to a detached garage starts leaking into the ground.

Also, some utilities require that there be a grounding electrode conductor running from the meter to the nearest grounding electrode. For electronic meters this is a little more critical in a high lightning area.