Mike Holt Newsletter

[elec_eng]

"Well-grounded principles" dated June 27, 2007 talks about the guidelines how to eliminate lightning damage with proper grounding.

Mike Holt had comments on Ufer ground. One of his comments was

NEC equipment grounding electrode are not intended to serve as the lightning protection grounding electrode.

He also said,

The study published in the IEEE August 1970 by Mr. Eugene J. Fagan and Mr. Ralph Lee evaluated steel rebar in concrete as a suitable 'grounding electrode.' Note: A grounding electrode as required by the NEC is not for lightning protection, so the study evaluated the rebar/concrete for surge and fault current, not lightning.

Summary, steel rebar is an excellent electrode as required by the NEC, but it is not permitted by NFPA Lightning Protection Code to be use as the lightning protection electrode.

He goes on and said,

A grounding electrode as per the NEC is to provide a path for inducted overvoltage to travel to the earth. Not a direct lightning strike.

I guess I am still not sure how a grounding electrode requred by the NEC is different from a grounding electrode as requred by NFPA 780.

How does the grounding electrod provide a path for inducted overvoltage to the earth?

 

[bphgravity]

A grounding electode as required by the NFPA 780 is a component of a system designed for the purpose of directing lightning currents to the earth through an intentional path.

A grounding electrode as required by the NFPA 70 is a NOT a component of a lightning protection system. This grounding electrode provides a means for bonding all metal parts of the electrical system to the earth at the structure so that the Ground Potential Rise of the earth and all bonded parts occurs equipotentially if a near strike occurs.

Induced overvoltages are not mitigated by a grounding electrode system of any kind. Only properly installed TVSS or other surge protection devices are designed for this purpose.

 

[megloff11x]

A direct lightning strike will usually wreak havoc no matter what you do. How well and fast the ground absorbs it depends on soil composition and moisture.

Tall structures are usually high in structural metal content and thus are better at dumping the energy than your wooden house and perhaps a lightning rod with a thin wire. They would prefer that the lightning not pass through the structure as it would be weakened, as shown in some of those pictures. Thus the lightning protection metal is usually exterior.

If the metal is continuous and thick enough and well bonded to the earth below, it can effectively shield the contents from the lightning.

Most lightning issues are not direct hits, but hits far away that result in transients on one or more incoming wires (phone, cable, internet, power). Thus your TV power is grounded but suddenly the coax return is at 2000V for a few microseconds. The grounding process of bonding all incoming returns to one point means that everything in the house rides the wave up and then down, including the structural steel and pipes, instead of resulting in a big potential difference between them for a very short time - but still long enough to cook your appliances or you. You don't want to grab a pipe and get shocked.

And as Bryan notes and I almost forgot, you also need transient Voltage surge suppression devices to dump spikes from the ungrounded side of signal and power wires to the ground.

We get far away hits and other transient events, every day, dozens of times a day. There isn't enough energy in these to melt rebar or shatter concrete, but there is enough to fibrilate you or cook your new Plasma TV if you don't mitigate through proper installation.

I believe NFPA 780 is for hardening something you expect to take a direct hit and that won't like it, like an oil refinery, power pole, or skyscraper. The regular code is for homes, which usually avoid direct hits because trees, poles, and other nearby things are taller.

I find it a bit confusing too.

Perhaps one of the lightning gurus should clarify, or correct me if my explanation has any errors.

Matt

 

[elec_eng]

They would prefer that the lightning not pass through the structure as it would be weakened, as shown in some of those pictures. Thus the lightning protection metal is usually exterior.

Matt,

Isn't the structure metal requred to be bonded to down conductors?

 

[don_resqcapt19]

The regular code is for homes, which usually avoid direct hits because trees, poles, and other nearby things are taller.

Lightning doesn't care what is taller....I worked on a lightning repair job where the lightning hit the underground phone line about 25' from the house. This was in the middle of the back yard and the power lines were on the back lot line.