Lightning 2

[tulum]

"When lightning strikes a severe over voltage on the order of 300kv may be delivered to the electrical system. Fifty percent of all strikes have a current less than 15,000A. However, currents of up to 200,000A have been recorded (SHEE, Pg. 18-43)." - summarized version.

I do not have to much electrical experience. However, I seem to be swamped with questions on it now that I am required to make many installation decisions in my plant. My question is as follows;

During a lightning strike, since the current only last for micro-seconds, is equipment usual only damage from overvoltages or does current play a role as well? What types of protection usually operate during lightning strikes? Obviously, arrestors, but do you find much instantaneous equipment tripping...or is the time to fast, and current to low, for usual trip settings (I don't set breakers, etc)?

 

[jraef]

There are several good websites devoted to the issue of lightning protection if you search for them. Some basic concepts to address your questions however are:

1) Yes, the majority of damage (by number of damaged units) will come from the voltage spike. There are plenty of very good devices available to limit this damage to manageable levels, but nothing will guarantee total protection. GE sells a very good line of supression devices called Tranquell

http://www.geindustrial.com/cwc/pro...lid=5&famid=37&catid=85&id=tvssoem&lang=en_US

but they are certainly not the only ones. A good overall search term is TVSS, Transient Voltage Surge Supressor.

2) Current plays a major role in that such a rapid rise to such extreme levels is generally explosive. The quantity of damage then, as measured by physical destuctive energy, will come from current. Whatever the lighning hits is usually vaporized or severely burned, depending on its size and resistance properties. The concept of Lightning Rods is based upon giving the lightning something easy to hit first, and being able to conduct it to ground without emitting shrapnel or starting a fire.

3) Protection is limited to what I mentioned above, deflection of the overall power to ground, and supression of the voltage surge that will take place in the surrounding area. Nothing currently available can act quickly enough to "trip" in time to save anything. Think about it. If the lighning is powerfull enough to jump the air gap from sky-to-earth, how can anything on earth hope to interrupt it?

Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion to the original posting marked ///\\
My question is as follows;
During a lightning strike, since the current only last for micro-seconds, is equipment usual only damage from overvoltages or does current play a role as well?
///Actually both, since the VxI* in Watts. The lightning strikes have to be taken a case-by-case bases. There are instances where the lightning strike intites combustible.\\ What types of protection usually operate during lightning strikes?
///Lightning Arresters, TVSS, etc.\\ Obviously, arresters, but do you find much instantaneous equipment tripping...or is the time to fast, and current to low, for usual trip settings (I don't set breakers, etc)?

 

[jghrist]

Overcurrent devices generally will not operate unless the surge causes a flashover to ground and a fault. Sometimes small fuses may blow from the surge current if the fuses are ahead of an arrester.

 

[jbartos]

Continue with my previous posting:
During a lightning strike, since the current only last for micro-seconds, is equipment usual only damage from overvoltages or does current play a role as well?
///Actually both, since the RE[VxI*] in Watts. The lightning strikes have to be taken on the case-by-case basis, meaning that each lightning strike has different parameters including a path, current, voltage, location, etc. There are instances where the lightning strike ignites combustible or flammable. The larger current magnitude, e.g. 200,000A, lasting micro-seconds can develop a force peak that can structurally destroy house, split tree branch from the stem, shatter a wrongly designed ESE air terminal, etc.\\ What types of protection usually operate during lightning strikes?
///Lightning Arresters, TVSS, etc.\\ Obviously, arresters, but do you find much instantaneous equipment tripping...or is the time to fast, and current to low, for usual trip settings (I don't set breakers, etc)?
///Some protective devices, e.g. fuses, circuit breakers, etc., have so-called energy-let-through characteristics. If such energy is high, beyond the let-through-energy design, the device can physically disintegrate. See IEEE Std 242-2001 Buff Book for "peak let-through current," "peak let-through energy," "let-through characteristics," etc.\\