Lightning, direct hit or not.

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aelectricalman

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Well this is the third time ive been out this week for the the same exact problem at three different locations, miles away. Lightning struck and burned all of the low voltage equipment in the home. The low voltage component of TV's, the cable boxes, the modems, the garage door openers (low voltage only), the phones, the computers sound cards, and audio and video, sound cards, you name it, but no line voltage is effected. I feel like as a contractor I should have a definate answer as to why this is the case. I feel like if lighting struck the home, this would be a sign of good grounding ( because there is no damage on the 240V system), but is it something more. In the past I have just told my customers lightning struck, this is the damage, and heres the bill.

I feel like you dont have to have a direct hit for this to happen, however. I feel like this is more of a matter of, you just cant save it all, rather than there is something wrong. What are some of your thoughts on this? In all of the cases, I found no indications of damage to anything other than circuit boards and LV. So, what is the most likely point of entry? Is this a sign of good grounding because the line voltage was not affected? Is it a case of taking one for the team? Thanks for your help!!
 
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Transistors don't tolerate overvoltage well. Little doses of energy at modest voltage (125% of device rating -- which, nowadays is 3 or 4 volts) at the PNP or NPN junctions of transistors and other solid state devices will simply punch through and turn the device into green goo, good only as a paper weight (and not a heavy one at that).

Most wiring and motors can endure 400 to 600% overloads for quite a while with no ill effect.
 
Entry through service, CATV, or phone but with the extent of damage I would guess service. Make sure the homes have a good ground. Put surge protection on TV's etc with sensitive electronics. Some even like whole-house protection but I have never found a need personally. If I were in an area prone to strikes I might.

Al got the rest.

Add: You might see if the utility's lightning arrestor standard is adequate for this area and that they don't have some blown arrestors.
 
I would check the cable and phone is grounded to the service GEC. I had one customer that lived next to a rock quarry, every time a storm came up he would loose every phone in the house along with the TV's and VCR. The cable and phone company brought their service in at the opposite end of the house, and had it attached to a lone 3' ground rod. I buried a #4 bare copper in the yard from the electrical service over to the phone and cable drop. Added a ground rod at both locations, the customer had no more trouble.
 
Lightning hit a storage shed 20' from my house last year.

It took a huge chunk out of a 2x6 and somehow traveled to the motion sensor in my basement.

It blew out that zone on the main board :mad:
 
Lightning hit a storage shed 20' from my house last year.

It took a huge chunk out of a 2x6 and somehow traveled to the motion sensor in my basement.

It blew out that zone on the main board :mad:

In Atlanta a couple weeks ago, lightning hit a brick chimney, blew a hole about 5'-6' across about 6' off the ground. There was a couple of kids sitting the living room that received minor injuries from flying glass and brick.
 
In Atlanta a couple weeks ago, lightning hit a brick chimney, blew a hole about 5'-6' across about 6' off the ground. There was a couple of kids sitting the living room that received minor injuries from flying glass and brick.

The exact same thing happened in Cleveland a few weeks ago. The lightning missed the top 15 feet of the brick chimney and hit at the firebox level. Sent two kids to the hospital.
 
Well this is the third time ive been out this week for the the same exact problem at three different locations, miles away. Lightning struck and burned all of the low voltage equipment in the home. The low voltage component of TV's, the cable boxes, the modems, the garage door openers (low voltage only), the phones, the computers sound cards, and audio and video, sound cards, you name it, but no line voltage is effected. I feel like as a contractor I should have a definite answer as to why this is the case. I feel like if lighting struck the home, this would be a sign of good grounding ( because there is no damage on the 240V system), but is it something more. In the past I have just told my customers lightning struck, this is the damage, and heres the bill.

I feel like you don't have to have a direct hit for this to happen, however. I feel like this is more of a matter of, you just cant save it all, rather than there is something wrong. What are some of your thoughts on this? In all of the cases, I found no indications of damage to anything other than circuit boards and LV. So, what is the most likely point of entry? Is this a sign of good grounding because the line voltage was not affected? Is it a case of taking one for the team? Thanks for your help!!


I'm going to try to give you a short explanation as to what happens when what is called a "Near Field point of Attachment" hits

I have been on many strikes like this, and it stumps most.

When lightning hits in a close vicinity of a structure, the current in the stroke develops a very large EMF pulse that couples to other metal objects in this field, this transformer like effect will induce a voltage/current on these metal objects just like the windings in a transformer. now, one of the biggest mistakes cable, phone and satellite installers do is install there equipment in a different location from the service, this will allow even move voltage to develop between the service conductors and the low voltage system involved. by having every system enter and exit a structure at the same location you can keep the voltage to a minimum but even then, current can still develop between two systems run separately in the structure. what happens is the EMF will treat each system as a conductor, all the wires in a single cable will act like one wire, and the difference of potential will be between these sets of cables not each conductor, this leaves SPD's totally useless, because they only protect between each conductor, and even if you do place them between cables the distance will probably be to great to be effective.

A good example of this is when a entertainment system is powered off one circuit and a powered sub woofer is powered off another circuit, the two circuits take different paths through the building back to the common service, when the EMF from a NFPOA happens voltage/current will develop across each set of cables as if there are a single conductor each of a circuit, this will place a difference of potential across the little audio cable that connects between the two systems, I have seen this audio cable burn up like fuse and vaporise across the floor.
No SPD can ever protect against this. this is a good reason to try to keep our runs close together as much as possible in high lightning areas.
And also all cables entering or exiting a structure should be at the same point or as close as possible to each other to have a canceling effect.

Lightning is a high frequency event and has to be treated like such.
 
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I'm going to try to give you a short explanation as to what happens when what is called a "Near Field point of Attachment" hits

I have been on many strikes like this, and it stumps most.

what happens is the EMF will treat each system as a conductor, all the wires in a single cable will act like one wire, and the difference of potential will be between these sets of cables not each conductor, this leaves SPD's totally useless, because they only protect between each conductor, and even if you do place them between cables the distance will probably be to great to be effective.

Won't each "conductor" also have a potential to ground? If there are SPD's between each conductor and ground wouldn't they then provide some level of protection? Provided the SPD remains functional they would then limit the maximum potentail that could exist between "conductors".

It is understood that when the level of the induced currents is great enough that even SPD's will blow open, but they do provide some level of protection - to a point.

If the SPDs are of a large enough energy rating and the impedance of the grounding system to them is low enough they are quite effective, again -up to a certain level of induced energy.
 
Won't each "conductor" also have a potential to ground? If there are SPD's between each conductor and ground wouldn't they then provide some level of protection? Provided the SPD remains functional they would then limit the maximum potential that could exist between "conductors".

It is understood that when the level of the induced currents is great enough that even SPD's will blow open, but they do provide some level of protection - to a point.

If the SPDs are of a large enough energy rating and the impedance of the grounding system to them is low enough they are quite effective, again -up to a certain level of induced energy.


The answer to what I highlighted in red above will depend upon the center frequency of the lightning stoke at the time of the strike, remember conductors at RF frequency's can be many times at a higher impedance that at 60hz or even DC, secondly is that a near field strike will produce an isolated voltage and current in these cables that is isolated from the lightning strike just like the secondaries are isolated from the primary's in a transformer, so the current developed will only circulate in the cables of this made secondaries, and will no longer seek the Earth path the original stroke was taking. so SPD's have no effect on these cables, only running the cables in a fashion that keeps the induce current from developing upon the cables is effective. this is why running the cables together as much as possible and creating a bulk-head window to enter and exit the structure is the only effective measure to block near field points of attachment.

There is a big difference between a direct strike and a near field strike,

Direct strike wants to return to Earth.
Near field strike will result in currents on metal or cables that form an isolated secondary that only return to this secondary thus isolated from Earth. this is why we see the damage to low voltage equipment that will have conductors that will complete the circuit of these made secondaries.

SPD's tied to an Earth reference will be of no effect.
 
In the case of the normal home we are talking about a grounded system. So when currents are induced into this wiring as the "isolated secondary" you mention, there still will be a reference to ground so these wires will develop a potential with respect to ground.

I do see what you are saying though with respect to all three conductors within a particular cable being elevated to the same potential and thus no potential difference between them. Thus no reason for a local SPD to break down until there is another path introduced between cable sets, each elevated to a different potential.
 
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