Lightning damage to lead-acid batteries

Hi,

Is it common to find damage resulting from a lightning strike to the lead-acid batteries in a solar system ?
and if so, what type of damage would be expected ?

Also, is there a way to determine if such damage has occurred to the batteries by looking at them or testing them ?

Thank you...

Joel

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning damage to lead-acid batteries

    if you cannot see any damage visibly, the only other thing you can do is to load test it per your battery manfacturer's recommendations. there could be an internal short ranging to no damage at all. if you don't go through or have a manufacturer's recommended load test you may do it yourself by a drain on it at a rate of 5% for a max of 20hrs. this should bring it down to 100% depth of discharge at the voltage point on the battery of 10.5volts. if your battery reaches that point say after 10hrs then it has lost 50% of its capacity. i don't like draining a battery down that far as any deep discharge takes away from battery life some so if your battery manufacturer has any alternative methods please follow them.
    if you do nothing at all then see if it is acting as per normally, but small differences from damage could then be missed. was there other damage to equipment and what makes you believe it was lightning?
  • System2System2 Posts: 6,290 admin
    Re: Lightning damage to lead-acid batteries

    Thank you for this quick reply...

    It was lightning all right... coz somebody witnessed it firsthand !

    The inverter fried as well as one of the solar panels. The plug over also popped up on two of the batteries.
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Lightning damage to lead-acid batteries

    Joel,

    ---OK, saw your post after I typed this--But much still applies... And, it sounds like you should probably review your grounding/system wiring to make sure that it is properly designed/installed...

    You are trying to determine if you have lightning damage, or are you more interested in preventing it?

    I don't live in an area where we have much of a chance for strikes, but I have designed equipment that needed to withstand surges associated with strikes.

    In general, I would guess that either the battery would show visible damage (blown off caps, cracked case, etc.) or it would probably be good... More likely would be damage to wiring, electronics (chargers, inverters, etc.) and possibly panels too... That damage may be visable, or only detected during operation.

    Following the grounding requirements for the National Electric Code (NEC--assuming you are in the US) and local building codes are good places to start. Usually, the point in grounding is to protect life, and secondarily, to protect equipment. In general, good safety practices for protecting against lightning is also pretty good at protecting equipment too.

    Grounding can be a complex, and at times, counter intuitive, as how to best properly ground equipment--especially for solar where you have the panels on the top of a home or on poles in a field, with the chargers/batteries located 100'+ away else where.

    Do you wish to start a discussion on grounding or are you just trying to figure out if you have some bad batteries--or possibly a damaged charger or inverter because of poor performance?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Lightning damage to lead-acid batteries

    Thank you Bill,

    But I'm really just looking to determine if the batteries have sustained some damage. Your comment regarding the grounding problem is welcome however.

    I do understand that adjustments and upgrades to the sytem will have to be made to safeguard it against lightning strikes. As it is configured now, the system has no lightning arresters and only poor grounding.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning damage to lead-acid batteries

    quoting from myself that was previously posted:
    "was there other damage to equipment and what makes you believe it was lightning?"

    maybe i made an assumption that you had proper grounding, but lightning can and does still hit and do damage even if following proper practices. lightning arrestors are good, but they don't necessarilly stop damage either as they can activate at certain voltage points that could be too late and high for your equipment. also be aware they can be little hand grenades when they activate and even if it may have stopped a devastating electrical blow to you or your equipment that the exploding of the device could do you and your equipment harm. this makes the placement of these devices very important so as to prevent such an occurance. these devices that can explode are the mov type protections. the gas discharge types are much better, but a smaller discharge on it will be passed rendering you and your equipment unprotected from any to follow as they protect once.
    there have been a few discussions on grounding and lightning already and i would suggest that you seek them out and read them. i'm also sure that we wouldn't mind a further discussion either.
    bb,
    i though of using the reverse breakdown voltages or piv of standard diodes as protective devices. what do you think of the idea? i use this method on my equipment as i have a 50piv 1amp diode across the pv output just prior to the controller. the controller is an sb50 and it cannot take 47v or more, but piv ratings aren't that exacting either so it may avalanche below or above their ratings. this i believe is better than seeing mov protection starting at around 250v or 300v for me and for others as well.
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Lightning damage to lead-acid batteries

    Niel,

    Realizing that a direct or near direct strike of lightning is a huge amount of energy and that it may be impossible to prevent damage to electrical equipment... This is how I would proceed.

    First, get a copy of the NEC and and local building codes and read the sections on grounding... To be honest, since I have worked on grounding from a different side of the issue (grounding for safety from shorts, ground loops, radio interference, esd supseptability, ship's grounding, etc.)... I will talk about the generic methods I would employ... --But follow the NEC/Codes first, for safety and insurance reasons.

    I would break the problem into several parts... The first part is to prevent (as much as possible) to prevent the lightning from getting into the electrical circuit. To do this would be to make sure that all wire sets/pairs (+/- DC, Line/Neutral/Line, etc.) should run parallel next to each other (including the ground wire too).

    Even better would be to give a twist or two of the wire bundle once or twice every foot (more is better for higher frequency control--but any twist is better than none). Most people are familiar with twisting wires for AC signals (audio, networks, antenna leads, etc.)--but this would also be good practice to protect against lightning induced differential currents.

    Best would be to place the bundles into grounded metal conduit. And, do not mix in one bundle/conduit both the DC from the panel with AC output from inverters and/or control wires (keep circuits separated as much as practical and if they come near each other, try to always cross at 90 degrees to each other). Also, avoid making big loops with excess wire or make loops around solar arrays (like around a metal pole from the ground to the array). These loops can behave like a large transformer and induce common mode current into the wiring. If you must store excess cable length, then store it as figure eights ("8").

    The idea here is to:

    A) prevent any differential voltage from entering the harness--i.e., you don't want the "+" lead in a 12 volt harness to be 300 volts higher than the "-" lead. By keeping all of the leads running together, there cannot be any large differential voltages developed (If that leads are, say several feet apart near a lighting or ESD hit, the current induced is not of the same polarity or current between the separated wires--think of a couple of loops of wires around a second wire--the basic definition of a transformer).

    B) that any voltages/currents induced are the same on all leads has the same polarity of voltage/current--i.e., induced common mode current (for example, +/- leads to a battery. I try and put +300 volts to "+" lead and -300 volts to the "-" lead, I could get (assuming there is enough current and impedance) upwards of 600volts across the battery (see post above as result). However, if the wires are close together, and only +300 volts is induced on both the "+" and "-" leads, then the battery sees zero volts induced across it terminals (and no damage) but 300 volts from battery case to ground--could still be a problem, but much less likely to cause a problem unless there is an arc from the plates through the case to the battery shelf.

    C) to prevent induced currents and voltages from crossing from one circuit to another (say an exterior Solar Array and the internal home AC circuits)... keeping wire at right angles and avoiding long parallel runs will help prevent induce current crossing from one circuit to another.

    D) twisting prevents differential voltages that are very hard to protect against equipment damage. Commom mode voltages are usually much easier to control damage. For example, an inverter would use a transformer to prevent common mode voltages from transferring from the input to the output. A simple battery charge controller would have no protection... A MPPT controller probably does not have internal common mode isolation--but it could be designed in by some smart manufacturer.

    E) using metal conduit to shield the energy from getting into the wires in the first place--and grounding the shield to direct the energy to someplace safe.

    AVOID Multiple DC grounding points in a circuit. For example, you connect the DC battery bus to earth ground near the input to the battery shed. Don't ground the DC bus again near the battery (or controller). What can happen is that you now have two parallel wires carrying the DC buss current. And, you may end up 1) carrying too much current on the ground wire and melting it and/or 2) violating the issue of running +/- leads next to each other and picking up induced current an injecting differential energy into your DC buss (example, next near strike may cause equipment damage).

    Ground loops are not always obvious and sometimes it is not clear how best to ground a large distributed system. For example a solar array 100' away from the battery shed. Do you DC ground the negative at the array and the battery "-" buss... The problem with multiple DC grounding is a lighting strike at the array will can now enter the DC harness on the negative lead at the array and its voltage will increase with respect to the ground at the battery shed. And this voltage profile will be different that that of the "+" lead... Now creating a differential voltage between the positive lead and the negative lead. And this is the easiest method to destroy your charger/batteries/inverters... I would normally suggest grounding the array metal frames and not grounding the "-" solar PV panel lead again (assuming grounded at the "shed").

    Regarding protection devices... I started to suggest where simple reversed biased diodes could provide protection--but I am not sure that they are a good idea (for example, could provide a discharge path at night and such). The use of Schokty Diodes to pass current at 45 volts--I am not sure that they would be fast enough to prevent damage, or large enough to do much good--and could possibly hurt if, for example, the diode failed shorted (fused the junction inside)--now it could be a dead short and overheat/fire from either solar panel power and/or battery power depending on where it was placed...

    Gas Discharge Tubes are pretty high voltage and (when used in a telephone circuit or other communications circuits) are protecting pretty high impedance/resistance wire. Almost every Solar/Battery system is heavy to very heaving wiring. I worry that a standard Gas discharge Tube would not be able to shunt the high currents available in with such low impedances...Also, I would worry that once a Gas Tube went into conduction, I would fear placing one from + to - on a 12-48 VDC buss as that it would not stop conducting until the voltage went near zero--which would only happen if 1) the solar panels lost sun or 2) the batteries fully discharged or 3) there was a fuse, circuit breaker or something else opened to interrupt the current/voltage.

    A MOV works like a the GDT above... A high voltage spike would force the MOV into conduction and would, normally, blow a fuse (in-line with the power source). Again, when we are talking about fuses of 10's to 100's of amps, we would be talking about some pretty large MOV's to trip the current protection devices (assuming that they are installed). And, like you said, MOV (and other such devices) should be contained as they can, and do, explode.

    Actually, Wind-Sun's grounding link is a very good place to start--I am no expert, but everything stated in their FAQ (and linked articles) exactly reflects my concerns with a few things I have seen in the NEC and represent best practices for grounding--basically using one DC (hard ground) connection in a circuit, and if needed, using multiple capacitors and arresters at various points in the circuit. Fuses and Circuit Breakers will not protect against lightning at all (lightning is much faster than any mechanical switch/device):

    http://www.windsun.com/Lightning_Protection.htm

    And here is a link to surge/capacitor arrestors:

    http://store.solar-electric.com/deliar.html

    At this point, I would recommend against any "home brew" lighting suppression systems and just go with the (somewhat expensive) devices designed for the task and wire them where recommended per the Wind-Sun FAQ.

    The one thing in the Wind-Sun FAQ is missing (in my humble opinion) is the recommendation of keeping common wire sets bundled/twisted and keeping unrelated bundles separate (control of differential and common mode noise/current injection). Again, I don't have any direct lightning experience, but this is a very common practice (bundle/twist/90 degree crossing) in many electrical engineering designs and should be helpful in Solar PV, Wind, hydro, and any, distributed power system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Lightning damage to lead-acid batteries

    PS: I am rather ambivalent to grounding of the DC side of a solar power system...

    As one of the links in the Wind-Sun Lightning FAQ stated, Europe runs large ungrounded systems vs US which runs grounded neutral systems. In the US grounding vs ungrounded had been debated for decades. Basically, the idea for grounding at least one leg in a power system was so that a circuit breaker would be tripped if the power leg were grounded.

    A system with a fully unreferenced power (floating) system is, in some ways safer. If a person touches any leg, there will be no shock (hence the use of isolation transformers in some instances).

    Properly laid out systems (parallel/twisted/conduit) are more important to me.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning damage to lead-acid batteries

    bb,
    the twisted wire is an excellent suggestion and i should've thought of it myself. i do this with my ladder line for my radio so it would do great for pv systems in rejecting much of the emp. let's face it in a direct strike don't hold your breath on coming out unscathed no matter how much you do.
    as to the diode idea this would be in parallel to the polarities so that + is to + and - is to - and no current flows unless the piv of the diode is exceeded and then it acts much like a zener. this is not in series so it does not allow current to flow through it at night and normally no current would flow at any time through the diode. it really is like placing a zener on the + and -, but the rating is the piv where it will avalanche at.
    i do know of multi grounds as i have 7 rods into the ground around my home and interconnected with #6 buried about 1ft below the surface. between the pvs, my radios, and the ac house wiring it was necessary to do this large ground. yup i had ground loops before and i still have one, but i know the cause of it and just haven't gotten around to curing it and isn't the fault of the rods being interconnected.
    as to floating grounds, if it is required by an inspector following the nec to have it grounded and not floating then this is redundant to argue. now there was solar guppy who did argue the case for not grounding his pvs and was allowed to not ground them. he is in fla and sees many strikes year round. he looked at grounding them as making them into a lightning rod and more apt to be damaged with a ground connection because it would attract the lightning. there are arguements too that a ground does diminish or eliminate the static feeler of a bolt. i saw first hand with an antenna i had that the ground wire was accidentally cut from hedge trimming and the very next storm that came by lit up my bedroom with a large bolt extending out of the disconnected antenna coax up to my wall outlet about 6ft away. the seperation in the ground wire was about 1.5ft so most of the energy jumped that gap so how much do you think it allowed into my bedroom to beam itself 6ft? it was about 3 inches in diameter and i'm glad i wasn't inbetween the outlet and the coax. once it was regrounded i never saw an event like that happen again. that proved to me that in general grounding works better than just floating metal in the air.
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Lightning damage to lead-acid batteries

    Niel,

    Sounds like you have nice lightning in your area! We just see a few thunderstorms every few years (cloud to cloud or cloud to open hills).

    I still believe in the arrestors--of course, they will clamp it to a few hundred volts (one would hope). Don't know what they do in Europe for their ungrounded 220 VAC 50 Hz distribution--do they use some sort of surge suppressors or something else??? Or, do they not have as much lightning...

    The article linked to the Wind-Sun FAQ said that is why double insulation (for electric tools/appliances without a ground plug) was developed (for Europe)... But even DI is not going to withstand much of a lightning strike without any arrestors... I can sure see the reasons for earth grounding too.

    I understand your idea of placing the diode's back to back to act like a clamp. I was thinking along the lines of a snubber (like back EMF suppression of a switched coil and protection of electrical components from reverse polarity. Your suggestion is more general--but I am not sure how much it would protect... Generally, in electronics (or building wiring) when you put a suppression device in, you use a fuse to the source (battery, AC, etc.). The suppression device (or accidental circuit fault) then causes (forces) an over current, the wires are sized large enough to handle the momentary transient, the fuse/breaker pops, and the circuit is protected against further damage. I just don't see the diodes handling the high current available without failing in some wild and wonderful way...

    Of course, Wind-Sun sells surge suppressors... Looking at the specs for the DC suppressor:

    Weatherproof Enclosure
    DIMENSIONS: 2-1/4" High, 2-1/4" Diameter
    Type of design: Silicon Oxide Varistor
    Maximum current: 60,000 amps
    Maximum energy: 2000 joules per line
    Maximum number of surges: Unlimited
    Response time one milliamp test: 5 nanoseconds
    Response time to clamp 10,000 amps: 10 nanoseconds
    Response time to clamp 50,000 amps: 25 nanoseconds

    And the AC suppressor:

    Rated Voltage - 250V single phase, 3 wire, residential. 60,000 amps maximum current - 2,000 joules per pole

    Those are values that the average diode is probably not going to survive...

    Now the suppressors are used to (usually) dump the energy to ground.... There is another way to manage voltage--the capacitor--and I wondered about using that instead in the DC circuit. And looking at Wind-Sun again, I was mildly surprised to see they offer those too:
    A surge capacitor is a device designed to absorb surges and/or reduce the steepness of their wave front. A capacitor is able to absorb and hold a charge of electricity, returning it to the circuit at a later time. Since the surge capacitor is always connected to the power circuit, current flows at all times. When a surge occurs, added current flows to the capacitor thereby lowering the intensity of the surge voltage. The amount of current the capacitor can absorb depends on the size of the capacitor, and the amount of voltage pushing the current. If the surge is of a low current relative to its voltage intensity, the capacitor will absorb it. If the surge has high current, the capacitor cannot absorb it.

    By contrast, our lightning arrestor takes no current from the line during normal operation. When a surge occurs, the arrestor turns on to provide a discharge path. When the surge is gone, the arrestor turns off. The arrestor will handle unlimited amount of current, although amounts exceeding 100,000 amps will generally damage the arrestor.

    The main advantage of a capacitor is that there is no time delay in turning on as it always conducts. The disadvantage is that the amount of current it can handle is limited to a few amps, depending on the surge voltage. For this reason, an arrestor should always be installed with a capacitor to protect it from intense surges.

    That would probably do a good job of protecting the electronics on the DC side of the house against fast rise times, low current, of nearby hits... I would hate to be the one to hope to get lightning struck to test the theory. :|

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning damage to lead-acid batteries

    a fuse is great, but in the case of lightning and emp voltages go into the thousands. do you think the gap in the 1.25in fuse might present would stop thousands of volts? i know it won't as there's a general rule of about 1in per thousand volts through the air. i don't remember where it was i heard that or it's validity, but i'm sure that's in the ball park. it couldn't hurt though as the voltage if under about 600v may be stopped with the gap the fuse presents. remember what i said it jumped before in 2 seperate places? that's a mucho much lot of juice.
    i'm not saying just to rely on any one thing as the more you do the better your chances are. when you get right down to it a diode will only limited protect you too. as when there were 2 places the lightning could travel in my example this divided up the potentials. if the ground wire didn't exist the full capacity of the strike would've flowed through the coax to my wall outlet. capacitors can add another small degree of protection, but only the initial portion of the spike as a prolonged spiked won't be dissipated well in a capacitor. to bleed off voltages from dc or ac that are higher than that which you are using isn't as difficult. as you know some components are prone to blowout at very small voltage levels so there's no such thing as overdoing it with lightning or emp protection unless you live someplace it just never happens. cloud to cloud too would emit large bursts of emp, but i mostly get cloud to ground. this not only presents the danger of a direct strike, but increases the emp potential significantly.
    to those that know not of emp this is electromagnetic pulse. you may have heard of this in conjunction with nuclear weapons that knock out phones and other electronic devices. if after that storm your fancy telephone you just bought(hardwired versions) didn't work it was more than likely one of 2 things happened. 1 is a direct strike sending that lightning through the wires or 2 the wires acted like an antenna picking up the emp and sent it to your phone whose components are prone to blowout especially the small integrated circuits. many of you out there may have fooled around in your pc or replaced things in it. they are prone to blowing out just with small static electric charges so you had to handle them carefully especially something like the cpu. some controllers use integrated circuits as well as inverters, tvs, etc. fyi, your cell phone is unlikely to blowout from this unless it is charging and gets the pulse through the wiring. i could go on, but i'll give it a break as i'm getting tired.
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Lightning damage to lead-acid batteries

    Good Evening Niel,

    Actually, I was agreeing with you that the fuses/breakers are basically worthless around lighting--and that is why I was concerned about the diodes--was that they would not handle enough energy and fail themselves (possibly shorted and shorting the batteries/solar panels), and even if they could pop the fuse/breaker, the plasma would just continue to conduct until the event was over (and possibly, even do a very nice job of continuing to arc across the fuse due to the DC of the battery bank).

    If it is a nearby strike/EMP, then the capacitors can actually do a very nice job on reducing the edge--which causes much of the capacitive damage in electronics (and inductive coupling too)... Used to to ESD testing of computers up to something like 10-15kV (both air gap and direct injection).

    Fuses/Breakers are only good if rated against the known capabilities of the source (for US 120/240 VAC home distribution that is 10,000 amps maximum AC of the pole transformer).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning damage to lead-acid batteries

    ok i guess i misunderstood that you were in agreement. actually the diode is quite apt to pop, but as with the fuses it will continue that arc as you had stated. i'd not recommend putting the diode at the battery potential, but if the pvs short out temorarilly that's ok as it's putting that high potential to ground giving you and the rest of your equipment a better chance. in the case of a strike or high enough emp the wires themselves in and around the pv are likely to fail as they are rated to 600v. if you place the diode in the junction box (of those not sealed because of mc connections) it may just save your pv from internally frying. 50piv for pvs up to say 40v ocv which covers most of them.
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Lightning damage to lead-acid batteries

    Also, for wiring and, probably the panels, if they are NRTL listed/registered devices, they are probably hi-pot tested at around 1,800 VAC (for the 600 VAC rated wire) and somewhere around, at least, 1,500 VAC (or ~2,100 VDC) for the panels...

    I am not sure about where you are suggesting to place the diodes for the solar panels? If you place the diodes PIVs between the +/- leads--probably not a big help as solar panels probably act as a big capacitor, plus they usually have series diodes (dead panel by-pass in at least one direction, and proably not a high withstand voltage in the other polarity / blocking diode).

    Also, if you place the diode(s) near the panel between the panel +/- and ground, you may just run the risk of bringing the (what were insulated to at least 2kV DC) DC panel connections to the level of the the local safety ground carrying the lightning strike--and just shunting around the 600 vac NRTL designed/hipot tested insulation (with real current via a "metal" connection--not just high edge-rate capacitive or inductive coupling). Which now just brings the common mode current/voltage spike down to the charger/inverter/battery junction where, if you have an locally earthed negative ground battery system (which because of distance is at a different ground potential wrt to the ground at the lightning strike), it converts the common mode pulse into a differential pulse.

    My thoughts would be to place the arresters, not near the panels, but near the "battery shed" to local earth (on both + and - leads). The idea being to keep the voltages "low" around the electronics and where humans live/work. The panels themselves should see mostly common mode voltage and, I would think, be damaged be either differential voltage conversion where the +/- leads are terminated differently (i.e., - is grounded and + is not). Placing arrestors at the entrance to the battery shed would hopefully both trip to ground and keep both + and - at same potential.

    When looking at lightning, I am using RF/High Frequency circuit design rules and transmission line theory... Rather than treating the problem just as a high current/voltage event (which it is too)... The RF/Transmission Line stuff can be addressed with physical protection and good design/contruction during installation--high voltage and high current protection is really something that comes with the design of the major components (panels, chargers, internal insulation/isolation, or not, between inputs/outputs, etc.) and the fact that protecting against 100's of amps at millions of volts, once it is in your wiring, is pretty difficult to prevent equipment damage.

    By the way, I have not seen much discussion of lightning rods and their grounding systems used to, help, protect solar PV/Wind installations... What are your thoughts on that?

    Niel, just looking for a what-if/experience discussion, not trying to start a grounding/religion...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Lightning damage to lead-acid batteries

    It is a bit surprising that you can take a hit that damages the batteries without damage to the inverter or charge controller.

    As to the debate about grounding DC side, although Xantrex/Trace recommends grounding the negative side of batteries I do not do it. I do not see the reason for giving a lightning surge another path to blow out inverter circuitry. I do have surge suppressors on the AC grid connections and ground the inverter case.

    I do not have PV panels however. For PV panels I do see the need to ground at least the metal frames of the panels. As was mentioned, use only a single grounding rod point for system. Even if lightning hits a tree several hundred feet away there is a ground voltage gradient for a wide area around the strike. Two grounding rods separated across this ground gradient can develop several thousand volts potential between the two ground rods.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lightning damage to lead-acid batteries

    you know rich that reminded me of the old cb days(early 70s). we had some nasty storms blow by and somebody i usually talked to in the county just south of me shut down and disconnected the coax just like i had done until it blew by. it was months later that i heard from them and they told me they were heading for the basement that evening because they thought it to be safer down there when the lightning hit their antenna. no damage to the radios occurred because it was disconnected and it was grounded properly. yup the antenna melted, but the best part that stuck in my head was that they said they were still on their basement stairs when it hit and that lightning was dancing all over the basement floor.
    you are quite right in saying a voltage potential will develope across 2 seperate grounds and that voltage could fry allot of stuff including a person. even the sago mine dissaster was caused by lightning hitting a pocket of methane thousands of feet below the surface. if anybody with a basement has a ground rod it absolutely must go below the basement floor by as many feet of ground rod as you can muster to help prevent what happened in the top story.
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