Lightning makes my saw blade move?

[Larry Edgerton]

Yesterday I was working on a machine while sitting at a tablesaw, this one a Minimax, when a lightning strike hit close by. I saw the saw blade move. I have seen this one other time on my Powermatic. The blade moves just about one tooth and backs up to its original position.

Any explanations from someone that actually understands what may be going on? Both saws have magnetic starters, but one is single phase 240, and one is three phase. If you do in fact understand is there any possibility that a strike could turn on the saw?

Need to know, don't want to cut off any more fingers.......

Larry

[Tim Cottle]

I personally go inside the house when we get the thunderboomers, or at least go to neanderland, and step away from the power tools. As far as reasons, lightning’ s high current means that the associated magnetic fields will radiate and cross couple energy inside the equipment, and maybe cause your blades etc to move. I don't know about turning a switch on, but I don't know if I want to play this old house roulette :)

[Jim Matthews]

Overvoltage from lightning strikes happen so quickly as to be considered instantaneous. Your service panel may not trip fast enough to stop the surge of power to any connected device.

If you have an induced current from a buried line through the panel, you could be seeing the result of arcing in the motor, which makes the commutator advance to the next pole.

I doubt this could defeat any mechanical interlock that allows the capacitor to discharge and start the motor turning. Still, unplugging any device not in use is a reasonable safety practice.

I wouldn't want to be in the middle of a cut, and have the motor stall - nor would I want to be working near a machine that spontaneously moves. Disconnection during a storm is an advisable caution.

[Dan Hintz]

lightning’ s high current means that the associated magnetic fields will radiate and cross couple energy inside the equipment, and maybe cause your blades etc to move.
That would be my explanation, too... though I didn't think the field would be strong enough to tweak the coil unless the lightning practically hit the tool itself. That's a lot of radiated energy to couple into a large motor's coil and move it.

[John Coloccia]

My guess would be physical vibration from the strike.

[Lee Schierer]

A nearby strike can induce voltage spikes in wiring, particularly underground wiring if it is near the strike. It might be time for you to consider a whole house surge protector.

[Ken Fitzgerald]

I'm with Lee on this one. While in the Navy and working air traffic control maintenance at NAS Meridian, MS, I had to deal regularly with damage to equipment from lightning. It's impressive. Direct strikes will leave serious damage to underground cables. I have seen connectors 4-5 inches in diameter melted due lightning and the strike was over a mile away. The lightning was observed by a crash crew sitting on the runway...it ran into a precision approach radar system via buried power cables melting a removeable connect 4-5" in diameter. Lightning is some serious power with it's own rules and physics as a result.

[Dan Friedrichs]

I'm with John - Larry, do you think it's possible that there's physical vibration of the ground causing it?

The only other two options are (1) induced magnetic field that couples into the motor itself (but as Dan said, that would have to be a huge field, meaning the strike was REALLY close nearby), or (2) a line voltage spike. The line voltage spike idea doesn't make a huge amount of sense, because you have the power switch to the saw off. The spike would have to be large enough to arc across the switch contacts (very unlikely - stuff would be blowing up all over the place if this were the case). Or - perhaps there's an EM field just strong enough to briefly trip the magnetic starter on.

But, as Ken said, lightning is weird stuff. It does things that we have a very hard time explaining.

[Neil Brooks]

We're spending roughly a grand, in our new house, to get a "whole house surge protector."

It will be interesting (plenty o'lightning in these parts) to see if that happens on my saw.

Unplugging it WOULD give you an idea if it was coming through the line, at least, but wouldn't preclude it being an induced field sort of deal.

Weird. If I "saw" that in my shop, I'd shut of the lights, go upstairs, and crack a beer.

YMMV :)

[David Hostetler]

Uh, so what you are saying is you leave your table saw plugged in when not in use? Unplug that sucker! Remember the old rule, no power, no problem.

[Larry Edgerton]

Uh, so what you are saying is you leave your table saw plugged in when not in use? Unplug that sucker! Remember the old rule, no power, no problem.

David, this is not a home shop, this is how I make a living. It would take me a half hour to unplug and plug everything in every day. If I quit working every time it rains I would loose tens of thousands every year.

My question was, can lightning start a saw with a magnetic starter. Seems the answer is no, but I think I will drop the blades when its raining just in case.

Thanks, Larry

[Ken Fitzgerald]

Larry....no body can predict reliably whether or not lightning can cause a magnetic starter to engage. As I stated earlier....I saw lightning melt a connector that wasw 4-5 " in diameter. I have large hands and the pins it melted were bigger than my thumb. Mind you the lightning struck the ground 1 mile away and ran in on buried cables.

Some can say it probably won't .....but they can't say with 100% reliability that it wont' start a magnetic starter....

[Larry Edgerton]

Ken

I am going with your theory. Its an unexplained possibility. I'll lower the blades in a storm. It is freaky to see that blade move......

Larry

[Mike Davis NC]

Lightning jumps a mile of air and you think it won't cross a breaker? Don't take chances with lightning.

Sometimes my phone will ring when lightning is too far away to see or hear. Just a "Ping" and I know what's coming. Unplug the computer and check the generator.

[Dan Hintz]

Lightning jumps a mile of air and you think it won't cross a breaker?
I think the point is if the incoming energy is strong enough to cross the air gap in a breaker, it's strong enough to do some damage. In other words, you'd notice it.

[Trent Shirley]

Am I wrong in thinking that these magnetic switches do not actually use a magnet to engage and require a physical push? I would think that once pushed the power activates an electromagnet that holds the switch closed and if power is disrupted then the circuit is broken, the electromagnet no longer produces a field and the spring loaded switch opens.
If my assumption is true then no matter how strong a magnetic field you get from the lightning it does not close the contacts on the physical switch and therefore there is no power to the saw.

I imagine the magnetic field generated by the lightning caused induced current in the motor windings causing a minor twitch to the blade but could never be strong enough to spin the blade fast enough to cause serious damage.

[Jason Roehl]

What about the magnetic starter become just charged enough to act as a capacitor, inducing a small amount of charge flow on the other side of the contacts in the starter, thus causing the blade to move a slight amount?

[Dan Hintz]

Am I wrong in thinking that these magnetic switches do not actually use a magnet to engage and require a physical push? I would think that once pushed the power activates an electromagnet that holds the switch closed and if power is disrupted then the circuit is broken, the electromagnet no longer produces a field and the spring loaded switch opens.
If my assumption is true then no matter how strong a magnetic field you get from the lightning it does not close the contacts on the physical switch and therefore there is no power to the saw.
Correct...

[Trent Shirley]

What about the magnetic starter become just charged enough to act as a capacitor, inducing a small amount of charge flow on the other side of the contacts in the starter, thus causing the blade to move a slight amount?

I have never read details about how these work but my assumpton has always been that it is not a starter but just a capacitor storing a little extra power for the next spin up to give the motor a little extra juice on spin up. But a capacitor charges as current is drawn across the circuit and does not discharge until it has a ground/return path so the lightning it not likely to either charge or discharge the capacitor.

Anytime a magnetic field passes through copper wire it induces current so a lightning strike producing a strong magnetic pulse can cause induced current in all the wiring of the saw but could not cause it to actually turn on the saws connection to the AC power. The magnetic pulse would have to be very strong and very close to have any effect though as magnetic fields dissipate very quickly with distance. It is recommended that people using hair dryers hold them about 4 inches from their head as the magnetic field generated by fan motor disipates within a few inches. Magnetic fields are suspected to cause long term harm to health but they dissipate so quickly that it is not generally a large risk except for large producers near your head or living right by large power stations and high voltage transormers. Whether there is any real risk is constantly debated but statistics show a lot of birth defects and developmental problems in children living near high EM fields.

Playing devils advocate on the saw magnetic switch issue, it is possible that a mechanical failure in the switch leaving the switch without any spring tension holding it in the open position could result in it powering on if enough current was induced in the circuit causing the electro-magnet in the switch to enage and pull the switch into a closed position. The odds are astronomical for something like this happening but I never discount possibility as it will always come back and kick your behind when your not prepared.
I once replaced my kitchen stove, first shutting off the only 220v breaker in the house then laying the stove down and going at it with a ratchet to release the terminal lugs of the power line. My ratchet came into contact with the body of the stove and sent a nice shower of sparks across the room and took a bite out of the bottom of my socket wrench. I spent some time investigating why there was power in the circuit even though the breaker was off and eventually found that one of the two 30 amp ganged breakers did not in fact disconnect power in the off position so there was still 110v going through the stove. I have never heard of a breaker failing that way but it taught me a lesson about making assumptions with electrical power and always using my tester before working.

[Wes Thom]

I am going with your theory. Its an unexplained possibility. I'll lower the blades in a storm. It is freaky to see that blade move......

First off, kudos for noticing something. And then having the curiosity to ask such a question. Important because I will later explain why you have described a serious problem.

Will a millimeter gap in a switch stop what three miles of sky could not? Of course not. A lighting strike could easily arc across that gap. Create a plasma path. And be done in microseconds. That is not enough energy or enough time to move a saw blade. But may have created plasma just like an inert gas becomes a conductor inside a fluorescent tube.

Once a fluorescent tube has no current, the plasma turns to gas. So tube conduct a low voltage current to maintain that plasma and to create light. Your switch did same during at least one cycle of AC power. Now it is not a microsecond current through the switch. Now a tens of millisecond (or longer) current that has higher energy maintains that plasma. The switch acts like it was closed.

Well, AC power eventually goes to zero. That is when the tiny plasma path might return to gas. Fortunate for your hand.

That plasma path and AC electric flowing through it is not your concern. You had surge energy inside and hunting destructively for earth. Fortunately some of the best protection inside every house is that inside every appliance. This time the surge was too tiny to do damage. But you were warned (because you took notice). Since you did not have a properly earthed 'whole house' protector, then energy was inside and hunting. Protection means that energy must never be inside. This time, it was too tiny to do damage. But learn from what you saw. Any wire that enters without first connecting to single point earth ground (either directly or via a 'whole house' protector) means your only protection is that inside every appliance. Eventually (typically once every seven years) a surge will be large enough to overwhelm protection inside some appliances.

Your cable connects directly to earth - best protection if the cable company did as required by Federal regulations and code. Your telephone wires have always had a 'whole house' protector installed for free. But your AC electric has nothing is you did not install a 'whole house' protector from more responsible companies such as Square D, GE, Intermatic, Siemens, ABB, or Leviton. A Cutler-Hammer solution sells in Lowes and Home Depot for less than $50. Options are many. Best protection costs the least amount of money - about $1 per protected appliance. And, of course, earthing must be upgraded to both meet and exceed post 1990 code.

BTW, induces currents from lightning do not have enough energy to move that. At best, energy content is only sufficient to push milliamps through an NE-2 neon glow lamp. To get that blade to move meant a current entering inside the building.

[Dan Hintz]

First off, kudos for noticing something. And then having the curiosity to ask such a question. Important because I will later explain why you have described a serious problem.

Will a millimeter gap in a switch stop what three miles of sky could not? Of course not. A lighting strike could easily arc across that gap. Create a plasma path. And be done in microseconds. That is not enough energy or enough time to move a saw blade. But may have created plasma just like an inert gas becomes a conductor inside a fluorescent tube.

Once a fluorescent tube has no current, the plasma turns to gas. So tube conduct a low voltage current to maintain that plasma and to create light. Your switch did same during at least one cycle of AC power. Now it is not a microsecond current through the switch. Now a tens of millisecond (or longer) current that has higher energy maintains that plasma. The switch acts like it was closed.

Well, AC power eventually goes to zero. That is when the tiny plasma path might return to gas. Fortunate for your hand.

That plasma path and AC electric flowing through it is not your concern. You had surge energy inside and hunting destructively for earth. Fortunately some of the best protection inside every house is that inside every appliance. This time the surge was too tiny to do damage. But you were warned (because you took notice). Since you did not have a properly earthed 'whole house' protector, then energy was inside and hunting. Protection means that energy must never be inside. This time, it was too tiny to do damage. But learn from what you saw. Any wire that enters without first connecting to single point earth ground (either directly or via a 'whole house' protector) means your only protection is that inside every appliance. Eventually (typically once every seven years) a surge will be large enough to overwhelm protection inside some appliances.

Your cable connects directly to earth - best protection if the cable company did as required by Federal regulations and code. Your telephone wires have always had a 'whole house' protector installed for free. But your AC electric has nothing is you did not install a 'whole house' protector from more responsible companies such as Square D, GE, Intermatic, Siemens, ABB, or Leviton. A Cutler-Hammer solution sells in Lowes and Home Depot for less than $50. Options are many. Best protection costs the least amount of money - about $1 per protected appliance. And, of course, earthing must be upgraded to both meet and exceed post 1990 code.

BTW, induces currents from lightning do not have enough energy to move that. At best, energy content is only sufficient to push milliamps through an NE-2 neon glow lamp. To get that blade to move meant a current entering inside the building.
I can't decide on what you're trying to say. Your second/third paragraph suggest arcing across the switch's unclosed contacts, but as I said earlier, if that was happening the amount of damage would be severe... you'd know it was happening. To top it off, he has seen this before, and that kind of cumulative damage would be quickly noticed via non-working equipment in the shop (and more likely, electronics in the home long before the beefy equipment failed). Any surge strong enough to ionize the air gap (i.e., plasma) would not go unnoticed.

[Phil Thien]

We're spending roughly a grand, in our new house, to get a "whole house surge protector."


Whole-house surge protectors should only run about $150 to $250, depending on brand/features. Installation cost should be negligible because they install it when they wire the panel.

I'd find out what they're installing for $1000 to make sure you're getting your money's worth.

[Dennis Ford]

A lot of guessing going on here. If the movement came from an induced current (seems reasonable to me), unplugging the machine will not prevent it from happening again. If a surge was strong enough to jump across the motor starter, I think some smoke would have been released (probably a lot of smoke).

[Wes Thom]

I can't decide on what you're trying to say. Your second/third paragraph suggest arcing across the switch's unclosed contacts, but as I said earlier, if that was happening the amount of damage would be severe...

Damage would not exist. You would not know a lesser transient existed. Protection inside all appliances would have made that transient irrelevant. The transient would have enough voltage to create that plasma. But be too short and have insufficient current to cause any damage. Don’t confuse this lesser transient with a future and more severe one that will do damage. This lesser transient was a warming that no one would have known had blade movement not been observed. It explains a one or two cycle application of AC power to that motor necessary to move the blade.

If an induced field or the transient strike alone was sufficient to move that blade, then extensive surge damage would have existed. Had the motor been unplugged, the plasma could not form across a millimeters gap in a power switch.

[Bill Huber]

If the movement came from an induced current (seems reasonable to me), unplugging the machine will not prevent it from happening again.

I guess that I just can not see it, if you unplug the machine how in the would would a lightening hit on the line casue it to happen again. The lightening would have to strick the cord laying on the ground to do anything. If the lightening hit the saw then I guess it could do it but I am sure it would take out the saw at the same time.

[Ken Fitzgerald]

If the machine is unplugged, for a lightning strike to induce a current in the motor and cause the motor to turn would require the lightning bolt to be awfully, awfully close and I'd be worried about flashburn if I was in the area.

[Charles Krieger]

I have a different theory on why the connector with thumb size conductors melted. When the lightning struck a mile away it possibly ionized the air around an exposed, metalic, area of the cable. Ionized air becomes a good conductor which possibly created a short between two legs of the power. Once shorted the power source supplied enough current to heat the point of most resistance, the connector. I think it is unlikely that unless it were directly struck by the lightning that the connector would fail from the energy of the strike. As has previously been mentioned lightning does strange things so I rule nothing out!

[Ken Fitzgerald]

Charles....the parts that melted were inside the insulation of the connector. The crew of the crash truck were sitting maybe 50' away on the same hardstand and observed the lightning strike. If as you say it was an air ionization, it would have been visible to them. I would expect ionized air to be very visible too!

[Ken Fitzgerald]

At NAS Meridian, MS thunder storms, and lightning were a frequent event starting very early each New Year. Lightning striking the ground and taking out power cables for AC power to the radar and other flight navigational aid electronics including runway lights was a fairly frequent event. I worked air traffic control maintenance and yet the civilian public works folks were responsible for maintaining and repairing those buried cables. While there, the public works guys came to us with a problem. They couldn't locate a damaged buried cable along the runway. Some of us put our heads together and we developed a "shop made" timedomain reflectometer using a known length of coaxial cable, a pulse generator and an oscilloscope. We used it often enough that we could pinpoint a damaged spot in buried cable...power cable...signal cable...video cable (radar) within 10'. Now you might say 10' isn't accurate but when you consider there could be 3 miles or 15,000 feet of cable between the air operations building and the bad spot in the cable, 10' is pretty good considering it was the early 1970's and what was available for test equipment then and the cost of same. The civilians at public works using a backhoe to dig considered 10' a pin point.

[Wes Thom]

I think it is unlikely that unless it were directly struck by the lightning that the connector would fail from the energy of the strike. Lightning is capricious when a basic electrical nature of the beast is not first learned. For example, a direct lightning strike is maybe 20,000 amps. How many amps can an 18 AWG (ten amp lamp cord) carry? Something less than 60,000 amps. Lightning is not the high energy event so often believed in myths. Even that small wire can conduct a direct lightning strike without damage.

More often, damage is created by other high energy sources. For example, a west coast radio station was severely damaged when lightning struck incoming AC wires. Due to a failed installation, lightning formed a plasma connection from a 33,000 volt primary wire to low voltage wires into the transmitter. Lightning did not provide that high energy. Lightning only created a path - a temporary short circuit. Damage was from the follow-through current sourced by an AC utility.

Colin Bayliss describes in "Transmission & Distribution Electrical Engineering" what many do not learn:
> Although lightning strikes have impressive voltage and current values (typically
> hundreds to thousands of kV and 10-100 kA) the energy content of the discharge
> is relatively low and most of the damage to power plant is caused by 'power
> follow-through current'. The lightning simply provides a suitable ionized discharge
> path.

A trivial surge too tiny to harm any appliance can create a plasma path across millimeters inside a switch. Lightning would not cause the blade to move. But one or two cycles from a high energy source (AC electric) might be enough to more that blade.

Numbers explain how a temporary plasma path could be created when a surge, too tiny to overwhelm protection inside all appliances, does create a tiny and temporary conductor inside a switch.

[Ken Fitzgerald]

I'd bet a cold microbrew you can't get an 18 awg wire to carry 60,000 amps.

[Wes Thom]

I'd bet a cold microbrew you can't get an 18 awg wire to carry 60,000 amps.

From Electrical Engineering Times of Oct 2007 entitled "Protecting Electrical Devices from Lightning Transients":
> ... consider that a bare 18 AWG (1 mm diameter) copper wire, in air, normally will conduct at least
> 10 amperes safely, with very low self-heating temperature rise. If the current slowly rises, the
> temperature will increase until the melting temperature of 1065° C (1950° F) is achieved at about
> 83 A. This same temperature could be reached "instantly" by an 8x20 µs pulse at a current of 61 kA.

8x20 us is a classic lightning pulse. As I said, something less than 60,000 amps.

Make it a Royal Weisse.


BTW, what frequency and power did you use for the TDR?

[Ken Fitzgerald]

Wes,

It's been 37+ years ago and I honestly don't remember what the frequency was. The power couldn't have been much because it was just a standard pulse generator we used for repairing test equipment and troubleshooting radar pulse circuits. Yes...in those days in the Navy we troubleshot to the component level. We "t-eed" a known significant length of coaxial cable and pulse from the pulse generator to the input of the scope. We could measure the time for the reflected pulse to appear for a time reference. We then "t'eed" in the bad cable and looked for the reflected pulse.....measured the time and calculated the distance. I had a note I carried for 30 years that had the propagation time for that specific coaxial cable and the formula for calculating distance just in case I needed to make one again. I just threw the notes away a few years ago. OF course the combination of PW and frequency was some of the limiting factors for accuracy.

I prefer Speight's Gold Medal ale but will settle for a Scottish ale......I said 60,000....not less than.....

[John McClanahan]

Larry, at the time you noticed the blade move, did the saw make any kind of sound? Like a click, buzz, hum, or was it silent?

[Charles Krieger]

Wes, you and I are on the same page. Thank you for clearly stating what I was attempting to say. My poorly worded statement was meant to convey that I didn't believe the energy of the lightning would be sufficient to destroy the connector with 1/2" conductors although I didn't rule out the possibility if the hit were direct that it could possibly fail. You have clearly shown that even a direct hit would not destroy the connector from the energy in the lightning strike alone, as you have clearly stated it would have failed as a result of Plasma/Ionization creating a path for AC supply current to cause the damage.

I don't understand 8x20 usec. Is that the same as 160 usec pulse? or is it 8 pulses of 20 usec each? If the latter what is the period between pulses?

[Ken Fitzgerald]

I might buy the ionization theory if the connector involved hadn't been located approximately 1 mile from the observed lightning strike. The crash crew was parked about 50' from the radar involved. They saw the lightning strike beyond the radar. The control tower observed the lightning strike. The air controllers in the radar room came into our adjacent maintenance shop to report the precision approach radar was inoperative immediately following the observed strike.

The military grade Cannon plug was approximately 4 inches in diameter. It contained only 4 pins each approximately 3/4" in diameter with some sort of plastic or phenolic insulating them. It burned the plastic and melted the pins. We couldn't remove the plug from the jack. We had to remove the panel in which the jack was mounted. We then sawed the jack apart just to observe what had happened....curiosity of some guys in their early 20s.

[Rich Engelhardt]

I think I will drop the blades when its raining just in case.

That's a habit I got into as a matter of SOP.
Unless I have the blade height dialed in for a specific reason, I always lower the blade whenever I'm between uses of the TS.

It had nothing to do with safety.
It's all because I'm a slob & I didn't want anything I laid on the TS to damage the carbide teeth on the blade.

[Wes Thom]

I don't understand 8x20 usec.
The waveform is defined by IEEE Standards such as C62.xx. If found in datasheets that describe how a protector might perform. The classic surge created by lightning is defined as a rise time of 8 microseconds and a fall time of 20 microseconds.

[Wes Thom]

I might buy the ionization theory if the connector involved hadn't been located approximately 1 mile from the observed lightning strike. Lightning is connecting from a cloud to earthborne charges maybe 5 miles distant. Therefore a shortest electrical path may be three miles down to earth. And four miles through earth to those charges.

Geology and other variations in earth can determine where and how those currents flow.

[Wes Thom]

It's been 37+ years ago and I honestly don't remember what the frequency was. I had heard many make crude TDRs that way. But never knew how accurate that measurement was. How many feet different between what the TDR reported verses where an actual fault was.

[Will Blick]

my guess is, the blade moved due to sound..... most TS blades are so free wheelin, you could blow on them, they would turn. Sound waves are powerful... and of course, some travel faster than the speed of sound. I see stuff in my shop move a lot, but I know its from jet fighters waaaay above traveling at supersonic speeds... no noise is detected due to the altitude they fly at, but large surface area objects get pushed, like hit with a huge air gun. Specially the thin alum. garage doors, when down, it seems like someone kicks them.... they move inward about 1", which makes an obvious sound.

Also, it could have been something as simple as.....the blade was buried in dust at the bottom, the noise loosened the dust down the chute, and the blade followed the dust....

Most strange events have simple explanations, but its human nature to seek the far-out reasons first....

the chances of a magnetic field from a lightening strike moving a blade is, IMO, very remote. These fields fall at the inverse of the square, which means, unless the strike was VERY close, I would rule that out immediately. and BTW, electric strikes send off large electric fields, not magnetic fields. Electric fields are induced by voltage, magnetic fields are induced by uneven current distribution..... where current is equal, in synchronous sine waves, there is NO induced magnetic fields from the conductors. Zilch.

Since the starter switch serves almost as a disconnect when off, I can't see any stray ground current making it through to the motor.

[Dan Hintz]

This lesser transient was a warming that no one would have known had blade movement not been observed. It explains a one or two cycle application of AC power to that motor necessary to move the blade.
Again, I'm not sure what you're suggesting... we're not talking about a temporary connection to the PoCo (later posts suggest maybe you are?), so how can you have "a one or two cycle application of AC power"? Lats I checked, lightning didn't exactly follow a 60Hz sine wave ;)

[Larry Edgerton]

Larry, at the time you noticed the blade move, did the saw make any kind of sound? Like a click, buzz, hum, or was it silent?

Not that I noticed, but there was a lot of noise because of the storm between the rain and the lightning strikes.

The same storm blew a hole in a aluminum flex gas line. The molten aluminum was blown out the hole from the inside, but it did not ignite.

Interesting reading here.........

Larry

[Larry Edgerton]

Ok, while this discussion is going on what is the opinion of lightning rods.

The reason I as is I am building a one room schoolhouse with a bell chase, and the framework will be steel or aluminum wrapped in Azek with a copper roof, so I was going to investigate lightning rods.

Can't see how they could work, but I am often wrong so what say you?

Larry

[Dan Hintz]

The reason I as is I am building a one room schoolhouse with a bell chase, and the framework will be steel or aluminum wrapped in Azek with a copper roof, so I was going to investigate lightning rods.

Can't see how they could work, but I am often wrong so what say you?
The idea behind them is to shunt the majority of the energy through a known path. By making the rod high than surrounding structures, as well as making it more pointy (comparitively speaking) for better build-up of point charge, you're inviting any local strikes to a designated area... kind of like if you know a thief is going to break into your house some day, you'd rather he came in through a wide-open door that led into an empty hallway and out the back door with a $100 bill attached to it. There's no guarantee the thief will not force his way into a side door or a door in the hallway, but you've prevented him an easy route. Same with the lightning... you can't guarantee it will not strike another area of the building or branch off halfway down the lightning rod system, but it definitely increases your chances.

Fixing lightning damage in a building properly protected is significantly less expensive than one that wasn't. You may only end up with some charred eaves in a protected building, where as an unprotected one will likely end up as a smoldering pile of rubble after the fire is finally put out.

[Pat Barry]

Are you certain it moved? I mean, there was a lightening storm, lots of flashing light. If you were glancing around the shop and maybe just caught a reflection of the flash it might appear that the blade moved but it was just an illusion. But, for the sake of argument, lets say it actually did move due to an electrical surge, then why would it move back? Do you think there was a reverse surge to make it move back? Keep in mind its an AC motor, there would have to be some highly coincidental phase fields to make it do anything. I just doubt that it moved. Now if you were watching it and had your total attention on it while this storm was going on then I might be more inclined to believe it. Call it a mirage.

[Neil Brooks]

Whole-house surge protectors should only run about $150 to $250, depending on brand/features. Installation cost should be negligible because they install it when they wire the panel.

I'd find out what they're installing for $1000 to make sure you're getting your money's worth.

Thank you, Phil. A thousand things to research, and I did NOT look into that one.

I will, now, though :)

[Steve Ryan]

Magnetic field from strike induced current in the winding causing the slight movement. Had an aluminum shaft umbrella in my hand at an airfield, slight drizzle. Dark on the horizon as storm apprached. Saw a lightning strike quite a few miles away and at same time my thumb tickeled. Turned my eye to my thumb and saw another flash, and at the same time saw a spark jump from the shaft to my thumb, about a half inch away. Yeah, closed the umbrella and ran for my car.

[Dan Hintz]

Magnetic field from strike induced current in the winding causing the slight movement. Had an aluminum shaft umbrella in my hand at an airfield, slight drizzle. Dark on the horizon as storm apprached. Saw a lightning strike quite a few miles away and at same time my thumb tickeled. Turned my eye to my thumb and saw another flash, and at the same time saw a spark jump from the shaft to my thumb, about a half inch away. Yeah, closed the umbrella and ran for my car.
A straight, aluminum pole picking up an electric field strong enough to ionize air over half an inch? You have so many things working against you there for creating a spark, I'm surprised the lightning bolt didn't just strike you directly.

[Steve Ryan]

A straight, aluminum pole picking up an electric field strong enough to ionize air over half an inch? You have so many things working against you there for creating a spark, I'm surprised the lightning bolt didn't just strike you directly.
I knew the storm was coming and could hear the rumble long before any light show. I was standing on a de commisioned army airfield and could see the storm many many miles off in the distance. How many million volts in a lightning strike? Damp shoes and a plastic handle on the umbrella. A friend had a lightning arrestor on his TV antenna (inside his house) and there were often juicey sparks at the gap with lightning way off in the distance.
Funny thing is that we were auto racing, rain or shine. About 20 of us scattered around the area. Call came over the radio to come in and take cover in our cars. Pickup truck went out to pick the farther workers up. On an open aluminum car trailer! No thanks, ran to my car in the rain.

[Chris Tsutsui]

A couple of ideas from an average joe that works in the cathodic protection field.

I believe it's current related and probably not magnetic though it's difficult to say because I have one other theory.

If I were to hit a tablesaw with a hammer, there's a chance the blade can move the way the OP says, thus lightning can often give a home quite a shake so it could be "physical" movement of the saw blade if the belt has some torque/tension.

Belts have a memory so it is not uncommon for me to move a blade a little bit, and watch it go back to its original position. This explains the saw blade moving back to its original position. ON my old delta jointer I could literally turn the cutter 30 degrees and watch it go back to its original position.

Another theory is the tablesaw is plugged into a wall with no surge protector or supressor. Current goes everywhere during that lightning strike so I want to say a little bit of current caused a slight motor torque that came from the power cord plugged into the wall. It just takes a little current spike to jolt the induction motor ever so slightly. If one were to use a defibulator on a motor then I also imagine there could be slight movement of the armature.

If you had a surge protector to protect your home's main panel, then this is what's readily available to isolate our machines from lightning problem.

What can also save your machines is a lightning protection system that channels the lighting from a lightning rod, through thick wires running down the building into copper grounding rods/mesh in the ground.

[John McClanahan]

I'm thinking the flashes of lightening through the windows caused a shift in lighting and shadows, giving the illusion of movement. Pat Barry has good points on the effect of DC current on a AC motor.

John