Lightening - ground currents

[Gerald Sargent]

Few realise that cables buried in the ground to outside observatories can result in severely
damaged equipment due to spikes induced by ground currents from lightening strikes as
much as a mile or so away. Computers, CCD Cameras, modems, motors for roll off roofs,
and many other devices can severely damaged, usually terminally.
One should use a "surge protector" at the observatory end, and ensure that a good ground
stake is attached to the third "earthing" conductor of their input to the surge protector.
I might add that indeed few electricians know anything about ground currents.

[mill]

OOOOOHHHHHH!!! I think most electricians know about ground currents and i am one of them.
Ground currents are mostly low after 20Mtr depending on the moist in the soil.
As you know, current will follow the path of least resistance and will not go into a insulated cable.
Good practice is as always to unplug any equipment from the power point when there is a thunder storm.

PS: if you put another earth stake at the obs power you might induce a ground loop.

[duncan]

Very good point made Gerald. Just had a quick look on the web and came up with this. Read the paragraph under "Ground Currents"

http://www.feb.se/EMFguru/EMF/ground-currents.html

Cheers:thumbsup:

[Hagar]

Hi Gerald, I am a qualified electrician with an engineering degree to boot and am quite aware of Ground currents and the dangers they impart on Astro and computer equipment but to add another earth stake is opening the door to a very dangerous situation. In the event of requiring an earth stake at your observatory you really should disconnect the earth which is in the cable leading out to the observatory and turn the observatory into a seperate MEN system (Multiple earthed neutral). You will also require a switchboard to provide the protection of your observatory cabling. By far the best way to protect any electrical or electronic equipment, be it in your observatory or your house during an electrical storm is to physically disconnect it fron all sources of supply and earth by unplugging it.
Much of the commercial surge protection equipment sold is only good for minor transient rises in voltage and provides little protection from a very close lightning strike or one on the local reticulation grid.
Please be careful adding extra earth stakes to your electrical installation without the advice of a qualified electrical contractor.

[Gerald Sargent]

I wrote my warning because I have several times suffered from the effect of lightening strikes at some distance, certainly more than a kilometer. My worst case took out a
computer, modem and router, I now use a correctly instally surge protector.
I also had some years ago when a I used a 4ft disk on the roof on and AZ-EL
computer controlled tracking systenm to track weather satellites. Here distant
lightening strikes induced currents in the coaxial out running from the low noise
giga-hertz amplifier at the centre of the dish to the main receiver in the house, this cable carried rf down and 12v dc up to the amplifier. The currents induced in the outer caused transient spikes on the inner that blew up the fet amplifier in the LNA. I cured this by the
proper use of faraday shields, and voltage regulators. So to say 20 meterrs as a limit for
ground currents of consequence did not apply inb any of these instances.

[stephenb]

All sound advice from an expert in the know :thumbsup:, thanks Doug.

I don't have an observatory, but we always unplug the more sensitive items leading up to thunderstorms - LCD TV, DVD, computers etc. It's a habit that should be second nature to everyone. There is no harm once the plug is out of the wall.


I wonder how many observatories out there have not had correct, legal wiring done on them? ;)

[allan gould]

I thought long and hard about power to my new observatory and reLised that with all the lightning strikes I've seen here in Brisbane that I would NOT put underground power in but instead run an extension cord to it. AgIn thanks for tour timely advice Gerald - it's much appreciated.

[westom]

[FONT=Verdana]I confirm what Gerald has posted with many technical reasons why and decades of experience. Appreciate that most electricians do not understand this. Experience is often found in amateur radio operators, rocket launch facilities, munitions dumps, commercial broadcasting stations, farmers, Telstra switching stations, and in science publications even 100 years ago.

An application note from one professional demonstrates two structures. Each must have its own single point earth ground. Any wire in every cable that enters either structure (overhead or underground) must first connect to that single point ground.:
[COLOR=#800080]http://www.erico.com/public/library/fep/technotes/tncr002.pdf (http://www.erico.com/public/library/fep/technotes/tncr002.pdf)

To make both single point grounds better, an interconnecting buried wire connects to both.

Both underground and overhead wires require the same protection as that app note demonstrates.

For example, any cable TV wire must connect short (ie 'less than 3 meters') to single point earth ground before entering the facility. Other wires (AC electric, telephone, ethernet) cannot connect directly. That is the only purpose of an effective protector. Make that same short connection to earth.

No protector is protection. So many educated only by myths will have difficulty. So it is repeated. Protection is not provided by any protector. The protector is only a connecting device. Either it is effective due to that short connection to single point ground. Or it may even contribute to damage of adjacent and powered off electronics.

Replace that antenna in the app note with an observatory. To perform proper earthing, the electrician must take special precautions to also conform to what electricians are taught - human safety requirements.

Disconnecting has a long history of failure. Even early 20th Century ham radio operators would disconnect their antennas. Put the antenna lead inside a mason jar. And still suffer damage. Damage stopped only when the antenna was earthed.

What is the only thing that provides surge protection? Well, what is surge protection? Protection is only and always about where energy dissipates. Either energy dissipates harmlessly in earth. Or dissipates destructively in nearby (even powered off) appliances. Protection is always about keeping surge energy outside the home or observatory.

Farmers can confirm what Gerald has posted. 15 meters from a cow is a tree struck by lightning. Therefore the cow is killed. Why? Surge protection is always about following the current. The current from a cloud three kilometers overhead must connect to charges four kilometers distant. The shortest path is not five km across the sky. Shortest electrical path is 3 km down to the tree, then four km through earth. The shortest path is also up the cow's hind legs and down it fore legs. Another concept rarely understood by electricians because the codes do not discuss it.

That cow suffered a direct lightning strike because it did not have 'single point' grounding. Had a buried copper wire surrounded the cow, then currents would have flowed through the wire; not through the cow. And yes, ground currents from nearby lightning strikes can be that massive; are really a direct lightning strike.

This famer's example is also why the term 'single point earth ground' is repeatedly used. And why Gerald's post so accurately demonstrates what was well understood even 100 years ago.

If building an observatory, then surge protection starts before footings are poured. Learn about Ufer grounds that were originally developed so that direct lightning strikes to munitions dumps cause no damage - no explosion. And yes, concrete is an electrical conductor.

Protection is always about where energy dissipates. No plug-in protector provides protection. Will not claim protection. Cannot dissipate a typically destructive surge. Is a ‘profit center’ because so many foolishly spend tens or 100 times more money on those ineffective devices.

The informed spend so much less money, upgrade earthing, and install a 'whole house' protector from more responsible companies (ie Clipsal). The informed learn why grounding for transistor safety must exceed what electricians install only for human safety.

Surge (lightning) damage is directly traceable to human failure. Effective protection has been well understood for over 100 years. A protector is only as effective as its earth ground.

[Gerald Sargent]

As I started this thread I had better make a correction. My attrention has been drawn to
the possibility that some of my suggestions may contravene regulations and put
insurance at risk. It is suggested therefore that anyone with concern about lightening
and associated ground currents get advice from a licenced electrician with experience
in this matter

[westom]

As posted earlier: Surge protection is always about both meeting and then exceeding code requirements for earth ground. Earth ground which is different from the safety ground found in wall receptacles.

For example, if the wire to ground has a ninty degree bend, then that wire meets safety codes and compromises surge protection. An example of how earthing can meet code and still not be sufficient for transistor safety.

[Hagar]

Westoms post makes one very relevant statement. That is "Protection is always about where energy dissipates. No plug-in protector provides protection. Will not claim protection. Cannot dissipate a typically destructive surge. Is a ‘profit center’ because so many foolishly spend tens or 100 times more money on those ineffective devices."

To follow a considered safe installation as detailed by the company listed above would add thousands of dollars to the cost of an observatory installation. This may be appropriate for a commercial installation it is overkill for a domestic installation. In most cases a good serge protector provides some protection but the best and cheapest method is just to pull the plug.. Physical isolation is the only proven method of eradicating surge or ground current risks. House contents insurance should cover the rest of the risk.

[jjjnettie]

The Belkin range of surge protectors come with insurance to cover this.
Though that doesn't make up for loss of data.

[westom]

More expensive solution is that Belkin protector. Serious and significant protection is routine with one 'whole house' protector and a $10 ground rod. To make their protection a little better, high reliability facilities spend massively on extensive grounding. Because that earth ground determines protection.

Costs approach zero if protection is installed when footing are poured.

For homes, one 'whole house' protector costs about $1 per protected appliance. How much per appliance for that Belkin with its mythical warranty? That Belkin has all but no earthing. It does not even claim protection in its numeric specs. Its warranty is so full of exemptions as to not be honored.

How to identify least effective protector? One characteristic: it has a big buck warranty. We all learn this from free markets. Since GM has a clearly superior warranty, then GM products are superior to Honda? Nonsense. Because GM's hyped warranty is so much better, then we can predict from free market history that GM products are the least reliable.

Why do high reliability facilities use 'whole house' protectors? Because that solution is effective. And may cost tens or 100 times less money per protected appliance.

More expensive and least reliable solutions are plug-in protectors such as that Belkin that does not even claim to provide effective protection in manufacturer's numeric specs. Listed in the previous post is a less expensive and clearly superior solution. The only solution implemented in facilities that must never suffer damage.

[GTB_an_Owl]

so where does an UPS solution stand in all this ?

geoff

[PeterM]

Interesting thread,

I go with Hagar totally and unplug the power from the wall and also from the scope and ccd camera whenever there is a storm around. Yes I have the Belkin protector also but I reckon a claim would be like pushing the proverbial up hill.
However, noting that concrete is a conductor what does this mean for my steel pier, dynabolted to a 3x3m x100mm slab, that carries the LX200 on top, say given a close (and how close) ground strike? Even though the power is unplugged at the wall and at the scope & CCD are my electronics still at risk? If so how else do you guard against this, other than SPECIFYING my equipment - (this is a VERY important word - it is such a get outline for insurers - "oh you don't have it specified") in my contents insurance policy.
Oh and by the way make sure you have told them the equipment is housed outside the house and send them an email confirming same.
PeterM.

[Ric]

A very interesting and informative thread.

I have what may be a dumb question. When I get around to building my observatory I intend to run it on solar power thus doing away with the need to have any cabling in the ground.

Would ground currents have any effect on solar powered items?

[westom]

The only UPS that really claims or provides protection is a building wide system that is located within meters of single point earth ground. Every plug-in UPS claims near zero protection. Since they forget to put numbers in a sales brochure, most automatically assume 100% protection.

Most UPSes connection electronics directly to AC mains. How does that relay stop what three kilometers of sky could not? It doesn't. Either a surge connects to earth before entering the building. Or it is inside the building hunting for earth destructively via electronics.

Ben Franklin demonstrates the concept in 1752. Lightning found earth ground destructively via wooden church steeples. To have not damage, Franklin made no effort to stop or block the surge. Franklin simply gave lightning a better path to earth. The lightning rod is not protection. The lightning rod simply connected lightning to protection - earth ground. Now energy dissipates harmlessly in earth; no in wood.

Protection is always about diverting that current. Always. Anything that would stop, block, or absorb a surge is also called a scam. You do not stop what 3 kilometers of sky could not.

[westom]

Ground currents are the same problem cited in that cow. First, you want no currents entering and leaving the solar array. If the array is surrounded by a buried ground ring, then voltages everywhere in that solar array are same - no current.

BTW, this is what farmers do to eliminate ground currents in a barn. Ground currents can cause a significant reduction of milk production from cows. Not just currents from lightning. Also stray currents from nearby AC distribution lines.

That previous example showed two structures - a building and an antenna. If the solar array is not part of the building, then the solar array must be treated as a second structure - just like the antenna in that application note.
http://www.erico.com/public/library/fep/technotes/tncr002.pdf (http://www.erico.com/public/library/fep/technotes/tncr002.pdf)

Two factors to remember. First, conductivity and equipotential are both made as best as possible so that no currents flow through the cow, building, or structure.

Second, resistance is not relevant. Conductivity is about low impedance. Wire resistance is defined mostly by wire diameter. Wire impedance is mostly defined by wire length and other factors (ie sharp wire bends). Surge protection is always about impedance. Impedance connects as low as possible to earth ground. Impedance between protectors and electronics higher. Impedance - not resistance - determines where surge currents flow.

[PeterM]

Hi Westom,

Thanks for your contribution very informative, I think I am reading that my steel pier bolted to the concrete (as noted above) even with everything unplugged, carrying the 'scope and electronics, is still at risk is that right?
PeterM.

[tlgerdes]

Remember that it is not only power cables, but any cable ie USB, serial, ethernet etc

North Sydney library found this out 15years ago, when a strike hit the parking lot 50 metres away and blew out the serial ports of 43 terminals. Smelt shocking:atom::cloudy:

[rogerg]

My LX200, and network hubs got fried by such currents in 2007. Took ages to repair the LX :( I always try to unplug things, always did before the event, but still don't get to it in time sometimes.

[MuntiNZ]

westom thanks for registring here specially to defend Gerald Sargent !
That was real good of you!! :rofl:

[westom]

Always depends on what the incoming and outgoing electrical path is. For example, a direct strike from the sky - would it pass through any electronics to obtain earth?

One IEEE paper comes to mind. Electronics were mounted on a communications tower so often struck in Germany. This box was to collect lightning data. Numerous strikes to the tower. Plenty of incoming electrical paths. No outgoing path to earth. Therefore no electronics damage.

Your setup has numerous (possible) paths. One might be a direct strike from the sky. Another might be currents as demonstrated by the cow. We try to avert all by grounding everything so that the most conductive paths are not through anything damaged. And earthing so that the outgoing path is always to that one earth ground.

As noted earlier, all conductors have impedance. For example, all ICs on your motherboard are grounded to a copper plane inside that board. Every IC has one pin always connected to that ground plane. But since currents are changing (not DC), then voltages all over that ground plane are different. IC ground pins all see a different voltages because even that copper ground plane has impedance.

Impedance is why nothing is conductive enough. And why we make all connections to the common ground as short and direct as possible. Impedance is why safety ground in wall receptacles is not earth ground. Impedance is why we can never make any ground conductive enough; why we also want earth ground equipotential.

Another concept - lightning rods. A lightning rod can intercept (divert) the surge to earth it on a more conductive path. So that more energy dissipates harmlessly in earth. A lightning rod typically has a cone of protection of abut 60 degrees.

Another technique can be observed on all high voltage transmission lines. At the very top may be one or two small wires. These so that lightning is connected to earth - does not continue to the next lower AC power wire. A catenary is another technique similar to lightning rods so that the most conductive path to earth is not destructive.

As that 20th Century ham radio example demonstrates, unplugging tends to be a less reliable solution. In part because so many items you assume are not conductive (ie wood, concrete, linoleum tile) can actually be electrical conductors. Just another reason why we spend more effort to divert (conduct) a surge to earth rather than try to block it (open switch, disconnecting). And why the best protection means no (minimized) current inside the building.

Too many secret and conductive paths exist inside a building. And we have no way to test for them except during the actual lightning strike.

If damage does happen, we start the investigation to discover why the path to earth was defective. Surge damage is always traceable to human error - in part because we really cannot test a protection system - only learn from other's mistakes.

Protectors are simple science. Art is earthing and making connections to earth. Earthing is mostly about using and enhancing existing systems - to exceed what is required by human safety codes to also do transistor safety. Most of the techniques (using better materials) were used even 100 years ago. Best I can suggest is to identify each potential incoming and outgoing current path. Current must not go incoming and outgoing through anything expensive and easily harmed. That means learning even some wall paints can be electrically conductive.

[PeterM]

Oh boy Roger that's rotten. Did insurance cover that? So you had unplugged the power at the wall and the scope and still got fried is that right? If so that is cause for concern living in here SE Qld.

PeterM.

[westom]

If light poles are powered by AC electric wires and every AC wire (in each cable) does not connect short to the building's single point earth ground where entering the buiding, then each overhead light pole is nothing more than a lightning rod connected to earth ground destructively via computers inside the building.

All computer ports already have significant protection. So that protection on each USB, ethernet, et al port is not overwhelmed, the surge must be earthed where that wire enters the building. If those overhead parking lot lights violated the single point grounding principle, then surges may find earth ground destructively via USB or ethernet ports - with or without surge protectors adjacent to those ports.

[Hagar]

Providing everything is disconnected you should be fairly safe Peter. A single leg connected to earth provides by far the best protection you can achieve. Step touch potential should be isolated to near nil.

I have both worked on and designed quite large commercial installations requiring best ground protection possible. These systems involved creating a ground mat under the building which comprised an earth mat of 0.2 Copper cable )over 1/2 inch thick and bare) welded with phoscalloy in a 1 meter square design. The corners of this mat are bonded to 3 meter long copper pipes driven into the sub soil and tested between using a resistance tester (Ducter). Every incoming supply cable is shielded and connected to the grid, the roof is connected to the grid and each metalic surface is bonded to this grid. Incomming supplies to the building are fed through discharge resistors and capacitors to reduce the possibility of a raise in potential above the normal ground levels for the area.
I have seen gates on Metal fences welded together and unable to be opened as a result of not having a solid earth bond between the gate posts.
Even with a very sophisticated system problems still occur and sensitive equipment can still fault because of a poor connection or dry sub soil etc. It doesn't take much.

Most commercial surge protectors only protect from a voltage rise in the supply cable and provide little or no protection from ground based potential rises.

Unplugging is the only sure protection.

[PeterM]

Thank you Doug, Gerald and all, really makes you think beyond hailstones, wind damage and theft/damage. As many have already built observatories the message I get when storms are nearby is unplug, cross your fingers and have a properly set up insurance policy.

PeterM.

[Hagar]

That's pretty much the best advice you can get. In most cases, unless you live in a remote area, the electricity supply authorities provide a good earth return system to their sub stations or terminal stations which tend to provide a good protection from ground currents. The are in most cases returned to these stations which have very elaborate protection systems to protect you the customer from major damage although the unthinkable is always possible.

You may well have guessed I work for one of the major electricity authorities and have for 38 years in numerous positions. In all cases the earthing and protection from earth faults is the major cause for concern and probably always will be.

[adman]

Just had a quote from an electrician to install a Clipsal 970RM removable module (an overvoltage arrestor) in switchboard is $255. I think that is a great price - but a little more than $1/appliance - at least at my house....

[rogerg]

Insurance could have but I chose not to. Initially I expected the repair cost to be less than my insurance excess, so chose not to go with it, in the end I could have saved $700ish by going through insurance. BUT, it was more the 8 months it took to repair the LX which was the pain in the end.

Network cable was connected between the hubs, so understandable that they would be fried by the cable (10m through ground, 10m through house, between the hubs).

The telescope was disconnected from the mains power. It was zapped either through the pier (which is concreted on to undergound rock) and mount housing or through the data connection to the PC. I'm not sure which.

Bit of a bugger. Practically it is very hard to prevent I think, just have to hope it doesn't happen too often :rolleyes:

[westom]

Other appliances that need protection include dimmer switches, RCD (ground fault detectors and other specialized breakers), furnace, smoke detectors, door bell, etc. Of all those, the one appliance that most needs protection during a surge is the smoke detector (or fire alarm).

[MuntiNZ]

Mate that would have to be a pearler of a strike to fry the smoke detecters!!!
All the ones I have seen in private places are those funny little hocky puck things you bung on to the celing with a 9v battery inside!!!!
Crikey you must have a ripper of a home fire system if the smokies are cabled in. :eyepop:

[Garyh]

Very interesting thread guys,
Less than 2 years ago at our previous residence we had a lighting strike maybe 100m away. It took out a video, DvD player one ceiling fan. Also took out some of my telescope gear in the observatory which was closer to the strike than the house even though the power (extension cable was unpluged. Damaged the Losmandy digital drive and fried a GPusb. Yet the pc was unharmed as well as my DSLR. Also on the tramsformer out the front a earth link was hanging off and not much later the fuse link melted through giving us a blackout for a day.

A question here...
At our new place the we have a ground stake each side of our place , one on the meter box yet another where the phone line enters which has its conductor buried with the phone cable to the stake which is also connected to our shed..This doesn`t sound very ideal? Maybe a buried conductor to the other stake would be better than to the shed?

Also would a ground stake on the observatory connected to the metal roof offer any protection?
Gary

[Karls48]

Two factors to remember. First, conductivity and equipotential are both made as best as possible so that no currents flow through the cow, building, or structure.

Second, resistance is not relevant. Conductivity is about low impedance. Wire resistance is defined mostly by wire diameter. Wire impedance is mostly defined by wire length and other factors (ie sharp wire bends). Surge protection is always about impedance. Impedance connects as low as possible to earth ground. Impedance between protectors and electronics higher. Impedance - not resistance - determines where surge currents flow.[/QUOTE]

Firstly, the cows get sometimes killed during near by lightning strike because of “ step resistance”. Being large animal the ground resistance between front and hind legs may be sufficient to develop voltage potential high enough for sufficient current to pass and kill the animal. Unless someone can train the cows to stand on one leg during thunderstorms, such live stock losses can not be prevented.
If anyone of you is misfortunate to have high voltage power line fall on the ground close to you – don’t walk away, hop away with your feet close together.

Lightning strike is a DC discharge. For such impedance and resistance are same thing.
Quote Wikipedia:
Electrical impedance, or simply impedance, describes a measure of opposition to alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC) there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.

Surge protectors and voltage spike arrestors do work but work best when they are integral part of equipment they suppose to protect. Voltage spikes up to few thousands volts get dissipated in varistors or shunted to the earth .

[westom]

Your building is 'designed' to make surge damage happen. The repeated term was "single point earth ground". Posted repeatedly because it is about as important as god's commandments are to religion.

With multiple ground stakes, you have no equipotential. You have set your household appliances to be damaged for the same reason that cow was killed.

One utility demonstrates how to kludge a solution. See their good, bad, and ugly pictures at:
http://tinyurl.com/yefm8n9 or
http://www.duke-energy.com/indiana-business/products/power-quality/tech-tip-08.asp

You must decide if an observatory is part of the home or is a separate structure. Then install surge protection accordingly. Either an observatory is a separate structure and is 'wired' according the application note at:
http://www.erico.com/public/library/fep/technotes/tncr002.pdf (http://www.erico.com/public/library/fep/technotes/tncr002.pdf)

Or the observatory must be integrated into the building's single point ground system.

Everything here was only about secondary protection. Each protection 'layer' is only defined by single point earth ground. Not defined by any protector. Primary protection is typically installed by the utility. Typically defined by an earth ground that all transformers must have - where distribution voltages are reduced to consumer (240 VAC) voltages.

Because lights work, then many utility workers (and homeowners) cannot bother to fix (inspect) that earth ground. Then a best path to earth for a surge 100 meters away is via household appliances.

Any conductive device (wire or pipe) that enters a structure must be integrated into the only thing that provides protection – single point earth ground. If not, then an ‘offending’ conductor simply gives surges more paths to find earth destructively via household appliances. Which appliances are damaged? Which appliance makes a better connection to earth?

For example, lightning confronted both a TV and VCR simultaneously. TV made a better connection to earth. Only TV was damaged. TV did destructively what a ‘whole house’ protector should have done without any damage. A failure directly traceable to the human who did not properly implement each layer of protection - each defined only by a single point earth ground.



BTW, resistance and impedance are radically different during lightning surges. Surges are radio frequencies; not DC currents as some have mistakenly assumed.

One joke in engineering school was the "DC spike". A classic oxymoron. One need only remember high school mathematics (calculus) to know a "DC spike" cannot exist. Or learn from history what Marconi did. It is that obvious. Due to surge radio frquencies, wire impedance is significant and critical; wire resistance is irrelevant.

[PeterM]

QUOTE "For example, lightning confronted both a TV and VCR simultaneously. TV made a better connection to earth. Only TV was damaged. TV did destructively what a ‘whole house’ protector should have done without any damage. A failure directly traceable to the human who did not properly implement each layer of protection - each defined only by a single point earth ground."

This is probably a dumb question Westom but from the above could I then assume that if I disconnected everything in the observatory to do with the scope but left an appliance ie dvd/player a fan? still connected in the observatory this might act as the sacrificial cow to give a connection to earth?
PeterM.

[Baddad]

Hi All, :)

Lightning strkes produce another hazard which has not been mentioned:

A nearby lightning strke produces a huge transmitted EMF (electro magnetic force or field.)

Sensitive electronic equipment will often fry even though it is unplugged. Fortunately there exists a device called Faraday shielding. The metal case around the computer, the metal case around most other equipments provides this.

Cheaper DVD players in plastic cases fry. As happened to us. (It was won as a prize)

A car provides a good shield, and provides good protedtion to the occupants. Also with commercial aircraft. It is common for airliners to be struck. The passengers are safe as the power travels on the "skin" of the aircraft.

So if you have anything that is unshielded ie some telescope Go-to instuments, a metal bag will do the trick. Does not need to be earthed. In fact best not to but unplugged of course.

Cheers Marty

[westom]

Might. Or might even connect the surge through those other disconnected devices. Or might do nothing. First the argument is subjective. Second, do you really know what is and is not conductive? That was another major point. A home, for example, is just filled with materials that are discovered later to be conductive. That is why the only effective solution (short of literally carefully constructing a completely electrically isolated building inside the building) demands doing everything necessary to earth a surge BEFORE entering the building.

It is so much easier, so effective, and so much less expensive to simply upgrade the earthing system. As noted in the beginning, surge protection is always about where energy dissipates.

[westom]

If EMF was so destructive, then a nearby strike causes all mobile phones, wrist watches, calculators, TVs, and automobile radios to fry. Reality. No such damage exists.

A longwave (maybe 50 meter) antenna suffered a nearby lightning strike. Many thousands of volts appeared on the antenna lead. Then an NE-2 (neon glow lamp) was connected. Those thousands of volts became tens of volts. The milliamp (or less) of current conducted by an NE-2 literally made that energy irrelevant. Why? Because EMF so hyped in myths is made irrelevant by the simplest solutions. (Many 1960s CB radios used that same solution.)

Auto and mobile radios were connected to antennas to maximize EMF on the most sensitive transistor inside that radio. The RF amplifier. And none were damaged. Because induced fields are so trivial once we add numbers to the myth.

In one case, lightning struck the building's lightning rod. Maybe 20,000 amps entirely on that wire from lightning rod to earth ground. One meter just inside the building was an IBM PC. It did not even blink. Those massive fields from the direct lightning strike (only a meter away) did not even crash a program. All electronics must be designed to make that (and more potentially destructive static electric disharges) irrelevant.

Destructive EMF exists only when numbers are not provided. When the claim is 100% subjective. If those fields are so destructive, then where are so many destroyed radios? Damage occurs when currents pass through that device (incoming and outgoing); such as a direct strike to utility wires 100 meters down the street.