BASIC GUIDES FOR PROTECTING YOUR TELESCOPE FROM A LIGHTNING SURGE

I will contribute what I know from experience.  The older I get the more my "experience" gets compounded, so drag up a chair and read a while.  Those of you who know it all, need not read any further.

First of all, you need to ask the telephone and electrical company nearest you if the "ground has proper grounding potential" for electrical discharge and lightning.  If it does, then your problem is minimized a bit; if it does not then you must take very special precautions because you MAY create the best ground that your nearby area has via the pier that you are putting in the ground.

Two things to remember:
1) lightning does not always come from the "air"; and,
2) your will get more surges and strikes from the ground than you will from lightning aloft.  Nearly all telescope electronic damage, and component (i.e., CCD cameras, computers, clocks) damage is a result of indirect ground surges during lightning storms than from aerial strikes.

Lightning may strike a tree, tower or even high ground two miles away from you attempting to find grounding.  If it cannot find suitable discharge, being the fun stuff that it is, it will travel through the source of the strike and through any medium, particularly wet and rock-laden ground, to the nearest "best" grounding potential point. This point may be an iron-rich lump of sandstone beneath the surface of the earth, a mobile home metal anchor, a large structure standing on the surface, OR worst case: your pier filled with metal rebar.


If you live on top of a hill or in one of those grounding-poor areas such as Arkansas Sky Observatory is on Petit Jean Mountain in Arkansas (we have taken five lightning hits in five years until I  inally figured out the cure), your observatory is going to be a prime target for both aerial and ground lightning surges, particularly if you do not design your pier properly.

1)  My first rule of thumb is simple:  install a complete shut-off system for the observatory, where all power can be disconnected from all equipment via a lever switch outside the building at the breaker box.  Remember that lightning and surges are also very common through phone lines; if you have your PC plugged into a modem, or even a phone connected to a land line in your observatory, you are inviting electricity into the privacy of your sanctuary.

2) Second rule:  even with the breaker totally disconnected from the electrical source (pole/transformer from utility company), you can and will likely be in danger of a ground surge during lightning storms; these will enter your system via the pier and not from outside power sources nor through direct hits ("strike").

3) Third rule:  knowing the second rule, never leave any wire whatsoever connected to the control panel of the telescope, even with all power disconnected; something so simple as the rs232 cable running from your PC to your coiled Autostar cable running to the Autostar resting on a nearby table or telescope base can act as an "antenna" for any surge traveling up the pier and looking for that juicy PCB behind the control panel for your telescope.

4) Fourth rule:  NEVER install lightning rods nor tall antennae close to your observatory for the intent of diverting lightning away from your building.  You will attract a lot of energy toward your location that might otherwise go somewhere else. Lightning rods should never be installed nor used to your observatory.

5) Fifth rule:  know how to build your pier, construction material wise, for your location.  If you are in a grounding poor region then you must take extra precautions to protect your telescope from ground surges which will "get your equipment" even when disconnected and unplugged.  If your telescope pier is metal OR if your concrete pier has metal rebar in it you must be very careful to protect....since most piers are similarly made, then let's look at how to protect.

NOTE that virtually all telescope mount/pier arrangements are going to transfer part of the support of the telescope to any pier whatsoever by means of a piece of metal; this can be either a metal pier from the ground or floor level to the base of the telescope, or a metal wedge from the top of a concrete pier to the base of the telescope. So....the fact that metal is going to be in your framework is a given.

In any case, high quality *Guaranteed" (i.e., such as the Radio Shack replacement guarantee) surge protectors should always be used; I recommend using a major commercial "Utility Line" surge box (this attaches to your main meter via your Utility Company and there is a monthly service fee for this) for the overall building, and a separate small surge protector strip for EVERY outlet into which your equipment is plugged.

PROPER CONCRETE PIER CONSTRUCTION

This discussion is based totally on my vast experience of 40 years of doing things the wrong way and then knowing how to never do it that way again.  So all of you lightning experts and electrical gurus can just sit back and listen because I have "been there and done that."  I know what works and what does not work with regard telescope systems in regard to lightning protection.  In some lightning-prone regions, proper protection can become expensive quickly and NOTE that surge protectors are always recommended, but they are certainly NOT assurance against hits.  In fact, nearly all equipment damage that I have seen and have reported to me has resulted with the surge protectors (and fuses in line) not even being tripped whereas the electronics inside a telescope or
other equipment are aptly fried.

A quick rule to remember for your observatory:  any surge that can stop a digital clock can damage your PCBs inside the telescope, and GO TO telescopes have at least four for surges to nest in.  If you keep having surges on your electrical line, or if you have lightning storms that repeatedly will require you to reset a digital clock, then you have potential danger for your computerized telescope and nearby equipment.

There is a precautionary step that can and should be taken with every concrete pier poured (more discussion will follow on the applications of this):  whether you your using Type A or Type B piers (below), always install TWO to FOUR 3/4" J-bolts (preferably one at each corner of the base pier, a few inches from the top surface, sticking outward like "arms") extending out the SIDES (horizontal) for either immediately or in the future creating "terminals" for an outside "grounding cage" to divert surges totally out of your observatory.  More on this later.

There are TWO types of concrete piers for telescopes:

A.  Those that are built to slightly above ground level onto which your metal pier will be bolted; the metal in turn will rise typically 30" or more above the surface of your observatory floor and that is what the telescope or its wedge will rest upon.

B.  The pier is designed to anchor to the ground AND to rise above the floor level and serve, in addition to being the ground anchor, as the riser pier between the floor and the base of the telescope or pier.

For lightning protection, I would always opt for "B" above.....concrete all the way to the base of the telescope.

** PIER A (substructure only, concrete base anchor for metal pier) :  This pier will go deeply into the ground, preferably below the frost line for your region and should flare outward as would a pyramid base into the ground.  THIS section is where the rebar or any structural enhancing steel should be placed, below ground.  However, the more rebar you are using the greater ground you are creating for lightning and ground surges to "find", particularly when the ground is saturated.  You must use some rebar or metal cage in the below ground construction, but use it wisely. Remember, "mass is good" in regard to the base, but it can be overkill.....you do NOT need three tons of concrete in the ground to support a 90 pound telescope.  Period.

For most modern amateur telescopes and those in small college observatories, a below ground anchoring pier should be no less than 3-feet square, flaring toward the bottom and no less than three feet in depth into the ground.  Ideally, this pier should come up about one foot above ground, this section out of which you will have nominally three (3) anchoring "J-bolts" of at least 1/2" diameter; 3/4" to 1" are better, and preferably about 12-15" deep inserted into the concrete during the curing process.

For a 3' deep anchoring pier, you should drive SIX (6) pieces of standard 6-foot rebar into the ground:  one driven at an angle outward from top to bottom inside of each "corner" of the below ground pier, inset with enough space to properly assure engagement with the concrete being poured around it.  The other TWO (2) pieces can be driven directly toward the middle of the anchor pier and straight down into the ground.  Drive each of these metal bars deep enough so that not one bar extends to within 6" of your large "J-bolts" and certainly never should the rebar make contact with these bolts.

That is all the rebar (and concrete for most applications) that is needed for PIER A.

For PIER A, added surge protection can be applied when fastening your metal extending pier (above ground) by placing very large nylon or Delrin Fender washers around each J-Bolt and allowing these to rest between the metal pier bottom plate and the concrete below it.  I also recommend using nylon bushings AROUND the extended threads of the J-bolts to isolate the metal threaded bolt from the inside metal of the drilled holes in the base of the metal pier, followed by a second top Nylon Fender washer before the large nut is applied to fasten the metal pier plate firmly in place.  These added isolators seem minor....but they can make a huge difference in the drop of surge potential from the ground, through the concrete below ground, and into the metal pier.

** PIER B (complete structure including interior rising pier to telescope, all made of concrete):  This is the recommend method for lightning prone regions. This method of pier construction offers advantages over a base pier (concrete anchor) with an extended metal pier above it.  Using concrete throughout the pier has added benefit in both stability and vibration suppression AND in reduction of surge potential if properly constructed.

For this discussion, we will assume a "model pier" of 36" above floor level to the point at which a telescope base/mounting plate or wedge will be attached.

The ideal concrete structure would have an anchor pier very similar to that described previously as "Pier A;" however, during the pouring process, all concrete MUST be poured at the same time to prevent differential drying which would greatly reduce the stability of the overall structure.  So planning a "PIER B" must be done very thoroughly before the mixing of any cement or before the concrete truck arrives.

The base section ("Anchor") in this example will be a minimum of 36" square with rebar placed exactly as previously described EXCEPT at the center; in this example, we will be using a total of EIGHT (8) rebar sections, each 6-feet long.  The corner sections are driven as described about half-way into the ground at an angle away from center; however the center section (about a six-inch diameter circle minimum) will contain a total of FOUR pieces of 6-foot rebar, each driven into the ground until they extend ONLY HALF WAY to the anticipated TOP of the overall pier.

In other words:  if your anchor hole extends 3 feet into the ground, and you want the anchor pier to be elevated above ground level by ONE FOOT to your floor level, that is a total of four feet for your anchor pier.  Your pier above floor level is going to extend (in this example) 36" more, so you will have four lengths of rebar, each extending TWO FEET above your anchor pier.  This allows the rebar to secure and strengthen the overall pier, both bottom and top sections, but also diminishes the proximity of the potentially-grounding metal away from the telescope system which will be sitting on top of the pier.

For the actual 36" pier that "shows" in the observatory, I always recommend simple Sonotube, available and any large hardware outlet, such as Lowes or Home Depot.  Minimum diameter should be 8 inches.  Ten inches is preferred, this being centered in the larger anchor pier below, and resting OVER the four extended lengths of rebar; those metal rods will be extending about two feet into the three-foot section of Sonotube.  The tube is placed into position immediately after the anchor pier has been poured and is still quite fluid; you must immediately set up a framework to support the Sonotube in two ways:

1) to assure that the Sonotube is perfectly perpendicular to the ground and level vertically using a good square and level; and,
2) to assure that the Sonotube does not SINK into the anchor pier and shorten your desired height above floor level.

Two by four lumber frames, previously constructed prior to pouring is recommended for this.  One frame can be designed to serve both purposes.

Quickly, the remaining concrete must be poured into the Sonotube prior to any curing of the cement already poured in the anchor pier.  Once again, prior planning dictates that you MUST have a bolt pattern template (thin plywood with anchor bolts already inserted is recommended) already made and ready to place on the very TOP of the Sonotube to allow for your anchors for the final wedge or pier/telescope mounting plate.  The anchor bolts used at the top of the final pier must NOT make contact with, nor come too close to, the rebar that you have driven into the ground and extending into the vertical Sonotube. Isolation here is a must.

Once the cement is poured into the Sonotube and your top template is in place, RE-CHECK the very top assembly and surface to assure that you are perfectly level at the top; if not, make slight adjustments to your Sonotube and 2 x 4 frame and secure firmly to assure that the cement cures to as perfect to level as possible.

Once cured, do NOT attempt to remove the outer cardboard of the Sonotube; leave it in place, as it makes an excellent base layer for a beautiful paintjob using Epoxy resin (marine) paint, the color of your choice.  Leaving the cardboard in place, if properly finished and filled where necessary, will provide a beautiful exterior finish to your pier.

You have just constructed a very safe (in terms of electrical surge) and rock solid telescope pier for nearly every small observatory application.

FURTHER SURGE AND LIGHTNING PROTECTION - The "Grid Cage"

Remember the four horizontal anchor bolts extending like arms from each corner of the Anchor Base Pier?  You can actually divert surges and direct lightning strikes away from your delicate and expensive observatory equipment if you connect these like you would terminals on a automobile battery to an external "Grounding Cage."

This method, highly recommended by telecommunications and electronics storage companies, is a bit on the expensive side,but will assure you of considerable added protection against surges.  The idea is to assume that you ARE going to take a surge or strike at the observatory at some point and no matter what precautions you have already taken, powerful electrical potential is going to enter your building.

So....you are going to essentially "get it OUT of your building" using this cage method.

On each "terminal end" of those J-bolts coming out of the sides of your anchor pier you are going to use two hex nuts and Fender washers.  Between each pair of nuts you will secure a length of #6 or #8 copper braided wire which will lead outward from your pier......these four lengths of braid will each join outside the four corners of your observatory to an external grid that you have buried in the ground using a common rented trenching tool.  The grid can be pretty much any kind of metal, even long welded lengths of rebar, but preferably thick copper rod is best, the kind that is used by electric companies to ground your electric meter at the service point of your home or observatory.  The underground rod, no matter what type of construction, is all connected in a huge square outside of your building....the farther away it is from the building and the deeper in the ground you can put it the better.  However copper is expensive and the cost will add up quickly.  Nonetheless, the grid must be at least 6 feet distance from each wall and no less than two feet in the ground.

The large copper or steel square will be connected entirely from each corner; at each corner a separate copper braid from each J-bolt will be connected via a heavy duty grounding wire connector assembly (available at any hardware store).  Once all this is connected, added protection can be assured by driving ANOTHER copper rod six feet or more into the ground straight down from each corner and either welding or connecting (via a wire connector) each vertical rod to the square cage once driven into the ground.

Once in place, bury the cage and forget about it.

You will sleep very well at night.....but only if you take every precaution I have just outlined.

Now, wasn't this a shocking bit of reading?
__________

Dr. Clay
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Arkansas Sky Observatories
Harvard MPC/ H43 (Conway)
Harvard MPC/ H41 (Petit Jean Mountain)
Harvard MPC/ H45 (Petit Jean Mtn. South)
http://www.arksky.org/

Copyright, May 2008
Arkansas Sky Observatory
(may be reprinted with permission and proper credit)

 


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