Can any of you electrical gurus provide me with a wire gauge size to use for grounding aircraft against static electrity while they are being worked on? I know to calculate wire sizes for length of runs, voltages, amps etc, but how on earth (pardon the pun) does one calculate for static electrity? I know the voltage can be extremely high, but the amperage is very low. To go to an extreme, I suppose a 20 gauge wire would be more than enough to drain any static built up by working on the airplanes, but let's be really realistic. The longest wire run would be about 40 feet if a central ground rod is used.
Talking of ground rods, how many, diameter and length? Or is this determined by using a mega and keep adding until a suitable gound is found? The concrete floor is not yet poured, so I have a couple of weeks to experiment. There will be about four copper rods embedded into the floor (in a shallow basin so as not to protrude above floor level) to which a ground wire can be clipped. Each rod will be wired in parrallel to the next and to the ground rods. Actually, each rod sticking up could be made about 3 or 4 feet long and be used as a ground rod itself.
Is there any advantage of using a half inch diameter copper pipe (larger surface area for better electrical dissapation) under the concrete floor instead of the usual ground wire? All connections will be soldered or brazed.

 

On our equipment in the Air force, all the aerospace ground equipment that we use to support the C-5 and pretty much any other aircraft has 1/4 inch diameter cable for static grounding everything.

 

The cabling used in the workshops over here is extremely small diameter because you will not generate a lot of static electricity while working on a plane- the problems come from flying the plane where the air is "rubbing" the plane and due to the fact that higher places in the athmosphere have a different potential than lower layers.
You will want to make shure that you do not only properly ground the plane but also the rest of the workplace, because you are really interested in all thing there having the same potential.
The standard grounding wire over here is only 10mm2 (AWG 8) but the only reason it's that strong is that it's on an automatic spooling device that will retract it while not in use and the lunatics we have over here will not stop pulling when the wire is fully extended. This is the only reason to go over 0.75mm2 which is approx. AWG18

HTH

AlpineRAM

 

1/4" cable that the USAF uses is more than adequate-- overkill, even.

For your use of less than 40ft, I'd say 16ga is plenty.

jh

 

1/4 inch and some 1/8 inch. definately overkill

 

Thanks for the answers so far. Sounds like there is no way to actually calculate for static electricity and it is all by experience and guessetimate. I was originally thinking about running #12 bare copper wire under the floor and to each ground rod and from the answers so far it sounds like that is more than adequate.
The air force always overkills on stuff like this and of course, cost is no object for them. (Have you seen the price of copper wire these days!!!!! )

 

Be careful about bare copper wire on the ground- it will corrode and then it's useless.

The calculation of the wire size isn't that complicated, even a full sized Airbus will just have a certain amount of static energy stored. And you are not concerned about the time it will take to dissipate the static electricity (Like whether it takes a femtosecond or 2 milliseconds) so the amperage can be calculated as being extremely low. The reason for the big ground wires is that they back up as a lightning grounding device for the parked plane.

AlpineRAM

 

To test your ground rods to make sure they are in the ground deep enough, run a single wire from a 120V (turned off for the time being) source through a 3 amp fuse. Tie the other end of the fuse to the driven ground rod. Now make the 120V source "hot". You should blow the fuse. If not, you will have to go deeper. They make regular grounding rods that have threads and can be coupled together. Cooper will corrode underground but it will take awhile so use at least a #4 or #2 cable. That is my 2 cents worth working high voltage substations.

 

The grounds are all inside a building so there is no danger of a lightening strike. They are there only for when a plane (fixed and rotary wing) is being worked on. I had wondered about the corrosion factor, which is one reason I proposed using half inch copper tubing (common water pipe) instead of wire, buried under the concrete. The other reason was that pipe exposes more surface area for grounding purposes.
A long time ago I had to install a ground system for another purpose and did not have anything to test it with, so came up with the idea of using an electric kettle. I filled the kettle with water and then timed how long it took to boil. then replaced the hot water with cold and hooked up one leg of the 110 AC to the new ground and the other to the hot wire on the plug. The kettle boiled in the same time, so figured the ground worked.

 

Mexstan, is this by any chance going to be a metal building? If so, simply ground the plane to the steel frame of the bldg... You won't find a much better ground available. Electrical service ground will (ought to)be bonded to the building steel too, plus there will be ample foundation bolts that are set in conductive concrete.
If you wish, some of these bolts could be intentionally bonded to the rebar to make additional use of the steel in the slab..
That kind of concrete/rebar grounding method is referred to as an "Ufer" ground.. A Mr. Ufer developed this method in WW-II for safety grounding ammo stg bldgs in areas of extremely poor conductivity soils. (desert sand, granite rocks, etc).

This is all WAY extreme overkill if this is just for static grounding..

But, if it is a personnel safety ground for maintenance on the ACft then yeah, bond it to a known GOOD ground conductor with a ground cable as heavy as the conductors in the extension cord.

K.

 

Yes, it will be a metal building. The footings have already been poured with big steel bolts sticking up. I do intend to bond everything to the building frame, but I do not trust the conductivity of the bolts embedded in the concrete. They may be OK, but I still want the assurance of some traditional ground rods. The last thing to happen will be the pouring of the concrete floor. Now the floor will have rebar in it, but that rebar cannot be trusted to have continuity from piece to piece unless I stand over the workers and watch them weld them together. In reality the rebar in the concrete will be tied together with wire. So with the ground rods plus the steel building frame, hopefully I will have overkill for the grounding.

 

Stan,

As to the "More surface area" on the pipe. This really only applies if you are grounding against RF. RF travels ON the wire while current travels through the wire. That is why when grounding antennas and transmitters you "NEVER USE ROUND FOR GROUND!" Flat stock or braid only.

For what you are trying to do, a couple of 10' rods driven into the ground and tested should be more then enough.

 

I know that - but I forgot! Shame on me, especially as I am a ham who used to play a lot with antennas. Scratch the copper pipe idea.
Thanks for reminding me about this.

 

Stan, wire-tied rebar is perfectly acceptable for use as a "Ufer" type concrete slab ground. They just run some copper cable on a diagonal and CadWeld the cable to a couple spots. Properly tied rebar is every bit as good as welded once it is in the cement. There was no real measurable difference in relation to the resistance to Terra-Firma. It even stands up to lightning strikes without blowing the concrete apart.

You could probably get a couple pieces of #4 solid (or larger)and lay them on the "X" diagonal and cadweld (or clamp) it to the rebar mat in a couple of places each. Then run the 4 ends up into the corners of the footings to connect it to the building steel base flanges. That ought to stand a bolt from Zeus and keep on smiling.

I installed a ground rod in my living room to take care of an RFI problem in my computer... I drilled a hole and drove in an 8' ground rod flush with the slab. I then drilled and tapped the flush end for 1/4"-20 threads and bolted a ground wire to my computer equipment. Easy to remove and is undetectable.

Heck, you could get fancy and drive a big groundrod in the middle of the "X" an inch or two below finished grade and bond it to the copper conductors. Then stuff a round piece of hard foam over the driven end of the rod and finish the slab flush. Then pull the foam to make a small ground rod "pocket" like seen on flightlines with the grounding ball terminal in the little green cup in the tarmac.
If you goof on the elevation of the ground rod, those concrete guys are going to want a piece of your hide...

K.

 

This is a false statement. When creating a unimpeded path to ground for ground faults, electro-static discharge, etc., the greater the surface area of the grounding electrode conductor the less impedance to earth exists.

Any copper wire, rod, plate, pipe or inclusive electrode that is of sufficient size or is listed by a listing agency for the purposes, and that is in direct contact with the earth makes a fine electrode. Any of the above that is encased in a concrete floor or footing also makes an even better electrical bonding with earth, a process called Ufer-grounding named after Herbert G. Ufer. However, grounding electrode conductors must be protected from physical and/or environmental damage, such as machines, chemicals, etc by a proper conduiting system and/or outer insulation if necessary due to corrosion.

This is not a very good test. There is actual equipment to determine the impedance of a grounding electrode system. Too many variables exist with fault currents, and static and lightning voltages to rely on this.

You may certainly rely on the wire-tied reinforcing rods and bolts holding down the steel columns. There is a lengthy explanation, but this all makes for a better grounding electrode system then ground rods. However, you may certainly supplement your system with rods.

Sizing grounding electrode conductors is most often a sum of the maximum fault currents to be imposed under NFPA 70. However, where static discharge or lightening is of a concern, you may want to consult the National Electrical Safety Code (not the National Electric Code) and moreso NFPA 77. If in doubt, I strongly suggest contacting a professional in this field to answer this question.

AJ Pulley
MA Licensed Electrician
MA Electrical Instructor
Member International Association of Electrical Inspectors