Ground rod question

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I know hte NEC calls for 8' of ground rod for house wiring.
This is a different application where I want to ground a piece of machinery for static discharge. It is not possible to get the rod into the ground more than 2 to three feet because it is on ledge. Aside from boring through solid rock for the extra five feet it would still probably not be the best contact. How effective is a shorter rod for static discharge? Is three feet better than nothing or not worth the bother?
No codes are involved, just trying to isolate electronics from static charges.
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On 10/14/2010 1:42 PM, Ed Pawlowski wrote:

Don't think you need to do this. I don't go through these extremes to get static off my body when working on computer innards.
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This is a machine that processes plastic. Much more static potential than you get in your body. Manufacturer recommends a ground rod.
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On 10/14/10 6:02 PM, Ed Pawlowski wrote:

I'd like to read about that machine. Usually, providing a path to the ground of an outlet is adequate because it takes very little current to prevent static buildup.
Often the discharge paths are through high resistance such as some glasses or rubbers rather than metal so that if a charge is built up, it will discharge gently in a second rather than violently in a microsecond.
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The machine is 480V 3ph powered, has three motors plus pneumatic controls and steam for the process. It handles up to 10,000 pounds of plastic a shift using vacuum to draw it into a chamber and a 5 hp blower to airvey it up to 100 feet away to storage. The controls are typical industrial panels with a logic center, screen, assorted timers and sensors. Picture two of these along with hoppers for raw material. http://www.hirsch-gruppe.com/homepage/com/geschaeftsfelder/maschinen_anlagenbau/produkte/vorschaeumer/preex_6000_XXL.php
We have two machines together and it is recommended to ground the chassis. The two units are about 20' long and 20' high. Moving plastic through the suction tubes and the ducting can generate a lot of static. I don't know the voltage, but I've seen improperly grounded equipment throw an arc over 12".
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On 10/14/10 11:11 PM, Ed Pawlowski wrote:

http://www.hirsch-gruppe.com/homepage/com/geschaeftsfelder/maschinen_anlagenbau/produkte/vorschaeumer/preex_6000_XXL.php
Wow! The equipment must have been very well insulated for the voltage to climb that high.
The principal is to have everything that might arc be able to drain to a common point. If a grounding rod were connected to the concrete floor, any person or equipment with a drain path to the concrete should have a drain path to the grounding rod. The resistance of the drain path could probably be a million ohms and still work.
If the rebar in the concrete is already bonded to the building's grounding system, ground surges from lightning should not be hazardous.
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With a ground a static charge couldn't build up in the first place, so the above doesn't make sense.
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On 10/15/10 7:56 AM, snipped-for-privacy@optonline.net wrote:

A few years ago, toll takers began getting shocks from drivers because new tires on some cars allowed the cars to build a static charge. One solution was to drag a grounding strip.
That would ground the car. Depending on the upholstery material and his shoe soles, the driver may get a shock when he gets out, either opening or closing the door. A static charge can build between two objects when they are insulated from each other, even for a moment.
The driver can probably discharge himself painlessly by touching the glass.
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On Fri, 15 Oct 2010 13:55:42 -0400, J Burns wrote:

Back in the 40's/50's/60's it was common to see grounding straps on the back of vehicles dragging on the ground. I guess it was to dissipate static on AM band radio.
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On 10/15/10 2:01 PM, A. Baum wrote:

I used to see straps in fuel trucks. In the 1960s I was warned not to touch the tires of a large aircraft because there were sharp wires sticking out. I guess a 707 sitting in a dry breeze could accumulate quite a charge.
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Not that you need it or anything, but I think you can lay the rod horizontally when bedrock is in the way, but you might have to use multiple rods, for whole-house grounding that is.
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On Thu, 14 Oct 2010 13:42:26 -0400, Ed Pawlowski wrote:

I've got two 4 foot rods at either end of my house and they serve well for reduction of static noise on my amateur radio equipment. They also serve as part of my low and medium frequency antenna counterpoise.
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wrote:

Driving rods that are not bonded to the grounding electrode system is an invitation to disaster.
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yes all grounds must be bonded, so you can drive a short rod if you want, but it must be bonded to the main ground sytem.
ground rod at main service bonded with incoming water line etc etc
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Exactly.
And that means that a "short rod" will just not make any difference.
There usually is a "single point" where your incoming electrical service neutral is "bonded" to ground.
If you have noise problems, running a separate conductor from that point to your "sensitive" equipment might reduce noise but likely that's not worth the trouble.
In some hospitals, on some circuits "they" run a separate ground in addtion to the protecting ground provided by the conduit and a protective ground wire. This "ground" will be slight less "noisy" than, say, the ground from the conduit. Most equipment has a slight leaking between the current carrying conductors and the chassis/ground. These can introduce some noise that other circuits can pick up.
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Yes, it's important and part of the code that any new rod must be bonded to the rest of the grounding system for the building.

From what I know of static electricity, that's probably true. But since it's a massive piece of industrial equipment that processes plastic material, why ask here? I'd contact tech support at the manufacturer and ask them.

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I did. They specify a grounding rod, but they don't know if a short rod works well or not. Lots of smart people here though.
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Ed Pawlowski wrote:

Use the 'green' wire on an outlet. Ground is ground is ground.
--
LSMFT

Simple job, assist the assistant of the physicist.
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Thee is no "green wire" as such. It is 480V 3ph with a transformer for the lower voltages required.
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Ed Pawlowski wrote:

Um, 480V 3ph or not, it is *supposed* to have a green ground wire. A white neutral wire may not be required, but a ground certainly is.
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