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
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.
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.
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
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.
On 10/15/10 7:56 AM, firstname.lastname@example.org 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.
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.
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.
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
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.
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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