Posted: Fri Apr 24, 2009 12:06 am
Section F. on Bill's page points to this link:
http://home.comcast.net/~rodec/woodwork ... myths.html
"...The issue, in a nut shell, is that with a charged conductor the charge is free to move, and thus the charges throughout the conductor can can join forces to make a strong spark. But, because charge on an insulator is not free to move, discharges only occur from a small area, leaving the majority of the charge behind. For this reason discharges from insulators are not as energetic as those from conductors. ..."
"I would also like to point out that you can not truly ground PVC; there is nothing you can do that will guarantee that a static discharge from the duct will not occur. While you can not actually ground PVC, the so called "grounding" seems to help anyway according to many accounts. Typically people add so called "grounding" using one or more of the following: a grounded wire inside the duct work, a grounded wire wrapped around the outside of the duct, or by having grounded screws poking though to the inside of the pipe. Interestingly enough, I've never heard of anyone wrapping grounded aluminum foil around the ducts, which for 4 inch PVC costs about 2.5 cents per lineal foot, or about a dollar to do 40 feet of ducts. Wrapping the ducts in foil will make your attempts at "grounding" much better than simply wrapping in wire. However, as discussed above, backing a PVC duct with a conductor is problematic. At least three effects may play a role in the so called "grounding": leakage currents and shielding for outside wires, and providing a short hop to ground for inside wires or grounded screws through the pipe wall. Leakage currents and the short hop to ground work to reduce charge build up. Shielding works to protect you from a discharge to your body, but does not reduce charge in the pipe. The effectiveness of the attempt at "grounding" will depend on many factors, and may be overwhelmed if the system is pushed too hard. "
And the "Myths" section at the bottom of the page is interesting... for example Myth No. 4:
4. The external ground wire works by reducing the static on the outside of the PVC. There is little or no static on the outside of the PVC unless you are rubbing the outside for some reason; the static is on the inside. The electric field due to the static charge in the pipe can cause a discharge on the outside, but this in no way means there is static on the outside. Indeed, if there were lots of static on the outside, say due to lots of charged dust floating around from poor quality filter bags, the static on the outside will be the opposite charge from the static inside since opposite charges attract each other. Thus the electric fields from the two will tend to cancel each other as discussed above in shielding. Outside static helps protect you! The charge on the external ground wire due to the electric field from the charge in the pipe will also be of the opposite sign. The outside static and the ground wire have the same sign; they repel each other! Static on the pipe is not drawn to the ground wire. There is no reduction in outside charge due to the external ground wire. And, if you believe that not enough charge will go through the pipe to be of help, how is it that the charge will go along the pipe? It is an insulator either way. "
http://home.comcast.net/~rodec/woodwork ... myths.html
"...The issue, in a nut shell, is that with a charged conductor the charge is free to move, and thus the charges throughout the conductor can can join forces to make a strong spark. But, because charge on an insulator is not free to move, discharges only occur from a small area, leaving the majority of the charge behind. For this reason discharges from insulators are not as energetic as those from conductors. ..."
"I would also like to point out that you can not truly ground PVC; there is nothing you can do that will guarantee that a static discharge from the duct will not occur. While you can not actually ground PVC, the so called "grounding" seems to help anyway according to many accounts. Typically people add so called "grounding" using one or more of the following: a grounded wire inside the duct work, a grounded wire wrapped around the outside of the duct, or by having grounded screws poking though to the inside of the pipe. Interestingly enough, I've never heard of anyone wrapping grounded aluminum foil around the ducts, which for 4 inch PVC costs about 2.5 cents per lineal foot, or about a dollar to do 40 feet of ducts. Wrapping the ducts in foil will make your attempts at "grounding" much better than simply wrapping in wire. However, as discussed above, backing a PVC duct with a conductor is problematic. At least three effects may play a role in the so called "grounding": leakage currents and shielding for outside wires, and providing a short hop to ground for inside wires or grounded screws through the pipe wall. Leakage currents and the short hop to ground work to reduce charge build up. Shielding works to protect you from a discharge to your body, but does not reduce charge in the pipe. The effectiveness of the attempt at "grounding" will depend on many factors, and may be overwhelmed if the system is pushed too hard. "
And the "Myths" section at the bottom of the page is interesting... for example Myth No. 4:
4. The external ground wire works by reducing the static on the outside of the PVC. There is little or no static on the outside of the PVC unless you are rubbing the outside for some reason; the static is on the inside. The electric field due to the static charge in the pipe can cause a discharge on the outside, but this in no way means there is static on the outside. Indeed, if there were lots of static on the outside, say due to lots of charged dust floating around from poor quality filter bags, the static on the outside will be the opposite charge from the static inside since opposite charges attract each other. Thus the electric fields from the two will tend to cancel each other as discussed above in shielding. Outside static helps protect you! The charge on the external ground wire due to the electric field from the charge in the pipe will also be of the opposite sign. The outside static and the ground wire have the same sign; they repel each other! Static on the pipe is not drawn to the ground wire. There is no reduction in outside charge due to the external ground wire. And, if you believe that not enough charge will go through the pipe to be of help, how is it that the charge will go along the pipe? It is an insulator either way. "
