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#72972 12/13/06 08:43 PM
Joined: Oct 2004
Posts: 276
T
trollog Offline OP
Member
This morning I was in a soon-to-be parking lot reparing some 2" pipes the
asphalt company's grader had mangled. The 2" sweeps were buried about 3 feet
deep so my coworker and I dug them up and were using a borrowed Rigid shop-vac to clean
out the mounds of fill dirt and concrete dust that had clogged the first few
feet of the pipe. To get past the bend of the sweep we duct-taped a three foot
length of 1" pvc to the end of the shop vac's hose, and then duct taped about 2" of 3/4"
smurf tube on the end of the 1" pvc and were poking it in like a giant
aardvark's snout down the sweep to get the dirt out. I was standing outside the
hole, and my co-worker was in the hole on his knees, hunched over the pipes
while the shop-vac did its work. Suddenly his shoulders jerked the way your
body jerks when you touch a live wire. I shut the shop-vac off and he told me he
just got shocked and saw a five inch electrical arc inside the sweep between the
wall of the sweep and the 1" pvc pipe stuck down it (it was about 7:45 am, broad
daylight). He said, "you take a
turn", so I got in the hole and continued on with the vacuuming. The shop vac
hose was hanging right next to my arm, and after about 5 seconds all the hair on
my arm started to stand on end. Shut the vac off, my hair lies down.. Turn it
on, hair stands right back up. I never got my shock or 5" arc, but thought it
was kind of interesting that a shop vac could build up that much static charge.
The weather wasn't particularly dry and the soil was damp. My guess is that the
dry cement powder filling the pipe might have helped things, but still, a first
for me.

Joined: Aug 2006
Posts: 356
Member
A GC that i work with had experienced the same thing. They were getting pretty good shocks from the plastic hose of the shop vac.
They told me to try and i said i will take your word for it. Their solution was to twist #14 copper on the PLASTIC hose and bond it to a near by grounded item. Faucet, building steel, frame of an appliance.
This bonding of Plastic hose to a grounded surface made it easier for them to work.

I thought it must be a static charge. But how come it does get build up when one does regular vaccuming.

Edward


Be kind to your neighbor, he knows where you live

Joined: Oct 2004
Posts: 132
M
Member
Google Van De Graph generator...you basically just built one! If you stand on an isolated base (mason jars with a pice of wood are nice) your hair will stand up!

The dust collection systems in workshops have a bonding wire wrapped around them to prevent this sort of thing happening.

Joined: Oct 2006
Posts: 745
E
Member
Strange as it may seem, air moving rapidly through a plastic pipe will allow static electricity to build up. Kind of like when you rub a balloon on your head and your hair stands up.

[This message has been edited by EV607797 (edited 12-13-2006).]


---Ed---

"But the guy at Home Depot said it would work."
Joined: Jul 2004
Posts: 9,677
Likes: 9
G
Member
Try sucking up toner beads. That is really a static machine. We had to use special conductive tools and hoses.
You can go over on the woodworker's BB and read about tricks they use to ground their dust collectors. They worry about explosions


Greg Fretwell
Joined: Mar 2001
Posts: 333
S
Member
The same thing can happen with an airless paint sprayer. There is a drain wire built into the hose to prevent this.

steve


Steve
Joined: Oct 2004
Posts: 276
T
trollog Offline OP
Member
I know about static electricity in spraypainting applications, just never thought air would present similar problems.. guess I was wrong there.

never thought about woodshop dust collectors, but that is interesting, a giant wet/dry vac with a much larger "hose".

At any rate, a memorable day at work just the same

Joined: Oct 2000
Posts: 4,050
Likes: 3
Member
There's a thread in the Photos area about Dust Collector systems:
https://www.electrical-contractor.net/ubb/Forum5/HTML/000312.html

Bill


Bill
Joined: Jan 2005
Posts: 91
H
Member
Had a related experience one time while we were involved in a manufacturing area retrofit. It was a 60,000 sq. ft. room with precast walls and bar joist type roof support system. They were making some changes to the concrete floor and were running a mini excavator and a skid loader inside. Because the area was enclosed they asked us if we could run some large process related exhaust fans to draw the fumes out. These babies were big and produced about eight air changes per hour.
As soon as we started the big fans all of the trades that were working on hi-lifts began to complain about getting very nasty shocks when they touched a bar joist. They all started touching it first with a piece of ready rod to discharge the static charge .
A couple of days later when we the concrete work was finished and we shut the fans off this problem went away.
From what I understand about static electricity I think that this was caused by friction. Friction can be caused by air molecules moving over a surface, even in a 60,000 sq. ft. room. In fact, isn't this how lightning develops?

Joined: Dec 2003
Posts: 886
H
Member
Take a look at the PVC piping for a central vac system that has been in service for awhile. Most that I've seen are covered with dirt thats been attracted by the static charge.

-Hal

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