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Blast Cabinet Static Electricity

Blast Cabinet Static Electricity

Blast Cabinet Static Electricity

(OP)
I learned from experience that grit blast cabinets will build up a static electricity charge and the operator can receive mild shocks. So, when I put a blast cabinet in my hobby shop I made sure it was grounded. I would still get a little shock once in a while. In the past week, I have been blasting two motorcycle fenders (steel)and got an occasional shock on my hands through the rubber gloves that protrude into the cabinet. Yesterday I got quite a few and not just in the hands. I started wondering what was suddenly different. I thought about my shoes of the day--a ratty old pair of Reboks. On the previous days I wore an old pair of Iron Age steel tow safety shoes. Nothing special--black leather slip-ons with a rubber sole. On a hunch, I switched shoes and the static shocks ceased. Anyone got a clue on why switching shoes ended the static shocks?

RE: Blast Cabinet Static Electricity

Might be the shoes, you could check to see if the soles are more conductive on the Reeboks, though you might not have a megger to do it with. Might be humidity, or how much your feet sweat in one vs. the other, and might be the rubber formulation again (one absorbs water more easily and changes conductivity). Might have been the ambient humidity, was it colder/drier yesterday than previously?

RE: Blast Cabinet Static Electricity

The charge is building up at the blast nozzle and transferring to your body.

Your shoes must be fairly good insulators. If you have a resistive path to earth, even one in GΩ, then the static charge will generally bleed away to earth. Chances are your other shoes have a slightly lower resistance to earth and were preventing the charge buildup. Dirt, sweat and damp lend themselves to making a conductive path, plus some rubber materials are inherently static dissipative.

You can try a drain earth on the blast nozzle itself connected back to the machine frame. A length of single-core flexible cable will be more than adequate if you ty-rap / zip-tie it to the blast hose to keep it out of the way.

RE: Blast Cabinet Static Electricity

Back in the day one of my dirty jobs was vacuuming fly ash out of the collection hoppers of electrostatic precipitators. I would get shocks bad enough my entire arm would go numb; initially it made no sense to me, as the units were in a state of shutdown and all the high-voltage precip components were isolated and grounded. Once I figured out that the exceeding dry fly ash friction was turning the whole apparatus into one mighty Van de Graaf generator, I took to always holding the 6" hose with only one hand while gripping the door frame of the hopper or some other steel component with the other; end of problem.

If grounding the blast nozzle doesn't do the trick, you could try some sort of bonding conductor between say, your ankle and ground - but use something like a rare earth magnet that will easily pull loose to form the actual connection point between your ankle and the conductor so you're not mechanically tethered to the machine - just in case of a fire or something like that.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Blast Cabinet Static Electricity

I agree with all of the above.
The cabinet is grounded. You are not.
Another possible solution may be one of the conductive wrist bands that are used for work on static sensitive electronic equipment.
They are fastened with Velcro I believe.
I encountered a similar situation working under 500,000 Volt transmission lines. Capacitive shocks rather than static, but when a large group of electrons abruptly enter or leave the body, both effects feel the same. We were a long way from town and we improvised with stranded #14 bare copper wire around one wrist and connected to the objects that were charging up and zapping us. (WE did not have a "Quick Release".)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Blast Cabinet Static Electricity

Safety shoes are very likely to have conductive soles as that is a very important safety feature in many environments, like around flammable vapors. Ideally the gloves should be slightly conductive and grounded. Of course, all metallic objects should be grounded but if you hold the object you are blasting without it touching the grate it will get a charge.

RE: Blast Cabinet Static Electricity

Where I work there is a requirement for the use of dielectrically resistive footwear in all electrical environments; different "pokes" for different folks...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Blast Cabinet Static Electricity

(OP)
Guys, thanks for the good tips. The no-brainer part of the solution is to wear the magic safety shoes. I can also readily fashion a ground cable for the blast nozzle. I just scrapped out a front brake cable that I can repurpose as a grounding cable.

RE: Blast Cabinet Static Electricity

Years ago I was called upon to design a small operating theatre for a four bed hospital in the Moskito Coast region of Honduras. (This little hospital serviced a wide area of savana and jungle with a population of about 10,000)
I bought the appropriate safety codes. The code required the flooring resistance, as determined by a specified test method, to be in a range that was high enough to prevent touch potential shocks but low enough to discharge static sparks.

As an aside, the hospital has an interesting history.
It was established by two Baha'i families as a faith based project. They operated the hospital for a number of years as an unprofitable, for profit enterprise.
It was a small but steady drain on the families finances.
Then hurricane Mitch hit Honduras. It was little know outside of the affected areas that Cuba sent a medical support team to aid in the relief efforts.
It was a well planned team;
Five doctors of different specialities, all of which useful in disaster relief.
Also five support staff. A lab technician, a radiologist, and some nurses.
About this time the Baha'i families were asking the Honduras Government to take over the running and support of their hospital.
Cuba agreed to send part of their medical team to the hospital and the Cubans took over manning the Hospital for a time.
Link

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Blast Cabinet Static Electricity

Is your gun properly grounded? What about the part you are blasting? Ground both of those properly and you won't have to worry about what shoes you're wearing or what wrist strap to put on for the day.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Blast Cabinet Static Electricity

(OP)
Gun is not grounded as mentioned in my previous post. The parts are grounded via the grate in the cabinet and these fenders were heavy, so even if I lifted one end, it the remainder was still sitting the grate.

RE: Blast Cabinet Static Electricity

Definitely ground the gun tip, then. Media flying/grinding past the tip and media flying/grinding past the part are the two main sources of static build-up... ground those out and you should have no further issues.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Blast Cabinet Static Electricity

My small blast cabinet has a conductive hose, when I had to replace it my Parker dealer had compatible materials.

Here is an article I found discussing this issue you may find helpful,

http://www.clemcoindustries.com/news_detail.php?id...

MikeL.

RE: Blast Cabinet Static Electricity

(OP)
Thanks, catserveng. That link was most interesting.

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