Grounding in hazardous atmospheres

On Sat, 22 Nov 2003 11:14:53 -0600, "weco" Gave us:

You could also core drill the floor, and drive a ground rod right at the machine site.

ESD is probably not too well covered by the power distribution boys.

What you need is an area around the machine that has a bona fide conductivity to both the machine, and ground. say ten feet...

Then, ALL personnel approaching the machine must have heel straps on.

THEN, you need posts with ESD mat, grounded surfaces on them, near the machine so that personnel can be doubly sure that they are discharged before touching any metal on or near the machine.

The problem here is that ANY arc, however small, is likely to be above the flash point temp of the gasses involved.

Who would risk it? Not me.

Oooops... see above. Standard ESD procedures should work, if you think of using high resistance mats to slowly discharge any accumulation on personnel, as THEY are the charged element, when not grounded. DO NOT use metal touch points as there would STILL be an arc. The safest hedge is the heel straps as they ground the person, even as they approach the machine, and with each step.

Merely walking across the room in insulated shoes can allow one to pick up several hundred volts of charge, right out of the air.

On Sat, 22 Nov 2003 09:25:44 -0800, DarkMatter Gave us:

Also... use good, flowing ventilation in the area to keep any whisps of vapor from accumulating into prospective pockets of pain.

One doc is nfpa.org NFPA 77 - Recommended Practice on Static Electricity

--s falke

For starters:

Generally, the thing that causes a concern with solvents is static elctricity when unloading the material, or moving it from Point "A" to Point "B".

Grounding shold be to bond the area to the same point, so that the entire system rises as one as faults may occur. (For instance, a lightning strike should see the entire area rise as one voltage level; trying to stay at Zero is both difficult and also may be causing a problem).

Put a cable which is tied to the main ground on the vehicle unloading the solvent, and also ensure that any pipelines moving the solvent are bonded well to your main ground system.

HR.

this is for me where the confusion begins

the words "bonded well" ..does that mean a heavy guage conductor to ground? for the main operating components of the machine where there are heaters or motors this makes sense, but for areas like the solvent drums I'm not sure it does. It almost seems like you have to evaluate which hazard is more likely. A motor fault wont be an issue at the drum if the motor is well grounded...static wont be an issue around the motor - assuming it's not in the explosive atmosphere... I'm going to look into the standard that has been suggested....hopefully they will have an answer, but it reall seems like this is not a "one method always works" situation....

do most of you guys use low impedance grounds in situations like this?- my understanding of well bonded... and if so what do you use as the rule for the ampacity of the ground conducter?

thanks for your thoughts

henry

Do you have a ground grid around your entire facility, or at least around the part where you are dealing with the solvent? I know some facilities do; I am assuming you would have this. (This would be so you could provide a common ground reference point to your entire facility; I thought it was common practice for the past 30 years or so.)

That is what I am talking about when I say you should be "bonding" to your ground system; be it the vehicle off-loading the solvent or the pipeline transporting it.

I suppose if you do not have a common ground grid for your facility, you could create a triad with 3 10' rods in an eqiliateral triangle 10' apart - or 3 20' rods in the same configuration 20' apart... And then tie your static ground to this.

(If you want, go off-line with this and we can talk one to one.)

HR.