Grounding in a flammable atmosphere

[blumenstein]

I am a Fire Captain with Salem Fire Department, and a HazMat Technician with Oregon?s Region 13 Hazardous Materials Team. I would like to correspond with anyone who is aware of regulatory information dealing with establishing a ground (to earth) that has greater than 25 ohms resistance (for special circumstances).

I am gathering information for a fire department-based class on grounding and bonding as it relates to grounding a damaged transportation tanker in the presence of flammable vapors.

I have been advised that a 10 to 100 megohm cable should be used to prevent a spark when grounding in the presence of flammable vapors (Graham Hearn ? Wolfson Electrostatics, University of Southampton).
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Presently we are testing ground rods placed at emergency scenes to assure that we have sub-25 ohm ground to earth (one for the damaged tanker and one for the rescue tanker), and use low impedance cables to connect each tank to ground (before we electrically bond the tanks together).

First, is anyone aware of a regulation that allows the creating of a high-impedance ground? Since it appears we don?t need a sub 25 ohm ground, it would be much easier to create a ground that had 500 ohms or less resistance (It will be faster and safer to create in the field).

Second, where would I find a 10 to 100 megohm cable suitable for connecting a damaged tanker to a ground rod? I am looking for something that would be durable and have a 50 to 100 foot reach.

Any assistance would be appreciated.

Larry Blumenstein, Captain
Salem Fire Department
blumenstein@comcast.net

 

[petersonra]

Re: Grounding in a flammable atmosphere

One would be inclined to think that such a cable could be made from a couple of appropriate length pieces of wire and a 10 megohm resistor.

I'm just trying to think why such a procedure would be necessary in the first place. If one were to connect the end at the HM side first than went over to the ground rod which is not in a hazardous area and connected it there, any spark would be away from something that might ignite.

<added>
I'm not sure why you would need a sub 25 Ohm ground anyway. Thats an arbitrary number someone made up years ago for an entirely different purpose, and I'm not sure it has any relevance to what you are doing, or for that matter to what it was intended for in the first place.

I also wonder if a ground rod does much for you. I guess you are trying to create an equi-potential area to work in thinking that reduces the chance of static creating a spark that might ignite some flammable material. Are typical road surfaces conductive enough that this would even work? If they are, there would be no potential buildup anyway.

I would think that your gear might be your worst enemy, unless it was somehow conductive and could bleed off charges. I did some work in an explosives plant once, and they required people in many areas to wear special shoes that had carbon particles imbedded in them and paint on the floor that also had carbon particles in it to dissipate any static buildup. No polyester clothing allowed, no nylon. All cotton, since it does not generate static charges.

[ February 25, 2005, 05:07 PM: Message edited by: petersonra ]

 

[blumenstein]

Re: Grounding in a flammable atmosphere

Thanks for the reply,

I'm sure that a 10 megohm resister and two lengths of wire would be just fine. Before I go and build a system, I would rather see if there is a commercial product out there; just trying to keep our City Risk Management happy.

Currently we ground to the damaged tank first and the ground rod second. Because a spark as small as 0.25mJ can ignite a hydrocarbon vapor, the potential between the grounding wire (not connected to anything)could yield a spark with enough energy to ignite a hydrocarbon vapor when the wire touches the damaged tank (my assumption here; I don't have any hard science to back it up).

You are correct that grounding a damaged tank in a flammable atmosphere is risky business. We worry about step potential, static charge on clothing, lightning, energized objects (some roadway paints will transmit an electrical charge for miles), and many other ignition sources.

I agree that a sub-25 ohm ground has nothing to do with our application. Unfortunately my State adopted that standard. I am currently in search of another industry that has abandoned the 25 ohm ground standard for a specific application; it will make asking for an exemption to this regulation easier if someone else (who has a degree in electrical engineering) has already broken that ground.

 

[don_resqcapt19]

Re: Grounding in a flammable atmosphere

Larry,
I'm also on a regional hazmat team, but we do not have a specific value for the bonding required to transfer flammable liquids. It is my opinion that a good bond between the two containers is more important than a connection to the earth. Our kit only has a short (4') rod, so it would never be less than 25 ohms. We will use the rod if needed, but are also looking into connecting our bonding conductors to the two tanks and to any available electrical grounding electrode that is in the area. One of the most important things in preventing static ignition would be the use of conductive transfer lines and bonding around the couplings in these transfer lines. The IEEE Green book and other sources state that a bonding connection of 1,000,000 ohms of less will prevent a static spark.
Don