Static Dissipation

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GOZ

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Maryland
I'm not sure if this is the right forum for this question. If not i kindly ask a Mod to redirect this post.

Let's start with a little backround. We currently installed an Engineered ( 10' x 12' ) building to store all of our flammables associated with our laboratory. Inside the building we had two ground points on adjacent walls. We are required to have all drums and containers that occupy the building grounded. Even carried containers that are being filled from hand pumps in drums. To make the grounding easier to access i ran copper buss bar ( attached at the provided ground points ) along the interior walls. From the Ground Buss i have ground straps that clip onto any container.


Now for my question. Management asked me if it is better to ground in series? Ground strap from buss to drum to container being filled. Or in parallel? Ground strap from buss to drum and ground strap from buss to container being filled.
 

don_resqcapt19

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Illinois
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retired electrician
It doesn't matter...a bonding path of one million ohms or less will prevent static build-up. The connection between the two containers when product is being transfered in much more important than a connection to earth.
 

GoldDigger

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It doesn't matter...a bonding path of one million ohms or less will prevent static build-up. The connection between the two containers when product is being transfered in much more important than a connection to earth.
Or, looking at it another way, the path details only matter if you expect to have a lightning strike inside the building while transferring. And in that case you have much bigger problems to worry about. :)
 

ptonsparky

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NE (9.06 miles @5.9 Degrees from Winged Horses)
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Electrical Contractor
Sorry OT. Makes me think of when we were kids and had to transfer gasoline from a drum, to a gas can then to the tank of the car during a thunderstorm. No hoses. I don't remember finishing the chore because when lightning hit a fence post a couple hundred feet away. I left. Dad could walk to work. The post was still smoldering in the morning with a good 2' of it gone.
 
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Rick Christopherson

Senior Member
It doesn't matter...a bonding path of one million ohms or less will prevent static build-up.
This is not necessarily true. It depends on whether the rate of charge is less than or equal to the rate of discharge. If the rate of charge is greater than the rate of discharge, you will get a static buildup. Granted, in this situation, the rate of charge is not likely to be very high. Nevertheless, the blanket statement can't always be made.

The connection between the two containers when product is being transfered in much more important than a connection to earth.
For the most part this is correct. That's because the charge buildup is actually a differential charge between the containers resulting from the transfer, and the net charge is near zero. In other words, the total charge of both containers does not change, but charge is just transferred from container to the other. The bleedoff prevents the differential from getting large.

However, if the net charge buildup for the isolated system of the two containers is non-zero, then you could have a problem. For example, if the pumping system that was used in the transfer were to introduce new charge into the sum of the other two containers, then your differential charge will stay zero, but the net charge of the whole system will be increasing.

The carpet walking example is a good analogy. Your feet would represent one container and your finger tips the other. There is no charge differential between the two, but your feet on the carpet are continually bringing new charge into the system (your body). You don't feel this new charge because there is no charge differential within the system, but when it builds high enough, you will feel the discharge when you touch the light switch.

Or, looking at it another way, the path details only matter if you expect to have a lightning strike inside the building while transferring. And in that case you have much bigger problems to worry about. :)
That is not the only purpose for a grounding system, and I really wish Mike Holt would stop teaching this limited view. It is the easiest explanation to accept, but it is short sighted. There is a lot more going on than just lightning. If it was solely for lightning, then there wouldn't be the need for equipotential grids around swimming pools, and various other grounding requirements that are not lightning related (such as the topic of this thread).


Back to the OP's question:

Now for my question. Management asked me if it is better to ground in series? Ground strap from buss to drum to container being filled. Or in parallel? Ground strap from buss to drum and ground strap from buss to container being filled.
Your question is actually related to failure-mode, in that it assumes minimizing the effects of a failure of the system. If no failure exists, then it does not matter.

So the best answer to your question is to look at the most likely cause for a failure. If your ground straps are permanently connected to your ground bus bar, then that is the least likely point of failure. Even if the straps are not permanently connected to the bus, the consistency of the bus's shape still makes these connections the most stable, as compared to connecting to various irregular containers.

Therefore, your connections at each container are the weakest points of the system, and should be minimized. Regardless which methodology you use, you still have two strap-to-container connections separating the two different containers. However, in the "series connection" scenario, you actually have one additional strap-to-container connection. So you have one additional point of failure than the parallel scenario has.

So all other things being equal, the parallel scenario is better (but see my "edit" below). It also means that one of the two strap-to-container connections between the two containers is more permanent, and is not re-made every time a new container is brought into the system.

The scenario that would give you the lowest failure rate would be to have a single daisy-chained strap, where the middle connection is not dependent on two strap-to-container connections. This would be a single strap that went from the bus bar, to the first container, and then to the portable container. You still have a failure if one of those strap-to-container connections fails, but you have eliminated one of the strap-to-container connections.

Edit: In re-reading your post, my last scenario might have already been what was being asked. So yes, I agree with that part of the question.
 
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GoldDigger

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That is not the only purpose for a grounding system, and I really wish Mike Holt would stop teaching this limited view. It is the easiest explanation to accept, but it is short sighted. There is a lot more going on than just lightning. If it was solely for lightning, then there wouldn't be the need for equipotential grids around swimming pools, and various other grounding requirements that are not lightning related (such as the topic of this thread).

Thanks Rick,
I did not intend to imply that there would not be other differences resulting from a longer or shorter ground path, or the presence or square corners in the wire path or anything else. I only intended to discuss the importance of path with respect to flammable fluid transfers. And even then your comments on the integrity of the path must be considered.

I too would like to see a clearer understanding on all sides of the difference in function and requirements for effectiveness between a grounding system, which includes fault clearing, and an equipotential plane which addresses primarily life safety issues whether a fault is cleared or not. The superficial similarities between the parts and methods of the two systems encourages confusion. :)
 

Rick Christopherson

Senior Member
The superficial similarities between the parts and methods of the two systems encourages confusion. :)
This is the $miillion statement!! :thumbsup: Yes, this is what causes many people on this forum to think one-dimensionally about the purpose of a grounding system. In my opinion, there is far too much emphasis on thinking about a grounding system that is comprised of multiple segments or parts. For example, thinking of it as separate components or even names upstream and downstream of the bonding point.

It is a "system", and it has multiple functions. When you start breaking it up into pieces and giving each piece a different name or different purpose, you quickly loose sight of the rest of the function(s) of the system.
 

GoldDigger

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When you start breaking it up into pieces and giving each piece a different name or different purpose, you quickly loose sight of the rest of the function(s) of the system.

And maybe that viewpoint is encouraged by the annoying complexity of the NEC requirements (made, I think, in an effort to produce a functional system by means of rules rather than design. Which is the mandate of the NEC after all.)
 

don_resqcapt19

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retired electrician
The one milion ohm statement is a general statement from the IEEE Green Book. The NFPA standard for static control calls for a path of 10 ohms or less.
 

don_resqcapt19

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Illinois
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retired electrician
...
That is not the only purpose for a grounding system, and I really wish Mike Holt would stop teaching this limited view. It is the easiest explanation to accept, but it is short sighted. There is a lot more going on than just lightning. If it was solely for lightning, then there wouldn't be the need for equipotential grids around swimming pools, and various other grounding requirements that are not lightning related (such as the topic of this thread).
...
But just like the static control, the connection to earth is not the important part of the pool equipotential bonding grid. We cannot drive the voltage that may be on the electrical system back to that of the earth, so we use a bonding system to raise the voltage of everything with in touch of the pool water to that of the electrical grounding system. There is often a few volts of neutral to earth voltage on the grounding system under normal conditions, and much more under abnormal conditions such as primary or secondary open neutrals and a number of other fault conditions.
 

iceworm

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EE (Field - as little design as possible)
.... We currently installed an Engineered ( 10' x 12' ) building to store all of our flammables associated with our laboratory. Inside the building we had two ground points on adjacent walls. We are required to have all drums and containers that occupy the building grounded. Even carried containers that are being filled from hand pumps in drums. To make the grounding easier to access i ran copper buss bar ( attached at the provided ground points ) along the interior walls. From the Ground Buss i have ground straps that clip onto any container.

Now for my question. Management asked me if it is better to ground in series? Ground strap from buss to drum to container being filled. Or in parallel? Ground strap from buss to drum and ground strap from buss to container being filled.

"The nice thing about standards is that there are so many to choose from...."

It doesn't matter. The standard I would recommend is API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents. Container filling is covered under 4.6.4 Portable Containers, 4.6.5 Nonconductive Equipment and Materials, 4.6.5.2 Nonconductive Portable Containers, and Fig 9, Bonding during Container Filling. API RP 2003 doesn't even mention a connection to earth for filling portable containers. If the management wishes, to ground the drums - great. But it does not appear to reduce risk of ignitions.

Considering the questions your management is asking, I recommend you buy the API spec, so you have it right in front of you when answering.

The keys points are:
1. For a conductive container (4.6.4 and Table 9)
a. Use a conductive nozzle in contact with the container. Hose need not be conductive​
b. If the nozzle is not in contact with the container, a bond wire is required between the drum and container​
c. If the bond between the fill pipe and container is 10^6 ohms or less nor bond required​
.
Non conductive containers are okay as well. There are some restrictions on temperature and fluid conductivity. 4.6.5.2 Nonconductive Portable Containers.
The key points here are (4.6.5.2, Nonconductive Portable Containers):
1. No bond required to the container(what a surprise)
2. The fill pipe needs to be conductive, bonded to the fill system, and always in contact to the liquid (inserted to the bottom of the container)

ice
 

sgunsel

Senior Member
Look at NFPA 77, Recommended Practice for Static Electricity. The maximum recommended resistance to prevent static buildup is 1 million ohms (1 megohm). In practice, this is conservative. You should be aware that bonding two containers together during liquid transfer is important to prevent a differential voltage from occurring between the containers that could result in a spark that could ignite flammable vapors. But failure to provide a connection to earth could result in a charged container (or two) that could spark to a conductive object that is a different potential, high or low. Most importantly, people are conductive objects and if electrically insulated from either earth ground, or from the charged container(s), or both - a spark capable of igniting flammable vapors could occur. This most frequently occurs when a worker is either wearing insulating footwear or is on a non-conducting surface and/or wears nonconductive gloves. Again, the recognized maximum resistance is 1 megohm which is adequate for static electricity control. Lightning is a different matter entirely. Bond all conductive objects, including personnel, and connect to a good earth ground.
 

bward

Member
Location
New York
You should be aware that bonding two containers together during liquid transfer is important to prevent a differential voltage from occurring between the containers that could result in a spark that could ignite flammable vapors.

I prefer to bond my portable container to the supplying container. That way, if the ground path between the supplying container and the grounding system were to fail, the 2 containers would be at the same potential.
 
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