In my last article I wrote about "grounding" and the types of hazards that can be introduced into a hazardous process involving flammable liquids and gases.  A worker moving through air will generate a static charge.  It is our hope that this charge will go to ground as fast as it is generated, thus resulting in  NO ACCUMULATION of a charge.  If the worker does not dissipate the charge to ground fast enough and the charge accumulates on the worker, when that worker reaches out and touches a grounded conductor at a different charge, a spark will jump from the worker to the grounded conductor.  It is this spark jumping from the worker to the grounded conductor that is the IGNITION SOURCE in many flash fires.  If there is a flammable atmosphere present where the spark occurs AND the spark is at least enough to meet the Minimum Ignition Energy (MIE), there will be a flash fire.  It is this spark we MUST control!

Sounds pretty scary - doesn't it!  A worker simply walking across the room can generate MORE THAN ENOUGH ENERGY to ignite almost all Vapor Clouds and Gas Clouds.  In fact, the energies needed to ignite these clouds is SO SMALL that we can NOT even feel the shock from the spark!  As I have said before, the shock we feel when we are shocked by a door knob in the winter time is around 25 millijoules (very generalized statement!).  Most of the flammable vapors and gases need 1 millijoule or LESS (most are <1mj) to ignite; so we could say that the shock we get from a door knob is nearly 25X the energy needed to be an ignition source.  So this "walking" is a SERIOUS HAZARD in Hazardous Locations, especially Class I locations where we are dealing with flammable vapors and gases. Companies spend millions classifying an area as a hazardous location thru buying specialized equipment that are intrinsically safe or explosion proof, yet when it comes a very basic hazard, there is very little understanding and thus very little action to control this hazard.  Although combustible dust(s) need a much larger charge to be an ignition source, they too deserve our respect and control of static electricity!  But I am going to focus solely on vapors and gases in this article. 

As noted in my previous article, we need a path to ground with little resistance to ensure that this static charge can EASILY and EFICENTLY go to "ground" rather than accumulate on the worker.  The first place to start is the flooring in the hazardous location.  This flooring needs to aid in taking the charge to ground; however, companies often do things to the flooring that make it less conductive. 

Let me also point out that EVERYTHING I AM MENTIONING in this article is based on the need for conductive flooring for flammable liquids and gases safety and NOT FOR ELECTRICAL SAFETY!  Having a floor that is TOO CONDUCTIVE can cause risk from electrical equipment, so there is a BALANCE that MUST BE achieved.

Let's not forget other sources of static generation in work areas. 

1) One of the most common sources, other than the worker, is a worker using a cart and that cart has plastic wheels.  As that cart rolls across the floor, it can generate HUGE CHARGES.  Even if the worker pushing it is grounded, most often this is not adequate to prevent the accumulation of a charge on the cart and when another worker touches the cart there is a spark between that contact.

2) how many have seen a forklift operator use plastic wrap or a trash bag over their cab when it is raining and then drive the Electrical Rated forklift into the Hazardous Location.  Sort of defeats the rating of the forklift with all that plastic wrap blowing in the wind!!!  Another issue we find on a lot of our audits is that the inspection sheet for the rated forklift does not include a check to ensure the grounding strap is attached and functioning.  This is a SAFETY CRITICAL device on many rated forklifts as this is how the static is grounded on these forklifts; thus the inspection sheet should include this as part of the shift inspection.

3) removing of shrink wrap or applying shrink wrap in a Hazardous Location is another one we see quite often.  We worked with a client many years ago that got dinged on an insurance inspection because they had a stretch wrap machine within the boundaries of their Class I Div 2 location.  Their solution... move the machine outside the area.  But this caused delays in wrapping pallets, so management bought some manual hand-held stretch wrap rollers.  We came along and wrote them up for the plastic stretch wrap in the area.  When we interviewed workers we asked them if they thought it was odd that the hairs on their arms would stand up when wrapping a pallet and they thought nothing of it.  I could tell at the close out later that week that the management team all the way down to the employees doing the work had some serious doubts about what we were telling them - we can ONLY hope they seek another opinion from a reliable and competent source.

So with all of this going on, where do we start our assessment?  I like to start with a test of the flooring to see what I am up against.  I do this regardless of construction or coverings involved, as we have seen bare concrete and metal grating have too much resistance to ground.  This can be done using a volt meter or as some call them, ohm meters; however, volt meters are NOT the best suited tool for this type of testing.  There are other types of volt meters or ohm meters that are made specifically for this type of testing.

When we measure the floor, we are measuring the resistance the floor presents as the path for the static electricity to go to ground.  Remember, we want the easiest and least resistance path to ground for our static charge.  We measure this resistance in a unit called an "Ohm".  Ohm is defined as the resistance between two points on a conductor.  A puddle of water will have a resistance to ground near ZERO Ohms and a thick high count shag carpet can have a resistance to ground of 1 Billion Ohms.  So there is our range... 0 to 1 billion Ohms - class dismissed!  Just kidding.  Seriously, we need to narrow our scale somewhat, as ZERO resistance (100) is TOO LOW as it can present other hazards should the worker come into contact with an electrical circuit and we do not need this low of resistance for our static charge to safely go to ground.  On the other hand, a resistance of 1 Billion (109) Ohms is WAY TOO MUCH.  So lets whittle this down...

ANSI defines "Static Dissipative" as anything with a resistance between 1 Million (106) Ohm's and 1 Billion (109) Ohms -  I call this the HIGH SIDE.

ANSI Defines "Conductive" as anything with a resistance LESS THAN 1 Million (106) Ohm's -  I call this the LOW SIDE.

This means that 1 Million (106) Ohm's is the half way point and separates the flooring into these two categories:  Static Dissipative and Conductive.

ANSI 20.20 ESD calls for a combined resistance of the floor AND the person (cumulative) to have a resistance LESS THAN 35 Million Ohms.  This is somewhere between 10,000,000 (107) Ohms and 100,000,000 (108) Ohms.  Electrical Safety Standards call for the floor to have a resistance of NO LESS than 25,000 (between 104 and 105) Ohms.  So now we have narrowed our range down to 104 to 108 Ohms.  Throw in some safety margins and we can narrow it down to 105 and 107 Ohms.  Now we have a tight specification we can test for and clearly establish as PASS/FAIL for our flooring.

PLEASE note that our NEW RANGE falls into BOTH categories:  "Conductive" and "Static Dissipative".  For flammable liquids and gases safety we WANT TO BE IN THE CONDUCTIVE RANGE:  100,000 (105) Ohms - 1,000,000 (106) Ohms. 

This is in line with NFPA 77, Recommended Practice on Static Electricity, 2007 Edition, which states:

7.6.2.1 ...Typical resistance to ground for flooring systems should be less than 108 ohms. ...

7.6.2.2  ...Resistance to earth through ESD footwear and conductive flooring should be between 106 ohms and 109 ohms. For materials with very low ignition energies, the resistance to earth through footwear and flooring should be less than 106 ohms...

7.6.2.4 Conductive footwear is footwear designed to have a resistance to ground through the footwear and the floor of less than 106 ohms.

Is your head spinning yet? 

Here is a video that makes this so simple, even I can understand it. In fact, Staticworx® is the leader in static electricity safety.  They have several other EXCELLENT videos that I strongly recommend for anyone learning about static control methods - in fact they have one that shows us how to actually measure for this resistance and the right and wrong devices to use.  You can view this video on their website.

A couple of DISCLAIMERS...

There are some flammable liquid guides and static control standards that may suggest lower or higher resistances to ground.  There is NO ONE ANSWER for this issue and MANY FACTORS have to be considered.  This article is meant to open the door to static control by discussing one source of static generation and how it can be controlled, but there are dozens of other factors, such as environmental factors, that can rapidly change the resistance! 

I do not consider my self an expert in this topic and therefore SAFTENG.net does not conduct testing of floors for proper resistance.  Expert advice should be sought from a well qualified individual, such as Dave Long @ Staticworx®, should you have a concern regarding your flooring.  In this article I am ONLY trying to lay a foundation for safety professionals to perform a hazard analysis of their processing and storage areas so that they can determine if a more thorough analysis and testing is needed. I am in NO WAY affiliated with Staticworx® and only recommend them from my past experiences working with them at my previous employers and having been a fan of their safety materials.

 
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