Most everyone who works around flammable liquids knows about bonding and grounding; but, do workers and engineers really UNDERSTAND the principles behind bonding and grounding. In this article, I will try to explain “grounding” in very simple terms in the hopes that more workers can understand exactly what they are trying to achieve when they use bonding and grounding on containers AND themselves! Some items of concern is the safety footwear the workers are using! Do your certified PPE Hazard Assessments specify any SPECIAL conditions for the safety footwear your workers are required to wear? Every heard of “Static Dissipative Footwear” or “Electrical Hazard Footwear”?
A couple of years ago, the SAFTENG team was working with a company as a 3rd party expert when we came across one of the worst set ups for transferring flammable liquids we had seen in our careers. A worker was badly burned doing a flammable liquid (Class IB) transfer that had been done millions of times over a 40 year period WITHOUT INCIDENT. On a cold February morning in the mountains of northern CA, a flash fire consumed the worker and the equipment she was using when she began to transfer around 150 gallons of the Class IB flammable liquid. The following is what we found in our investigation.
Workers assumed that a container sitting on the “ground” was a “grounded container”. There were several factors that worked against this assumption and ultimately proved to be a SERIOUS ENGINEERING error. Over the years the company became more serious about housekeeping and at some point decided to paint the floor so it could be more easily cleaned and present the work area in a more positive light during tours by prospective customers. The epoxy floor covering became an INSULATOR to ground!!! At some point, later on, a worker slipped on the coated surface and the facility put down anti-slip mats to improve traction on the coated floor. These mats acted as another INSULATOR to ground!!!! Some time later, another incident involved a female worker who was injured in an accident, in which it was determined the point of contact was too high for her. The company purchased a three step stool for the workers who needed a small boost to reach a valve and hose connection. The three step stool was plastic, another insulator from ground!!!! It took us all of 15 minutes to determine that the ignition source was the worker herself. Although thought to be grounded, we demonstrated that all three insulators PREVENTED the worker from being grounded and as she walked across the floor she became charged. She was insulated from ground by the FLOOR COATING, RUBBER anti-slip mats, and the PLASTIC step stool. As she reached to grab the pan of toluene, a flash fire consumed the 15-gallon pan, also made of plastic.
Workers need to understand that when they move, they generate a static charge. When they are “grounded”, the charge does not accumulate on them, but rather goes to ground; and hopefully, the resistance to ground is MIMINMAL so that the charge can go to ground as quickly as it is generated. But when workers are INSULATED from the “ground” the charge will accumulate FASTER than it can dissipate to the “ground”. Keeping in mind that most flammable vapors have an EXTREMELY LOW Minimum Ignition Energy (MIE), even the smallest static charge MUST be treated as a SERIOUS HAZARD.
Some workers can even INCREASE their risk by the footwear they use! We routinely find workers using EH rated safety footwear and not knowing what the EH stands for. After all, they got it from the company APPROVED Shoe Vendor or Shoe Mobile on site, so it must be OK!?!?!? The EH stands for ELECTRICAL HAZARD and this type of safety footwear is designed to PREVENT the worker from becoming grounded – a KEY safety feature for electrical workers, BUT a SERIOUS HAZARD for a worker handling flammable liquids or even walking in a Class I Hazardous Location. The worker in the Hazardous Location should be wearing safety footwear with a “SD” rating, NOT an EH rating. (NOTE: how many certified PPE Hazard Assessments go into the actual type of safety footwear the user should be using – your assessments should specify!) Static dissipating footwear is designed to dissipate (reduce) the amount of static electricity that accumulates on the worker. Static dissipating footwear actually conduct static electricity through the linings and insole, cement, and out-sole and into ground. Static dissipating footwear will have “SD” clearly visible on the ASTM label sewn inside the safety footwear (ASTM F2413-05).
There are some ABSOLUTELY CRTICIAL items regarding the PROPER USE of SD footwear that MUST BE MANAGED in a PPE Program and annual training. Workers MUST NOT add any kind of insole to the shoe, except a specially designed Static Dissipating insole. (This would even include the odor control inserts, but the “gel shock absorbing” inserts are the WORST!) If an everyday insole is added to the static dissipating footwear, static electricity will NOT flow through the insole. When the insole becomes non-conductive; the shoe will NOT dissipate static electricity. Also, the floor and the sole of the shoe MUST REMAIN CLEAN and free of dust and other foreign matter to maintain good contact between the sole and the floor (e.g. “ground”) This allows the static electricity to dissipate, or flow through the sole and into ground. (This assumes that floor is “grounded” and Static Electricity will flow through to the floor material)
A general rule of thumb for ALL hazardous locations (Class I Locations) is that ONLY CONDUCTIVE materials are used – NO INSULATORS allowed in the area! We want the static charge to have the easiest path to ground and any INSULATOR that we place between the charge and the ground ONLY INCREASES our risk. Items like floor mats, rugs/carpets, floor coatings, layers of dust and debris on the floor, cardboard taped to the floor, plastics containers, plastic funnels, are ALL INSULATORS and may be just enough to prevent the static charge from going to ground!!!!
One way that a company can help reduce these hazards is to use wrist straps that workers use while working around flammables. These straps work when they are USED, properly installed, and cared for. But they are NOT 100%, as many times we observe workers NOT using them or they are broken or in some cases not installed properly making the resistance to ground too HIGH for them to actually work! Another way some companies manage these risks is by installing a steel plate in front of the vessel where the worker will be handling/standing the flammables. This is meant to ensure that the worker is PROPERLY grounded; however, if the plate is dirty or coated with materials that are insulators or the employee is wearing the wrong type of footwear, the plate is USELESS!
Lastly, when setting up a flammable liquid transfer, ensure that the bonding and grounding is set up properly. The whole idea behind bonding and grounding is that EVERYTHING the flammable liquid comes into contact with is at the SAME CHARGE! The bonding and grounding allows this, as everything will be electrically connected and thus essentially the same electrical circuit. With everything at the SAME CHARGE, there will be NO SPARK JUMPING to something of a DIFFERENT charge. It is this spark jumping we are trying to AVOID! We often see improper set ups where the two containers involved are using DIFFERENT GROUNDS and are NOT bonded to one another. This may sound like a good idea and may even look right to a novice, since “bonding and grounding cables” are being used, but it is WRONG and does little to control the static accumulation during the transfer. I will say that two separately grounded containers are better than NO GROUNDED container, but the key is that we try and EQUALIZE the charges throughout the ENTIRE transfer system by BONDING the two containers involved to each other. Depending on how far apart the containers are and the surface they are sitting on, their “ground” could be considerable different, thus allowing a spark to “jump” and become an ignition source.
Please see my other Flammable Liquid Safety Info Post for more on safely handling flammable liquids or feel free to contact me with questions or concerns.