Bonding & Grounding Controlling Static Electricity

8/12/2019 Bonding & Grounding Controlling Static Electricity

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A few years ago, I investigate an incident

involving a re that resulted from thetransfer of a ammable liquid mixture. A driver of a chemical supplier was deliveringa load of product to a customer. Te productconsisted of a solvent solution containingtoluene and other ammable ingredients. Asthe driver was lling a 350-gallon metal (tote)container from a top nozzle, a re suddenlyerupted, causing burn injuries to the driver.He suffered second- and third-degree burnsto about 20 percent of his body.

Te investigation of this incident determinedthat a spark, resulting from static discharge,ignited the solvent vapors. Tere are two basictechniques to protect against the dangers ofstatic electricity - grounding and bonding. Tese techniques should be strictly followedin areas where ammable and combustibleliquids are stored, dispensed or used. Tis article describes howstatic discharge occurs and how bonding and grounding helpprevent potential res due to electrostatic discharges.

St at ic El ect r icityStatic electricity results from the interaction of dissimilar materials. Tis can occur when materials are rubbed together, such as in theclassic example of walking across a carpet on a dry winter day while wearing woolen socks. However, static discharges can alsodevelop when a liquid passes through a pipe, through an openinginto a tank, and/or when mixing or agitating the mixture. Teliquid is moving different electrons from one to another, and thefriction of electrons creates static electricity.

An electrostatic discharge (also called a static spark) is a dischargeof electricity across a gap between two points not in contact,

2011 J.E. Spear Consulting, LP

resulting from a difference in electrical potential. Te sparkproduced when the electrical charge jumps across the gap usuacontains enough energy to ignite ammable vapors if they are concentrations that will sustain combustion.

Te generation of static electricity cannot be totally eliminatedbecause it is normally present at every interface. However, theare ways to reduce the potential for static charge build-up whetransferring ammable liquids. Te two most important ways toprevent static sparks are bonding and grounding.

BondingBonding is done to eliminate the difference in electrical potentibetween two or more objects. An adequate bond between two more conductive objects will allow the charges to ow freely betwobjects, resulting in no difference in electrical potential. Bondi

BY JEROME E. SPEAR, CSP, CIH

C O N T R O L L I N G S T A T I C E L E C T R I C I T Y

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2011 J.E. Spear Consulting, LP

Gr oundingGrounding an object serves a differentpurpose than bonding. Bondingeliminates the difference in electricalpotential between containers that arebonded together, but it will not eliminatethe potential difference between an

object and the ground. o ensure that astatic charge will not create a spark asa result of this difference, a conductivepath must be provided to the earth. Aproper ground will provide a means forcontinuously discharging a charged,conductive body to the earth.

Grounding may be achieved by attachinga wire conductor between the containerand a water pipe or the full length ofan 8-foot long copper clad steel rodembedded in the ground. otal resistanceto ground should be kept below onemega-ohm. When using a buried rod,resistance is affected by soil moisture. It isimportant that the grounding system bechecked to ensure that there is continuityand proper resistance.

Rate of St at ic Dischar geElectrical charges can build up in ammable liquids whenthe liquids ow through piping systems or when they areagitated in their storage containers as a result of mechanicalmovement or splashing. Te proper bonding and groundingof the system is often enough to control this static build-up.However, if rapid ow rates are used to transfer the liquidinto a storage tank or container, high electrical potentials canoccur on the surface of the liquid in the tank.

Te rate of accumulation of static charge may be much greaterthan the liquids ability to transfer it to the grounded metalstorage vessel. If the accumulated charge in the containerbuilds up enough, a static spark may result when the liquidlevel approaches a body with a different potential. Tis kind

of static situation can be controlled by reducing ow rates,avoiding violent splashing in the tank, and allowing for timeuntil the static charges dissipate. Splash lling can be avoidedby using a ll pipe designed in accordance with NFPA 77.

In ReviewIn review of the incident described earlier, the driver failedto connect the bonding cable to the metal container. Tedriver also created a high rate of static discharge due to theproducts splashing during lling. Tese errors may be due

to lack of training, lack of company procedures, and/or lackof enforcement and management oversight. Te containeralso did not have a proper ground. However, the containerbelonged to the customer and was located at the customerssite. When transferring ammable materials at a multi-employer worksite, some planning and coordination may benecessary to ensure that safe bonding and grounding practicesare followed.

Additional guidance for controlling static electricity may befound in NFPA 77, Recommended Practice for Static Electricity .

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Bonding & Grounding Controlling Static Electricity