GROUNDING AND BONDING
Recognition and Protection from Ignition Hazards
Lightning, Static Electricity and Stray Current
By Tony Rieck
T.R. Consulting, Inc.
Site workers may be exposed to flammable and combustible liquids contained in drums, tanks, piping and other containers. When these containers leak or these materials are released into the environment due to a spill, the potential for fire and explosion hazards is real. Potential ignition by lightning, static electricity and stray currents must be considered and proper grounding and bonding techniques must be employed.
This article will cover:
It is important to note that bonding and grounding are not the same. Bonding is the connection of two or more conductive objects to one another by means of a conductor such as a wire. Grounding, also referred to as “earthing”, is a specific form of bonding wherein one or more conductive objects are connected to the ground by means of a conductor such as a wire or rod. Thus, proper “grounding” of objects (conductors) in the field will normally incorporate both bonds between objects and a specific bond to the earth (ground).
For example, when applying paint with an acetone carrier (Class IB Flammable Liquid) to a steel plate using a spray application system, proper grounding and bonding are required. The paint passing through the nozzle of the spray gun will generate a static charge and the acetone will cause flammable vapors to evolve into the area where the painting is being conducted. Thus, the spray gun must be bonded to the metal plate and the metal plate must be bonded to the ground (grounded) in order to prevent potential ignition of the flammable vapors generated by the acetone in the paint.
Not all bonding is accomplished by use of wires. The human body can accumulate a static charge of significant energy. When the work entails working within a large steel structure, such as a tank or silo, if that structure is properly bonded to the ground (grounded), the use of static dissipating boots may be sufficient to bond the worker to the grounded surface. Static dissipating boots are boots manufactured from an elastomeric material (a material that when stretched returns to its approximate original shape) that is a conductor of electrical current. Thus, the electrical charge flows from the wearer, through the boots and onto the grounded surface instead of accumulating. This differs from dielectric elastomeric materials used in the manufacture of many boots that act as insulators. Boots manufactured from dielectric materials will insulate the wearer from the ground and cause a static charge to accumulate until the wearer is close enough to a conductor that the charge can arc to the other conductor. This method of static protection is used in oxygen manufacturing plants, for example. In these plants, oxygen can become trapped in the clothing of a worker, creating a flammability hazard. The simple act of removing the clothing can cause a static discharge. To eliminate this potential hazard, the use of conductive flooring and conductive (static dissipating) footwear has long been practiced within these facilities.
Additionally, humidification and ionization are methods of preventing static energy accumulation.
Electricity can move freely over the surface of a conductor such as copper, but can flow through or over the surface of non-conductors (insulators) only with great difficulty or not at all. Examples of insulators are air, rubber and glass. When electricity is present on the surface of an insulator and it is prevented from flowing onto another surface (isolated by another non-conductor) or when electricity is present on a conductor that is in contact only with non-conducting materials, it is called static electricity and that material or body is “charged”. This charge can be either negative or positive.
All materials are comprised of atoms. These atoms are composed of electrons, protons and neutrons. The electron is a negatively charged particle and the proton is a positively charged particle (neutrons have no charge). In a neutral or unchanged material, electrons are present in equal numbers and their charges cancel one another. When an event separates some of the electrons from one surface and they are transferred to another surface, the surface from which the electrons were removed has a positive charge and the surface to which the electrons transferred is negatively charged. An excess or deficiency of as few as 10 electrons for every million atoms will cause a surface to be considered very strongly charged.
Perhaps you remember this phrase from school, “For every force, there is an equal and opposite force.” This is exactly true for static charges as well. Static electricity is not generated it is made free. Somewhere, there is an exact opposite charge. Like charges will repel one another like the like poles of a magnet and opposite charges will attract one another.
When two conductors are isolated from each other by a dielectric union or dielectric bushing, the ability of the conductor to accumulate a charge is increased. Unlike a spark from an insulating material that will only release a charge from a small area, all of the static energy accumulated on a conducting material can release in a single spark. Thus, releases of static charges from conducting materials can be significant as the ability of a spark to produce ignition is largely governed by its energy, which will be some fraction of the total energy that is stored.
In order for the release of static electricity to pose an ignition hazard, four elements need to be present:
Some mechanisms that can effectively generate static electricity include:
An example of effective isolation would be an above ground storage tank set on a concrete pad. The concrete pad insulates the tank from the ground allowing the tank to accumulate static charges while being ventilated or filled.
Adequate ignition energy simply means that the spark that is produced has to provide enough energy to ignite the fuel in the atmosphere. For example, a match may have enough energy to ignite small twigs, but the ignition energy from a single match will be insufficient to start a large log on fire. Acetylene and hydrogen require very little energy to ignite. The necessary ignition energy fro propane or methane is somewhat higher. Combustible dusts and ignitable fibers require significantly more energy to ignite than do vapors and gases.
The spark must also occur in an atmosphere that is within the flammable range of the fuel present and that contains enough oxygen to support combustion. For example, the flammable range for gasoline is generally accepted to be between 1.4% and 7.8% in air. When oxygen levels are less than 10% (normal oxygen in air is about 20.9%), even gasoline vapor concentrations within the above flammable range cannot be ignited. Therefore, in an atmosphere containing gasoline vapors, the electrical spark can only result in atmospheres containing more than 10% oxygen and where the concentration of gasoline vapors in the atmosphere is between 1.4% and 7.8% (not taking into account oxygen enriched atmospheres where smaller or greater gasoline vapor concentrations may be ignited).
When all of the above conditions are met, and the differential in electrical energy becomes great enough or an electrically charged body is brought close to a grounded or oppositely charged conductor (creating a spark gap), an arcing occurs and the (once) static charge becomes a source of ignition.
There are two phenomena associated with electrical storms that can act as potential ignition sources of flammable atmospheres and of vessels storing flammable and combustible liquids. The first is called direct-stroke lightning. Objects in the path of direct-stroke lightning can be severely damaged due to heat, mechanical forces and direct ignition of flammable and combustible materials. The second is indirect lightning currents. During an electrical storm, as heavily charged clouds move slowly over an area, an opposite charge is induced on the surface of the earth and other objects. When an insulated conductor, such as a tank on an insulating pad, becomes charged through this induction process and lightning strikes nearby, this trapped charge can be suddenly released in a discharge to the ground, creating a potential ignition hazard.
Response personnel should never be on site during electrical storms. Several points about lightning and lightning protection are, however, important. A tank which is properly protected from damage due to lightning will be more than adequately protected against hazards due to the formation of static electricity due to site operations. However, especially with older facilities, it is advisable to ensure the viability of the protection. Protection of metallic tanks and other metallic structures that are insulated from the ground is usually accomplished by bonding and grounding. Protection of structures made of insulating materials is usually accomplished by installation of lightning rods, overhead wires or conducting masts.
Stray current is simply electrical current traveling in places that it is not intended to be. Some of the most common sources of stray currents emanate from cathodic protection systems, railroad lines, faults in power circuits and welding operations. Stray current is especially problematic because, when current is traveling in places that it is not intended, the presence of an electrical current is unexpected and easily overlooked. An assessment of the site should be made by a qualified individual to assure that potential stray current sources are identified and that any necessary steps to eliminate the potential for stray current to act as an ignition source.
Two common sources of stray current activity are especially common to sites where chemicals are stored and transported (common sites for response activities). Where chemicals are stored in metal tanks (especially underground) or where they are transported through pipelines, cathodic protection systems are often encountered. Additionally, at a railroad tank car loading or unloading terminal served by a pipeline, the pipeline may be at a different potential from the rails due either to stray current in the rails or in the pipeline.
In the case of the pipeline terminal, prior to breaking tank car connections, it is important to assure that at least one rail is bonded to the pipeline to protect against stray current arcs. When the tank car loading rack is located on a spur line, the spur line should be assessed for the presence of insulating joints and to ensure that rail equipment does not bridge the insulating joints (providing a bond to a potentially energized rail).
Where cathodic protection systems are in place, it is advisable to consult with someone familiar with the power source, underground cables and other components of the cathodic protection system prior to the onset of work activities or excavation in the area. It is especially important to note that turning off the power to an impressed current cathodic protection system will not immediately eliminate the potential for ignition. In impressed current cathodic protection systems, the currents will persist for some time following deenergizing of the system due to the polarization effects on the buried metal structures and piping.