Static Electricity and grounding in the industrial workplace.
Static electricity refers to the presence of a non-neutral electric charge. Static electricity can be created when two objects of different material come into frictional contact with each other, resulting in the migration of electrons, known as the triboelectric effect.
The difference in charge between the two objects is referred to as the potential difference or voltage. The more negatively charged object will want to disperse its excess electrons to a more positively charged object and thus equalise the charge between the objects.
Static Electricity Hazards in Industrial Environments
Should two objects having a different electrical potential be placed in close enough proximity to each other, and if the difference in voltage is sufficiently high, a discharge of electrons, commonly known as a spark, can occur. This discharge equalizes the potential between the two objects in the same way as if they were connected together by a wire.
Not only can sparks pose a serious risk of fires and explosions where ignition of flammable chemicals, fuels, or other materials is possible, there is also the risk of explosion in environments where there is lot of dust or fine powder, such as flour mills.
Static hazards can be minimised by taking appropriate safety measures to control accumulation of static charges. One of the most important ways to control electrostatic build-up is by properly bonding and grounding equipment and containers.
The majority of electrostatic build-up in industrial environments is the result of operations which involve friction such as:
- Powder or liquids flowing through pipes, hoses, valves, etc.
- Blending or mixing.
- Spraying or coating.
- Filling operations.
- Conveyor systems.
Piping Systems, Filling Operations and Fluid Transit
Static electricity is generated when a low conductivity liquid such as oil or fuel flows in a non-conductive pipe. This is especially dangerous when filling or un-filling tankers with flammable liquids. A negative charge accumulates on the pipe wall, whilst a positive charge is carried away by the liquid. The non-conductive pipe cannot dissipate the electrostatic charge. Should flammable air be present inside the pipe, it can be ignited by a discharge, typically near the end of the fill pipe.
Spraying and Coating Operations
The coatings industry uses electrostatic charges to apply substances such as paint and powder coatings to surfaces. This process is commonly used in the painting of cars and appliances. The spray gun adds a positive charge to the paint or powder, whilst the part to be painted has been grounded. The grounded part attracts the charged particles, resulting in an even coating with minimal waste.
Proper grounding in this application is of utmost importance, as the generated charge can cause sparking and ignition. If the substrate is not effectively grounded, ignition can occur at the spray gun, setting fire to the highly charged jet of particles.
Whenever two materials make contact and separate, an exchange of electrons takes place. This includes when a person is walking or moving. Arms rubbing against clothing and shoes in contact with the floors surface can generate an electrostatic charge which accumulates in the body. This is called ‘walking body voltage’. The second that person touches an object, any potential difference between the persons body and the object will be discharged.
The generation of static electricity can not be stopped, but its accumulation and dispersal can be controlled by correctly designed machinery, pipes, and filtration systems, in addition to utilising appropriate bonding and grounding equipment. To help prevent build-up of static charges in conductive equipment, the resistance path to earth must be minimised.
Bonding and Grounding
Bonding and grounding are effective methods of reducing the accumulation of static electricity and therefore minimising the possibility of electrostatic discharge or ignition. Bonding is the process of connecting two objects together, whilst grounding is the process of connecting an object with the earth.
Bonding connects two or more objects together using wires and connectors, so as to equalise the potential between them. Sparks cannot occur between objects having the same electrical potential.
Grounding (or earthing) is the safest way to dissipate an electrostatic charge. Conductive objects are connected to the earth using wires, clamps and clips.
Good connections are vital in providing an effective conductive path between objects and to earth.
In potentially hazardous or flammable situations, all objects that are conductive but isolated from ground by non-conductive items (such as gaskets, hoses, pipes, probes, spray nozzles, etc), should be bonded. Should an item become isolated from either a ground or bond, it can become charged enough to cause a static spark.
Grounding Clips, Clamps and Assemblies.
The conductivity of items such as drums and tanks can be affected by paints, coatings, build-up of product or corrosion. These coatings can prevent the dispersal of electrostatic charges. The solution is to use grounding assemblies with clips or clamps that piece paints or coatings, to create a good metal to metal connection. The photo below shows one type of Mueller Electric grounding assembly, having a paint piercing clamp on one end and a copper clip on the other end.
Mueller Electric manufacture many different types of clamps, clips and assemblies, which are selected based on the items and materials to be bonded and grounded.
Some important criteria to consider when selecting a bonding / grounding assembly include:
- Does the object being grounded have a paint or other coating that requires piercing?
- In what environment is the assembly to be used; how rugged does the assembly need to be?
- What type and size of clip is required?
- Are the objects being grounded stationary or do they need to move?
- What length of wire is required?
- Should the wire be insulated or non-insulated?
Static has a tendency to accumulate more in dry environments, because any moisture in the air can help disperse the static charge on an object. Increasing the humidity in an industrial environment is not always feasible, but it is an option to consider. It is advisable to maintain a humidity level above 60% though the use of industrial humidifiers.
Antistatic additives to liquids such as fuel, can increase conductivity, helping to reduce electrostatic build-up.
Materials, Flooring and Clothing
Conductive flooring, shoe soles, and special clothing can help dissipate static charges from a person as they walk or move around. The type of container should be a safety consideration when storing and handling flammable materials. Insulated and non-conductive materials increase the risk of static accumulation.
Controlled Filling Speed and Relaxation Time
The higher the flow rate of liquids, the higher the generation of static charges. Once dispensing is completed, it is best to wait before doing anything further to the equipment, such as opening lids, moving objects or cleaning. This relaxation time depends on the type of liquid.
Static electricity is an invisible hazard in many production, manufacturing and industrial environments. To protect workers, the equipment and the facility, proper bonding, grounding and other antistatic measures are of critical importance.
Mueller Electric is an Industry leader in the field of bonding and grounding and offers a wide range clips, clamps and custom grounding and bonding assemblies, to met the majority of industrial applications.