EDITOR ’ S CHOICE AN INVISIBLE HAZARD
MUELLER ELECTRIC
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 .
Mueller Electric grounding assembly bonding the drum to adjacent equipment .
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 discharging tankers with flammable liquids . A negative charge accumulates on the pipe wall , whilst a positive charge is carried away by the liquid . The nonconductive 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 .
MOVEMENT
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 .
STATIC CONTROL
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 nonconductive items ( such as gaskets , hoses ,
38 PECM Issue 54