Static Electricity

Static Electricity
What is Static Electricity?
What Are Some Sources of Static Electricity?
What Are the Hazards of Static Electricity?
How Can Static Electricity Be Controlled?
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What is static electricity? What causes it? How is it hazardous? And, most importantly, how can it be controlled? All of these questions will be answered in this health and safety guideline.
What is Static Electricity?
At its simplest, static electricity is an electrical charge that cannot move. It is created when two objects or materials that have been in contact with each other are separated. When in contact, the surface electrical charges of the objects try to balance each other. This happens by the free flow of electrons (negatively charged particles) from one object to the other. When the objects separate, they are left with either an excess or a shortage of electrons. This causes both objects to become electrically charged.
If these charges don't have a path to the ground, they are unable to move and become "static". If static electricity is not rapidly eliminated, the charge will build up. It will eventually develop enough energy to jump as a spark to some nearby grounded or less highly charged object in an attempt to balance the charge. A good example of this in everyday life is lightning. Lightning is produced by a discharge of electricity from one cloud across an air gap to another cloud or between a cloud and the earth.

What Are Some Sources of Static Electricity?

Static electricity is commonly produced when:
• liquid flows through a pipe or hose, or through an opening in a pipe or hose
• spraying or coating
• blending or mixing
• filling tanks, drums, cans or pails
• dry powdered material passes through chutes or pneumatic conveyors
• non-conductive conveyor belts or drive belts are moving

Figure 1: Common Sources of Static Electricity

People can also accumulate static charges generated by clothing or footwear. This is most likely to happen in dry atmospheres, such as heated buildings in winter.
What Are the Hazards of Static Electricity?
The main hazard of static electricity is the creation of sparks in an explosive or flammable atmosphere. These sparks can set off an explosion or fire. The danger is greatest when flammable liquids are being poured or transferred.
For static electricity to be a hazard, four conditions must be met:
1. There must be a means for a static charge to develop.
2. Enough energy must build up to cause ignition.
3. There must be a discharge of this energy (a spark)
4. The spark must occur in an ignitable vapour or dust mixture.
How Can Static Electricity Be Controlled?
Most static electricity control measures provide ways for the static charges to dissipate harmlessly before sparks occur.
Some ways to prevent static charges from accumulating on materials are:
• bonding and grounding
• humidification
• ionization of air
• static collectors
• additives
Bonding and grounding
Bonding and grounding are common controls for static electricity. Bonding is connecting two or more conductive objects with a conductor, such as a copper wire, that equalizes the potential charge between them. (See Figure 2.) Bonding is also connecting various parts of equipment and containers that are electrically separated by, for example, gaskets or caulking compounds. Note that bonding does not eliminate the static charge.
Figure 2: Bonding Two Conductive Objects

Grounding is connecting one or more conductive objects directly to the earth using ground rods, cold water copper pipes, or building steel. Unlike bonding, grounding drains the static charges away as quickly as they are produced.
Static grounds must not be made to:
• electrical conduit systems
• plastic pipes
• gas or steam pipes
• dry pipe sprinkler systems
• lightning rods
Connectors for bonding and grounding, such as copper wire and clamps, must provide a good conductive path. To ensure this:
• remove all dirt, paint, rust, or corrosion from areas where connections are to be made
• use connectors that are strong enough for the job
• use flexible connectors where there is vibration or continuous movement
• connect metal to metal
Figure 3 shows some typical examples of connectors. See Table 1 for minimum specifications for bonding and grounding equipment. For more information, consult your safety supplier.
Figure 3: Some Examples of Connectors


Table 1: Minimum Specifications for Bonding and Grounding Equipment
Bonding Wires 4.1 mm (#6 AWG) copper wire
Bonding Connectors 12.7 mm (1/2") copper bonding straps, solder lugs, pressure connectors or bolts and star washer
Bonding Conductors 19 mm (3/4") copper braid with pressure-type ground clamps
Grounding Wires 5.2 mm (#4 AWG) copper wire
Ground Connectors At the equipment: approved connectors securely bolted to the machine. Solder lugs are acceptable, but pressure connectors are preferable At the earth; an approved ground clamp attached to a ground electrode
(Source: Ontario Ministry of Labour, Engineering Data Sheet 10-0 "Static Electricity," June 1987)
When dispensing flammable liquids, both bonding and grounding are required. Ensure that the receiving container is bonded to the dispensing container before pouring the liquid, and that the dispensing container is grounded (see Figures 4 and 5).
Figure 4: Bonding and Grounding while Dispensing

Bonding and grounding are not necessary when plastic (high density polyethylene) containers are used. This is because high density polyethylene is non-conductive. However, all metal parts on the container must be bonded to the fill pipe or the container being emptied.
Bonding or grounding will not eliminate the surface charge on vessels holding flammable liquids. To prevent static in these cases, make sure that the nozzle is touching the bottom of the vessel so that the liquid discharges horizontally. Also, lower the rate of flow. These two measures will prevent the free fall and turbulence that generate static.
Figure 5: Bonding and Grounding while Top Loading a Tank Truck

Humidification
A relative humidity of 60% to 70% at 21°C (70°F) may prevent paper or layers of cloth and fibers from sticking together. A high relative humidity, however, is no guarantee against the accumulation of static electricity. Therefore, don't rely solely on humidification as a control measure in areas where there are flammable liquids, gases, or dusts.
Ionization of air
Ionization is where a neutral atom or charge loses or gains electrons. Ionization can be used to control the accumulation of static electricity on non-conductors. The air between the surface of an object that has the potential to accumulate static and a nearby grounded object is ionized. Since the ionized air is conductive, the accumulated charge conducts through the ionized air space to the grounded object.
Devices such as high voltage AC neutralizers, radioactive neutralizers, or open flame neutralizers can be used. Open flame and high voltage AC neutralizers are restricted to operations using low volatility materials. These are slowly evaporating materials, such as inks, that have high flash points (above 37.8°C or 100°F).
Static collectors
Devices that collect static electricity can be used on moving belts, plastic film, and similar non-conductive materials. Some examples of static collectors include: needle pointed copper combs; spring copper brushes; and metallic tinsel bars.
A static collector works by its closeness to the source that generates the static electricity. If a discharge occurs, it is captured by the highly conductive collector; this prevents long hot sparks. To be effective, collectors must be properly grounded.
Additives
Another control is the use of anti-static additives (as in fuels). The additive increases the conductivity or lowers the resistance of the liquid. It also reduces the time it takes for the static charge to leak through the wall of the container and to the ground.
Controlling static electricity on people
Controls to prevent or reduce static from building up on people include:
• conductive flooring
• conductive clothing and footwear (to allow the charge to be conducted away; these items must be free of dirt and other contaminants)
• cotton or linen clothing instead of wool, silk, or synthetic materials