ElectroStatic Discharge The mystery and how to live with it By Ray Richter Static Electricity Lightning is probably the most recognizable effect of static electricity. Generating millions and millions of voltage and we all know the damage lightning can do to property and people. Another recognizable effect of static electricity is the shock you receive when you slid out of a car in dry weather conditions and feel a zap. The human body feels a shock when the voltage is higher than about 3,500 volts. Static Electricity Here's another problem with the usual "static electricity" concept. First, think about everyday matter. Molecules have protons and electrons, unbalanced electron shells always try to balance. "STATIC ELECTRICITY" IS CAUSED BY FRICTION? WRONG. "Static" electricity appears whenever two dissimilar insulating materials are placed into intimate contact and then separated again. All that's required is the touching. Chemical bonds are formed when the surfaces touch, and if the atoms in one surface tend to hold electrons more tightly, that surface will tend to steal charged particles from the other surface immediately as they touch. "STATIC ELECTRICITY" IS CAUSED BY FRICTION? WRONG. One surface now has more electrons than protons, while the other has more protons than electrons. When the surfaces are later separated, the regions of opposite charge-imbalance also get separated. For example, when adhesive tape is placed on an insulating surface and then peeled off, both the tape and the surface will become electrified. No friction was required. Static Electricity (cont) Walking over a carpet can generate 35,000 volts. The Electrostatic Discharge (ESD) from this voltage can cause pain. The discharge is not life threatening but it still hurts. Static Electricity (cont) The static electricity that concerns the electronics industry is the discharge that you can neither feel nor see. The static electricity that concerns the Petroleum industry is the discharge that you may not feel but may see. Difference is the voltage and POWER (watts) Spark Gap mm gap 0.5 1 2 3 4 5 Min volts 2850 4350 7350 10350 13350 16350 Any voltage above the minimum is converted into current. This where the potential power comes in. Static Electricity Clothing Their measurements for different clothing and various car-seats give impressively high voltages, and this occurred at humidity levels above 50%. The voltages should be MUCH higher at, say 5% R.H.! Nylon clothes: 21,000 volts (Ouch!) Wool clothes: 9,000 volts Cotton clothes: 7,000 volts Static Electricity and Daily movements Table 1 TYPICAL ELECTROSTATIC VOLTAGES* RELATIVE HUMIDITY EVENT 10% 40% 55% Walking across carpet 35,000 15,000 7,500 Walking across vinyl floor 12,000 5,000 3,000 Motions of bench worker 6,000 800 400 Remove Material from plastic tubes 2,000 700 400 Remove Material from vinyl trays 11,500 4,000 2,000 Remove Material from Styrofoam 14,500 5,000 3,500 Remove bubble pack from Material 26,000 20,000 7,000 Pack Material in foam-lined box 21,000 11,000 5,500 Triboelectric Series Acquires Human Hands a more positive charge + Asbestos Rabbit Fur Acetate Glass Mica Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel Wood Amber Sealing Wax Hard Rubber MYLAR Nickel Copper Silver UV Resist Brass, SS Gold, Platinum Sulfur Acetate, Rayon Celluloid Polyester Styrene (Styrofoam) Orlon Acrylic SARAN Polyurethane Polyethylene Polypropylene PVC (Vinyl) KEL F Silicon Teflon Silicone Rubber Acquires a more negative charge - "STATIC ELECTRICITY" IS ELECTRICITY WHICH IS STATIC? NO! Instead, 'static electricity' is a collection of different electrical phenomena; phenomena where... The amounts of positive and negative electric charge within a material are not perfectly equal. Where voltage is high and current is low. "STATIC ELECTRICITY" IS ELECTRICITY WHICH IS STATIC? NO! Where electrical forces (attraction and repulsion) are seen to reach across space. Widely spaced objects may attract or repel each other. Hair might stand on end! Where electric fields (as opposed to magnetic fields) become very important. (Electric fields are also called "electrostatic fields" or "e-fields." Static Electricity Electrostatics is about "charge," and about the attract/repel forces which electric charge creates. The motion or "staticness" of the charge is irrelevant. After all, the forces are still there even when the charges start flowing. And charges which are separated or imbalanced can sometimes flow along, yet the "static" effects are undiminished when the current begins. In other words, it's perfectly possible to create flows of so-called "static" electricity. Quick Note of Interest The workers on high tower power lines are more at risk from the static buildup on the wires than from the power in the lines. The static buildup (from wind blowing across the wires) can cause burns and falls even when the power in the lines is off. ESD DISCHARGE Controlling your Discharges The control of electrostatic discharge is an important aspect in the manufacturing, packaging and Shipping of products. The primary method of control is to ground or use static-bonding (bring to the same potential) all conductors that come in contact or near proximity to the product ESD DISCHARGE Controlling your Discharges These conductors include humans, tools, ESD mats, other electronic devices, boards, connectors, packaging, etc. There are other components to a good ESD Control program including, removal of unnecessary insulators, adding shielding, ionization, environmental controls, provide training, education and top down compliance. ESD DISCHARGE Take time to Discharge Movement times (averaged) from typical operations. Reaching Grabbing Lifting Relocate Landing Time (ms) 455 153 231 924 247 Std. Dev. (ms) 48 11 61 137 73 The table above shows how long it takes someone to discharge their static buildup. The times is from the start of the movement to the end of the movement. Grounding and/or Bonding Grounding: Applying a grounding wire to a container to eliminate a difference in static charge, grounding changes the potential between an object and ground. Bonding: Use of a wire between two drums to eliminate the static charge potential between objects. Safe Handling of Flammable and Combustible Liquids Explosive Limits Explosive limits involve the concentration level of the given chemical in the air. In popular jargon, a vapor/air mixture below the flammable limit is too lean to burn or explode. A mixture above the upper flammable limits is too rich to burn Most flammable liquids are volatile, meaning they evaporate quickly, and can reach a concentration in air that could lead to an explosion. Flash Point The flashpoint of a fuel is the lowest temperature at which it can form an ignitable mix with air. At this temperature the vapour may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is defined at which the vapour continues to burn after being ignited. Neither of these parameters is related to the temperatures of the ignition source or of the burning fuel, which are much higher. Flammable and Combustible Liquids Flammable Liquids Combustible Liquids Any liquid having a flashpoint below 100o F (37.8o C) or lower. Any liquid having a flashpoint at or above 100o F (37.8o C). NFPA Classes IA, IB, & IC NFPA Classes II & IIIA &B Flammable and Combustible Liquid Classifications Flammable Liquid - any liquid having a flashpoint below 100o F (37.8o C) or higher, the total of which make up 99 percent or more of the total volume of the mixture. Flammable liquids shall be known as Class I liquids. Class I liquids are divided into three classes as follows: Class IA - shall include liquids having flashpoints below 73 F (22.8 C) and having a boiling point below 100 F (37.8 C). Class IB - shall include liquids having flashpoints below 73 F (22.8 C) And having a boiling point at or above 100 F (37.8 C). Class IC - shall include liquids having flashpoints at or above 73 F (22.8 C) and below 100 F (37.8 C). Flammable and Combustible Liquid Classifications (Cont.) Combustible Liquid - any liquid having a flashpoint at or above 100o F (37.8o C). Combustible liquids shall be divided into two classes as follows: Class II - shall include those with flashpoints at or above 100 F (37.8 C) and below 140 F (60 C), except any mixture having components with flashpoints of 200 F (93.3 C) or higher, the volume of which makes up 99 percent or more of the total volume of the mixture. Class III - shall include those with flashpoints at or above 140 F (60 C). Flammable Liquids Flashpoint Temperatures Triethalemine 17o F Enamel Reducer 20o F Barsol A-2316 54o F Isopropyl Alcohol 45oF Acetone Solvent 0o F Methanol 45o F Barsol A-2343 56o F MEK 23o F Barsol A-3447 0oF Xylene 79o F N-Butanol Toluene 45o F 36o F Sec. Butanol 72o F Glycol Ether 94o F Gasoline -50o F 142 Solvent 66/3 610 F Refueling Incidents movie Here is the breakdown of the 20 accidents involving portable containers for which we have first-hand reports: 13 accidents occurred while fueling metal containers either in the plasticlined bed of a pickup truck (12) or the carpeted bed of a pickup truck (1). o 4 accidents occurred while filling plastic containers on the plastic-lined bed of a pickup truck. 1 accident occurred while filling a plastic container that was formerly in the back of the plastic-lined bed of a pickup truck. Continued from prev. page 1 accident occurred while filling a plastic pail on the ground. 1 accident occurred while filling a metal container on the carpeted floor of an automobile that had been driven with the windows open. Can flammable or combustible liquids be hazardous to my body? The most obvious harm would be the danger of a fire or explosion. Flammable and combustible liquids can also cause health problems depending on the specific material and route of exposure (breathing the vapour/mist, eye or skin contact, or swallowing). The Material Safety Data Sheet and the supplier's labels on the containers should tell you about all the hazards for the flammable and combustible liquids that you work with. Prevention Transporting and proper material handling procedures are typically the weakest link in an ESD program. Many companies drop their guard when it comes to correct packing, handling, and storage techniques. Adequate protection requires attention to the details. Prevention (cont) Function of packaging, handling, and storage products is to limit possible impact of ESD from: 1. triboelectric charge generation • (Use Grounding and/or Bonding) 2. direct discharge • (Use Discharge stations, Prevent vapours) 3. electrostatic field • (Control humidity, use ION generators) Prevention (cont) Putting ideals into Practice 1. Packaging/Transfer Areas • Remove all non-ESD packaging from area: Standard bubble wraps, packing peanuts, plastic bags, Styrofoam. • Purchasing should insist that all ESDS items be packaged in a shielded package. Faraday cages • Repackage if necessary • • • • Use of ESD labels / tape Heel grounders / ESD floor in transfer areas Wrist straps for static bonding Static Bonding clamps and grounding a must do Prevention (cont) Putting ideals into Practice 2. Storage • Grounded shelving areas with conductive paint. • Grounding with Ganged Bonding on metal drums. • Keep areas clean, reduce dust build up. Metal shielding above stored containers for dust control. ESD Protection in the Plant a. Awareness - Training / education - Signage b. Top down - participation by all levels operators/supervisors/management - Must have participation from all levels. Everyone must understand cost savings and recognize ESD as QC issue. - documentation of the reality of ESD