Electrostatic Phenomena and Trouble/Accidents
Static electricity measurement and countermeasures
Generally speaking, static electricity is produced due to contact/separation of the surfaces of objects, deformation, ion deposition, radiation, alteration, chemical changes, and similar processes. Static electricity causes electrostatic phenomena such as mechanical action (attraction, repulsion) typified by the coulomb force, electrostatic induction, and electrostatic discharge, and has long been a cause of trouble/accidents. Recently, progress in the area of semiconductor elements has resulted in extreme vulnerability to static, e.g., in cases like the GMR heads of hard disks, and measures for dealing with this have become a pressing need. (See Fig. 1 below.)
Fig. 1: Trouble/accidents due to static, measurement, and countermeasures
Characteristics of static
- Charge movement speed: Extremely slow in the case of an insulator
- Grounding does not work.
- Charge movement speed: Extremely fast in the case of a conductor
- Results in sparks with high energy.
- Current, amount of charge: Comparatively small with an insulator. Potential: Extremely large
- Hard to measure. Induction occurs.
- Current, amount of charge: Both potential and amount of charge are large after induction
- High-energy spark discharge occurs.
- Occurrence of static: Occurs comparatively easy
- Can occur anywhere, so industrial problems occur easily.
- Electric fields due to static: Have mechanical action
- Objects cling, cannot be separated, or float up.
- Effects of humidity: If humidity is high, static occurs less easily and thus is often not a problem
- Often becomes a major problem in the winter.
Measurement of static
To prevent trouble/accidents due to static, it is necessary to first ascertain the static charge situation. Measurement of static is very important for this purpose. Measuring static is not easy and is unlike measurement of ordinary electricity for reasons like the following:
- Static has high voltage and minute current, and thus cannot be measured with ordinary instruments
- Measurement systems disturb and change the original state of the object of measurement
- In many cases, charge is spatially distributed
For these reasons, various measurement techniques specific to static are used. Measurement methods include: measurement of electrification charge for understanding the charge state, measurement of surface potential produced by charge, charge decay measurement for understanding charge leakage characteristics, and resistance measurement as a rough indicator of leakage characteristics.
Measures to counter static trouble/accidents
Electrostatic charge is not the occurrence of static itself. It is a state represented by the difference between the occurrence of static and its leakage (alleviation). Generally speaking, there are many cases where it is difficult to suppress the occurrence of static, and thus methods for quickly achieving leakage or neutralization of any static that has occurred are a key point for preventing charge, and various anti-static devices and materials have been developed. The following are the three basic measures for preventing electrostatic charge:
- Grounding (only in the case of conductors)
- Improving conductivity (humidification, anti-static agents)
- Charge neutralization (effective in all cases)
Grounding is the most basic anti-static measure for metals (conductors). Grounding is effective because the human body also belongs to the category of conductors. The human body in particular suddenly produces static due to friction with clothes, and potential abruptly rises due to changes in electrostatic capacity brought on by the act of walking. To prevent such static charge, it is necessary to wear a grounding wrist strap or electrically conductive shoes. Grounding is basically only useful for conductors, and is ineffective for preventing static charge of insulators (dielectrics). Therefore, the typical approach is to use humidification or an anti-static agent to improve the dielectric constant, or to use a static eliminator such as an ionizer. In the case of an insulator, another measure is to improve conductivity, and it is particularly effective to reduce surface resistance. An easy means of accomplishing this is to increase humidity in the air. The effect appears when relative humidity is maintained at least 50–60%. However, condensation, rust, and mildew can also be expected in a high-humidity environment, and there are other problems such the effect of high humidity on products, the drop in work efficiency due to discomfort of workers, and the appearance of impurities if pure water is not used. Practical methods have also been developed for mixing anti-static agents into the inside of insulators, and for realizing conductivity through surface coating. However, these methods are not always applicable, and in many cases, reliance on charge neutralization cannot be avoided. Charge neutralization using air ions (employing an ionizer) is the most effective measure, and is widely used to eliminate static of insulators. Our ZAPPII product achieves static elimination by delivering ions even to deep-set locations that previously could not be reached with an ionizer. Ionizers are used as an anti-static measure in all industries, and they have a tremendous impact on anti-static measures. There are other methods for dealing with static besides ionizers, but combined use with ionizers is particularly effective. Optimal anti-static measures can be achieved through installation and use of ionizers based on the advice of a static electricity specialist (ESD coordinator). Other methods of countering static are as follows:
- Self-discharge static eliminators (generally called anti-static brushes)
- Installation of industrial humidifiers
- Grounding of the human body (wrist straps, etc.)
- Changing to a conductive floor or adopting conductive shoes
Also, the following are typical measuring instruments for managing anti-static measures. *Our company's measuring instrument products are indicated in parentheses.
- Static measuring instruments (DZ4, DSF601/W)
- Surface resistance measuring instruments (MEG101)
- Shoe resistance measuring instruments (Shoe Tester II)
- Decay measuring instruments (for resins: HONESTMETER, for ionizer evaluation: CPM301, DP)
- Institute of Electrostatics Japan (ed.), Static Electricity Handbook (New Edition), Ohmsha, 1998 (in Japanese).
- Research Institute of Industrial Safety, Ministry of Labour (ed.), Static Electricity Safety Guidelines, RIIS-TR-87-1, Mar. 1988 (in Japanese).
- Research Institute of Industrial Safety, Ministry of Labour (ed.), Structural Standards for Static Electricity Products, RIIS-TR-91-1, July 1991 (in Japanese).