Associate Professor Neils Jonassen authored a bi-monthly static column that appeared in Compliance Engineering Magazine. The series explored charging, ionization, explosions, and other ESD related topics. The ESD Association, working with In Compliance Magazine is re-publishing this series as the articles offer timeless insight into the field of electrostatics.
Professor Jonassen was a member of the ESD Association from 1983-2006. He received the ESD Association Outstanding Contribution Award in 1989 and authored technical papers, books and technical reports. He is remembered for his contributions to the understanding of Electrostatic control, and in his memory we reprise “Mr. Static”.
~ The ESD Association
Reprinted with permission from: Compliance Engineering Magazine, Mr. Static Column Copyright © UBM Cannon
Several methods enable reduction or negation of the damaging effects of static charges on conductors.
A previous article (“How Fast Does a Charge Decay?” In Compliance Magazine, July 2012) argued that, from a physicist’s point of view, it does not make sense to talk about removing static electricity. It was demonstrated that the rate of charge neutralization, usually called charge decay, depends on the resistivity and permittivity of the materials involved. This article discusses the practical principles used in abating the nuisances and risks of static charge distributions on conductors, as well as the differences between charged conductors and charged insulators.
The first thought that comes to mind in this context is, “Isn’t it possible to avoid static charges altogether (i.e., prevent charges from being separated)?” In situations in which friction between two solid materials is essential for charging, a reduction in the degree of friction reduces the rate of charge separation. In the case of charging by flow of insulative liquids, a reduction in the flow rate also reduces the charging. In addition, because spraying of almost any kind of liquid often results in charge separation, free jets of liquids should be avoided whenever possible (for instance, by keeping the flow rate low when filling containers until the tip of the filling tube is immersed in the liquid). However, these are probably the only examples in which the actual magnitude of the charges separated can be affected.
Nevertheless, there are quite a few remedies and procedures on the market that claim to reduce or remove static electric problems by reducing charging. It is likely, however, that the efficiency of these methods depends on an increased rate of neutralization or recombination of the charges separated, rather than on an actual reduction of the charging rate.
Most static charge removal processes do not involve actual removal of an electric charge from the charged object. The exception is charged conductors. If a negatively charged metallic conductor is connected to ground by another metallic conductor, the excess charge (electrons) may flow to ground through the metallic connection. In all other situations, the neutralization consists of oppositely charged carriers, either ions or electrons, being drawn to the excess charge. The field from the neutralizing charge superimposes the original field, and the resulting reduced field is then interpreted as a reduction or removal of the charge.
The basic rule for fighting the unwanted effects of static electric charges is to ground all conductors that might become charged or exposed to induction from other charged objects. Ungrounded charged conductors can produce discharges ranging from weak current pulses that may harm only the most sensitive electronic components to energetic sparks that may cause explosions and fires. Direct (i.e., very low impedance) grounding is rarely necessary.
Charging of Persons
The charging of persons walking on an insulative floor covering was treated in detail in “Charging by Walking” (CE, March/April 2001). It was demonstrated that the theoretical maximum charge separated in one step, Δqmax, is about 4•10–7 C, leading to a mean value of the maximum charging current, im, of about 1 µA. Experimental results, however, suggest that more reasonable values would be qmax » 3•10–8 C and im » 6•10–8 A.
If a person has a total decay resistance (from floor and shoes) of R, the person’s maximum voltage can be written as
Vm = im • R (1)
Equation 1 assumes uniform charge separation. However, the charge Δq is separated in the time it takes to lift the foot from the floor. During this time, Δt, the body voltage grows to a value ΔV that can be written as
In Figure 1, Vm and ΔV are plotted as a function of R for im = 6•10–8 A, Δt = 0.1 second, and C (one foot) = 100 pF. It therefore appears that at a decay resistance of 1 GΩ the mean maximum voltage Vm may be as high as 60 V, while the one-step voltage ΔV is about 180 V.
The constant value of ΔV (see Figure 1) for R > (approximately) 1010 Ω is a case of bulk charging, a process in which a capacitive system receives a charge Δq in a time that is short compared to the time constant RC of the system. In addition to walking, examples of bulk charging of people include rising from a chair with an insulative cover and sliding across a car seat.
Figure 1: Maximum voltage, Vm, and one-step voltage, ΔV, as a function of the decay resistance R.
Bulk charging results in a sudden rise ΔV in the voltage of the system, given by
∆V = ∆q (3)
It follows from equation 2 and Figure 1 that ΔV decreases with the decay resistance R for given values of charging time Δt and capacitance C. In many cases and in many industries, ΔV and Vm can be kept at sufficiently low levels with sufficient reliability by choosing floor coverings and footwear to yield decay resistances in the range of 10–100 MΩ.
In the electronics industry, grounding through footwear and a floor covering may prove inadequate. This is especially true when dealing with MOSFETs and similarly sensitive components in which a current pulse from a person charged to, say, 100 V can be destructive.
Although the idea of keeping a person at zero voltage by tying him physically to a ground point with a conductive wire may seem odd and impractical, this is nevertheless an accepted procedure in many areas of the electronics industry. The gadget employed for this purpose is a wrist strap, which consists of a band or chain, similar to an expandable watchband and made of metal and conductive plastic or conductive fibers, and a strap that connects the band to ground. The strap is made of either solid conductive plastic or multistrand wire. Normally, the strap includes a series safety resistor of 1 MΩ for minimizing the shock from accidentally touching a live wire while being tied to ground via the strap. For the normal household peak voltage of 160–170 V, the maximum current through the person would be less than 0.2 µA, well below fatal values.
Although wrist straps appear to be simple devices, their use involves a series of problems to be considered, including intermittent skin contact with loose-fitting bands, bad skin contact caused by excessively dry skin or too much body hair, and sloughing of the band material resulting in contamination of electronic components. In addition, the strap should be grounded carefully to a separate ground terminal; the grounding should not be left to a chance connection through an alligator clip hooked onto a potentially suitable point.
Other Movable Conductors
While the grounding of stationary conductors like machinery and metallic tube systems is a straightforward and usually simple problem, movable conductors like trolleys and chairs constitute the same type of scenario as do mobile persons. The solution is also similar. Just as people are kept at a safe low voltage by a combination of footwear and floor covering, so are movable conductors kept at a safe low voltage by the use of conductive wheels.
There are two distinct differences in the electrostatic behavior of conductors and insulators. The first difference is that a charged conductor can dissipate all the energy stored in its field in a single discharge or current pulse, neutralizing its entire charge. The second is that a charged conductor needs only to be connected to ground from a single point of its surface through a suitably conductive path to have its charge eventually neutralized.
A discharge from a charged insulator, on the other hand, neutralizes only part of the charge and hence dissipates only part of the energy stored in the field. Furthermore, charges on an insulator cannot be removed by connecting the surface of an insulator to ground.
Figure 2a shows a charged plane insulator A with a charge of 10–7 C. In Figure 2b, the insulator is brought in contact with a grounded conductor B. When conductor B is removed in Figure 2c, insulator A turns out to have retained most of its charge. If the charge in Figure 2c is less than that of Figure 2a, it is because the approach of the grounded conductor B may have caused ionization between B and A. (See “Charges are Forever,” In Compliance Magazine, February 2012.)
Figure 2: Results of charged plane insulator A being brought in contact with grounded conductor B.
This article, the first of two in a series, discussed different techniques for abating the damaging effects of static electricity on conductors. The next article will discuss several methods used with insulators, along with the pros and cons of the individual methods.
|Niels Jonassen, MSc, DSc
worked for 40 years at the Technical University of Denmark, where he conducted classes in electromagnetism, static and atmospheric electricity, airborne radioactivity, and indoor climate. After retiring, he divided his time among the laboratory, his home, and Thailand, writing on static electricity topics and pursuing cooking classes. Mr. Jonassen passed away in 2006.