If the resistance is increased 100 GΩ, not only is the voltage generated increased to 10 kV, but on cessation of the current, the voltage will take 10 seconds to fall to 37% of its initial value. The presence of this voltage for such a long time could lead to the person experiencing shocks on touching something or discharging to cause some problem.
In ESD control, a different definition of charge decay time is usually used in standard measurements, and often the time for charge to reduce to one‐tenth of its initial value is measured (Figure 2.3). This value is theoretically equal to 2.3τ.
In practice, the charge decay time is often measured from the starting voltage down to a certain threshold voltage, e.g. 100 V. Polymers may have time constants of many tens or hundreds of seconds, or even days under clean dry conditions.
In practice, the simple model does not always correspond well with material behavior. Measured charge decay curve may depart considerably from the ideal exponential, and the measured time “constant” varies with measurement conditions. Often with high resistance materials the decay time lengthens as the surface voltage drops and may become very long at low voltages.
2.3.4 Conductors and Insulators Revisited
In many engineering fields, conductors are often thought of as materials such as copper or aluminum that have very low resistance or resistivity (see Section 1.7), much less than 1 Ω. In ESD control, materials that have a much higher resistivity than this may be thought of as conductors. In practical electrostatic control, materials and equipment are often defined as conductors or insulators based on either a measured resistance or a charge decay time, or both. The model of Figure 2.1 can be used to explain this.
As charge generation rate (current I) in static electricity is often low, even a relatively high value of leakage resistance R (Figure 2.1) may pass the current to give low voltage, V = IR. In ESD control, a resistance of 1 MΩ (106 Ω) could be considered quite conductive and would reduce the electrostatic voltage in the previous example to 1 V. As an example, in a case where the charge generation currents normally experienced in practice are expected to be no more than 1 nA, calculations can be made on this basis. Alongside this, it may be wished to limit voltages to some level, e.g. 100 V. Given these constraints, the model and Ohm's law show that resistances up to V/I = 102/10−9 = 1011 Ω would be acceptable.
In an application (e.g. electrostatic hazards avoidance in industrial processes) where higher charge generation is expected, the allowable resistance may be considerably smaller (IEC 60079‐32‐1).
A second way of looking at the matter is to decide how long a transient charge built up on a material or object may tolerably be allowed to remain without problems occurring. This may be evaluated in terms of the charge decay time. If a conductor has capacitance around 10 pF, resistance to ground of 1011 Ω will give a charge decay time of one second, and in the absence of charge generation a stored charge will reduce to only 5% of its initial value within three seconds. In manual assembly and handling processes, this will usually be fast enough to avoid problems. For materials, this decay time corresponds to a permittivity of 10−11 Fm−1 and resistivity of 1011 Ω. The permittivity of air is around 0.9 × 10−11 Fm−1, and many plastics are around 2 × 10−11 Fm−1. The presence of higher capacitance or material permittivity, or a requirement for faster charge decay, may lead to a lower maximum acceptable resistance.
2.3.5 The Effect of Relative Humidity
Water is an electrically conducting material. Moisture from the air forms a thin layer on the surface of many materials and can contribute to their apparent electrical conductivity. Some materials, especially natural materials such as paper, reduce by orders of magnitude in their resistivity as relative humidity increases from dry conditions.
As material surface resistance is increased under dry conditions, electrostatic charge build‐up is often greatly enhanced. Some ESD control materials use additives to attract moisture to a polymer surface and provide static dissipative behavior. These materials may not work well at low humidity. As a rule of thumb, electrostatic charge build‐up is generally increased for humidity less than about 30% rh.
The external atmospheric humidity varies daily with the climate and weather, in a range from below 10% rh (cold and dry winter conditions) to 100% rh (fog). The atmospheric relative humidity often has a large effect on material resistance, especially for materials that have resistance above about 1 MΩ. The effective resistance and charge decay times can be reduced over several orders of magnitude with increasing relative humidity for some materials.
Air relative humidity is a strong function of temperature and reduces as temperature increases for a given moisture content. Relative humidity is approximately halved by a 10 °C rise in temperature, if no moisture is added or removed. If, as in winter, cold air is brought indoors and heated, very low relative humidity can result. Hence, ESD problems can be seasonal and occur often in winter. Even in a room where the relative humidity is controlled, dry local microclimates can form where there are heat sources such as equipment, especially if air circulation is restricted.
Table 2.3 The effect of humidity on typical electrostatic voltages (MIL HDBK 263).
| Action | Voltage observed | |
| @ 10–20% rh | @ 65–90% rh | |
| Person walking across carpet | 35 000 | 1 500 |
| Person walking across vinyl floor | 12 000 | 250 |
| Person working at bench (not grounded) | 6 000 | 100 |
| Vinyl envelope | 7 000 | 600 |
| Polythene bag picked up from bench | 20 000 | 1 200 |
| Chair padded with polyurethane foam | 18 000 | 15 000 |
A view of the effect of relative humidity on static electricity in daily life is indicated by the following typical voltages (Table 2.3) given by MIL HDBK 263 as observed at different ambient humidities. These are indicative and cannot be used to predict voltages occurring in real situations.
2.4 Conductors in Electrostatic Fields
2.4.1 Voltage on Conducting and Insulating Bodies and Surfaces
Like charges repel, and in a conductor where charges are free to move rapidly, charge will rapidly move to the outer surface to minimize their proximity to each other. After charge has redistributed, the voltage on all parts of the conductor is equal (equipotential). This must be so – current flows