Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 52

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.