rel="nofollow" href="#ulink_f739a022-3653-5de5-ac65-ab6b4784c97e">Table 1.3 Example of how meanings of conductive, dissipative, and insulative can vary with context in static control in other industries (IEC 60079‐32‐1:2013).
| Object | Measurement | Conductive | Dissipative | Insulative |
| Material | Volume resistivity (Ωm) | <105 | ≥105 to 109 | ≥109 |
| Clothes | Surface resistance (Ω) | <2.5 × 1010 | ≥2.5 × 1010 | |
| Footwear | Leakage resistance (Ω) | <105 | ≥105 to <108 | ≥108 |
| Gloves | Leakage resistance (Ω) | <105 | ≥105 to –<108 | ≥108 |
| Floor | Leakage resistance (Ω) | <105 | ≥105 to <108 | ≥108 |
1.7.4 Point‐to‐Point Resistance
In ESD control, it is convenient to make simple measurements to evaluate the surface properties of a material or item of equipment. One simple way of evaluating a surface is to place two electrodes on it and measure the resistance between them. The electrodes are often cylindrical in form. This is called a point‐to‐point resistance measurement. Standard test methods based on this approach are often used. Examples of point‐to‐point resistance test methods are given in Chapter 11.
1.7.5 Resistance to Ground
As explained earlier, in ESD control work, voltages on conductors are often eliminated or controlled by providing an electrical connection for the charge to pass to earth (ground). It is often required to know the resistance from an object or surface to ground to help understand the charge dissipation paths. This is known as resistance to ground. Examples of measurement methods for this are given in Chapter 11.
1.7.6 Combination of Resistances
In practice, the resistance of a ground path may be due in part to several components. If these are effectively in series (Figure 1.4), the effect is to add the resistance of all component contributors R1…Rn to get a total resistance Rtot.
If resistance of ground paths is in parallel (Figure 1.5), they are combined as
Figure 1.4 Resistances in series.
Figure 1.5 Resistances in parallel.
1.8 Capacitance
The voltage V on a conductor is related to the stored charge Q as
The variable C is the capacitance of the conductor. In electrostatics, any conductive object has capacitance; it is just the relationship between the stored charge and the object's voltage.
In practice, the capacitance of an object can vary with proximity of other conductors and materials (see Chapter 2).
A charged capacitor stores energy. The energy W stored in a capacitance C at voltage V is given by
This can also be expressed as
An object in free space (with nothing in the near vicinity) still has capacitance. For a spherical conductor of radius r in air or a vacuum, this capacitance C is
In practice, the capacitance of an object may be due in part to the proximity to several objects. If these are effectively in parallel (Figure 1.6), the effect is to add the capacitance of all component contributors C1…Cn to get a total capacitance Ctot as
If capacitances between objects are in series (Figure 1.7), they are combined as
Figure 1.6 Capacitors in parallel.
Figure 1.7 Capacitors in series.
1.9 Shielding
The term shielding is used in ESD control in a different way to other disciplines, especially EMC and radio frequency work. Shielding definitions and tests used in ESD control are often highly specific to the standards used. Typically, the term is
