control. So, it is often convenient and is common practice to electrically connect all conductors to earth. Earth is also known as ground, and earthing is also known as grounding.
The terms earthing and grounding can have different meanings and requirements in different contexts or industries. An electrical engineer may require an earth resistance less than an ohm. A plant engineer may earth bond two items of plant, requiring a resistance less than 10 Ω. An electromagnetic compatibility (EMC) engineer may require an extremely low impedance to be maintained from direct current (DC) to hundreds of MHz or even GHz. To an ESD control practitioner, a resistance to ground <109 Ω at dc may be sufficient.
In practice in ESD control, there are various types of ground that can be used. In the ESD standards IEC 61340‐5‐1:2016a and ANSI/ESD S20.20‐2014, the term grounding is used to mean any of the following:
Connection to electrical earth (the safety earth wire of a mains electrical system)
Connection to a functional earth (e.g. an earth rod driven into the ground)
Connection to an equipotential bonding system
1.6 Power and Energy
Energy is the ability to do work. Physics recognizes many types of energy – heat, light, gravitational, mechanical, and of course electrical.
Mechanical energy expended is the product of force and the distance moved. If a force qE is applied to move a charge q over a distance s between points A and B, the work done, WAB, is
Energy (work) expended, W, is also the product of power P and the time duration t that the power is applied.
The electrical power expended is the product of voltage V and current flowing I.
So, the electrical energy expended is
1.7 Resistance, Resistivity, and Conductivity
1.7.1 Resistance
Electrical resistance is the ratio between the dc voltage applied to a circuit or material and the current flowing through it, given by Ohm's law.
1.7.2 Resistivity and Conductivity
1.7.2.1 Surface Resistivity and Surface Resistance
Surface resistivity is defined as a material surface property. It is based on the theoretical resistance of a square of material surface with sides of unit length, with a voltage applied to two opposing sides of the square (Figure 1.2). In theory, the current flows across the surface of the square. For a material of surface resistivity ρs with linear electrodes of width w placed parallel on the surface a distance d apart, the surface resistance Rs measured between the electrodes is
where d = w, which reduces to ρs = Rs.
The unit of surface resistivity is ohms (Ω). In some industries, it is quoted as ohms per square (Ω/sq). This reflects the property that the value of the surface resistance measured with a square electrode pattern (d = w) is the same, no matter what the dimension of the side of the square is.
In practice, standards exist for measuring surface resistivity using concentric ring electrodes (IEC 62631‐3‐2 (International Electrotechnical Commission 2015), IEC 16340‐2‐3, ANSI/ESD STM 11.11 (EOS/ESD Association Inc. (2015a)). This is further discussed in Chapter 11.
Figure 1.2 Surface resistivity definition.
Surface resistance is a resistance measured between two electrodes on the surface of a specimen. The electrodes may be of any convenient form. Sometimes this measurement is made using electrodes designed so that conversion from surface resistance to surface resistivity is a simple calculation. In ESD control practice, conversion to surface resistivity is often not needed, and the surface resistance result obtained with defined standard electrodes is used directly.
1.7.2.2 Volume Resistance, Volume Resistivity, and Conductivity
Volume resistivity is a bulk material property based on the resistance of a cube of material with sides of unit length, with a voltage applied to two opposing faces of the cube (Figure 1.3).
The volume resistance Rv measured through a material of volume resistivity ρv using electrodes of area A is given by
where t = A = 1, or t/A = 1, which reduces to ρv = Rv.
The unit of volume resistivity is ohm meter (Ωm). The volume resistivity of a material is often simply referred to as its resistivity.
In practice, standards exist for measuring volume resistivity using concentric ring electrodes (IEC 62631‐3‐1 (International Electrotechnical Commission 2016c), IEC 61340‐2‐3 (International Electrotechnical Commission 2016b), ANSI/ESD STM 11.12 (EOS/ESD Association Inc. 2015b))
Figure 1.3 Definition of volume resistivity.
Volume resistance, Rv, is a resistance measured between opposing faces of a material. The electrodes may be of any convenient form. Often this measurement is made using electrodes designed for volume resistivity measurement so that conversion from volume resistance to volume resistivity is a simple calculation. In ESD work, conversion to volume resistivity is often not needed. The volume resistance obtained with defined standard electrodes is used directly, saving the effort of calculation. Examples of surface and volume resistance
