The primary objective of earthing (or grounding as referred to in North American standards) is safety, to reduce risks to both human life and installations. The principles of electrical earthing are agreed upon internationally, though in practice, there are differences as to how these principles are best achieved.
Electrical systems were not always earthed. The first systems were unearthed ones with no earth reference at all. Even though such systems still exist in specific areas, they are the exceptions rather than the rule and by and large, some form of earthing is adopted for all power systems.
Electrical earthing requirement could be expounded into these five main purposes:
- To reduce the risk of personnel shock
Under normal or fault conditions on a plant, the resistance of return paths must be sufficiently low so as not to produce unsafe voltages. Fault currents may be calculated with a target earth loop impedance to prove personnel safety aspects are satisfied.
There must likewise be concerns for low voltage installation considering that the heat generated from low-voltage systems at high currents can cause injury in the form of burns. - To operate electrical protective devices
Occasionally an electrical fault will occur inside a piece of equipment such as a switchboard or motor that causes a conductor to touch the casing or frame. In most power systems this type of fault would cause a much larger than normal current to flow in the conductors. This current would flow through the casing or frame and in so doing would usually cause serious damage to the conductors, their insulation and casing metalwork due to sparking or arcing. The damage will usually increase with time.
Effective earthing provides suffcient faults current to operate electrical protection relays reducing further damage to the electrical system. - To guard against lightning surges
Lightning generate a vast static charge that build up to an extremely high voltage. Lightning protection conductors are installed not to handle the vast discharge current flow but to conduct substantial but smaller currents to prevent higher voltages from developing preventing insulation breakdown. - To control electrostatic discharge
Static electrical charge is caused by the forced separation of molecules of non-conducting materials. Movement of the material or friction against the material can cause the charging effect. When wrenched apart, a surface charge builds up a high potential difference between the separated materials. The size of the charge depends on the violence with which this separation occurs. The non-conducting materials act as a dielectric, and allow the accumulation and storage of the charge as in a capacitor.
Earthing prevents static build up which discharge can produce ignition, explosion or fires. - To minimize electrical interference
Electrical noise can be generated by non-linear loads, likewise by parasitic capacitances of non-shielded electrical equipments.
Earthing will reduce the coupling capacitance effect and conduct stray currents back to the source of supply.
Earthing Paths
The term earth path tends to be defined by what is connected to it. It is therefore more logical to describe the earth by the circuit types from which it is gathering stray currents. Hence power, instrument, computer, structural and terrestrial earths are more meaningful names but their function requires clear definition in plant safety documentation so that their use is not misunderstood. There is little concern that the risk of ignition will be increased by incorrect earthing but the operational aspects of systems may be severely hampered.
Two common and more meaningful terms to distinguish different earth paths are
- dirty earth
the return path conductor that can carry the prospective fault currents from electrical equipment. As a result the conductor is likely to shift in potential by a greater amount (with respect to the neutral of the supply system which is the ultimate reference). Fault currents are inseparable from the parasitic capacitance currents, which will flow along the same route, although they will be at a much lower level. - clean earth
the return-path conductors, which cannot carry fault currents from electrical equipment. This, by definition, is impossible to control or guarantee since any circuit to earth will conduct fault currents back to the return of the source of supply.
Earthing and bonding
There is a subtle but essential difference between earthing and bonding, which must be understood.
- Earthing
is where a low-impedance path is provided in order for return currents to operate electrical protection devices such as fuses and overcurrent trips in an appropriately short time. - Bonding
is where voltage differences between electrical conducting parts are eliminated.
Definition of terms
- Bonding
- The electrical interconnecting of conductive parts, designed to maintain a common electrical potential.
- The permanent joining of metallic parts to form an electrically conductive path, which will assure electrical continuity and the capacity to conduct safely any current likely to be imposed.
- Common-mode noise
The noise voltage that appears equally and in phase from each current-carrying conductor to ground. - Crest factor
Ratio between the peak value (crest) and rms value of a periodic waveform. - Critical load
Devices and equipment whose failure to operate satisfactorily jeopardizes the health or safety of personnel, and/or results in loss of function, financial loss or damage to property deemed critical by the user. - Distortion factor
The ratio of the root mean square of the harmonic content to the root mean square value of the fundamental quantity, expressed as a percent of the fundamental – also referred to as total harmonic distortion. - Dropout
A loss of equipment operation (discrete data signals) due to noise, sag or interruption. - Dropout voltage
The voltage at which a device will release to its de-energized position (for this recommended practice, the voltage at which a device fails to operate). - Equipment grounding conductor
The conductor used to connect the non-current carrying parts of conduits, raceways and equipment enclosures to the grounding electrode at the service equipment (main panel) or secondary of a separately derived system (for example, isolation transformer) – (this term is defined more specifically in the NEC, Section 100).
- Failure, degradation
Failure that is both gradual and partial. Note: In time, such failure may develop into complete failure. - Flicker
A variation of input voltage sufficient in duration to allow visual observation of a change in electric light source intensity. - Form factor (periodic function)
The ratio of the root mean square value to the average absolute value, averaged over a full period of the function. - Forward transfer impedance
An attribute similar to internal impedance, but at frequencies other than the nominal (for example, 60 Hz power frequency) – knowledge of the forward transfer impedance allows the designer to assess the capability of the power source to provide load current (at the harmonic frequencies) needed to preserve a good output voltage waveform – generally, the frequency range of interest is 60 Hz – 3 kHz, for 5 – 60 Hz power systems and 20–25 kHz for 380–480 Hz power systems. - Ground (Earthing)
A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth.
Note: It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground currents to and from each (or the conducting body). - Ground electrode
A conductor or group of conductors in intimate contact with the earth for the purpose of providing a connection with the ground. - Ground electrode, concrete encased
A grounding electrode completely encased within concrete, located within and near the bottom of a concrete foundation or footing or pad, that is in direct contact with the earth. - Ground grid
A system of interconnected base conductors arranged in a pattern over a specified area and buried below the surface of the earth – the primary purpose of the ground grid is to provide safety for workmen by limiting potential differences within its perimeter to safe levels in case of high currents, which could flow if the circuit being worked became energized for any reason or if an adjacent energized circuit faulted – metallic surface mats and gratings are sometimes utilized for the same purpose – this term should not be used when referring to a signal reference structure. - Ground impedance tester
A multi-functional instrument designed to detect certain types of wiring and grounding problems in low-voltage power distribution systems. - Ground loops
A potentially detrimental loop formed when two or more points in an electrical system that are nominally at ground potential area connected by a conducting path such that either or both points are not at the same ground potential. - Harmonic distortion
The mathematical representation of the distortion of the pure sine waveform. - Isolated equipment ground
An insulated equipment grounding conductor run in the same conduit or raceway as the supply conductors – this conductor is insulated from the metallic raceway and all ground points throughout its length. It originates at an isolated ground type receptacle or equipment input terminal block and terminates at the point where neutral and ground are bonded at the power source (this term is defined more specifically in the NEC [2], Sections 250–74 and 250–75).
- Isolation
Separation of one section of a system from undesired influences of other sections. - Linear load
An electrical load device which, in steady-state operation, presents an essentially constant load impedance to the power source throughout the cycle of applied voltage. - Noise
Electrical noise is unwanted electrical signals that produce undesirable effects in the circuits of the control systems in which they occur (for this recommended practice, ‘control systems’ is intended to include sensitive electronic equipment in total or in part). - Non-linear load
Electrical load that draws current discontinuously or whose impedance varies during the cycle of the input AC voltage waveform. - Non-linear load current
Load current that is discontinuous or is not proportional to the AC voltage. - Notch
A switching (or other) disturbance of the normal power voltage waveform, lasting less than a half-cycle, which is initially of opposite polarity than the waveform and is thus subtractive from the normal waveform in terms of the peak value of the disturbance voltage – this includes complete loss of voltage for up to a half-cycle. - Over-voltage
An rms increase in the AC voltage, at the power frequency, for durations greater than a few seconds – see Swell and surge. - Phase shift
The displacement in time of one periodic waveform relative to other waveform(s). - Power quality
The concept of powering and grounding sensitive electronic equipment in a manner that is suitable to the operation of that equipment. - Recovery time
Time interval needed for the output voltage or current to return to a value within the regulation specification after a step load or line change – also may indicate the time interval required to bring a system back to its operating condition after an interruption or dropout. - Recovery voltage
The voltage that occurs across the terminals of a pole of a circuit interrupting device upon interruption of the current. - Shield
As normally applied to instrumentation cables, a conductive sheath (usually metallic) applied over the insulation of a conductor or conductors, for the purpose of providing means to reduce coupling between the conductors so shielded and other conductors that may be susceptible to, or that may be generating, unwanted electrostatic or electromagnetic fields (noise). - Shielding
Is the use of a conducting barrier between a potentially disturbing noise source and sensitive circuitry – shields are used to protect cables (data and power) and electronic circuits – they may be in the form of metal barriers, enclosures or wrappings around source circuits and receiving circuits. - Slew rate
Rate of change of (AC voltage) frequency. - Surge reference equaliser
A surge-protective device used for connecting equipment to external systems whereby all conductors connected to the protected load are routed, physically and electrically, through a single enclosure with a shared reference point between the input and output ports of each system.
References
- Electrical Installations in Hazardous Areas
Alan McMillan - Practical Electrical Equipment and Installations in Hazardous Areas
Geoffrey Bottrill - Practical Grounding, Bonding, Shielding and Surge Protection
G. Vijayaraghavan