During the early development of electricity, scientists discovered that the earth could be used as a grounding mechanism to scale down the negative impact of errant electrical power. This is because it offered a zero conductive value. Earthing means electrically connecting an object to an earth electrode. Earth grounding mechanisms are used in most electrical systems. Installers and designers often wonder which grounding system to use. To make the right decision, you should understand the pros and cons of each system.
In solid earthing, grounding conductors are connected to the earth with no intentional added impedance in the circuit. The neutral wire of a generator transformer is directly connected to the earthing system of the substation or facility. A neutral in earthing is a conductor that carries the current and has no electric charge. In case a fault shuts down a production process, back-up generators are used and are solidly grounded. The magnitude of the current depends on the fault location and the fault resistance. The single- phase earth fault current in a solidly earthed system may exceed the three-phase fault current. The advantages of solid grounding are that you can easily identify faults, and it also has good control of transient over-voltage, from neutral to ground. The limitation with this process is the likelihood of escalation from a single-phase fault to a three-phase fault, as well as severe flash or arcing hazards caused by the high line-to-ground fault current. There is also the unplanned interruption of the production process when a fault shuts down.
A resistor can be connected between a transformer's neutral point and the station's earthing system to limit the fault current. Resistance earthing is when one or more of these neutral points are connected to the earth through a resistor made of metal, such as a wire. The resistor increases the resistive part of the earth, thus improving the earth fault detection. The advantages of resistance earthing are: limited over-voltage by using the neutral point resistance, reduced phase-to-earth faults, limited hazards caused by arcing grounds and a low level of faulty ground current. However, a lot of energy is lost in neutral ground resistance, and the process is very costly.
A neutral point reactor is connected to the earth with an inductive reactor or reactance coil to reduce the reactive part of the fault current. The current generated by the reactance coil in the fault current compensates for the capacitive component of the single-phase fault current. This process is called reactive grounding. The characteristics of reactive grounding are the same as those of solid earthing. The advantages of this system are that it allows high impedance fault detection and the small reactive earth fault is independent of the phase-to-earth capacitance of the system. The disadvantages include the risk of extensive active earth fault losses and complicated relay protection. It is also a costly system to implement.
Isolated Neutral System
An isolated neutral system or ungrounded system is where all transformer neutrals are unearthed. The earth and the unearthed neutral are connected through high impedance equipment. Like voltage transformers and surge arresters, isolated neutral systems were mostly used in the 1940s and '50s. The main advantage is that an isolated neutral system provides limited capacitive connection to the earth because of the small earth fault currents. Unfortunately, when the capacitive connection is too weak, detecting the earth faults becomes difficult; and when too strong, it generates extensive earth fault currents. There is also the risk of over-voltages; therefore the use of this system is restricted to low and medium voltages.
- "Grounds for Grounding: A Circuit-to-system Handbook";Elya B. Joffe & Kai-Sang Lock; 2010
- "Transmission and Distribution"; U.A. Bakshi and M.V. Bakshi; 2009
- "Electric Power Transmission and Distribution"; S. Sivanagaraju; 2008
- Electrical Construction & Maintenance Magazine: "The Basics of Grounding Systems"; Jack Woodham, P.E.; 2003