eARTHING BASICS CONTINUED....,
The TN Networks are of 3 types. TN-S, TN-C, TN-CS. Figure 1 describes the nature and functioning of these networks.
Networks with Consumer Earth
Under the above nation system,if the second character is a ‘T’,then it refers to a local earth connection of the consumer at his/her premises that is independent of the earth connection at the generator (See Figure 2). Such networks can be further classified into TT and IT networks.
Comparison of Earthing Systems
From Table 1 we find that from the safety angle, the TNS is seen to be the safest. The TNS is also most expensive system due to the addition of a separate PE Conductor.
At the other end the earthing system with the least cost is the TNC, which is also the least safe. In the TNC system the C suffix stands for the ‘combined ‘ PEN Conductor that fulfills the functions of both a PE and an N conductor. Inspite of the cost benefit, the TNC is rarely used, as potentially hazardous rise in neutral potential (due to flow of return currents) could be observed on the surface of the consumer devices connected to such an earth.
Problems with the Existing Earthing System in India
The TT earthing system is currently being used by the state distribution companies in India particularly in rural areas, with overhead wiring.
Howewer, a survey has indicated that approximately 60 percent of the consumers do not have an earth provision at their premises. And amongst those who do, the earthing system is ill maintained. This means that the earth network reduces to a TN-C. Ironically, the TN-c is the very network that is rarely used in other countries. All the aisadvantages of this potentially dangerous system are frequently observed in India.
Some of the utilities that were established during the British era, like the Torrent Power in Ahmedabad have underground cables with the TN-S. due to weathering, the sheaths of these underground cables have corroded and, therefore,have stopped providing good earth connections. Thus the installations where ‘bad earths’ are found have got converted to TN-C-S.
In this section, we shall see how an inappropriate choice for an earthing system coupled with a total disregard for Indian Electricity Rules can prove to be a recipe for Disaster
2. FUNCTIONAL EARTH & SYSTEM EARTHING
A functional earth connection
Safety issues with TN-C System
In a TN-C system, the RCD devices are quite vulnerable to unwanted triggering from contat between earth conductors of circuits on different RCDs or with real ground. If the threshold settings of the RCDs are increased, then they are far less likely to detect an insulation fault. It is also unsafe to isolate the neutral core in a TN-C System, and hence the RCDs should be wired to interrupt only the live conductor and not the neutral, a convention that is not followed.
On an event of a neutral break, in a single phase system, all parts of the earthing system beyond the neutral break would rise to the potential of the L Conductor. In case of an unbalanaced three phase system, the potential of the earthing system would move towards the most loaded live conductor. In the Indian consumer premises, we frequently observe the use of plug/socket connections and flexible cables where there is a high probability of contact problems. Instead of a 3 Pin plug/Socket where live (L) and neutral (N) could get interchanged. The use of TN-C ot TN-C-S system should be banned in such premises. For instance, in the UK, cables where there is a high probability of contact problems. Instead of a 3 Pin plug/Socket where live (L) and neutral (N) could get interchanged. The use of TN-C ot TN-C-S system should be banned in such premises. For instance, in the UK, the use of TN-C-S is banned while TT system is recommended for outdoor wiring.
Disregard for the Electricity Rules
Rule 29 of the Indian Electricity rules, 1956, mandates that the electric supply lines and apparatus should be sufficient rating for fault currents, and that the same shall be constructed, installed,protected,worked and maintained to ensure safety of human beings, animals and property. Why then do we commonly hear of fatal accidents and damage to property. We still carry in our mind the vivid picture of an engineering student from our institiute at Vallabh vidyanagar who got electrocuted due to earth fault when his bicycle got too close to a guy wire (Used to keep an electric pole upright). We dedicate this paper to this innocent young student.
How often do see a danger notice with skull and bones? Rule 35 of the IE rules mandates that such notice be affixed in conspicuous locations of medium or high voltage installations. Similarly, rule 31 requires that the electric utility provide a cut out on every service line in the network and at the customer premises , Just as a rule 33 requires that the utility providea and maintains an earth terminal for the consumer’s use at an accessible position in his/her premises, Earlier,the utilities would make lame excuses such as, ‘ cut-outs are convenient points for energy pilferage’; or else they would take resource to admitting that their utility is sick. However, of late, after the setting up of the electric regularity commissions, the health of the utilities has seen a dramatic improvement. Isn’t it time that the authories connected with electricity start giving human lives and their property the due importance?
Which Earthing System to use
In the United states and Canada, the TN-C-S system is used, in argentina, Autralia (MEN) and France (TT), the customers must provide their own ground connections, Japan uses TT Earthing in most installations. UK has shifted from the TN-S to the PME. Modern homes in Europe TN-C-S earthing system. Norway is shifting from an IT to a TN-C-S system. TT networks are commonly used when the cost of a local earth connection, typically in older properties or in rural areas.
Even in the developed countries, until the mid 1990s, power outlets generally lacked protective earth terminals. Therefore, devices needing an earth connection used the supply neutral. Some used dedicated ground rods. Many appliances had polarized plugs to maintain a distinction between live and neutral, but using the supply neutral for equipment earthing was highly problematical. Live and neutral might fail or be improperly installed. Even normal load currents in the neutral might generate hazardous voltage drops. For these reasons, most countries mandrated dedicated protective earth connections that are now almost universal.
Adopting Correct earth Measurement Tool for India
To simplify our decision making process and choose the right earthing system for India, We first to a more basic question. Which would the right measurement tool for India?
Observability, Testability, & controllability (OTC) Issue
In a tropical country like India, Where humidity is high, the electrical systems corrode and need frquent maintenance. How do we know that the earthing system is functional! Does the system facilitate easy testing? These questions would get answered if we address the issues of observability, Testability and controllability (OTC). A laborious test procedure that involves disconnection of the earthing system for it to be tested or which requires additional earth spikes to be driven into the soil before taking the earth measurement is a deterrent to its usage. This is one of the main reasons why earthing systems, in developing countries, are harly ever tested. The measurement of earth has dropped to such pathetic levels, that today, it is even difficult to find a working earth measuring instrument with the local utilities.
A Faulty insulator on an unearthed pole could prove to be hazardous to lives. If the pole is earthed, then the potential would drop to near zero, however, leakage currents would now flow through the earth. It is necessary to check for such leakage currents and to identify and correct the fauly insulator. If this is not done, then the continued flow of leakage currents would cause the earthing to deteriorate. Also the utility would lose precious energy through this fault. In the following section we will describe an elegant,simple to use clamp-on earth tester which has the ability to measure the earth resistance or the earth (leakage) currents without disconnecting the earth wire