[TECHNICAL - ARTICLE] POTENTIAL BALANCING TT System L1 L2 L3 N Serdar Paker serdar.paker@emo.org.tr L1 L2 L3 When examining grounding systems in buildings, it is observed that the rule of creating an equipotential busbar is not strictly adhered to, and the concept of potential balancing, which is the most important philosophy of the current Regulation on Grounding in Electrical Installations, is not adopted. For example, demands and manufacturing processes continue for separate grounding for UPS, computers, tomography, lightning, and natural gas. Instead of separate grounding, the regulation requires that all grounding lines be gathered in an equipotential busbar to be created in the building. The situation is even more dangerous in natural gas main line grounding. Especially in buildings without grounding and without a residual current device (RCD), it is unthinkable to secure the natural gas main line with a single rod resembling a kebab skewer. In buildings with such separate grounding systems, the opportunity to ensure electrical safety by completing any missing grounding, equipotential busbar, and residual current switch components during gas supply inspections is missed. Our colleagues who inspect or measure grounding in buildings with separate grounding systems must be very careful about the application of the regulations. Measurements and inspections should be carried out from the perspective of the entire building in terms of protection against electric shock and fire risks. For example, measuring only the separate grounding system, especially without disconnecting it from the installation and without any comparison, and then interpreting a value smaller than a predetermined resistance value as "suitable," is not a responsible approach and is contrary to the current regulations. The basis of network grounding systems is that in the event of a short circuit in the wiring, the fault current travels from the source, activating the circuit breaker protection element on the fault path and interrupting the current. In our country, consumers receiving low-voltage energy from TEDAŞ networks are required to connect to a submersible (TT) system. In this widely used TT system, the ground itself forms part of the loop; that is, the circuit is completed through the ground. Therefore, the effectiveness of grounding in a TT system is very important. In TN systems, the entire loop circuit is formed from high-conductivity paths such as phase and protection lines, and grounding and earth are not included in the circuit. In TT systems, the resistances of the groundings entering the loop significantly increase the total loop resistance, therefore the resulting fault current cannot grow too large, the circuit breaker protection elements operate late, or in some cases, they do not operate at all. For example, a 5-ohm grounding in the building, when added to a 3-ohm star point grounding in the transformer substation on the loop, creates an 8-ohm resistance. If other line resistances and transformer impedance are ignored, the loop current becomes 230 V / 8 ohms = 29 A. For protection to be provided, this current is expected to trip the circuit breaker protection element, opening the circuit. However, in circuits protected only by fuses, for example, a 16A fuse with a B tripping curve, since it trips magnetically at 80A, instantaneous tripping will not occur, and according to the thermal curve of the fuse, the circuit will not trip for a long time, and the danger will continue. If the fuse is of type C, the time will be even longer. As the grounding resistance in the loop increases, the tripping time of the circuit will lengthen. After a certain value of 15 ohms, the total loop will not trip, but the current will continue to flow. This dangerous situation can only be eliminated by using a residual current switch (known in the market as a leakage current relay). For this reason, all relevant regulations mandate the installation of a residual current switch when using a grounding system. This is because the protection element will trip and provide protection when its threshold current (30mA) is exceeded. In this context, it can be said that the protection gap created in a grounding system is completed by residual current switches. The situation where two residual current switches are connected in series, as required by regulation, brings up the issue of fault limitation, i.e., selectivity. In the tripping characteristic of residual current switches, unlike fuses, an inverse time characteristic (i.e., the situation where the tripping time shortens as the current increases) is not defined. In residual current switches, the circuit trips instantaneously when the tripping threshold is exceeded, regardless of the magnitude of the fault current. For example, in a 500mA fault current, both the 30mA and 300mA switches are expected to trip. However, it's not expected that the 30mA switch will trip first. More precisely, it's impossible to predict which of the two switches will trip first. Therefore, to ensure that the switch closest to the fault location trips first, the 300mA switch behind it must be a selective type. Because these switches operate with a delay, they give the fast-acting switch ahead an opportunity to operate. For engineers responsible for implementing current regulations in projects and installations, it would be a more scientific approach to ensure the healthy operation of facilities if they did not hesitate to apply these regulations, avoided including clauses in special specifications that contradict the regulations, and relied solely on or requested support from the regulations in controversial situations. (17 October 2007)
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