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Low-voltage power distribution system grounding analysis

12/22/2011

When you select Fatech Electronic (CHINA) LTD surge protection device(surge protectors, SPD) for different grounding system, it is necessary and important to condering the grounding system type. Please let us give you a brief introduction of low-voltage power distribution grounding system.

The basic construction of electricity power supply system to be called (380V) three-phase three-wire and (380/220V) three-phase four-wire system, but these are not very strict terminology content. International Electrotechnical Commission (IEC) made unified regulations, known as TT systems, TN systems, IT systems. TN system is divided into TN-C, TN-S, TN-C-S system. Here a variety of power supply system referred to above to make a brief analysis.

 
According to IEC provides a variety of protection, the term concept, low-voltage distribution system divides into 3 ways by grounding systems different, which namely TT, TN and IT systems, as described below.
 
1. TT system
TT mode power supply system is the way that the metal casing of electrical equipment will be grounded directly to the protection system, known as protective ground system, also known as the TT system. The first symbol T means that the power system directly grounded neutral point; second symbol T said metal shell and the normal load equipment's metal parts are not charged directly connected with the earth, and has nothing to do with how the system ground.
 
The characteristics are as below,
1) When the charged metal casing of electrical equipment (line touch shell or equipment damage insulation leakage), due to a ground fault protection, can greatly reduce the risk of electric shock. However, the low-voltage circuit breaker (auto switch) does not necessarily trip, resulting in leakage of equipment enclosures to the ground voltage above the safe voltage, is a dangerous voltage.
 
2) When the leakage current is relatively hour, even if not necessarily blown fuse, so the leakage protection needed for protection, should not be trapped in the TT system, 380/220V Power Supply System.
 
3) TT system earthing consumption of steel and more, and difficult recovery, costs of working hours, fee material.
 
2. TN system
TN supply system means that power supply system is the metal casing of electrical equipment and metal parts not charged the normal work of the zero line with the phase of the protection system, called then zero protection system, with the TN said. Its characteristics are as follows.
 
1) Once the device appears live shell, then zero leakage current protection system can rise to (220V) short-circuit current, the current large, is a TT system, many times, in fact, a single relatively short-circuit fault, the fuse of the fuse Will fuse, low voltage circuit breaker action immediately release the trip, the faulty equipment power, more safety.
 
2) TN system can save material, it works in the country and is widely applied in many other countries, and we can see more advantages than the TT system. According to the protection of the neutral line whether separated, TN mode power supply systems is classified as TN-C and TN-S.
 
a. TN-C system
TN-C mode power supply system is connected with the work of the zero line double as zero-line of protection, protection can be called the neutral line, can be expressed NPE.
TN-C power supply system following features.
1) TN-C system lines when using leakage protection, leakage protection behind the ground must be removed all duplicate, or GFCI not close; Moreover, the zero line of work are not under any circumstances break. Therefore, the practical works in the zero line only allow leakage protection on the side of a repeat ground.

2) TN-C mode power supply system is suitable for basic load balancing phase (220V no load) condition.
 
b. TN-S system
TN-S power supply system is the way to the zero line of work for the protection of line N and PE strictly separate the power supply system, called TN-S power supply system, shown in Figure 1-4, TN-S power supply system following features.
 
1) The system normal operation, for the protection of online no current, but the zero line of work are unbalanced current. PE line on the ground there is no voltage, so the metal casing of electrical equipment, then protection is then zero line of protection in the PE on a dedicated, safe and reliable.
 
2) The neutral line of work is only used for single-phase lighting load circuit.
 
3) For the protection of PE not allowed to break lines, but also not allowed to enter a work GFCI zero line.
 
4) The use of Route leakage protection, leakage protection may not be a repeat ground, while the PE grounding lines are repeated, but without leakage protection, so the TN-S system can also be installed on the supply mains leakage protection.
 
5) TN-S method safe and reliable power supply system, for industrial and civil buildings such as low-voltage power supply system. Before the construction of "leveling"(electricity, water supplies and roads and the horizon - must be TN-S mode power supply system.
 
c. TN-C-S system
TN-C-S system in the way of construction of temporary power supply, if the first part is (no 220V load) TN-C mode power supply, and construction specification construction site must be TN-S mode power supply system, The system can be part of the scene after the separation of the total PE cable distribution box,. This system known as TN-C-S power supply system. TN-C-S system is characterized as follows.
 
1) the neutral line of work for the protection of line N and PE phase Unicom, after the main switch box circuit current imbalanceis large,protected electrical equipment connected by the zero line of zero potential. PE line behind the main switch boxes not current,t hat is, these action of the wire is no voltage drop, therefore, TN-CS system can reduce the electrical equipment enclosureto th eground voltage, however, can not completely eliminate this voltage, which depends on Wire unbalanced load current size and N lines in the total length of the line before the switch box. Unbalanced load current increases, N and ver ylong lines, equipment enclosureson the ground the greater the voltage offset. Unbalanced load current is not so much required, and the line should be repeatedin the PE ground.
 
2) PE line scan not, underany circumstancesinto the leakage protection,because the end of theleakage protection circuit will prevent leakage protection before the class trip causing wide spread power outages,specification: There are zero then no cascaded protection of any of the zero line Switches and fuses.
 
3) The addition of PE in the total box office lines and N lines connecting to other than the sub-boxes at the N line and were not linked PE cable, PE online not allowed to install the switch and fuse, and the connection must besecure.
 
Through the above analysis, TN-C-S power supply system isTN-Csystem in the temporary work around. When the three-phase power transformer grounding work well, when the three-phase load is balanced, TN-CS system results in the construction of electricity is feasible in practice
 
3. IT system
The first letter I means the power supply system ground side does not work, or through the high impedance grounding. The second letter T, said load side ground fault protection of electrical equipment.
 
IT way power supply system in power is not very long distance, high reliability power supply, and good security. Place to allow the general power outage, or power continuously demanding areas such as continuous production equipment, large hospital operating rooms, underground mines and other places. Conditions in underground mines are relatively poor power supply, cables, easy to dry. IT means the use of power supply system, even when the power is not neutral ground, once the equipment leakage, leakage current is still a single relatively small, will not disrupt the balance of supply voltage, so the neutral point than the power supply system is also secure.
 
However, if the distance is long in the power supply, power supply line capacitance of the distribution of the earth can not be ignored. Figure 1-6 shows the load short-circuit fault or leakage to the equipment enclosure charged, the leakage current through the earth form a loop, protective equipment is not necessarily action, which is dangerous. The power supply only keeps safe when the distance is not too long. This power supply is rare in the construction site of our company 35KV, 10KV, 6KV IT system using this method. IT is obvious shortcomings of the way, single phase line, the other two to the line voltage relative to ground voltage, over-voltage on the electrical equipment required is very high. Last year, three-wire overhead line construction is the construction unit severing the second phase to ground station 6KV system B, A, C both increased relative to the voltage 1.732 times the voltage transformer caused the second catalytic 6KV burned by over-voltage, the main fan trip, affecting normal device Production.
 
Summary of power lines symbol
1) The International Electrotechnical Commission (IEC) power supply under the symbol, the first letter means that the power (power supply) system in relationships. Such as the T that is directly grounded neutral point; I mean that all live parts of the insulation (not ground).
 
2) The second letter indicated that the exposed metal parts of electrical equipment on the ground relationship. T indicates that the device, such as chassis ground, which with any other system not directly related to access locations; N said load by then zero protection.