According to the iec provisions of the terminology of the various protective grounding methods of the concept of low-voltage distribution system according to the different grounding methods known as tt system, tn system, it system. Among them, tn system is further divided into tn-c, tn-s, tn-c-s system.
The following is a brief introduction to the various power supply systems.
1, tt mode grounding power supply system
tt grounding mode is the metal casing of electrical equipment will be directly grounded protection system, known as protective grounding system, also known as tt system. The first symbol t indicates that the power system neutral point is directly grounded; the second symbol t indicates that the load equipment exposed metal conductive part not connected to the charged body is directly connected to the earth, and how the system is grounded has nothing to do. All grounding of the load in a tt system is called protective grounding, as shown in Figure 1-1. This type of power supply system is characterized as follows.
(1) Because the three-phase load is unbalanced, there is an unbalanced current on the operating zero line and a voltage to ground, so there is a certain voltage on the metal case of the electrical equipment connected to the protective wire.
(2) If the working zero line is disconnected, the shell of the leakage equipment with protection zero is charged.
(3) If the phase wire of the power supply touches ground, the enclosure potential of the equipment rises, spreading the dangerous potential on the neutral wire.
(4) tn-c system trunking on the use of leakage protector, the work of the zero line behind all the repeated ground must be removed, otherwise the leakage switch can not be closed; and, the work of the zero line in any case must not be disconnected. So, in practical terms the working zero wire can only be allowed to have a repeated ground on the top side of the earth leakage protector.
(5) tn-c mode power supply system is only applicable to three-phase load basic balance.
4, tn-s mode power supply system
It is the work of the zero line n and special protection line pe strictly separate power supply system, called tn-s power supply system, as shown in Figure 1-4, tn-s power supply system features the following.
(1) when the system is in normal operation, there is no current on the special protection line, only the working zero line has an unbalanced current. pe line has no voltage to the ground, so the metal casing of the electrical equipment is connected to zero protection on the special protection line pe, safe and reliable.
(2) the work of the zero line is only used as a single-phase lighting load circuit.
(3) special protection line pe is not allowed to break the line, also not allowed to enter the leakage switch.
(4) the use of leakage protector on the trunk line, the working zero line shall not have repeated grounding, and pe line has repeated grounding, but not through the leakage protector, so tn-s system power supply trunk line can also be installed leakage protector.
(5) tn-s way power supply system is safe and reliable, applicable to industrial and civil buildings and other low-voltage power supply system. In the construction project work before the "three passes and one leveling" (electricity, water, road and ground leveling - must use tn-s mode power supply system.
5, tn-c-s way power supply system
In the construction of temporary power supply, if the first part of the tn-c way of power supply, and the construction specification must be used on the construction site of the tn-s way of power supply system, you can use the construction of the dotted line after the section of the power distribution box out of the pe line, as shown in Figure 1-5. This system is called tn-c-s power supply system. tn-c-s system features are as follows.
(1) when the metal casing of the electrical equipment is electrified (the phase wire touches the casing or the equipment insulation is damaged and leaks), the risk of electric shock can be greatly reduced because of the grounding protection. However, the low-voltage circuit breaker (automatic switch) is not necessarily tripped, resulting in leakage equipment shell to ground voltage is higher than the safety voltage, is a dangerous voltage.
(2) when the leakage current is relatively small, the fuse is not necessarily able to fuse, so also need leakage protector for protection, so tt system is difficult to promote.
(3) because the tt system grounding device consumes more steel, difficult to recover after installation, labor time, material. Now some building construction units with electricity, the use of a special protection line to reduce the installation of grounding devices to reduce the amount of steel, as shown in Figure 1-2.
The dotted line in the figure after the wiring method using construction power distribution box, the new addition of a dedicated protection line pe line and work zero line n separate, which is characterized by: first, *** with the grounding line and the work of the zero line has no electrical connection; second, normal operation, the work of the zero line can be current, while the dedicated protection line no current.
From the above analysis it can be seen that tt system is suitable for places where the grounding protection is very decentralized.
2, tn mode power supply system
This power supply system is the metal casing of electrical equipment and the work of the zero line connected to the protection system, called the zero protection system, with tn said. The characteristics of this power supply system are as follows.
(1) when the equipment appears shell charged, the zero protection system can leakage current rises to the short-circuit current, this current is very large, is tt system of several times, in fact, is a single-phase to the ground short-circuit fault, the fuse will fuse, low-voltage circuit breaker will be immediately acted on the off-trigger and tripped, so that the faulty equipment is disconnected, relatively safe.
(2) tn system saves materials, man-hours, in our country and many other countries are widely used, can be seen than the tt system has more advantages. tn mode power supply system, according to its protection zero line is separated from the work of the zero line and is divided into tn-c and tn-s and so on two kinds.
3, tn-c mode power supply system
It is the use of the working zero line as the zero protection line, can be called the protective neutral line.