July 06, 2023
1. IT system
An IT system is a system in which neutral of power supply is not earthed, but exposed conductive parts of electrical equipment are directly earthed. IT systems may have a neutral, but IEC strongly discourages use of a neutral. Because if a neutral line is installed, a ground fault will occur at any point of N-line in IT system and system will no longer be an IT system.
The IT system connection diagram is shown in Figure 1.
Fig. 1. IT system connection diagram
IT system capabilities
When a first earth fault occurs in an IT system, earth fault current is only capacitive current of non-faulted earth phase and is very small. to ensure continuity of power supply; - in case of a ground fault, voltage relative to ground increases by 1.73 times; - it is necessary to equip 220 V load with a step-down transformer, or it is powered exclusively by an external power supply - Install an insulation monitoring device. Places of use: increased requirements for power continuity, e.g. emergency power supply, hospital operating room, etc.
When distance to power supply of IT power system is not very long, reliability of power supply is high and security is good. Generally, it is used in places where power outages are not allowed, or places where strict uninterrupted power supply is required, such as electric steelmaking, large hospital operating rooms, underground mines, etc. The power supply conditions in underground mines are relatively poor, and cables exposed to moisture. Using IT power supply system, even if neutral point of power supply is not earthed, in event of equipment leakage, leakage current of one phase to earth is still small and will not upset power supply voltage balance, so it is safer than system with earthed power supply neutral. However, if it is used at a great distance from power supply, distributed capacitance of power line with respect to ground cannot be ignored. When load has a short circuit or leakage current electrifies body of equipment, leakage current forms a bridge through ground, and protective equipment does not necessarily operate, which is dangerous. It's only safer if power supply distance isn't too far. This type of power supply is rarely seen on a construction site.
2. TT system
The TT system is a system in which neutral point of power supply is directly earthed and exposed conductive parts of electrical equipment are also directly earthed. Grounding neutral of power supply is commonly referred to as working grounding, and grounding of open conductive parts of equipment is called protective grounding.
In TT system, these two bases must be independent of each other. Equipment grounding may be that each equipment has its own independent grounding device, or several devices may share one grounding device.
Wiring diagram of TT system as shown in Figure 2
Fig. 2. Connection diagram of TT system
Main advantages of TT system
1) It can suppress overvoltage of low voltage power grid when high voltage line and low voltage line overlap or insulation breakdown occurs between high and low voltage windings of distribution transformer. .
2) It has a certain leakage capacity due to overvoltage from a lightning strike of a low-voltage power grid.
3) Compared with low-voltage electrical appliances that are not grounded, in event of an accident with electrical case, earthing voltage of case can be reduced, which reduces danger of electric shock to a person.
4) Since ground current is relatively large, when single-phase ground is grounded, protective device (leakage fuse) can work reliably and fault can be repaired in time.
The main drawback of TT system
1) If lightning strikes low-voltage and high-voltage lines, overvoltages of direct and reverse conversion of distribution transformer may occur
2) The protective effect of earthing of low-voltage electrical appliances is not as good as that of IT systems.
3) When metal case of electrical equipment is charged (the phase line touches case or insulation of equipment is damaged and leakage occurs), risk of electric shock may be greatly increased due to grounding protection. decreased. However, low-voltage circuit breaker (circuit breaker) may not trip, causing voltage of earth leakage device to be higher than safety voltage, which is a hazardous voltage.
4) When leakage current is relatively small, even if there is a fuse, it may not blow, so leakage protection device is required for protection, so TT system is difficult to promote.
5) The earthing device of TT system consumes a lot of steel, it is difficult to recycle, it takes a lot of time and materials.
Using TT system
Because grounding device is close to equipment in TT system, it is unlikely that safety grounding line will be disconnected and can be easily detected.
The enclosure of equipment of TT system is not charged during normal operation, and high potential of enclosure will not be transmitted to entire system through PE line when a malfunction occurs. Thus, TT system is suitable for powering voltage-sensitive data processing equipment and precision electronic equipment, and also has advantages in applications in hazardous locations, such as explosive and fire hazardous areas.
The TT system can greatly reduce short circuit voltage on leaky equipment, but it generally cannot be reduced to a safe level. Therefore, TT system must be equipped with a leakage protection device or an overcurrent protection device, and former is preferred.
The TT system is mainly used for low-voltage consumers, that is, for small consumers that are not equipped with distribution transformers and introduce low-voltage power supplies from outside.
3. TN system
The TN system is a system in which neutral of power supply is directly earthed and exposed conductive parts of equipment are directly electrically connected to neutral of power supply.
In a TN system, exposed conductive parts of all electrical equipment are connected to protective conductor and to earth point of power supply, which is usually neutral point of power distribution system.
One point of TN system power supply system is earthed directly, and exposed conductive part of electrical device is connected to this point through a protective conductor.
The TN system is usually a three-phase network with earthed neutral. Its characteristic is that exposed conductive part of electrical equipment is directly connected to earth point of system. When a short circuit occurs, short circuit current forms a closed circuit through metal wire. A metal single-phase short circuit is formed to create a short-circuit current large enough for protection device to reliably operate and clear fault.
If working neutral line N is re-grounded, when shell is short-circuited, some of current may be shunted to repeated grounding point, which will make protective device unable to work reliably or refuse to work, and error will be increased.
In a TN system, that is, in a three-phase five-wire system, N wire and PE wire are laid separately and insulated from each other, while it is PE wire instead of N wire connected to electrical enclosure. Therefore, most important thing that we care about is potential of PE line, not potential of N line, so re-grounding in TN-S system is not re-earthing of N line. If PE line and N line are grounded together, since PE line and N line are connected at re-earth point, there is no difference between PE line and N line in connection between re-earth point and operating earth point of distribution transformer. The neutral current flowing in N line and PE line is shared, and part of current is shunted through repeating earth points. Because of this, it can be considered that there is no PE line on front side of repeating ground point, only PEN line, consisting of original PE line and parallel N line, and advantages of original TN-S. system will be lost, so PE line and N line cannot be connected together.
In TN system, it is divided into three forms: TN-S system, TN-C system, and TN-C-S system, depending on whether protective neutral line is separated from operating zero line.
(1) TN-C system
The connection diagram of TN-C system is shown in Figure 3.
Fig. 3. TN-C system connection diagram
In a TN-C system, functions of PE line and N line are combined, and a conductor called PEN line performs both functions at same time. On electrical equipment, PEN wire is connected not only to load neutral, but also to open conductive part of equipment. Due to its inherent technical shortcomings, it is now rarely used, especially in civil distribution of electricity, TN-C system is in principle not allowed to be used.
TN-C System Specifications
1) When body of equipment is charged, zero connection protection system can increase leakage current to a short circuit current. In fact, this is a single-phase short circuit to earth. The fuse will blow. shock or circuit breaker will trip, so that faulty equipment will be shut down, which is safer.
2) The TN-C system is only applicable to situation where three-phase load is basically balanced. If three-phase load is unbalanced, there will be an unbalanced current on operating zero line and voltage to earth, so electrical equipment connected to protection line. The metal case has a certain voltage.
3) When working zero line is turned off, body of equipment connected to zero is charged under voltage.
4) If phase line of power supply is grounded, potential of equipment case will increase, which will lead to propagation of a dangerous potential on neutral line.
5) When a leakage circuit breaker is used on main line of a TN-C system, all heavy duty earths behind working neutral line must be removed, otherwise leakage breaker cannot be closed, and all re-grounds behind working neutral line must be removed, otherwise it will leak. The switch cannot be closed, and working zero line cannot be turned off under any circumstances. Therefore, in practice, working neutral line can only be re-earthed on top side of leakage circuit breaker.
(2) TN-S system
The connection diagram of TN-S system is shown in Figure 4.
Fig. 4. TN-S system connection diagram
The neutral line N in TN-S system is same as in TT system. The difference from TT system is that exposed conductive parts of electrical equipment are connected to neutral of power supply through PE line and share earth body with neutral of system, instead of being connected to their own dedicated earth body, neutral line (line N) is separated from line protection (PE line).
The biggest feature of TN-S system is that after separation of N line and PE line, there can no longer be any electrical connection at neutral point of system. Once this condition is violated, TN-System S will no longer be installed.
TN-S System Specifications
1) When system is working normally, there is no current on dedicated protection line, but there is unbalanced current on working neutral line. The PE line has no ground voltage, so zero protection of metal shell of electrical equipment is connected to a special PE protection line, which is safe and reliable.
2) The operating zero line is only used as a single-phase lighting load circuit.
3) It is not allowed to disconnect dedicated PE protection line, and it is also not allowed to enter leakage switch.
4) The leakage protection device is used on main line, so leakage protection device can also be installed on main power line of TN-S system.
5) The TN-S mode power supply system is safe and reliable, and is suitable for low-voltage power supply systems such as industrial and civil buildings.
(3) TN-C-S system
The connection diagram of TN-C-S system is shown in Figure 5.
Fig. 5. TN-C-S system connection diagram
The TN-C-S system is a combination of TN-C system and TN-S system. In a TN-C-S system, power supply section uses TN-C system. Since there is no electrical equipment in this section, it only plays role of transmitting electricity. At a certain point near electrical load, EN line is divided into separate N lines and PE lines. From this point on, system is equivalent to TN-S system.
TN-C-S system characteristics
1) The TN-C-S system can reduce motor case voltage to ground, but it cannot completely eliminate voltage. The magnitude of this voltage depends on unbalanced load and length of line. It is required that unbalanced load current is not too high, and PE line must be grounded multiple times.
2) The PE line under no circumstances can enter leakage protection device, because operation of leakage protection device at end of line will trigger previous leakage protection device and cause a large-scale power outage.
3) Except that PE line must be connected to N line in main unit, N line and PE line must not be connected to each auxiliary unit, and switches and fuses cannot be installed on PE line.
Actually, TN-C-S system is an adaptation of TN-C system. When three-phase power transformer is in a good earthing working condition, and three-phase load is relatively balanced, effect of TN-C-S system on building energy practices remains good. However, when three-phase load is unbalanced and there is a dedicated power transformer at the construction site, a TN-S power supply system must be used.