Function of split meter ready board neutral line, harm of neutral breaking and repeated grounding

TN low-voltage split meter ready board system is widely used in China. The line from the neutral point of the transformer is called neutral line, also called neutral line, which is mainly used to connect single-phase 220V load and carry single-phase current and three-phase unbalanced current. Reduce the neutral point potential drift of the load.

In TN-C TN-C-S, there are also grounding and neutral protection functions.

The impedance of the N line of the split meter ready board is on the milliohm level, and its load neutral point unbalanced voltage is the voltage drop of the N line current on the N line impedance. Its value is very small. Even if the three-phase load is seriously unbalanced, it is enough to clamp the load neutral point potential on the power supply neutral point potential.

On the ohmic level, the grounding resistance is hundreds of times greater than the impedance of N line, so it is impossible to clamp the load neutral point potential to the power supply neutral point potential.

1. Therefore, when the three-phase load is seriously unbalanced after the N line is broken, it is inevitable that the load neutral point will shift seriously.

(Each outgoing line of the low-voltage cabinet will lead out a neutral line from the neutral bar, and then branch at the end. There are countless neutral lines in the system. If a neutral line breaks, the damage will be different depending on the position of the broken line. At this time, if the three phases are unbalanced)

The load neutral point will shift to the phase with large load, and the voltage of the phase with large load will decrease; However, when the phase voltage with small load increases, the more serious the three-phase load imbalance is, the greater the load neutral point displacement will be. (The equipment forms a loop through the repeated grounding points of the neutral line)

2. If the phase line is short circuited to the ground when the neutral line is broken, the displacement of the neutral point will be greater.

3. After neutral breaking, the shell leaks electricity, which is easy to cause electric shock.

In the neutral line break accident we encounter, the voltage of the phase with large load decreases by 30-60V, making the incandescent lamp red, and the fluorescent lamp and home TV do not start; The phase to phase voltage of small load can rise to about 300V, which greatly exceeds the rated voltage of household appliances. If the fuse does not fuse at this time, the household appliances can be burned

Summary:

1. The neutral line transmits three-phase unbalanced current. Since the three-phase unbalanced current exists, it will inevitably lead to the drift of the neutral point.

The N-wire impedance is milliohm level. The unbalanced current caused by the unbalanced voltage is very small, and the neutral point potential is clamped to neutral potential. Therefore, the neutral line is not allowed to break, and the grounding voltage cannot be too high.

2. In TN-C system, the neutral line not only acts as the neutral point potential clamp, but also acts as the protection line. The neutral point cannot be disconnected. If it is necessary to disconnect, the contact used to disconnect the neutral wire must be closed before other contacts are closed, and then disconnected after other contacts are disconnected. At this time, if the neutral line is broken, the phenomenon of shell contact leakage occurs again. The equipment enclosure voltage is close to the phase voltage. Vulnerable to danger. Therefore, the neutral line of such system must be repeatedly grounded. (PE line of TN-S system can play a clamping role, so the neutral line can be disconnected)

Repeated grounding can realize two types of protection:

1) Reduce the voltage of the leakage device housing to the ground.

The voltage Ujd from the shell of the leakage device to the ground is equal to the voltage drop U generated by the single-phase short circuit grounding current Id in the neutral connection part. After the repeated grounding, it plays a shunt role.

2) Reduce the risk of electric shock when the neutral wire is disconnected (also reduce the leakage voltage of the housing)

The shell to ground voltage Ujd of the leakage device is close to the phase to ground voltage U, and after repeated grounding, UO and UC are both lower than U.

Setting principle of repeated grounding:

The terminal of the trunk line and branch line of the overhead line and every 1km along the line, the neutral line is repeatedly grounded;

Where cables and overhead lines are introduced into workshops or large buildings, the neutral line shall be repeatedly grounded (except for those not more than 50m from the grounding point);

In the power network where the grounding resistance of power equipment grounding device is allowed to reach 10 Ω, the grounding resistance of each repeated grounding device shall not exceed 30 Ω, and the repeated grounding shall not be less than 3.

The repeated grounding of neutral line allows the use of natural grounding body;

Low voltage lines powered by the same transformer or low-voltage bus should not be protected by grounding or neutral at the same time;

Repeated grounding refers to the neutral line in three-phase four wire system. It plays a certain role and has certain setting principles.

Three phase five wire PE wire shall be grounded nearby the equipment.

Compare the grounding fault of neutral grounded system and neutral ungrounded system:

1. Ungrounded neutral system

1) In case of case contact grounding fault, if the equipment is not grounded, the equipment enclosure voltage will rise to the phase voltage, which is very dangerous after human contact.

2) If the equipment is grounded, the human body will be grounded and connected to the equipment after contact. It can be seen that the smaller the grounding resistance of the equipment, the smaller the current flowing through the human body.

3) To limit the current flowing through the human body, the grounding resistance of the equipment shell must be controlled, which is generally less than 4 ohms.

4) Leakage current is too small to trip quickly.

5) After shell contact or single-phase grounding, the other two phase to ground voltages rise to 380V, which may burn out 220V equipment. (The three-phase phase voltage remains balanced) The neutral point is not grounded, and the leakage current returns to the power supply through the leakage capacitor, and the current is very small.

Excessive capacitive current will cause intermittent arc and overvoltage. Therefore, the arc suppression coil is generally used. At this time, the leakage current is equal to the sum of the current passing through the leakage capacitor and the current passing through the arc suppression coil. They compensate each other, reduce the fault current, avoid oscillation, and even extinguish the arc.

As the grounding capacitance current is not large, and the line voltage between the three phases is still balanced, there is no impact on the load power supply, so it can still operate for a period of time, which is the advantage of the system.

2. Neutral grounding system

1) When there is no repeated grounding, the voltage of equipment enclosure is the voltage drop caused by short circuit current.

2) In case of repeated grounding, the grounding resistance at the repeated grounding point is connected in series with the grounding resistance of the power supply to divide the voltage and reduce the enclosure voltage.

3) The leakage current is large and can trip quickly.

4) After shell contact or single-phase grounding, other two phases can still approach the phase voltage and prevent system oscillation. Electrical equipment and lines only need to consider their insulation level according to the phase voltage.