The first part: protect the earth
Grounding: for different purpose, to pass through the earth wire electrical installations in some place or grounding body, and the electrical connection in the earth to do good.Grounding work refers to the power system for running need or a one-point earthing of the equipment, such as transformer neutral point grounding directly;Grounding protection means to avoid personal accident getting an electric shock to the one point grounding of electric equipment, such as metal enclosure grounding of electrical equipment, etc.
Considering electrical equipment under the premise of not grounding, power neutral grounding and grounding two cases.
1. The power supply of neutral point grounding grid directly, the dangers of electrical equipment is not grounded
As shown, when the abnormal electrical equipment insulation shell belt, the body touch the equipment shell, the path of the current flows through the human body is: equipment shell - > - > - > power neutral point earth body.Assuming the body resistance is 1700 Ω RB, grounding resistance is 4 R0 Ω, the size of the current through the human body:
Beyond the scope of security, resulting in personal injury.
2. The power in the neutral point grounding grid directly, the dangers of electrical equipment is not grounded
If electrical equipment malfunction, shell belt electricity, the body touch the equipment enclosure, the path of the current flows through the human body is: equipment shell - > human - > other two phase line to ground capacitance - > other two phase power supply.Current through the human body the size of:
Its value depends on the cable to the capacitive reactance.In general, this situation may cause a person to get an electric shock.
Under the premise of electrical equipment grounding, respectively considering power neutral grounding and grounding two cases.
1. TT system: power supply neutral point grounding directly, the protection of the electrical equipment of the metal shell by the respective line PE directly grounded three phase four wire system low voltage power distribution system.
When the electrical equipment enclosure charged, assumes that the ground resistance RE = R0 = 4 Ω, human body resistance RB = 1700 Ω, at this point in the power circuit fault current is:
Even line when equipped with circuit breaker, usually is not enough to play overload protection function of the circuit breaker, so not to cut off the power circuit, this will make electrical equipment enclosure long-term existence of voltage 110 v, when the body touch the electric shell, for the current through the human body:
Can cause personal injury.
2. IT system: power supply neutral point grounding, and the protection of the electrical equipment of the metal shell by the respective line PE directly grounding three-phase three-wire system of low-voltage distribution system.
In IT systems, when the electrical equipment insulation damage make shell charged and human touch the shell, current from grounding body and at the same time through the human body.Because the body's resistance to many larger than grounding resistance Re, current flows through the body's current through the grounding body is than much smaller, the electrical equipment grounding protection, protection of personal safety.
Above all, protect the earth is mainly used in neutral point grounding grid (IT) system, work principle is the shunt resistor in parallel connection circuit.So the grounding resistance value is crucial for the protection effect.
The second part: protection by zero
The working principle of zero protection
Electric equipment to work normally, when the zero line voltage (or very small), electrically neutral due to electrical equipment shell connected to the mains zero, human touch device shell and there was no danger."Shell" occurs when the motor and other electric equipment failure, phase line to zero wire short, short circuit current enough to install in the power cord of the fuse or circuit breaker short circuit protection function, so as to cut off power supply.
Note: when a device with shell "fault" to fuse or circuit breaker to cut off the power supply of the time interval, the current through the human body at this point as shown:
Not considering cable resistance, obviously the electrical equipment enclosure to ground voltage is 220 v, if the body is very dangerous when it touches a shell;If considering cable resistance, including resistance R Φ is phase line, RN zero line resistance, assuming zero phase wire cable wire cable of 2 times, equipment shell voltage:
At this time the body will bear the voltage of 147 v, when it touches a shell is also dangerous.In general, the effectiveness of the protection by zero lies in the line of short circuit protection device can quickly cut off power supply.
Neutral grounding of TN system: power supply, load equipment by metal shell protection line to the ground of low-voltage distribution system.
Pick three forms of zero protection
1. TN - C system
The distribution system of the zero line N and the protection line PE for the same thread, become PEN line.
The problem: if the PEN point P disconnected, after the point P equipment shell, as a result of the load neutral point offset, possible dangerous voltage.More seriously, if the point P after a device with shell "fault", even if the lines are installed on the circuit breaker or fuse will not move, lead to the back of the equipment enclosure with phase voltage for a long time, very dangerous.
2. TN -s system
TN the zero line of N - S system and the protection of line PE is set apart, all equipment shell and public PE line connection.
current and work to become zero;PE line to protect the zero line.Zero N disconnected will only affect the normal use of single-phase load, will not make other equipment shell charged (premise is to protect the PE can't disconnect).
3. TN - C - S system
