This article focuses on:no electricity in the zero line
《Let’s say a lamp, he must be connected to a fire line a zero line, the current from the fire line through the bulb to the zero line to form a circuit, etc. will light, right, but why is there no electricity on the zero line?》
A: From the questioner’s point of view, for single-phase lighting lines, zero line no electricity is estimated to be the bulb ? internal filament has been completely burned out, even if the current flow from the fire line to the head of the point, no longer able to form a closed circuit to continue down.
no electricity in the zero line
Single-phase AC power is branched all the way from the three-phase four-wire power supply line, because the output of the three-phase four-wire system for L1, L2, L3 three fire line (phase) and star connection Y neutral point leads to a line called zero line, indicated by the letter N. For safety reasons, there is a grounding protection device in the transformer casing, under normal circumstances it is connected to the laying grounding network together with the neutral zero line, and the grounding resistance value is required to be less than or equal to 4Ω, because the earth is an infinite bulk fluid, it can ensure that the grounding point is always as one is zero potential. See the figure below
For the zero line has no current this problem? We consider it from two aspects, the first is the single-phase circuit, (no electricity in the zero line)single-phase circuit in this case, we analyze the circuit will, see the following figure shows that
Single-phase AC load of white woven lamp belongs to the resistive load, in addition to inductive load and capacitive load. Here only say white woven lamp load, under normal circumstances if the white woven lamp is on, the zero line is certain to have current, it is exactly the same size as the current value on the fire line.
no electricity in the zero line
We can use the AC clamp meter respectively in the case of non-stop fire line and zero line to measure the AC current between them, no doubt the same. (See the figure below)
The reason why the zero line is bare, why people touch the zero line do not feel the electric shock, which is because the three-phase four-wire system or three-phase five-wire system in the line of the zero line or protective grounding device are well connected to the earth, the wire even if there is a certain resistance value, but relative to the human body resistance, the human body resistance value is much larger than the wire resistance value. Even if the three-phase load is seriously unbalanced, then the zero line on the charged situation, its potential difference is not too high, (no electricity in the zero line)this potential difference on the human body resistance, there is a very small weak current, the human body does not feel it at all.
However, due to other reasons caused by the zero line short open, that will not work, when you touch the point that has been disconnected, the load in the fire line although no closed circuit is formed, your human body touches the fire line through the zero line, equivalent to the human body in series in the line, but also can form a closed circuit, any one more than the human body perceived current or electric shock current, is particularly dangerous.
In a three-phase four-wire circuit, the load must be calculated by “vector diagram”, according to the law of contact current and the law of circuit voltage in the AC current.
In AC circuits, both voltage and current are sinusoidal alternating variables, and therefore must be expressed as vectors when applying the nodal current law and the circuit voltage law. Thus, the nodal current law can be expressed as follows: In a sinusoidal AC circuit, the vector sum of the incoming nodal currents is equal to zero. This is expressed by the formula Σi=0 and specifies that the current flowing into the node takes a positive sign and the current flowing out of the node takes a negative sign. The circuit voltage law can be expressed as follows: In any circuit of a sinusoidal AC circuit, the vector sum of the electric potential of each supply is equal to the vector sum of the voltage drops on each component. Expressed in the formula ΣE = ΣU, and the selected winding direction with the same power supply potential and voltage drop to take the positive sign, and vice versa to take the negative sign. See the figure below.
In order to illustrate the problem, the following example: there is a three-phase four-wire connection of high-power resistance furnace, the resistance value of each phase of the electric heating wire Ra = Rb = 4Ω, Rc = 2Ω, the supply voltage is 380V, connected to the zero line N. (no electricity in the zero line)Try to ask this a resistance furnace in the phase current, line current and zero line current is how much each?
According to the relationship between phase voltage and line voltage in electrical engineering is Uxg = Uxn/√3 = 380/1.732 = 220V Uxg in the formula indicates the phase voltage, Uxn indicates the line voltage.
Then calculate Ia=Ib=Uxg/Ra=220/4=55A according to Ohm’s law.
iC=Uxg/2=110A.
According to the star connection, each phase current and the corresponding phase voltage is the same phase, in the star connection, the line current is equal to the phase current. So the line current is: Ia = Ib = 55A, Ic = 110A.
At this time, the zero line current formula is equal to the vector sum of the phase current, that is, Io = Ia + Ib + Ic. Calculate the zero line current Io, you can first seek Ia and Ib vector sum, see the vector diagram above. From the vector diagram can be obtained: Ia,+Ib = 55A, and with Ic is the opposite direction, therefore, Io = Ic +55 = 110-55 = 55A, and at this time the zero line current Io and Uc is the same phase. It must be noted that the current of the zero line, (no electricity in the zero line)although there, but it is through the wire will be zero line current with the transformer output of the neutral point to together, to participate in the opportunity to continue to work. That is to say, only completely clear it must be calculated using the basic concept of vector diagram to be able to calculate.
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