Working principle of live working - Solutions - Huaqiang Electronic Network

SMD aluminum electrolytic capacitor

First, the ground potential operation principle The position diagram and equivalent circuit of the ground potential operation are shown in Figure 1-2.
The operator is on the ground or on the tower, and the human body potential is at the same potential as the earth (pole). At this time, the current through the human body has two circuits: 1, the charged body → the insulated operating rod (or Other tools) → the human body → the earth, forming a resistance loop; 2, the charged body → the air gap → the human body → the earth, forming a capacitive current loop. Both loop currents flow through the body into the earth (rod). Strictly speaking, not only the capacitive current exists between the working phase wire and the human body, but also the capacitive current exists between the other two phase wires and the human body. However, the capacitance current is related to the size of the air gap. The farther the distance is, the smaller the capacitor current is. Therefore, the effect of the other two-phase wires can be ignored in the analysis, or the capacitor current can be considered as an equivalent parameter.

Since the body resistance is much smaller than the resistance of the insulating tool, that is, Rr< I=I'+I"
among them
I'=UPH/R
I′′=UPH/XC
The epoxy resin-based insulating material used for live working has a high electrical resistivity. For example, the volume resistivity of the 3640-type insulating pipe is greater than 1012 Ω·cm under normal conditions, and the insulating resistance of the tool is 1010 to 1012 Ω or more. For 10kV distribution lines, the leakage current I' is
I'=5.77/107≈0.5(μA)
In other words, the leakage current is only microampere.
In indirect operation, when the human body and the charged body maintain a safe distance, the capacitance between the human and the charged body is about 2.2×10-12～4.4×10-12F, and the capacitive reactance is
XC=1/(ωC)=1/(2πfC)≈0.72×109～1.44×109(Ω)
Then the capacitor current is I′′=5.77×103/(1.44×109)≈4(μA)
That is, when indirect operation, the human body capacitance current is also micro-ampere. Therefore, the vector sum of I'+I" is also micro-ampere, which is much smaller than the perceived value of human body current of 1 mA.
The above analysis and calculation show that when the ground potential operation mode is applied, as long as the human body and the charged body maintain a sufficient safety distance and the tool with good insulation performance is used, the leakage current and the capacitance current through the tool are very small (micro-ampere level). Such a small current has no effect on the human body. Therefore, it is enough to ensure the safety of the workers.
However, it must be pointed out that the performance of the insulating tool is directly related to the safety of the operator. If the surface of the insulating tool is dirty, or the inner and outer surfaces are wet, the leakage current will increase sharply. When it is increased above the human body's perceived current, there will be an electric shock or even an electric shock. Therefore, keep the surface of the tool dry and clean during use, and pay attention to proper storage to prevent moisture.

Second, the intermediate potential working principle The position diagram of the intermediate potential operation and the equivalent circuit are shown in Figure 1-3.
When the operator stands on the insulating ladder or the insulating platform, the operation with the insulating rod is the intermediate potential operation, and the human body potential is the intermediate potential of a certain suspension which is lower than the electric potential and higher than the ground potential.
When working with the intermediate potential method, a capacitor C1 is formed between the human body and the wire, and another capacitor C2 is formed between the human body and the ground (the tower), the resistance of the insulating rod is R1, and the insulation resistance of the insulating platform is R2.

The operator separates the grounding body and the charged body through two parts of the insulator, and the two parts of the insulator together act to limit the current flowing through the human body, and the air gap is combined to prevent the charged body from discharging through the human body to the grounding body. The combined gap consists of two air gaps.
In general, as long as the insulation level of the insulation operating tool and the insulation platform meets the requirements, the insulator composed of C1 and C2 can limit the leakage current to the micro-ampere level. As long as the two air gaps reach the specified working gap, the capacitor circuit consisting of C1 and C2 can also limit the capacitive current through the human body to the micro-amp level.
It should be pointed out that when the intermediate potential method is used, the voltage of the charged body to the ground is shared by the combined gap. The potential of the human body is a floating potential, and there is a potential difference between the charged body and the grounding body. During the operation:
(1) Groundwork personnel are not allowed to transfer items directly to intermediate potential workers by hand. This is because: 1 If the metal tool is directly contacted or transferred, the electrostatic shock phenomenon may occur due to the potential difference between the two; 2 if the ground operator directly contacts the intermediate potential person, it is equivalent to shorting the insulation platform and is insulated. The resistance R2 of the platform and the capacitance C2 between the person and the ground tend to be zero, which not only may cause the leakage current to increase sharply, but also because the combined gap becomes a single gap, air gap breakdown may occur, causing electric shock and death to the operator.
(2) When the system voltage is high, the space field strength is high, and the intermediate potential workers should wear shielding clothing to avoid people's discomfort caused by excessive field strength. However, in the live working of the distribution line, due to the low space field strength and the dense power equipment of the power distribution system, the space operation gap is small, and the operator is not allowed to wear the shielding suit, but should wear the insulation suit to perform the operation.
(3) The insulation platform and the insulation rod should be inspected regularly to maintain good insulation performance. The effective insulation length should meet the requirements of the corresponding voltage level. The combined clearance should be about 20% larger than the single gap of the corresponding voltage level.

Third, the principle of equipotential operation causes the human body to have numbness and even death due to electricity, not the level of the potential of the human body, but the size of the current flowing through the human body. According to Ohm's law, when the human body is not exposed to an object with a potential difference at the same time, no current flows through the human body. In theory, the operator with the same potential of the charged body has the same potential, and the current flowing through the human body is zero, so the equipotential operation is safe.
When the human body and the charged body are equipotential, if the two hands (or two feet) are in contact with the dotted wire at the same time, and the distance between the hands is 1.0 m, the potential difference acting on the human body is the voltage drop on the wire. If the wire is LGJ-150 type, the resistance of this section is 0.00021Ω. When the load current is 200A, then the potential difference is 0.042V, and the body resistance is 1000Ω, then the current through the human body is 42μA, which is much smaller than the human sense current. 1000μA, the human body does not have any discomfort. If the operator is wearing a shielded suit, because the shielded suit has a bypass current, the current flowing through the human body will be smaller.
In equipotential operation, the most important is the safety protection into or out of the equipotential process. We know that there is an electric field in the space around the dotted wire. Generally, the closer the distance from the dotted wire, the higher the spatial field strength. When an electric conductor is placed in an electric field, a charge opposite to the polarity of the charged body is induced on the side close to the high-voltage charged body. When the operator enters the charged body along the insulator, the insulation resistance of the insulator itself is sufficiently large. The leakage current through the human body will be small, but as people and the charged body gradually approach, the induction effect becomes more and more intense, and the local electric field between the human body and the wire is getting higher and higher. When the distance between the human body and the charged body is reduced to a field strength sufficient to cause air to liberate, a discharge will occur between the charged body and the human body. When the human hand approaches the live wire, it will see the arc happening and produce a squeaking discharge sound, which is the reason that the electric energy is converted into sound, light and heat during the neutralization of the positive and negative charges. When the human body is completely in contact with the charged body, the neutralization process is completed, and the human body and the charged body reach the same potential. During the process of achieving the equipotential, a large transient capacitor discharge current will occur, and the equivalent circuit is shown in Figure 1-4.
In Figure 1-4, UC is the potential difference between the human body and the charged body. This potential difference acts on the capacitor C formed by the human body and the charged body. During the transition of the equipotential, a discharge loop is formed. Corresponding to the moment when the switch S is turned on, at this time, only the body resistance Rr is limited, and the initial value Ich of the inrush current can be obtained by Ohm's law, that is,
Ich=UC/Rr

For transmission lines of 110 kV or higher, the initial value of the inrush current is generally about ten to several tens of amperes. It can be seen that the initial value of the inrush current is large, so the operator must wear a full set of shielding suits to contact the wires through conductive gloves or equipotential transfer lines (rods). If you touch the wire directly, it will cause strong irritation to the human body, which may cause electrical burns or cause a secondary accident. Of course, since the inrush current is a pulse discharge current, the duration is short, and the attenuation is fast. The shielding suit can have a good bypass effect, so that the inrush current flowing directly into the human body is very small, and the continuous flow capacity of the shielding suit is relatively small. Large, transient inrush current will not cause any damage to the shielding suit. Generally speaking, the conductive gloves are used to contact the charged wires. Since the human body wearing the shielding suit is close to the charged wires, it is equivalent to the two plates of the capacitor, and the induced charge is increased, so the inrush current is also large. If the operator uses the potential transfer line (rod) to connect, the human body can maintain a large distance to the wire, so that the induced charge is reduced, and the intermediate current is also reduced, thereby avoiding the influence of the equipotential transient current on the human body.
When the operator is off the high potential, that is, when the person is separated from the charged body and has an air gap, the two plates of the capacitor appear, and the electrostatic induction phenomenon occurs at the same time, and the capacitor is recharged. When the gap is so small that the field strength is high enough to cause the air to liberate, a discharge will occur between the charged body and the human body, and an electric arc will occur and a squeaking discharge sound will be emitted. Therefore, each time the working position is moved, if the human body does not maintain the same potential as the charged body, the charging and discharging process will occur. When the equipotential worker approaches the wire, if the action is slow and keeps the wire at a critical distance from which the air gap is easily broken down, then the air insulation breaks down and sometimes recovers, and the energy exchange between the capacitor C and the system occurs repeatedly. Some of these energy is converted into heat energy, which may cause some of the wires of the conductive glove to be blown. Therefore, it is necessary to move into the equipotential and the detachment equipotential.
The time of the equipotential transition is very short. When the hand and the wire are gripped, the rush current is reduced to less than 1% of the maximum value after about a few microseconds, and the equipotential enters the steady state phase. When the human body is equipotential with the charged body, it is like a bird resting on a single wire. Even if the human body has a bright spot in contact with the electric conductor, since the voltage drop between the two points is small, the current flowing through the human body is at the micro-ampere level, and the human body does not have any discomfort. From the analysis of the above operating principle, the equipotential operation is safe, but in the process of equipotential, the following points should be noted:
(1) When an operator enters a high potential by means of an insulating tool (hard ladder, ladder, hanging basket, boom, etc.), the insulating tool should perform well and maintain an effective insulation length corresponding to the corresponding voltage level, so that the human body can pass through the human body. The leakage current is controlled at the level of the micro-ampere level.
(2) The length of the combined gap must meet the requirements of relevant regulations and standards, so that the discharge probability is controlled below 10-5.
(3) When entering or leaving the equipotential, it is necessary to prevent the impact of transient inrush current on the human body. Therefore, in equipotential operation, the operator must wear a full set of shielding equipment to implement safety protection.