Contact potential differenceTwo differentMetalResulting from contactpotentialPoor, related to factors such as metal material and contact surface temperature.
Concept of contact potential difference in grounding system:
Contact potential difference: groundingshort-circuit currentWhen flowing through the grounding device, a distributed potential is formed on the ground surface. The potential difference between two points at a horizontal distance of 1.0m from the ground to the equipment and a vertical distance of 2.0m from the equipment shell, frame or wall to the ground is called contact potential difference.Grounding mesh center pairGrounding gridThe maximum potential difference of the grounding electrode is called the maximum contactpotentialPoor.
Chinese name
Contact potential difference
Foreign name
contact potential difference
Nature
Potential difference between two electric layers
Conditions
The distance between two objects is less than 25 × 10-8 cm
The potential difference generated when two different metals contact each other, that is, the potential difference between the electric double layers.When two objects are in close contact and the distance between them is less than 25 × 10-8 cm, there will be double electric layers and contact potential difference.The value is related to the properties of the two metals and the temperature of the contact surface, but not to the size of the contact surface and the length of the contact time.After the two metals contact,Escape workThe smaller metal increases the potential due to the loss of electrons, and the metal with larger work escapes decreases the potential due to the increase of electrons. There is a potential difference between the two.
When any conductor is inserted into the electrolyte solution, the equilibrium state of the original solid and liquid two-phase medium will be affected by each other.
Taking the insertion of metal zinc into zinc sulfate solution as an example, polar water molecules and metal surfacesZinc ionThey are attracted to each other and arranged directionally on the metal surface;At the same time, under the attraction of water molecules and the impact of continuous thermal movement, zinc ions tend to break away from the lattice, so there are two situations: one is that the electrons in the metal attract the metal ions in the solution to make the ions deposit on the metal, and the other is that the metal dissolves into the solution.
Which of the two situations mentioned just now plays a leading role varies with the materials, but no matter which one plays a leading role first, it is the result of the weakening of one side and the strengthening of the other, leading to the final two phases tending to balance.Just as there is a temperature difference between two things, they will interact to reach the equilibrium state after contacting for a certain time.
Of course, in the electrochemical process, this balance is dynamic, so a certain residual charge distribution is formed on the metal surface and the solution interface.There is charge and distance (even though the distance is very short). Isn't this a capacitor?This is also called double electric layer.Capacitors, of course, have electric fields.
The potential difference between the two ends of the electric field is the main source of the electrode potential.
Then, two conductors are inserted into the electrolyte solution at the same time. Of course, a double electric layer is formed at the interface between each electrode and the solution, generating a certain electrode potential. Note that this is the potential, not the voltage.Just as the potential energy of water is not the same concept as the water pressure.
Calculation of maximum contact potential difference
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When the insulation M of the electrical device is damaged, the short circuit current flowing through the grounding electrode is Id.If the grounding resistance Rd of the grounding electrode, the voltage Ud=Id · Rd generated at the grounding electrode, and Ud is generally called fault voltage.Generally, the impedance of the grounding wire can be neglected, and the potential on M is Ud.When a person is in the dispersion zone, the ground potential of the person is U φ.At this time, if a person contacts M, the fault voltage Ut generated by the contact is Ud-U φ.When a person stands on the ground, the resistance of one foot's shoes, socks and ground is Rp. When a person contacts M, the two feet are in parallel, and the comprehensive resistance is Rp/2.Under the action of Ut, Rp/2 is connected in series with human body resistance RB, then the current flowing through the human body IB=Uf/(RB+Rp/2), and the voltage that the human body bears Ut=IB · RB=Uf · RB/(RB+Rp/2).When the insulation of the electrical device is damaged, the contact voltage Ut between the hands touching the electrical device and the feet touching the ground is related to the distance between M and the grounding electrode.The closer M is to the grounding electrode, the larger U φ is, the smaller Uf is, and correspondingly, the smaller Ut is.When people are outside the dispersion area, U φ=0, Uf=Ud, Ut=Ud · RB/RB+Rp/2), Ut is the maximum value.Since the position where people stand in the dispersion area is related to U φ, the contact voltage is usually calculated when standing 0.8m horizontally from the electrical device and 1.8m vertically when hands touch the electrical device.If the electrical device is outside the dispersion zone, the above horizontal and vertical distances need not be considered when calculating the contact voltage Ut[1]。。
Example calculation
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Generation of contact potential difference
When the insulation of any motor is damaged and short circuit occurs to the shell, because their shells are connected to the same grounding electrode, the three motor shells get the same grounding voltage U as the grounding electrode.When human hands touch any motor shell, they will bear contact voltage between hands and feet, and the farther away from the grounding body, the higher the contact voltage.The contact potential difference is mainly caused by the short circuit current of the power system, and may also be caused by the lightning current.In order to avoid personal injury caused by contact voltage, the outer edge of the grounding grid should be closed and each corner of the outer edge should be made into an arc.If it still cannot meet the requirements, the water average pressure tape can be laid.Gravel or asphalt pavement can be laid along the edge of the grounding grid where people often pass through, and pressure equalization tape can also be laid.According to relevant Chinese standards, in case of single-phase grounding or two-phase grounding at the same point in a large grounding short-circuit current system, the potential difference between the ground and the equipment shell (or the frame connected to the shell) at 0.8m from the horizontal distance of the ground to the equipment, or between the wall and the vertical distance of 1.8m from the ground, shall meet the following formula: ρ is the soil resistivity (Ω m) of the ground where people stand,T is the duration of grounding short circuit current (s).When a single-phase grounding occurs in a small grounding short-circuit current grounding system, the fault is generally not cut off quickly, and the duration of the grounding current is relatively long, so the allowable value of the contact potential difference must be reduced.At this time, Et should meet the following formula: Et=50+0.05 ρ[1]。
For places with poor conditions, such as mines and paddy fields, the allowable value of Et should be appropriately reduced[1]。
