Discussion on common problems in installation and

Discussion on common problems in the installation and operation of leakage protector

as an important backup protection means of safety technical measures, the installation of leakage protector (RCD) in low-voltage power has a significant effect on preventing personal electric shock casualties, electrical equipment damage accidents, grounding faults and electrical fire accidents. In order to make the leakage protector play its due role, it is necessary to strengthen the technical training of installation and daily maintenance personnel to ensure correct selection and wiring. At the same time, the operation management and regular test of leakage protector shall be strengthened. However, there are still some problems in the installation and operation of leakage protector, which affect the normal operation of leakage protector. Therefore, it is necessary to analyze and discuss some common problems

1 Selection of RCD sensitivity

the author once found that some electrical installers installed RCD with action current of 10 mA or 6 Ma in places with high risk of electric shock such as bathroom. The reason is that because the bathroom is wet, only RCD with small action current can make personnel get away from the power supply in time in case of personal electric shock

in fact, according to International Electrotechnical standards, when the AC power frequency current through the human body does not exceed 30 Ma, the human body will not die due to ventricular fibrillation, which is not directly related to the humidity and contact voltage of the human body. Therefore, the International Electrotechnical standard stipulates that RCD with action current not greater than 30 Ma shall be adopted in all provisions against personal electric shock. The author believes that it is not good to choose a fast RCD with high sensitivity and action current of no more than 10 Ma in the above places. The reason is that 10 Ma and 30 Ma RCD have the same effect on preventing personal electric shock, which can prevent people from dying due to ventricular fibrillation. 10 Ma RCD is expensive and not suitable for wide use; And its rated non operating current is only 5 mA. In wet places such as bathrooms, due to the large leakage current of the line, it is easy to cause frequent misoperation and power failure. Finally, people often short-circuit or disassemble the RCD, which does not change the structure and working state of the original machinery and equipment, so that the line loses the ground fault protection and causes greater danger

the selection principle of RCD should be: the performance indexes such as rated voltage, rated current and breaking capacity of RCD should adapt to the line conditions (unbalanced leakage current, electromagnetic interference, etc.); The type of RCD should adapt to the power supply line, power supply mode, system grounding type and the characteristics of electrical equipment, rather than one-sided pursuit of RCD with small action current

generally, the rapid RCD with action current less than 10 Ma is selected only in the occasions where serious secondary accidents may be caused after electric shock, or in order to protect children or patients. For class I hand-held electric tools, a fast RCD with action current of 10 ~ 30 Ma shall be installed according to the danger of the workplace. The actual conditions of RCD manufacturing should also be considered when selecting the action current. For example, the operating current of pure electromagnetic products is difficult to be below 40 Ma, so electromagnetic RCD with high sensitivity should not be pursued. In the case of graded protection, the selection of action current should also consider the need of graded protection selectivity. The total protection should be equipped with RCD with low sensitivity or a little delay. For lighting lines, hierarchical protection should be adopted according to the size and distribution of leakage current. High sensitivity RCD is selected for branch line and medium sensitivity RCD is selected for trunk line

2 scope of RCD to prevent electric shock accidents

many people believe that as long as RCD can operate normally, electric shock accidents can be prevented. In practical application, RCD will not act in some cases:

(1) phase zero or phase electric shock

when the human body contacts phase zero (or phase), the human body resistance is equivalent to a load. At this time, although the human body stands on the ground, after the electric shock current passing through the human body is shunted, especially the detection of tensile strength, most of the circuit is formed by phase zero (or phase) wires, The electric shock current is only a small part of the distribution transformer through the earth, and the current can not make the RCD act. The general requirements for residual current operated protectors (GB) stipulates that "leakage protectors cannot protect against the risk of electric shock caused by simultaneous contact with two wires of the protected circuit"