Reasons and solutions for the electrification of metal aluminum box of instrumentation equipment

When the notebook is connected to a two-phase power supply, there is a common problem of enclosure leakage. Touching the metal part of the notebook with your hand will feel numbness; the metal enclosure of the computer host is also charged when the ground wire is suspended. Other metal enclosure electronic This phenomenon also exists in a large number of devices, such as household tablet computers, washing machines, refrigerators, etc. The occurrence of the above problems is obviously because the enclosure of the device is "electric". So why does the metal casing carry "electricity"? How to prevent the enclosure of the instrumentation equipment from being charged and hurting people? The analysis of these two issues is as follows:

1. The reason why the instrument enclosure is charged

1. The electrical instrument enclosure is normally charged

It is normal for most of the metal enclosures of electrical instruments to be charged. This kind of "electricity" is generally called induced electricity or dangling voltage, and the measured voltage is the induced voltage. Some people also call this "electricity" leakage current or leakage current. Its characteristics are: the voltage is higher than 48V, and the current is very small. Usually there is no direct harm to human safety, but some people who are particularly sensitive to electricity have a feeling of numbness. What is the reason for the "electricity" in the casing of the electrical instrument?

The induced voltage mentioned above refers to the voltage difference between the metal casing of the electrical instrument and the ground when the normal electrical instrument is isolated from the ground. The impedance caused by the insulation resistance and the capacitive reactance of the distributed capacitance between the internal circuit of the electrical instrument and the casing of the electrical instrument. In addition, the casing of the electrical instrument and the ground also have an equivalent impedance. The instrument enclosure is equivalent to the distributed capacitance and capacitive reactance of the earth in parallel. According to the characteristics of the circuit in series, the ground voltage of the electrical equipment enclosure can be obtained.

How is the contact current generated by the power adapter transmitted to the notebook? Whether it is a power frequency power supply or a switching power supply adapter, generally there will be an equivalent capacitor on the high-voltage and low-voltage side of the main transformer, and there will also be a small amount of leakage current. The voltage of the notebook, and the negative pole is the functional ground of the notebook, which is used to connect the  aluminum enclosure of the notebook, the functional ground of the USB interface, and the functional ground of the HDMI interface, and this negative pole is often connected to the ground of the adapter). Then this creates conditions for the contact current of the adapter to be passed to the notebook. This situation cannot be avoided. The solution is to increase the copper shielding layer in the middle layer of the transformer to isolate and ground, but this effect cannot be completely shielded, and there is a little leakage. In fact, most of this induced electricity can be completely ignored.

Under normal circumstances, most of the contact current generated by the adapter will be transmitted back to the ground through the grounding wire of the adapter; a small part is transmitted to the ungrounded casing, ungrounded data cable, etc., so the human body cannot feel it; and when the grounding is poor Or when there is no grounding, the touch current generated by the adapter cannot escape through the ground terminal of the power adapter, and will be transmitted to the metal enclosure of the notebook and the USB cable; when a person touches the notebook enclosure, the touch current will flow directly to the ground through the human body. This creates the feeling that the subject says. In order to prevent the danger of touching the current, it is necessary to limit the current size. This is an important security test when testing adapters. As long as it is a qualified product, the contact current value will be within the safe range.

Any equipment powered by AC (mainly adapters containing Y capacitors, switching power supplies, etc.) will inevitably generate touch current, but most of them are drained by the ground; when testing the touch current of the equipment, disconnect the ground wire. For testing, that is to say, even if there is a situation like the subject, there will be no danger.

2. The electronic enclosure of the electrical instrument is abnormally charged

According to the national standard for electromagnetic compatibility (EMC, EMI) and safety requirements. The main content of EMI design is the design of noise filter. The use of switching power supply filters can effectively suppress the noise in the power grid from entering the equipment (power supply), and can also suppress the noise generated by the equipment (power supply) from polluting the power grid. In order to suppress these noise pollutions, a filter circuit must be used. In order to suppress industrial common mode interference, it is necessary to connect two capacitors to the ground on the two input lines L and N respectively. One of the major functions of capacitors is to block direct communication (generally high noise and high disturbance are high frequency signals, which can be approximated to understand the main AC signal). If it is grounded, this part of the interference signal flows into the ground to achieve the effect of filtering. If not grounded, if a person touches it, it will flow into the human body. Also known as the leakage phenomenon. (The voltage of this part is AC, and the static state may generally be as high as about 110V).

It is stipulated in the national standard GB4943 (safety of information technology equipment) that relevant electrical appliances (including power supplies) must be grounded. If the computer case is not grounded, and the power supply is not grounded, this part of the electricity will not be released, causing the case to be charged.

However, due to safety regulations, this leakage current will not exceed the human body's tolerance limit, that is, generally, the electricity will not kill people, and these two capacitors will generally not be short-circuited (generally, a medium that will self-burn in a short-circuit is used). The solution is to ground well. If the ground cannot be grounded, the ground wire can be separated from the capacitor, or the capacitor can be removed (EMI will increase. Of course, it is possible to replace these two capacitors with common mode inductance, but the cost will be much higher), this is Most computer casings are charged with electricity.

The high-voltage side and low-voltage side of the switching power supply (that is, the two sides that the optocoupler crosses, generally there will be a clear distance between the two sides of the circuit board) There is usually an EMI capacitor (some power supplies may also be connected in parallel with a high-resistance resistor (rare)) , if there is leakage current on the power supply side, leakage will also occur.

The metal casing of an electrical instrument is electrified because it is connected to the power cord or the insulation of the electrical instrument is damaged for some reason, which is called abnormal electrification of the electrical instrument casing. At this time, the voltage of the metal casing of the electrical instrument to the ground is almost equal to the power supply voltage, which is a great threat to the safety of the human body.

It can be seen from the above analysis that the metal enclosure of the electrical instrument is abnormally charged, especially when the internal insulation of the electrical instrument is short-circuited, once the human body touches the Industrial Control enclosure of the electrical instrument, the voltage applied to the human body is 220V AC voltage, which is fatal. .