Popular science on household leakage paths: the physical laws of resistance attenuation and ground failure

Introduction

When it comes to "leakage", many people intuitively think that it is a short circuit in the wire. In fact, the real leakage is often not caused by direct contact between two wires, but by the resistance of the insulating material declining over time due to aging and moisture, causing the current to "accidentally" flow to places it should not go. The core physical law behind this phenomenon is Ohm's law: Current (I) equals voltage (U) divided by resistance (R). When the insulation resistance drops from the original megaohm (MΩ) level to several thousand ohms (kΩ), even if the voltage remains unchanged, the leakage current will increase significantly, resulting in the risk of electric shock or even fire. This article will help you understand the causes, paths and protection methods of household leakage from a physical perspective.

Physical principles of leakage

Ideal insulating materials (such as PVC, rubber) have extremely high resistance, up to 10⁶ Ω (1 MΩ) or more. However, as the service life increases, the insulation layer will age due to environmental factors such as heat, oxygen, and ultraviolet rays; more importantly, moisture will penetrate into the insulation material and form conductive channels. Data show that when the ambient humidity increases, the surface of the insulator will absorb moisture, causing the surface conductivity to increase and the insulation resistance to decrease. A 20-year-old wire with an original insulation resistance of 1 MΩ may drop to 10 kΩ due to long-term moisture exposure. At this time, the leakage current will increase 100 times, and the risk of electric shock will rise sharply.

In addition, temperature also affects insulation resistance. The resistance value of general insulating materials at 70°C may be only 10% of that at 20°C. This explains why wiring near heat sources (e.g. electric water heaters, ovens) is more likely to leak.

Illustration of common paths

Leakage current does not appear out of thin air. It requires a "path" to flow to the earth or the human body. The following are the two most common leakage paths in the home:

Water seepage in the wall: capacitive coupling effect

When the wall is damp due to rain or pipe leaks, a "capacitance" will be formed between the wires in the wall and the wet wall. Alternating current can "jump" from wires to wet walls through capacitive coupling, and then flow through the wall to the earth. This leakage path is not easily noticeable to the naked eye, but when measuring the line-to-ground insulation resistance with a megger, you will find that the value drops significantly.

Socket corrosion: oxide film conductive mechanism

If the metal contacts of the socket are exposed to humid environments for a long time (such as bathrooms and kitchens), an oxide film will form on the surface. This oxide film itself is an insulator, but when moisture continues to invade, tiny conductive channels will be formed inside the oxide film, allowing current to leak from the live pins to the socket shell or ground. This kind of leakage is often accompanied by phenomena such as heating of the socket and peculiar smell.

Self-test science methods

If you want to know whether the circuit in your home is safe, the most direct tool is a "megohmmeter" (also known as a megger).

Megohmmeter detection steps

1. Turn off the main power supply and completely disconnect the line to be tested from the load (electrical appliance).
2. Connect the "L" end of the megger to the line conductor and the "E" end to the ground wire or metal pipe.
3. Shake the handle at about 120 rpm for 10 seconds to read the stable insulation resistance value.

According to standards, the insulation resistance of low-voltage electrical installations should not be less than 0.5 MΩ (for equipment and lines in operation, not less than 1 MΩ/kV). If the measured value is lower than 0.25 MΩ, it means that the leakage has reached a dangerous level and must be repaired immediately.

Trigger conditions for circuit breakage of leakage protector

Modern residences are commonly equipped with leakage protectors (leakage circuit breakers), whose operating principle is to detect the difference in current between the "inflow" and "outflow" circuits. When the difference exceeds the set threshold, the protector will open the circuit in a very short time. There are two common operating currents: 10 mA and 30 mA: the 10 mA protector has higher sensitivity and is suitable for humid and high-risk environments such as bathrooms and kitchens; the 30 mA protector is used in general situations and can strike a balance between safety and avoidance of malfunctions.

It is worth noting that leakage protectors are not omnipotent. If the leakage path does not pass through the protector (such as leaking directly from a live line to the ground), or the leakage current does not reach the action threshold, the protector may not trip. Therefore, regularly testing the insulation resistance with a megohmmeter is the most fundamental way to prevent leakage.

In summary, the root cause of household leakage lies in the attenuation of insulation resistance. By understanding Ohm's law, mastering the use of megohmmeters, and correctly selecting leakage protectors, you can effectively improve the electrical safety of Shuangbei old houses and avoid electric shock and fire accidents.