If an ordinary oscilloscope is not isolated from the mains, will the neutral or live wire be short-circuited to the ground?

I recently made a mistake testing the operation:

Test scenario: Take power directly from the mains socket and connect it to the 3W non-isolated switching power supply circuit board. Use an oscilloscope to test the output voltage. At this time, the oscilloscope takes power directly from the mains through another socket.

Test consequences: The moment the oscilloscope was connected to the negative pole of the output, the leakage protection switch tripped! ! !

After consultation and analysis, I realized that I did not understand the "relationship between the ground wire of the oscilloscope, the neutral line, the live wire and the large address of the mains power", which led to the wrong test operation. This operation not only caused the mains power to trip, but also may cause the mains power to trip. It will burn out the oscilloscope!

The following picture is taken from Guangzhou Zhiyuan Electronics Co., Ltd.'s "How to Safely Measure Mains Power with an Oscilloscope". The picture shows the relationship between the live wire, neutral wire and ground wire:

Live wire (L): Also called phase wire, it is provided by a power station or substation with a voltage of 220V. It is dangerous for human body to contact it;

Neutral line (N): Provides a loop for the live line and is grounded at the power station or substation; because it is grounded remotely, the potential at the user end of the residential building is not necessarily zero and may carry weak electricity, but it is relatively safe;

Ground wire (E): Zero potential reference point, connected to the earth at the user end of a residential building, zero voltage, absolutely safe.

Wrong measurement method:

As shown in the figure below, "Ordinary oscilloscopes are not isolated from the mains. The metal end of the casing and the negative end (ground) of the probe are both connected to the ground wire. When the oscilloscope is used to directly measure the neutral and live wires, the neutral or live wires will be indirectly connected. Short circuit to the ground (equivalent to the red dotted line in the figure)".

When using an oscilloscope to test a circuit board that is directly connected to the mains power, although it is not directly testing the mains power, in the same way, the mains power will be connected to the ground wire of the oscilloscope through the circuit board line, and then connected to the earth's ground wire E, which will also cause The neutral or live wire is short-circuited to the ground, which is very dangerous.

For this scenario: the oscilloscope is directly connected to the mains, and the circuit board is connected to the AC Source. At this time, the circuit board is measured, and even the output voltage of the AC Source is measured, but there is no fault protection. This is because the AC Source itself is an isolated power supply. , its output AC power has actually been isolated from the mains, so when the oscilloscope ground wire is connected to it, there will be no short-circuit loop, causing faults.

Based on the above wrong method, we may think of artificially disconnecting the power ground wire of the oscilloscope or using an isolation transformer to power the oscilloscope to disconnect the test loop. However, this method is a "floating ground" test. Likewise, Not recommended. As shown below:

Recommended measurement methods:

"AB pseudo-differential measurement"

When using an ordinary passive probe to measure the mains power using the "AB" method, the negative ends (ground) of the two-channel probes should be connected to the power ground wire, the probe probe (positive end) of one channel should be connected to the neutral wire, and the probe probe (positive end) of one channel should be connected to the neutral wire. The probe probe (positive end) of one channel is connected to the live wire (as shown on the left in Figure 5), and the measurement difference between the two channels is the mains waveform.

High Voltage Differential Probe Measurements

Use a high-voltage differential probe to measure the mains power. It doesn’t matter if the live wire and neutral wire test points are connected forward or reverse. The ground of the measuring end and the ground of the oscilloscope are isolated through high impedance inside the probe, which will not cause short circuit problems. This is the best The recommended method is safe and convenient, but the price will be more expensive.