A "How to use oscilloscope safety test to connect the mains circuit board" problem

Recently made a mistake Test action:

test Scenario: direct from the mains socket to access the 3W non-isolated switching power supply circuit board, using an oscilloscope to test the output voltage, when the oscilloscope through another outlet directly from the mains power

test Result: in the oscilloscope to the output negative moment, the leakage protection switch trip!!!

After consulting, analysis, just understand oneself because did not understand "oscilloscope ground and mains 0 line, the FireWire also has the relationship between the big address" to lead to the wrong test operation, this operation will not only let the mains trip, but also may burn the oscilloscope!

such as the interception of Guangzhou Zhiyuan Electronics Co., Ltd. "How to use oscilloscope safety measurement of mains", the chart showing the relationship between FireWire, 0-wire and ground:

FireWire (L): Also known as Phase line, provided by power station or substation, Voltage 220V, human contact will be dangerous;

0-Wire (N): Provide a circuit for the FireWire, grounding in the power station or substation end, because of the remote grounding, so in the residential building user terminal potential is not necessarily zero, may be weak, but relatively safe;

Ground (E): 0 Potential reference point, in residential building users to connect the earth, 0 voltage, absolutely safe.

Wrong method of measurement:

For example, "The ordinary oscilloscope and the mains are not isolated, the metal end of the shell and the negative end of the probe (ground) are connected with the ground, when using an oscilloscope directly to the 0-wire firewire measurement, will be indirectly 0 or the line of fire to the ground short circuit (equivalent to the red dashed line in the picture)".

Using an oscilloscope to test the circuit board directly connected to the mains, although not directly test the mains, but the same truth, the electricity will pass through the circuit board line and the oscilloscope's ground connection, and then connected to the Earth's ground e, the same cause 0 line or FireWire short-circuit to the ground, very dangerous.

For this scenario: the oscilloscope directly connected to the mains, circuit board AC Source, at this time to measure the circuit board, and even measure the output voltage of AC source without fault protection occurs, because the AC source itself is an isolated power supply, The output of the AC power supply is actually isolated from the mains, so the oscilloscope ground and its phase will not produce a short circuit loop, resulting in a failure.

Based on the above error method, we may think of the oscilloscope's power line to be artificially disconnected or use the isolation transformer method to power the oscilloscope, to disconnect the test loop, but this method belongs to the "floating" test, also not recommended. Such as:

Recommended methods of measurement:

"A-b pseudo-Differential measurement"

Using the common passive probe application "A-B" method to measure the mains, the two-channel probe should be connected to the negative end (ground) to the power line, a channel probe probe (positive end) connected to 0 lines, the other channel probe probe (positive end) connected to the FireWire (5 left), the two channels of the measurement difference is the City radio.

High Voltage differential probe measurement

The use of high-voltage differential probes to measure the mains, FireWire and 0-wire test point back and forward are not OK, the inside of the probe through high-impedance way to the measurement side of the ground and the oscilloscope to isolate, will not cause short-circuit problems, this is the best recommended way, safe and convenient, but the price will be more expensive.

A "How to use oscilloscope safety test to connect the mains circuit board" problem