How to safely measure high voltage with a LOTO oscilloscope?
In this article, we define voltages above tens of volts, which exceed the safety voltage for the human body or the range of the oscilloscope, as high voltage, and use 220V AC electricity as an example for discussion.
The oscilloscope has a standard square wave for self-test and probe compensation, usually 1K Hz. Our USB oscilloscope also has this standard square wave, with an amplitude of about 1.5V. The default is a standard square wave, and when needed, it can also be set to a PWM wave with an adjustable frequency duty cycle.
I'm off topic. Most of the problems I encountered with AC power damage during technical support were caused by newbies who were curious and connected to the AC power. So I have to emphasize that the oscilloscope has its own square wave function, so newbies should not test the AC power right away. Experienced users are very cautious and will not do this.
The most common questions asked by new recruits are:
Why does the mains power always trip when I measure it with an oscilloscope?
Why did my oscilloscope burn out when I was measuring AC power?
First of all, we need to understand the power supply lines and principles of the mains electricity, which will help with safe electricity use and safe measurement.
Live, Neutral and Ground
The specification of my country's municipal electricity (residential electricity) is AC 220V@50Hz. The power supply line consists of a live wire, a neutral wire and a ground wire. Their relationship is shown in Figure 1.1.
Live wire (L): Also called phase wire, provided by power station or substation, with effective voltage of 220V, which is dangerous for human contact;
Neutral line (N): provides a loop for the live line and is grounded at the power station or substation. Since it is remotely grounded, the potential at the user end of the residential building is not necessarily zero and may carry weak current, but it is relatively safe.
Ground wire (E): zero potential reference point, connected to the ground at the user end of a residential building, zero voltage, absolutely safe.
Figure 1 The relationship between the live wire, the neutral wire, and the ground wire
First, confirm the oscilloscope range:
For example, for most LOTO oscilloscopes, the standard probe has a range of +-5V at the ×1 position and +-50V at the X10 position. Some models have a range of +-20V at the ×1 position and +-200V at the X10 position.
But this range is still not enough, because the mains 220V actually refers to its equivalent value. The peak-to-peak voltage is around 600V.
So can we just use a 100:1 high voltage passive probe? Theoretically, yes, but in practice we may still run into problems due to using the wrong measurement method.
Wrong measurement method:
When using a LOTO USB oscilloscope on a desktop computer or a laptop with a power adapter, the metal end of the housing and the negative end (ground) of the probe are connected to the USB ground wire, which is connected to the computer ground and then to the ground of the power strip.
In this case, when an oscilloscope is used to measure the neutral wire and the live wire directly, the neutral wire or the live wire will be indirectly short-circuited to the ground wire (equivalent to the red dotted line in the figure), which is very dangerous. This is also the most common incorrect measurement method.
Figure 2 Incorrect measurement method
Floating measurement method is not recommended:
As shown in Figure 3, using an isolation transformer to power a USB oscilloscope on a desktop computer can disconnect the measurement loop and achieve "floating" measurement. When a laptop is connected to a LOTO oscilloscope without a power adapter, the oscilloscope ground is also floating.
Figure 3 Floating measurement method
When measuring floating ground, the metal end of the oscilloscope housing and the ground end of the probe have the same voltage level and are short-circuited with the high voltage end being measured. Therefore, when measuring the mains, if you neglect to connect the oscilloscope's grounding clamp to the live wire, the oscilloscope and computer housing will have a voltage of 220V. Once the human body touches the oscilloscope housing, the current will flow through the human body into the ground, which is very dangerous!
Figure 4 Safety hazards of floating ground measurement
Resistor voltage division measurement method is not recommended:
Some engineers who are familiar with strong electricity may think of dividing the 220V mains power into a relatively low voltage through a resistor and then measuring it. This avoids the risk of contact with high voltage.
As shown in the figure above, two resistors are used to attenuate the live wire and the ground by 100 times, and then an oscilloscope is used to measure:
This approach is the same as using a 100:1 high voltage passive probe:
As can be seen from the above figure, this method can be successfully measured. The reason why it is not recommended is that it has high requirements for the operator, who must be very familiar with the high-voltage circuit being measured and be careful not to make mistakes. Because if the wires are connected in reverse or not connected properly, there may be catastrophic consequences.
From a safety perspective, it is very safe to use the "AB" method and a high-voltage differential probe to measure AC power using an ordinary oscilloscope. They are introduced below.
" AB " pseudo-differential measurement
When using an ordinary passive probe to measure the AC power using the "AB" method, and using an appropriate probe attenuation ratio to limit the voltage within the oscilloscope range, the negative terminals (ground) of the two-channel probes should be connected to the power ground wire, the probe tip (positive terminal) of one channel should be connected to the neutral wire, and the probe tip (positive terminal) of the other channel should be connected to the live wire (as shown on the left of Figure 5). The measured difference between the two channels is the AC power waveform.
This approach is also possible, but be careful that both probes are not grounded. Although this can avoid the problem of hot ground and reverse connection, you still need to be careful not to exceed the range of the oscilloscope. We recommend another method, which is to use an isolated differential module to measure as described below.
Isolated Differential Module Measurements
When using an isolated differential module to measure AC power, it does not matter whether the live and neutral test points are connected in the forward and reverse directions. The probe uses magnetic and electrical isolation to isolate the ground of the measurement end from the ground of the oscilloscope, which will not cause a short circuit problem.
The isolated differential module that comes with the LOTO oscilloscope is not as expensive as the traditional high-voltage isolation probe, which costs two or three thousand yuan. It is only about one hundred yuan, but it can measure high voltage of +-800V in an isolated differential manner.
IDM01 isolated differential module is the best recommended method, safe and convenient.
We filmed a demonstration video of two probes measuring 220V mains electricity as follows:
As an important measuring instrument, we should use the oscilloscope correctly and safely. Especially when measuring some high-voltage signals, we must understand the relationship between various signal lines in order to correctly connect the measured signal and ensure safe measurement.
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