USBscope50 USB PC Oscilloscope
Frequently Asked Questions : Isolation
Why is isolation important for a PC oscilloscope ?
When you connect a measuring device to a PC, generally the ground of the instrument is wired to the ground of the PC. If your PC is earthed, and you accidentally touch the ground clip on the instrument to an earth referred voltage, like the output of a power supply, or mains voltage for example, you will short circuit that voltage via the instrument and the PC. This can lead to damaged equipment, damaged PC, dangerous currents flowing and so on.
An even more hazardous situation arrises when the PC isn't earthed. You connect your instrument and connect the ground clip to a high potential (perhaps by mistake). Then the whole PC is at that elevated potential. This is very dangerous as it presents a shock hazard for you when you touch the PC.
The USBscope50 avoids this hazard by isolating the ground clip from the PC's ground. If you measure with a DMM between the USBscope50's ground (on the BNC) and the ground on the USB shell, there is no direct current path (there is a small AC path caused by capacitance in the power supply circuit of the scope, but this is negligible).
So with the USBscope50, if you connect the ground clip to some potential, that potential is neither shorted out via the PC nor is it conducted to the PC to make the PC sit at a hazardous potential.
Other USB scopes do not offer this important safety feature
What does the 300V Cat II isolation rating mean?
This means that for a Cat II installation (domestic power outlets, lab use and so on) the USBscope50 can be used with a potential difference between the ground clip and the PC's ground of 300V RMS (or DC), which in the above types of environment could see voltage transients of up to 2500V. This does not mean the scope will keep working properly with such large voltage spikes across its isolation circuit, but it does mean that the scope will isolate the PC (and you) from seeing the large and dangerous potentials.
If the scope has a 300V Cat II rating, does this mean I can meausre mains voltage which could be 340V Pk-Pk?
The isolation rating means that there can be 300V RMS (or DC) between the ground clip and the PC's ground (and for safety considerations this applies to the probe tip too). But the USBscope50, with a x10 probe, can measure up to +/-300V i.e. peak-peak this is 600V. The scope itself will be easily capable of measuring the 240V RMS level because this equates to about 340V pk-pk, and the scope can handle 600V pk-pk.
Also remember that the x10 probe includes a 9MegOhm resistance, which works with the scope's 1MegOhm input resistance to attenuate the signal by a factor of 10. So the 240V RMS signal on the probe tip turns into a 24V RMS signal at the scope's input.
The 300V Cat II rating really can be thought of as the maximum voltage that can be tolerated between the ground clip and the ground of the PC that you are using i.e. the allowable difference in potential between the unit under test and your PC. It's not directly to do with the measurement range of the USBscope50.
So the answer to the question is "yes", there's no problem with measuring 240V RMS with a x10 probe.
What are the safety implications of measuring large voltages?
Always read the USBscope50 Safety advice that can be sound in the User's Guide. Always follow this advice. You are also advised to select scope probes that are at least 300V Cat II rated and you should be aware that fully insulated scope probes are a good idea for such measurements because the ground clip and the BNC's ground could all be at a hazardous potential.
If I stack two or more USBscope50's, are the channels isolated from each other?
No, they share a common ground potential. But they are all still isolated from the PC.
