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usb isolation

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usb isolation

Postby NathanUnger » Sat Apr 15, 2017 4:52 am

Hello,

Do I need to isolate my picoscope 4444 USB from the desktop computer even if I am working on mains circuits?
What if I am using four TA271 adaptors which have only one BNC instead of two?
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Re: usb isolation

Postby Martyn » Fri Apr 21, 2017 7:26 am

If you are measuring Mains Circuits then you need to use appropriate CAT rated probes such as our PicoConnect 442
If using the TA271 then you will need to use an appropriate High Voltage Active Differential Probe
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Re: usb isolation

Postby checio » Sat Apr 22, 2017 11:30 pm

You may use some workaround to islolated USB - use laptop powered from battery. The truth is that laptop supplies are also isolated, but internal common mode cap's can make 127V floating potential.

I use to measure mains voltages (400-600V) with 100:1 probe TESTEC TT-HV250 (Uin max=2,5kV). For safety reasons, DUT is powered through isolating transformer.
If you haven't such transformer you must be very careful. Do things slowly, thinking twice. Always determine which wire is mains neutral and connect this as probe ground. Remember to not short live wires through probes ground.
A few days ago I had to maesure idle power consumption of fixed device. Current transformers was too insensitive. I've measured this through shunt resistor, both probes was referenced to neutral. Nothing special but if you're hurry you'll probably be shocked.
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Re: usb isolation - more questions

Postby NathanUnger » Mon Apr 24, 2017 5:00 am

Thank you for your reply.

- Will my computer be electrically isolated by the Pico4444 if I am measuring voltages under 30 volts that are ground referenced to the mains?

Here's an example of what I am trying to do.
I have an old machine with 4 analog meters on it showing different parameters. It is plugged into 220VAC.
I want to connect the Pico4444 with one channel to the back of each meter. None of those voltages exceed 30V at any time, but I need to keep them isolated from each other.
My computer is plugged into 110VAC. The neutrals on these two mains circuits (the 220v and the 110v) are not always exactly the same because of uneven loading, etc. Will my computer be isolated and protected by the Pico4444 differential front ends?
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Re: usb isolation

Postby Martyn » Mon Apr 24, 2017 5:53 am

checio wrote:If you haven't such transformer you must be very careful. Do things slowly, thinking twice.

We would add that if you have even the slightest doubt about what you are doing, then don't continue and seek advice.
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Re: usb isolation

Postby Martyn » Mon Apr 24, 2017 5:59 am

Please email your request in to support@picotech.com we want to make sure that you fully understand the safety aspects of any answers that we give.
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Re: usb isolation

Postby checio » Tue Apr 25, 2017 12:08 am

NathanUnger wrote: ... but I need to keep them isolated from each other.
The neutrals on these two mains circuits (the 220v and the 110v) are not always exactly the same because of uneven loading, etc.

You answered your question. Probes ground are connected to oscilloscope chasis, it will short your meters grounds and equalisation currents may flow. You must use separation or differential probes.

As you said, measuring different DUTs powered from mains is very tricky. You may measure DUT1 in cabinet 1 and DUT2 in cabinet 2. Theoretically both are referenced to PE wire but... when you have high loads, even small potential differences, less than 1VAC, may cause dozens of ampeers in PE wires. You may guess what will happen to your scope chasis. This is common when you have PLC in one cabinet and VFDrives in another. Theoretically you measure 24V, but must be aware of grounds, upss
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