USB isolation still impt when scope is floating?

[ksat44]

Hi, guys. I'm pretty green here, so please be kind 

I was wondering if USB isolation would be important (or even necessary) in a USB scope/ laptop combo where the laptop would be run solely off its batteries and never plugged into something like a wall outlet for charging. Isolation, in my understanding, seems to help remove ground loop issues, but I'm wondering if there's anything in addition to that it's designed to do.

Thanks for any feedback.

[Ian.M]

A typical laptop certainly wont be safe to handle with high voltage on its chassis.  Some even have conductive cases.  If anything it would be more dangerous than floating a conventional scope which is well known to be occasionally lethal. 

OTOH, if you are only working with low voltages (under +/-50V from ground) in low energy circuits (i.e. no high current supercaps or big battery banks), at low frequency,  it can provide isolation.

However if you get anything wrong, or forget to unplug that laptop PSU, you could not only blow the scope but the laptop hosting it as well.  Assuming you avoid injury, if you've got a recent full backup you are only out the cost of both + the time to acquire a replacement, reinstall all your applications and restore your data.  If not you'll be cursing the day you chose to be cheap and didn't get an isolated differential probe!

[Brumby]

Such a situation has little direct risk to equipment - but there is the potential for you to be at risk.

If you connect your USB scope to a device under test (DUT) and that device has the potential to present a significant voltage (referenced to earth) such as a mains powered unit, then the USB scope could potentially be raised to that voltage.  If there is no USB isolation, then the laptop will also be raised to that voltage.  The equipment will continue to function normally and there will be no indication that this is the case.

In this situation, if you make electrical contact between your test rig and an earthed article - you could receive a shock.

In the case of an earthed measurement system (eg traditional scope) - if the voltage exceeds its capabilities, something will go phhhhhttttt.

[Brumby]

+1 to what Ian.M said ... especially this:

... you'll be cursing the day you chose to be cheap and didn't get an isolated differential probe!

When the voltage potential^* starts getting up there, the isolated differential probe is the correct way to deal with such situations.

^* Yes, I meant to say that

[rstofer]

I'm just guessing without ever trying it but I would imagine a floating scope could have some voltage on the BNC connectors relative to earth ground.  That's why the scope is being floated in the first place.  This same BNC ground is connected to circuit ground which will eventually be connected to USB ground inside the scope and inside the PC which will elevate the laptop ground above earth ground.  This seems like a horrible situation.

In my view, if a scope needs to be floated, the operator is making a potentially fatal mistake.  Get a differential probe or use Math->Ch1-Ch2 differential measurement if the scope has such a thing.

For my Rigol DS1054Z, the procedure is on page 6-45

https://www.batronix.com/pdf/Rigol/UserGuide/DS1000Z_UserGuide_EN.pdf

[ogden]

In my view, if a scope needs to be floated, the operator is making a potentially fatal mistake.

Yes indeed. Never ever float instrument, especially when it is designed to be powered from AC mains. By floating scope you can kill yourself or get into jail for a loooong time when you kill somebody else.

[rstofer]

Using an isolator to provide protection from accidental mistakes is always a good idea.  Especially when working on motor drivers and such where the Vmotor could get shorted to the USB circuitry.  Protect the PC!

[ksat44]

Thank you for the very thorough replies.

In my situation, I would be using the (as yet to be purchased) scope primarily in an automotive setting, where voltages are typically low, though something like inductive feedback on an ignition coil primary could change that.

After doing some searching on the site here, the consensus seems to be the e Owon VDS-1022/VDS-1022i are halfway competent scopes for their price and the issue I had was whether it was important spend the 30 or so extra for the isolated model.  Obviously, from your responses, it appears that it is.

Just one more question on the original topic: Would USB isolation also offer some noise protection for a scope which might be introduced due to the laptop being on a charger at the time of use? I've noticed some people online complain about their handheld battery powered scopes throw readings off slightly when they're used whilst they're on a charger. Removal of the charger makes the noise goes away.

[Gyro]

USB isolation certainly breaks the ground loop that can occur between the circuit ground and the laptop charger (or desktop mains lead). In addition to protecting the PC and its USB port, yes, it eliminates this potential source of noise.