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My first scope is arriving but my home (old) doesn't have mains grounding

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Fergo:
Hey everyone!

I recently ordered my first scope (DS1054Z) and it's arriving soon. In the meantime, I watched tons of videos about how to proper use the scope and, specially, how to safely use one. After watching Dave's video about not blowing up your scope, which is heavily focused on grounding issues, I was a bit unsure about my situation. I live in an old building where the mains outlets don't have the ground pin.

 :-BROKE

What is the safest way to power the scope and make safe measurements in this situation?

I will mostly use the scope to probe low voltage such as uC/TTL/Arduino signals and maybe some 12V signals. From what I saw, I should be fine if the DUT is DC or battery powered, but I will mostly use the DUT connected to my computer with USB (as I do a lot of serial stuff), which can provide some risks as the USB negative is theoretically tied to earth (theoretically because my computer is not grounded either, as the sockets don't have the ground pin).

Unfortunately, moving to a more "electrically safe" home is not an option at the moment, and since I live in the 21st floor, neither it is to get proper grounding on my outlets.

Thanks in advance. I really appreciate your help.

Fergo

mindcrime:
You could get a USB isolator that uses something like opto-isolation. Something like this

https://www.adafruit.com/product/2107

or this:

https://www.sealevel.com/product/iso-1-oem-seaiso-embedded-single-port-inline-usb-isolator/

could work for you.

There are also opto-isolated USB hubs, but they aren't cheap. For example:

https://www.sealevel.com/product/hub7i-optically-isolated-7-port-usb-hub/


You might also be able to get an electrician to set you up with at least one properly grounded outlet if there's a good earth ground to be had somewhere in the house.  Would cost a little bit, but it might be worth it.

Fergo:
Thanks for the quick reply. I'll get one of those USB isolators then.

And regarding the scope itself, anything I should be specially aware of since I'm also plugging it on an unearthed socket?
The building where I live is actually in the process of renovating all the electrical infrastructure, but that will take some time.

Thanks!

mindcrime:

--- Quote from: Fergo on August 08, 2019, 10:00:42 pm ---Thanks for the quick reply. I'll get one of those USB isolators then.

And regarding the scope itself, anything I should be specially aware of since I'm also plugging it on an unearthed socket?
The building where I live is actually in the process of renovating all the electrical infrastructure, but that will take some time.

Thanks!

--- End quote ---

They do make (at least for US style outlets) little adapters that plug into a 2 pin socket and give you a 3 pin socket, and then there's a green wire (or a grounding tab) you're supposed to connect to an earth ground.



In many cases, the center screw that holds the face plate on the outlet is actually an earth ground and you simply connect to that and you're good to go. BUT, there is no guarantee that that center screw is actually properly grounded.  So be very careful of making any assumptions regarding any of this.  For example, if the center screw *isn't* grounded you may think "I'll just run some wire to this copper water pipe"... now you have to be sure that the copper water pipe is actually grounded, and that the wire you use is rated for the correct voltage and that the resistance of the wire run isn't too high, that the wire doesn't get physically kicked loose, or chewed by a dog, cat, child, etc. etc.

Disclaimer: I AM NOT AN ELECTRICIAN and am NOT necessarily recommending you do anything involving outlets and mains voltage.  If you do it, I'd recommend consulting an electrician or at least some of the more experienced members here for advice first.  There may also be quirks related to the way wiring is done where you live, etc., that I'm not familiar with. 

Edit: just re-read your post and saw about living on the 21st floor and all. I was thinking you were in a house, just an older one without the earth ground outlets. Yeah, I wouldn't do anything involving the outlets without consulting an electrician who's licensed in your area.

Ian.M:
This has been gone over many times before on these forums.   Basically, if you use a scope and many other items of mains powered test equipment on ungrounded circuits, there is a significant risk of safety hazards (why nearly all scope manufacturers nowadays tell you *NEVER* 'float your scope' - I wont rehash the horror stories and risks in this post), and a likelihood that their performance will be impaired due to leakage current and noise pickup, and a medium to high risk of equipment damage if you probe or work on any devices under test that are connected to any signal that has a true ground (e.g to any part of your home entertainment system if you have cable, or communal satellite and/or TV antennae.)

A *LOT* of the problems can be mitigated by equipotential bonding.   Basically you create an equipotential zone where all 'grounds' connect together even that's floating with respect to true ground + the use of a GFCI (RCD in 'europespeak') to trip the mains supply if there is significant leakage current from it.  Within the zone, everything works as-if it was grounded but you have to be careful of external signal connections e.g. aerials, cable, network cables and phone lines that go anywhere outside the zone as a hazardous voltage difference may exist between them and equipment 'grounded' to the zone.

For a single workbench not near other exposed metalwork, or a wet zone (e.g. near sinks, baths, showers etc.), with an insulating floor or floor covering under and around it (i.e. no bare reinforced concrete floors), that can be as simple as plugging in an extension socket strip consisting of three pin sockets with a common ground into a GFCI protected socket or adapter ad plugging all your equipment into the socket strip.   If you need more than one socket strip (not daisychained)  each must be on a GFCI protected circuit and you must join their grounds.   

Its important that *ALL* metalwork that would normally be grounded that there is any risk of coming into contact with near your bench must be tied to your bench ground so if you have pipes, a kitchen sink or 'white goods' appliances near your bench, you *MUST* talk to a locally qualified  electrician to find out how an equipotential bonding zone can be set up without breaching local electrical codes, or if you'll need to relocate your bench.

Tempting as it is to ground to a metal water pipe, that becomes a severe hazard if done without a full understanding of your local electrical code.  e.g. If there is a high resistance joint anywhere below you in the building, (or if a plumber patches in a length of plastic pipe to fix a leak without connecting an adequate bonding wire across the plastic section), a fault in any mains appliances or equipment using the water pipes for ground, or simply with a metal case in contact with a water pipe, could result in all plumbing above the faulty joint becoming live.   As you cant control or even know if another tenant has a dodgy fridge or cooker pushed up against a water pipe, *NEVER* trust a water pipe ground in a multi-occupancy dwelling (or anywhere else where *YOU* don't have full responsibility for and control of all plumbing from the water supply company stopcock onwards)!

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