-
My first scope is arriving but my home (old) doesn't have mains grounding
Posted by Fergo on 08 Aug, 2019 20:50 -
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.
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
-
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.
-
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! -
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!
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. -
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)! -
since you going into this business. maybe its time to...
not just to protect you, but to protect the IC's from yourself...ymmv. -
since you going into this business. maybe its time to...
I assume you missed the part about living on the 21st floor.
not just to protect you, but to protect the IC's from yourself...ymmv. -
I assume you missed the part about living on the 21st floor.
Sorry, could not resist
-
I assume you missed the part about living on the 21st floor.
right. maybe burying the rod in the concrete floor may help? it should be better than nothing, no? -
right. maybe burying the rod in the concrete floor may help? it should be better than nothing, no?
You'd only be moderately dead instead of very dead.
-
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).
In such "low voltage works" case get grounded multi-outlet adapter (splitter) where you plug all your equipment including scope and computer. Obviously you have to avoid any works related to mains voltages.
-
Thanks for all the suggestions everyone. They've been very insightful (well, with the exception to that plastic bag ground perhaps
).
I am aware that my situation is far from ideal. As a hobbyist, I wanna avoid doing stupid things as I don't have much electrical knowledge, despite messing with electronics for quite some time now (don't even know if that makes any sense...). Anyway, as I mentioned, things should get much better next year, when renovations are planned to start.QuoteA *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.
You mean to use something like this 2 prong RCD to plug the equipotential power strip to my mains?
https://www.amazon.com/OAONAN-Replacement-Interrupter-Protection-Conditioner/dp/B073PRH8H3/ref=sr_1_3?keywords=OAONAN+GFCI+Replacement+Plug+Assembly+2-Prongs&qid=1565318806&s=gateway&sr=8-3
I was also giving my situation a little bit more thought and two ideas popped up:
1) I have a 120V UPS 2000VA that is not being used for anything atm. What if I disconnect it from the mains so it becomes active and use it to power the scope and eventually everything that I'd be testing, including the PC and it's USB interface?
2) If I decide to go with the option of using the scope plugged into the mains and have to do some probing on an Arduino connected to a PC via USB: using my notebook while on battery power instead of my desktop computer should be fine, right? Since the device being tested - including it's power source (the notebook) - are isolated.
Again, thank you all for the replies.
KR,
Fergo.
-
Hi.
Since your electrical system does not provide Earthing :
None of your equipment will be earthed/grounded no matter what gadget you use/try (only creating an Earth/Ground will work).
You should be able to use the oscilloscope without the fear of accidentally grounding a circuit that is not grounded and damaging YOU or the scope or the circuit.
Any equipment with a metal case is a potential hazard (if a fault causes live mains to touch the case). Fully plastic enclosed equipment is safe(ish).
Just think carefully what you are going to do to the circuit you are measuring with the scope before you start (it is a good habit to form).
Try not to perform measurments on mains electricity while you have no ground.
Stay safe and happy
-
Without knowing how the local utility company implement their Earthing system, sinking a wire into soil, concrete or plumbing fixtures may or may not achieve much,
and or score floating voltage drama.
Maybe the dude next door is playing with electricity too after watching some backyardesque Youtubes
perhaps going for it with coat hangers and leaky dumpster grade arc welder that escaped its overdue rendezvous with the metal scrap yard
AFAIK+EMMV: in a proper Earth/ground system, a UPS in battery backup mode will 'usually' supply 60 volts on both live pins to give 120 volts,
and NO EARTH/ground, unless it sources it from the UPS chassis and or output third pin out of the wall socket.
i.e. even with a UPS in backup mode via battery, you still have a non-Earth referenced floating live electrical system that can still bite you, hopefully via 'inverter to output transformer'
but not with the full force of a power station feeding a street transformer
(but chances are still good for an unplanned demise from the Earth, and consequent burial in it..)
How about a 120v isolation transformer with two 60v secondaries with a center tap between the two acting as the third pin ground-earth reference,
for a 'better than nothing, hey might even work..' portable 'ghetto Earth'?
NOTE: I will delete the above tip if it's not the 'balanced transformer' set up I was thinking of.
Otherwise, lose the shorts and flip flops,
and enter Float City with gloves, glasses, long sleeves, rubber soles
..and someone nearby with a fully charged phone
-
Without knowing how the local utility company implement their Earthing system, sinking a wire into soil, concrete or plumbing fixtures may or may not achieve much,
electric shock happens when there is current path from contact back to the source. in the case of inside the building, it means the building is acting as conductor to ground and back to wherever the current came from. if no return path, this is just open circuit and contact will float along with the electric potential that is being contacted. another food for thought...
and or score floating voltage drama.
-
If that building is heated with a central hot water heater and metal pipes, you can use the metal pipe as a makeshift ground. It won't be a high quality ground, but still better than no ground at all.
-
If that building is heated with a central hot water heater and metal pipes, you can use the metal pipe as a makeshift ground. It won't be a high quality ground, but still better than no ground at all.
People already mentioned the issue with that. You don't know who else in the building might put tension on those pipes, potentially exposing yourself to new and unseen dangers. -
You said the building is currently being renovated. Maybe you could ask one of the electricians guys when you see them how the existing system is grounded. Or play a dummy and tell the landlord you are worried about your washing machine not being grounded and let them give you an answer.
-
If that building is heated with a central hot water heater and metal pipes, you can use the metal pipe as a makeshift ground. It won't be a high quality ground, but still better than no ground at all.
No. Many modern piping is not metal anymore. Plus if the pipe suddenly fails to be a proper ground the pipe could be live. You shower would be live. Your faucet would be live.
If you own the building and have control over everybody working there, and there are RCD's on everything... then perhaps... but 21st floor means probably not owning the building.
Anyway... I dealt with this.
Make sure you have an RCD/GFCI, and make sure your DUT power supply is referenced to the scope potential before connecting the DUT. Ian talked about this.
The hazard is in connecting two floating things, you will get equalization current, this can vaporize LQFP packages depending on the capacitance behind it.
The slight problem though is the stingy to severely uncomfortable feeling if you are somehow a bit grounded and you touch your equipment receiving all Y-cap leakage current of your entire bench with equipment.
-
Thank you!
Anyway... I dealt with this.
Make sure you have an RCD/GFCI, and make sure your DUT power supply is referenced to the scope potential before connecting the DUT. Ian talked about this.
The hazard is in connecting two floating things, you will get equalization current, this can vaporize LQFP packages depending on the capacitance behind it.
No RCDs, only regular breakers like this one in my apartment's switchbox (not sure if that's the name, but is the place where you have all the circuit breakers and stuff... English is not my first language). RCDs only became mandatory here in 1997 and my building is from early 70s.
Replacing the regular power plug of an extension block with something link this can provide a safer work enviroment?
https://www.amazon.com/OAONAN-Replacement-Interrupter-Protection-Conditioner/dp/B073PRH8H3/ref=sr_1_3?keywords=OAONAN+GFCI+Replacement+Plug+Assembly+2-Prongs&qid=1565318806&s=gateway&sr=8-3
Thanks!
KR,
Fergo -
RCD/GFCI needs an Earth/ground reference to work,
otherwise an imbalance between the Live 1 and Live 2 conductors (Active and Neutral) during a fault won't do anything after passing through the internal sensing coil
They are not magic, they need a solid three wire system AND be tested with an external RCD/GFCI tester to ensure they will work.
The TEST button on RCDs tests the RCD internals, it does not guarantee a trip on a real incoming fault,
nor do most users test it regularly (if at all) anyway.
Don't learn it the hard way
I would suggest to have a local MCB breaker and or glass fuse in a plastic CAT friendly box (not the meow kind) to power your gear, so it trips before the main breaker,
and maybe saves you and or the gear
especially if the RCD isn't in the mood to play 'save the mancave dude"
-
equipotential as Ian.M posted is the term, i googled "localized ground" and got disappointed. attached is probably okaish for lab that needs "localized virtual ground". a warning beeper may help to safety when the virtual ground raised to alarming level, this should make your lab a partial faraday cage. ymmv.
-
There is too much bad information in this thread. And unless you are able to provide solutions for TT, IT, and TN distribution systems, do not say anything. Perhaps Dave should find an engineer from an accredited body to explain this stuff in one of his interview videos.
North American jurisdictions all have something to similar to article 250 of the NEC (NFPA70) for grounding/PE interface requirements. In Europe, reference your harmonized national standard per IEC60364, and BS7671 for the UK. Read the appropriate reference now. -
Whatever the local distribution 'system' is, I'd be very hesitant to assume anything electrical has been done right in any 'built to a price' multi-storey building
with 'hurry up/anything goes' and or 'make it up as you go...' snafu Earthing,
much less any real 'R' testing to see if RCD and circuit breakers will work on a fault on ALL floors.
i.e. if you wanna play with electronics involving AC, better get a leg up on safe electrics that power it first
-
I am not an engineer but I am an electrician. Some people have weird ideas in this thread. An RCD or GFCI is a very good idea since if current that flows out through L1 and doesn't return on L2 wil cause it to trip. This is the danger with having no group d at the out let since you can become a path to ground just by touching the metal enclosure of the device. The danger in not having a grounding means at your outlet is that you could have damage or a fault in your device or dut and energize something that shouldn't be, if supplied from a grounded outlet this would cause your breaker or fuse to trip and deenergize the circuit. Since you don't have that the next best thing is to detect when current is flowing outside of the intended circuit and deenergize at that point. This is what an RCD/GFCI will do.
Sent from my SM-G965W using Tapatalk