Oscilloscope Safety - Page 1
Forum section: electronics / beginners

author: Suaro
author_rank: Newbie
timestamp: September 09, 2022, 01:53:25 pm
content: I've watched Dave's "How NOT to Blow Up Your Oscilloscope!" video many times.This was a real eye opener for me and glad I found it.I've been studying electronics on my own for about 6 months now, and recently purchased tools (nice oscilloscope, bench power supply, function generator, etc...) to get real hands on and away from circuit simulations. I don't want to blow anything up and certainly don't want to get shockedThere's one part of Dave's video that I'm a little confused on and hoping someone might be able to provide feedback.The last part of the video, he talks about the bonus , where an Arduino is the DUT and plugged into PC/Laptop via USB.Under this scenario, because the PC is powered by 3 prong AC into the wall , the arduino's ground is earth referenced, which can cause potential risk when probing around with your oscilloscope. Understood. If not sure, unplug the PC from the wall , the USB connected arduino (or any other similar device ) will not be earth referenced in any way. Safe.In my case, I have a Surface Laptop. The AC power cable that plugs into the wall only hastwoprongs (no earth reference center prong). The little transformer box that sits in the middle between the power cable and pc also only has two wire connector as visually observed.If I plug my Surface notebook into the wall, then connect the arduino to the PC via USB, then check a ground pin on the arduino against earth referenced cable plugged into another wall (i.e. connect meter on center (earth ) of the ac cable plugged into another wall outlet and the ground on the arduino, I see low resistance (~200k ohms). How can this be ? How can a loop path exist via earth between arduino and earth ground ?While doing the same test above, I simply unplug the Surface Laptop from the power source, and the meter reads 0L - no path exists.Would love some insight into how 2 prong AC adapter is earth grounded.Thank you so much for any clarifications!

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 09, 2022, 02:09:04 pm
content: First observation: 200K ohms is not a “low” resistance, it’s quite high.You stated that when you remove the plug of the Surface power supply, the 200K resistance become infinity. Sounds right. Next step is to take the ac plug of the Surface in your hand and measure the resistance across the two blades of the plug. You’ll find your answer there.Ultimately, it requires an understanding of how US outlets are connected to the distribution points of line, neutral and ground.

author: Suaro
author_rank: Newbie
timestamp: September 09, 2022, 03:00:00 pm
content: Thanks for the quick reply.Sorry about that, the resistance is high, but its the existence of a path that is mostly concerning me.Essentially, I was thinking the two prong plug would allow the PC to float - thus preventing any ground path from USB connected arudino to earth ground but that's definitely not the case here.When I measure the resistance across the two terminals from the Surface power plug, my meter reads 0L (infinite )Not sure what to make of this.I'll investigate the point you have below about understanding US outlets, distribution, neutral , ground, etc... to see if I can fully understand what's happening here. For now, I'll just leave the Surface unplugged when probing around arduino and circuits that are connected to it.Thanks again!

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: September 09, 2022, 05:54:05 pm
content: The path is through the neutral connection in your circuit breaker panel.

author: Suaro
author_rank: Newbie
timestamp: September 09, 2022, 08:12:05 pm
content: Thanks!

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2022, 10:01:59 pm
content: You might find it useful to read some of the references (especially the Tek documents) inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/They are based on hard-won experience, and are considered and thoughtful - unlike the well-meaning quick and too-short replies you find on forums.

author: ledtester
author_rank: Super Contributor
timestamp: September 09, 2022, 10:38:14 pm
content: It seems that there shouldn't be a problem connecting USB ground to earth ground. After all, when you connect two USB devices you are connecting the two grounds together and the supply of one of the devices might very well be earth grounded.

author: EPAIII
author_rank: Super Contributor
timestamp: September 10, 2022, 10:32:53 am
content: To expand on that a bit further, in the US and probably other countries, the neutral and the SAFETY ground pins on a 115 Volt outlet are BOTH connected to ground. They just take different paths. Actually, at the first fuse or breaker panel from the pole power, the neutral and the safety ground are connected or "bounded" together. And a ground wire goes from there to a ground rod just outside of your house. It is after that point in the house wiring that they become separate.The reason for this is the two different purposes these two conductors serve. The neutral is there to carry ALL the current in the circuit back to the earth ground which the transformer on the pole is also connected to. This completes the circuit and all circuits must be completed.On the other hand, the safety ground is there to insure that any current that may start to flow in the outer case or skin of any electrical appliance or device will not allow that outer skin to become energized above ground. And it also insures that if current does start to flow through it, the fuse will blow or the breaker will trip; shutting the circuit down before anyone is shocked OR before any of the wires in the walls get hot enough to start a fire in the case of a short. When everything is normal, there is NO current in the safety ground.In spite of these two conductors both going back to a ground buss in that first panel and also being connected to ground there, they can and OFTEN DO have different potential or Voltage readings when referenced to a solid ground. This is because the neutral carries the full current of the circuit and Ohm's Law then converts that current to a Voltage according to the amount of resistance in the wire between where you measure and the actual ground. E = I x R. This difference in Voltage between neutral and safety ground is normal and completely expected in a working circuit.Your computer's power supply is probably a switching supply and may or may not have an isolation transformer. So there may be a DC path from your house wiring to the interior of the computer. The way the NEC (National Electric Code) reads is if there is no safety ground then there must be double insulation between the user and the line Voltage. That is two different layers of insulation, both of which are sufficient to protect that user from shock. Different manufacturers may do this in different ways and some of these ways are not readily apparent.Although present day equipment will adhere to the code, older devices may not. In my youth I worked on radio and TV receivers that had one side of the line connected directly to the chassis with non-polarized plugs. If you removed a cover you were potentially exposed to a shock hazard. It was a 50/50 chance depending on which way the plug was inserted in the socket. But most such equipment has been scraped a long time ago. Well, most of it anyway. A 1 to 1 isolation transformer and a ground strap was used to work on such equipment.Quote from: Ground_Loop on September 09, 2022, 05:54:05 pm

author: nightfire
author_rank: Frequent Contributor
timestamp: September 16, 2022, 01:38:48 am
content: When the electric network has ground reference, in lots of cases the Neutral and PE wires are grounded in the fuse box in a house/apartment.I cannot speak for the systems used in the US, as I am familiar with the german way of building distribution networks.The residential system I just read about differs a bit from the industrial 4wye/3phase with grounded midpoint, which resembles the european TN-S system a lot.In US-households there probably is the chance to get some single-Phase system with Neutral in the middle, which then is earthed.This means, that 120V will be L-M, which is also earthed. M is the middle tap, which basically behaves like a N(eutral) from the view of a user.(But is technically different from the way in europe single-phase power is brought to the outlet- in most modern buildings you get as standard in germany a 3-phase TN-S system, which is split in the central fuse box in your house into the single-phase circuits with RCCB and RCD that are required by actual standards)When a notebook attached to a charger shows some galvanic connection to earth, the charger is not really isolated from the USB ports.When it comes to measurements like this, when the DUT has some earth connection, lots of people recommend the use of isolation transformers, I even have seen some youtube vids where some oldtimers propagated the use of 2 isolation transformers, one for the DUT, the other for the scope....But unless you are using a scope with floating inputs, like the Fluke scopemeter series, there is the question of earth connections and the corresponding currents.