Making sure I do not blow up my Oscilloscope or worse, get hurt.

[Tech77997]

Hello,

I am more or less a complete beginner with electronics and I am getting started with learning the basics. So to understand my context I am not necessarily designing anything right now, rather I am reading The Art of Electronics and I want to go through examples, build basic circuits on my breadboard, and investigate and study them with my oscilloscope.

I have watched Dave's video (Dave if you are reading, thank you so much for this great video!), here about How to Not Blow Up Your Oscilloscope here: https://www.youtube.com/watch?time_continue=1136&v=xaELqAo4kkQ&feature=emb_logo

I want to make sure I am taking the best safety precautions I possibly can before I begin my studies. The power supply I have is this one: https://www.amazon.com/Tekpower-TP3005T-Variable-Linear-Alligator/dp/B00ZBCLJSY

In order to avoid blowing up my oscilloscope and getting hurt (in the context of what Dave's video was mentioning about not causing a dead short to ground), I believe that for my purposes and for the tools I have, I need to make sure to not connect my circuit-under-test to the green ground (Earth) pin on my power supply, but rather only utilize the black and red pins. This way, my CUT is isolated and floating, and the dead short to ground phenomenon will not occur and I will be safe to use my oscilloscope probes anywhere on the circuit and not necessarily be concerned with the placing of my ground lead of my scope probe.

However, I believe I also need to make sure that when I use multiple scope probes to investigate my CUT, I need to make sure they are all in reference to the same point.

I am so grateful you for your time inaddressing my following concerns:

• Am I understanding this correctly?
• Is there anything else I should do?
• For my purposes, do I need to purchase an Active Differential Probe? or Isolation Transformers? Based on my purposes and if I always make sure my CUT is not connection to Earth, I believe from the video I am understanding that no, I do not need these things but I would like to be absolutely sure.
• Also, am I safe to assume that my power supply will not somehow be connected to earth ground even when not using that green ground pin? I tried my best to purchase a highly rated power supply so I am thinking that concern about that would be paranoia. But I like to inquire to professionals about that, as always.

I really appreciate all of your responses and look forward to hearing from you. Thank you for your time and for your experienced insight. I learn a lot from this great forum!

[bob91343]

There is far too much confusing misinformation about grounds.

A ground is what a reference point is called.  Originally it was a pole shoved into the earth but we seldom do that any more.

The third wire in the electrical outlet, green, isn't intended to pass any load current but just to be a reference point for safety reasons.  If you connect all your equipment grounds to it you should be safe.  That translates to connecting to the chassis or ground of the equipment you are using, and usually the black terminal on stuff that uses terminals.

The long, third pin of the power plug is connected to the chassis of the equipment.  Inside the electrical system of the house, that pin goes to the metal conduit and any other large metallic parts.

You won't blow anything up if you make sure all your gear is grounded to this system.

Exceptions occur.  If you want to float equipment you will run into trouble if you don't know what you are doing.  A portable DVM has no power cord so you can connect either wire anywhere you like without a problem, although you need to respect high voltage.

The old precaution of keeping one hand in your pocket is a good one but that's hard to do when you are moving equipment around.

You need experience and a few jolts to learn about this stuff.  I know of no instance where anyone got electrocuted from 120V but I suppose it could happen.  Long ago I lost count of how many times I have felt currents through my body, and I'm still here.

[rstofer]

You have to connect the probe ground somewhere and it is usually the black post of the PS or some other ground in the project.  But as soon as you do, you have connected Earth Ground to your device ground through the BNC connector and the probe ground clip.

None of this stuff is a problem - right up until you start messing with switching mode power supplies using mains voltage.  I'm not going to go into this because there are several different ways to melt things and you shouldn't be getting to these projects for quite a while.

Just make sure your scope ground clip is ultimately connected to the black binding post of the PS.  Not necessarily at the PS but at an equivalent spot in your circuit.

It isn't that hard, don't overthink it!

[JustMeHere]

  I know of no instance where anyone got electrocuted from 120V but I suppose it could happen.

It doesn't take much of a Google search.  This is why GFCI came along and is required in bathrooms and kitchens.

[JustMeHere]

Also, am I safe to assume that my power supply will not somehow be connected to earth ground even when not using that green ground pin?

The neutral and ground are bonded in the circuit breaker panel.  The purpose of the ground wire in your house is to provide an alternate path back to the panel.  Without this, if the metal body of a device were energized and you touched it, you may provide the path to ground (and the neutral) through the floor or earth.  The ground wire should trip the breaker, but it is not guaranteed. 

With your power supply unplugged and all of the capacitance inside of it discharged, probe it with your multimeter.  Probe in Voltage mode first.  This will help determine if there's any power left anywhere.  You can then probe in continuity mode.  See if you get a low resistance from any of the outputs to the prongs on the power cord.  (I think Dave mentions this in the video.)

[bob91343]

I did a google search and it's problematical about electrocution.  There are other ramifications that apparently can lead to serious problems but direct electrocution doesn't seem to be one of them.  And all the documented cases I found involved electricians working on power poles on ladders.

So I maintain that I have never heard of an electrocution from 120V, especially if you limit it to a layman in his workshop.

[james_s]

You are overthinking this. It's good that you are conscious of the risk of blowing up your scope and/or injuring yourself but the fact is, as long as your circuit is being run from batteries or from an isolated power supply (as virtually any DC power supply is) then you have nothing to worry about. Stay out of the inside of things that are wall powered and you won't blow anything up.

Regarding electrocutions on 120V, it is rare but it can and has happened. It usually takes some special circumstances, such as standing barefoot on a damp concrete floor or being in the bath and reaching for an electrical device. It was more common back in the era of the "hot chassis" AC/DC radios, say a knob is missing and someone reaches out of the shower, steadies themselves by grabbing the metal faucet and touches the bare shaft on the radio. It can also have consequences for people who have heart conditions or other existing problems that make them more vulnerable. It's best to treat mains current as lethal even if the risk of actually dying is small. 

This article https://www.electricianshangout.com/can-i-be-killed-by-110-volts/ claims that over 90% of electrocutions are from 120V residential however I don't see any source cited so take that with a grain of salt.

[chephato]

Regarding electrocutions on 120V, it is rare but it can and has happened. It usually takes some special circumstances, such as standing barefoot on a damp concrete floor or being in the bath and reaching for an electrical device. It was more common back in the era of the "hot chassis" AC/DC radios, say a knob is missing and someone reaches out of the shower, steadies themselves by grabbing the metal faucet and touches the bare shaft on the radio. It can also have consequences for people who have heart conditions or other existing problems that make them more vulnerable. It's best to treat mains current as lethal even if the risk of actually dying is small. 

This article https://www.electricianshangout.com/can-i-be-killed-by-110-volts/ claims that over 90% of electrocutions are from 120V residential however I don't see any source cited so take that with a grain of salt.

I'm a beginner too and if were to work on mains AC voltages I'd only do it with a large rubber mat underneath my feet (like the ones at the gyms) and on a wooden desk and a silicon mat on top. Though this is just my guess it would keep me safe.

I know a master electrician who does house work and he always likes to scare people by picking up a bare hot wire and making believe he is being electrocuted while a few seconds later he explains that there is no path back to neutral or ground to close the loop, hence nothing happens to him. Although his buddy is a little bit weird in the head so maybe he was the guinea pig that tested it first.