DIY protective earth grounding

[Mephitus]

I live in an ancient fourplex. It's so old that the hot water plumbing and electrical conduit had to be retrofitted in. Because of this, I was worried about potentially blowing up my scope because of common ground feedback loop. (thanks for the warning Dave!) And so I tested my wiring and found all the breaker sections are grounded to the same common. Leading me to conclude that it would probly be a good idea to make dedicated earthed grounding to protect my testing gear.

All of the information I can find on proper earth grounding is for electrical building code standards (grounding out to the foundation rebar, etc). While nice to know, I cant exactly make such major modifications on a rental. My lab is too far away to ground to the plumbing also. Luckily I am in a partially sub-level floor with a window within a couple feet. Which leaves me with 2 questions in the end:

1: Is this a good idea, or simply a waste of time?
2: How do you recommend I should go about this safely?

[nanofrog]

I'm not totally sure what you're asking, so a couple of questions.

Is the electrical conduit and/or metal box grounded at all (i.e. has proper 3 prong plugs that are fully wired up per code, or at least is grounded at the box, so they can be added)? Or is there no ground at all, and you need a way to ground the outlets (run wire to a proper ground point)?

Or is everything grounded properly at the outlets, but you're just concerned about a loop? 

Some photos might help as well. 

[Mephitus]

Sorry for not being clear. I really needed some caffeine when I wrote that...

The wiring in my apartment seems to have all the electrical outlets ground connected in parellel before the primary ground at the breaker box. (First pic) I am worried about creating a ground loop when using my scope to probe anything that also uses the mains ground and potentially causing damage to either the scope or the circuit under test. (pic 2) [As Dave warned about here: http://goo.gl/VzjNPi ]To avoid this, I am wondering if it would be a good idea to make a seperate dedicated ground that would be independant of the mains grounding, thus avoiding a potential grounding loop. (3rd pic) My idea is to create a connection directly from the ground terminal inside the scope to an earthed ground I would build just outside a nearby window.

My question is 2 fold:
1: Under such wiring conditions, do I still need to worry about potentially creating a ground loop when probing circuits?
2: If this is something I need to address; is my concept for a dedicated ground for the scope a good/bad idea? How would I go about building one safely?

[CatalinaWOW]

Calling it a ground loop confuses the issue Dave is talking about.

If your circuit under test has a connection back to neutral or the earth mains, and if you connect your ground clip to a point in your circuit under test that is not at ground potential AND has significant energy content ugly things will happen.

If the circuit does not have significant energy content, you will significantly alter your circuits operation and confuse your readings.

The problem is not in the house wiring - it is in trying to connect two points that are at different electrical potential, and there is no universal solution for this.  Isolation transformers, ground isolators and other patches can eliminate many but not all possible problems.  Thinking about what is happening, either on paper or in your head and verifying that you have created no unintentional circuits is the only solution.  Remember that there are wires (connections) in power strips, test connections, the device under test and also electrical connections in chassis, arms, and other mechanical apparatus.

The term ground loop should be reserved for a somewhat different problem - when you have different currents flowing in wires that are all nominally at the same potential.  Since the wires have some finite resistance, the actual potential at the points of connection will be different.  In circuits/systems with large currents the differences can be many volts.  This is the situation in manufacturing plants where hundreds of amps may be flowing through the neutral lines.  It can also be a problem in high accuracy measurements where the millivolts of difference that result from small current differences in different legs of the ground circuit

[Richard Crowley]

1) It is almost certainly illegal for you to make any significant changes/additions to the mains power wiring in any property that you do not own and occupy. Even if you own the fourplex, you likely are not permitted to work on the wiring unless you are a licensed electrical contractor because it is a rental unit occupied by others.
The reason for that is pretty obvious. They don't want ayone making dangerous wiring decisions unless they have some sort of assurance they know what they are doing. Prevents buildings from burning down, killing occupants, etc.  Yes, certainly many of us are more familiar with the concepts than many licensed electricians, but we haven't demonstrated that to the local government authorities.

2) Have you actually MEASURED the ground integrity to see if you even have a problem at all?

3) Even if there IS a problem with the green-wire safety grounds for the power outlets, if your test gear (oscilloscope, et.al) is connected to the same outlet as the device under test (DUT), why would there be a problem?

4) Can't you simply use a power strip on your test bench and ensure everything relevant: your test gear, and the DUT, are all plugged into the local power strip.  We are assuming that the power strip at least connects all the green-wire safety ground pins together.  Whether or not that eventually drains to the crust of the planet is a second-order effect.

[Mephitus]

CatalinaWOW: thanks for the clarification. I think I have a better grasp of how I missunderstood the situation.

1) It is almost certainly illegal for you to make any significant changes/additions to the mains power wiring in any property that you do not own and occupy. Even if you own the fourplex, you likely are not permitted to work on the wiring unless you are a licensed electrical contractor because it is a rental unit occupied by others.
The reason for that is pretty obvious. They don't want ayone making dangerous wiring decisions unless they have some sort of assurance they know what they are doing. Prevents buildings from burning down, killing occupants, etc.  Yes, certainly many of us are more familiar with the concepts than many licensed electricians, but we haven't demonstrated that to the local government authorities.

2) Have you actually MEASURED the ground integrity to see if you even have a problem at all?

3) Even if there IS a problem with the green-wire safety grounds for the power outlets, if your test gear (oscilloscope, et.al) is connected to the same outlet as the device under test (DUT), why would there be a problem?

4) Can't you simply use a power strip on your test bench and ensure everything relevant: your test gear, and the DUT, are all plugged into the local power strip.  We are assuming that the power strip at least connects all the green-wire safety ground pins together.  Whether or not that eventually drains to the crust of the planet is a second-order effect.

1: I would never touch any of the house wiring. I wouldnt even replace a light-switch, let alone futzing around with how the mains ground is setup. Don't worry, I am a "measure twice, then cut" kind of person. I wanted to see if how I understood the situation was correct or not. I now realize that I still have much more to learn before I can fully understand how the negative terminal output and grounding are differentiated in a practical sense.

2: When I test continuity on the ground pins for all of my electrical sockets for one breaker section to another, (Using a Greenlee DM-820) they all return a positive reading. I do not know if this is how it is intended to be or not. I fully admit my ignorance here. I just dont want to blow up my scope if I failed to take precautions.

3: After watching the EEVBlog episode about "how NOT to blow up your scope" (Link) I just wanted to head off any possibility that I may run into such a situation. I will re-watch it again to see how I got confused.

4: If the dut and scope share the same ground, I was concerned that this would create a voltage feedback and potentially cause damage to the scope or dut. It seemed to me that an obvious fix would be to ground to something else. For example, I have found people grounding out to plumbing instead.

[Kleinstein]

IF you measure on ground connected circuits, its allwas a good idea to have the scope and DUT powered from the same outlet. No problems with ground loops or how you call it anymore, no matter how bad the installation is.

If you are in doubt with the wireing you may do a quick check, e.g. by testing between the PE contacts of different outlets: there should be a very low voltage (a few mV, if any) and low resistance (e.g. less than 1 Ohm unless long distances)  between. Do the same test between an outlet and something like water lines (if metal), or other lines that should be grounded - here the the resistance might be slightly larger.

Especially doing measurements, its a good idea to have a working FI / GFC circuit. If in doubt use one to plug in the outlet you use for experiments.

[Mephitus]

Kleinstein: Thanks for the clarification. To make sure things are safe, I will go snag a GFCI plug then. Everything would already be connected via power strips. I just dont trust it's circuit protection 100%

[CatalinaWOW]

It appears that you are on the right track.  Be careful and think it through.

One difficulty is that it is difficult to actually understand the wiring in an old building with tracing it all, something that you probably don't want to do.  Between changes in electrical code over the years and great variety in competence of the people who worked on it over the years some very strange things occur.  You can come to some very dangerous conclusions if you assume that something conforms to current practice in much of the world (a neutral wire and ground wire that are tied together at the point of service, and also tied to earth ground).

Among the fails I have found in buildings I have occupied:
  1.  Neutral not tied to earth ground.  50 to 70 VAC to water pipes and other earth ground points
  2.  Earth and neutral tied.  Then exposed metal lighting fixtures tied to the hot line. 
  3.  Hot and neutral reverse in electrical outlets.  This one is extremely common.
  4.  Ground connection in electrical outlets not connect to anything.  This one is also very common.

Items 3 and 4 are easy to test with a go/no tester widely available at low price, or with a little more difficulty and less safety with your DVM.  If you find any of these faults you should be able to get the building owner to correct them.