Electronics > Beginners

Lab with NO grd, what about gear?

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bd139:
If you connect it to an isolation transformer, there's still mains voltage across two points. There's just no path between one terminal and ground through you.

FriedMule:
I can see how that would protect me!
What about a common wire so all gear have same voltage like a common ground?

CatalinaWOW:
First I will apologize to the safety rules zealots.  Second, I will emphasize that if you don't know exactly how all of this works you can do something dangerous by accident.  So be very thoughtful while reading this.  And then I will proceed with some opinions.

There are two reasons for the earth ground in our test gear and supplies.  One is for safety.  The other is to provide an element of electrostatic shield which can reduce capacitively coupled noise in our systems.  It may also provide a lower impedance path than other grounds directly reducing simple ohms law noise from ground currents.

a.  The safety arguments are valid, but you can limit risks adequately to satisfy most people without earth grounding.  The most important protection is against hot to case shorts.  These are actually uncommon, and can be further mitigated by periodic checks of insulation integrity.  Even when such a short occurs, health hazard is only serious when there is a path to neutral or earth ground through the body of the operator.  In a normal indoor location standing or sitting in a chair you have a very high resistance path to any of the danger paths.  A higher risk occurs from contact with metal benches and the like, but again periodic isolation tests can mitigate the risks.

The only time significant likelihood of injury occurs is when this path is low impedance - in a bathtub, on a wet floor or other circumstance.  And even then, only if the occurances mentioned previously that make tying your ground to a water pipe an inadequate solution are present.

Remember that there is no such thing as completely safe.  Things which comply with electrical codes can still hurt you.  They are somewhat more safe than some practices that violate the codes.  And the codes are designed to keep things safe enough for children and knuckle dragging adults.  You may well be safer operating your equipment outside of approved code but with your eyes open and brain active than the general public is while operating code approved installations.

b.  The noise issues may or may not be important to you.  It depends on what you are doing and how you have set up your equipment.  But if it is an issue it is easily resolved by connecting the grounds of all of your instruments together.  If your instruments have chassis grounded BNC (not uncommon) it is merely a matter of hooking them up with BNC or other shielded grounded connector.  This slightly increases the safety risk because it effectively combines the chance of a failure in any of the connected instruments.  But this chance is low, and you did decide to periodically check isolation didn't you?  Now we just arguing about inspection interval and how much risk you will accept.

FriedMule:
CatalinaWOW hmm how risky I want it to be?!

I am willing to know that any lead with voltage and current can hurt me, but I am not willing to get Einstein hair by touching any of my gear!
In other word I want to treat it as anyone would do a blender ore something like that, you don't get hurt by using the machine normally, but don't be an idiot and stick your hands down to the knives!! :-)

So I do get an isolation transformer, make a common wire (floating ground) and everything is fine?

EDIT: About noise, I would low to get as little noise as possible, I will be working a lot with LF analog circuit but will be "fighting" common noise and HF from interfering.

CatalinaWOW:

--- Quote from: FriedMule on October 16, 2018, 07:32:45 am ---CatalinaWOW hmm how risky I want it to be?!

I am willing to know that any lead with voltage and current can hurt me, but I am not willing to get Einstein hair by touching any of my gear!
In other word I want to treat it as anyone would do a blender ore something like that, you don't get hurt by using the machine normally, but don't be an idiot and stick your hands down to the knives!! :-)

So I do get an isolation transformer, make a common wire (floating ground) and everything is fine?

EDIT: About noise, I would low to get as little noise as possible, I will be working a lot with LF analog circuit but will be "fighting" common noise and HF from interfering.

--- End quote ---

Think it through.  In normal situations you will be fine, with and without the isolation transformer.  The isolation transformer provides protection against one class of faults, not all faults.  It also intoduces new possible faults to the system such as winding shorts. 

The ore grinder analogy is a bit strained, but much of the discussion here is not about sticking your hands in the blades, but about what happens when the shield that stops you from doing that fails.

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