Electronics > Beginners

Hello! Question on multiple GFI for test bench.

(1/2) > >>

speedman2049:
Hello everyone!

Currently a long time lurker (from Canada) and decided to finally register (should have done this a while ago)!

Just wanted to say thanks for this site and it's members as it has been invaluable to me and others and has helped many to venture forth into electronics engineering.
As for myself i have a decent knowledge of electronics as I've fixed game consoles, audio amplifiers etc.. by replacing capacitors, resistors, fuses etc...


I have a question in regards to testing a device that connects to 120v mains input (here in Canada the ground is connected to neutral).
If i have a true isolation transformer (no ground on secondary side) and the DUT is connected to it and i have a oscilloscope connected to my mains wall plug, would there be any advantage in adding a extra layer of safety by using 3 GFI plugs, one for the isolation transformer's input, another for the isolation transformer's secondary output and one for the oscilloscope plugged to the wall?

I tried searching here and online but didn't see anything (unless i missed it) so i was curious on if this would be of any good.

Thanks again and would like to see what you guys think!

Take care!

ArthurDent:
This is the type of question that is asked all the time here and there is a similar active thread discussing it now. On a GFI for the scope, I say no. It isn't a problem so why add a GFI to the power for it? The GFI will not interrupt the ground that is carried through to the test lead ground so any problem will still exist. The best answer is differential probes.

https://www.eevblog.com/forum/testgear/is-this-safe-way-to-measure-220v-on-oscilloscope/

speedman2049:
I see!
 
Ok, thanks for the reply!

I guess differential probes are the way to go then!

Ian.M:
You need to understand how modern GFCIs work.  They don't detect current in the ground conductor, they detect loss of current from the live circuit by measuring the difference between the Line and Neutral currents.

An isolation transformer prevents upstream GFCIs from operating due to a downstream fault.   There can never be an imbalance between live and neutral (except if the transformer fails) as the current into one end of any winding must equal that out the other.  This also means that as the secondary isn't grounded, no downstream GFCI can trip due to any fault.

Furthermore the scope wont trip its GFCI if you pass ground current through its probes and chassis.   

However you should have a GFCI protected feed to all test equipment, as that protects you from some internal faults in their power supplies.

Also note that when using an isolating transformer with a floating secondary, as soon as you clip a scope probe ground to any point in the D.U.T, you are at risk of electrocution: If you make contact with any point that is at a high voltage with respect to the point you grounded, you will get a shock, with nothing to stop it carrying on till you are a crispy critter except your physical reaction and whatever inadequate (from the human safety viewpoint) over-current protection may be present.

OTOH differential probes wont improve your safety when working on a live device.  They exist to protect your scope and the D.U.T. and to facilitate making relative measurements that would be difficult or impossible to make safely with traditional ground-referenced probes.    GFCIs aren't meant to permit routine work on a live device, they are a last resort safety measure that *IF* they operate as designed, and you don't have any pre-existing condition that increases your susceptibility to electrocution, will usually save your life.

IMHO you need safety training that is outside the scope of what can reasonably be provided on a web forum.  We cant assess your hands-on competence and discover any bad habits you may have - that would need the physical presence of a qualified instructor training you.

Electro Detective:
Tread carefully mate  :scared:  it took me a while to -UNLEARN- years of halfassed ejucation about isolation, GFCI/RCD and how  ground/earth/return paths ACTUALLY WORK in the real world. 

I had to brush up on domestic and industrial electricals, transformers and a ton of other stuff I won't bore you with. Electronics and electrical safety are different ball games.

I've observed that most people into electronics are clueless about electricals, and vice versa 

For starters GFCI will not work on the secondary/output of a true isolation transformer.

i.e. if the GFCI works, it means your secondary is not properly isolated,

...so you may not get zapped, but your gear might get a nice BANG on a short  :-BROKE


Every hook up should be a caution filled learning experience,

assume nothing, trust nothing and no one,

if you want to avoid BANGS  :o

and credit card abuse  :-[





 

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod