Hello! Question on multiple GFI for test bench.

[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    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 


Every hook up should be a caution filled learning experience,

assume nothing, trust nothing and no one,

if you want to avoid BANGS 

and credit card abuse 





 

[speedman2049]

Thx for the reply guys!

If i'm working on a piece of equipment that needs to be powered from the mains, i always make myself aware of all contact points in the dut that contains that voltage before the transformer and be extra vigilant. Always have to be extra cautious when working with devices connected to the mains as they must always be respected.

I have fixed plasma tv's, audio amps and crt tv's, all of which had to be energised at one point to test which contains high voltages (especially crt tv's with high voltage flyback and high kv etc...) and high energy capacitors. As long as you have a good fundamental understanding and you don't make yourself part of the circuit by willy nilly going in and start pocking around without knowing what you are poking, you'll avoid getting the darwin award lol.

Still, i'm always am a bit paranoid when working on any mains voltage devices (try to best avoid it unless absolutely needed, otherwise my dc bench power supply) and wanted to add a extra layer of safety.

Thanks again guys for the replies, always willing and wanting to learn anything i can!

[Ian.M]

*IF* you use an isolating transformer, differential probes and can keep the D.U.T. entirely free of any ground connection then you gain safety, as you can touch any single point in the D.U.T. at an voltage less than the least isolation voltage rating of the probe, isolating transformer etc. without serious risk of electric shock (HV high frequency switching circuits can still shock you due to the inter-winding capacitance of the isolating transformer).  However keeping the D.U.T ground-free is difficult.  Even a simple TV aerial will introduce a ground unless you use an isolating adaptor. 

Also unless you have equipped your transformer with a ground fault monitoring system (e.g. introduce a DC bias relative to ground via a pair of series 1Meg resistors to the center tap of the secondary, and monitor the current), you get absolutely no warning that there is a stray ground path and HV circuit nodes can now give you a lethal shock.

Without such a monitoring system you need the self-discipline to check for leakage with a Megger between D.U.T. 0V or chassis and ground before switching on the supply, *EVERY* *TIME* you do so, and to *NEVER* make any additional connections with the supply on.