Electrical safety GFCI VS isolation

Thread Starter

electronis whiz

Joined Jul 29, 2010
I was watching a video on youtube they talked about grounding, isolation, and GFCI type devices.

They mention one thing I never thought of which is that with isolation GFCI devices are useless. Isolation protects from a normal shock, but if you got across both isolated output there's no protection even from a GFCI on the primary side. They said this is why if testing live gear you should use the one hand rule. I understand this, but brought up interesting question.

In theory at least it seems it would be better to be unisolated and behind a GFCI. May be a bit more risk of shock, but it would be minor either way. Whereas if were isolated and contacted both you have nothing except a fuse or breaker. I know neither are perfect, but GFCI seems the lesser of the evils for most cases.

Say you were testing some live gear what method would you trust more?
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Joined Nov 18, 2012
Interesting thought. I have the same question as well. I am thinking that if I have a GFCI in my wall and I am working on say a television and I am on the mains, if I happen to come in contact with two points, won't the GFCI trip at something like 20mA. If that is the case, why does everybody say you should have an isolation transformer to do the same job. Never have understood this ?

Thread Starter

electronis whiz

Joined Jul 29, 2010
I'm not sure how true it is, but I have also heard if you contact both sides and it's GFCI protected the GFCI is useless then also. I understand the logic there because you kind of bypass ground due to resistance of the wire in comparison. To me I would think that if you weren't isolated and protected by a GFCI not all of the current would go back down the wire. There should be some difference there between them that it would trip. Maybe that is true, but I think if threshold of trip was low enough my theory would also be valid.


Joined Jul 18, 2013
Say you were testing some live gear what would method would you trust more?
This covers basically two areas of responsibility, those of say the general public where usually by rules and safety recommendations are spelled out in the regulations for the jurisdiction they live in, via the likes of NEC, UL etc.
The equipment is made as safe as possible without due regard as to what their knowledge of the safety subject should be.
IOW for the average consumer etc, the rules are set out and clear and their exposure to risk is covered by the above mentioned regulating bodies, the difference comes in those that actually work on equipment that can cover a scope the ordinary person or consumer does not work in, the protection and awareness is therefore different.
An extreme example, I have worked on power distribution of 4kv, there are safety rules and recommended measures in place when working with these types of voltages, and it is up to me to know and be familiar with the safety recommendations in place when working on such, I would disregard them at my peril.
The main idea of a GFI is where there is either a normally weak natural ground path and/or where the use of double insulated equipment is used, for example you dropped your mains powered hedge trimmer or hair dryer in a pool of water, any high voltage leakage should trip the breaker.


Joined Apr 24, 2011
A GFI device works by creating a transformer with the hot and neutral lines on the primary wound such the destructively interfere. So the secondary voltage is the difference in currents.

So yeah, you an grab hot and neutral and cook your brains out and still not trip a GFI. You may take out a fuse but not the GFI.


Joined Mar 14, 2008
The gist is neither a GFCI or an isolated supply will protect you if you get across hot and neutral or the two isolated hot leads.
If you touch only one lead to ground then you are protected either way.
The difference is that you are depending upon the reliability of the GFCI tripping in the unisolated case.

I tripped one of my GFCI breakers once when I accidentally touched a hot prong on a plug as I was plugging it in. Possibly saved my life but I still felt quite a jolt that I don't want to experience again. :eek:

Thread Starter

electronis whiz

Joined Jul 29, 2010
Everyone asks me how how many times i've been shocked. I say nothing besides static and batteries. There like really!. Yet to get one from an outlet or a capacitor. I test something that's live I almost always use one hand rule. Inless net to probe with meter then will use two then grab at closest point to the wires.


Joined Jan 15, 2015
Understanding GFCIs from our friends at NEMA is a good read on the subject and explains the operation of a GFCI in detail. They explain the differential current transformers mentioned and how things come together. Since the difference between current in and current out should be the same regarding the load if you place yourself between the hot and neutral you will indeed begin to slowly cook. Anyway, the link is a good read on how GFCIs actually work. :)

Idk is look into that more, my friend is a journeyman and I've read up enough myself to know that in most instances (aka codes been this way for a while ) neutral and earth are tied at the box. If what u said was true it would seem they offer no protection what so ever. Aka I find it unlikely to be the case cause u can find tons of stories of "luckily all I heard was a click"