I always like to retell this story whenever the opportunity arises.

We moved into a new house one year after it was built. In one bedroom I setup my computer workstation. The computer was plugged into one wall outlet, the printer in another. Every time I tried to connect the serial RS-232 cable between the computer and printer, the GFCI at the service panel would trip.

Tracing through which breaker serviced which rooms and outlets, I discovered that the outlet to the computer was on one circuit while the outlet to the printer was on a different circuit with the GFCI feeding a Jacuzzi in a totally different part of the house.

This took me awhile to figure out why the GFCI was tripping. At this point I noticed that the ground connection at the GFCI was clipped. I rightly guess that there is a fault somewhere along the house wiring. After some time-consuming investigating, using a binary search technique, I found the fault behind one of the wall outlets, again in a different part of the house. The neutral was touching the ground wire.

Here is the scenario that I can only imagined happened. The electrical installer wired the house. At some point he (gender neutral) turned on the main breaker and the GFCI tripped. It was too close to quitting time for the day. Instead of resolving the issue, he clipped the ground connection. GFCI no longer tripped.

When I connected the serial cable between the printer and computer, I completed the ground connection and the GFCI did its job, i.e. it tripped because it detected a ground fault.

The lesson learned: you cannot always trust a licensed qualified electrician.

 

The licesed electrician is in business to make money, and mostly, doing a minimum adequate job makes the money in the shortest time. They are not in the business to assure that the customer will be happy after the check clears. That is for folks who depend on referrals from happy customers to provide new clients.

 

Last word on this one.
What amazes me is that domestic properties never come with circuit diagrams.
This wouldn't or shouldn't be the case in industry and more so in electronics where the circuit diagram is part of the generally, PCB build.
Even new builds don't have a set of drawings.
The real problem would be in updating information.
My property built 30-years ago didn't come with any drawings.
A few years back, tired of guessing where things went, I went round with an installation tester, starting at the consumer unit (fuse box), tracing every stretch of cable from fitting to fitting, testing each stretch and documenting it, and what a convolulted route it was.
I lifted floors where I could and installed inspection covers - digital camera very useful for looking here - every hidden junction box was brought out to the surface.
At some stage, someone knew where everything was and what was used.
If you did that in industry, where I worked, you would be crucified.
The other thing that got you in a heap of trouble was to use the draw string in a cable duct and not replace it for the next person.
I do that at home and every time I pull a cable through, usually CAT5e these days, it comes with a draw string.
Ducting for cables and services should be part of a new build, like the old steel conduit systems. What we call twin and earth has a lot to answer for - it makes installers lazy.
On the reversed GFCI, if marked at all, the ins and outs are often given slightly obscure names like source and load or L1 and L2.
But in the box or printed on the side of the equipment will be a diagram. If all else fails, the writing on the front, like ON and OFF can be a clue. If that fails, test it offline to be absolutely sure or contact the manufacturer.

It is entirely probable that the person doing the wiring in that house, prior to flipping it, was not even close to a qualified electrician, or that they were even able to read. Telling black wire from white wire does not take a lot of skills or smarts.

 

The other side of the coin is the "Building Inspector" who may or may not have any inkling as to what is actually in the relevant electrical code for their jurisdiction.

 

It is entirely probable that the person doing the wiring in that house, prior to flipping it, was not even close to a qualified electrician, or that they were even able to read. Telling black wire from white wire does not take a lot of skills or smarts.

Registered, sticker on the van, licenced to print money.
Done the course, paid the fees, 99.99% pass rate.

 

Registered, sticker on the van, licenced to print money.
Done the course, paid the fees, 99.99% pass rate.

I was describing the one who did the work in the very southeast corner of Warren Michigan. Most others would be better qualified.