No ground, no print
This must have been in the late '80s. A customer had a LED printer, I think it was a NEC LC866, but I may misremember the name and model. Anyway, one day it started acting up. It would print one or two pages, then shut down completely. Over and over. I had to unplug and replug the printer to make it reset and start up again, every time.
So my first try was to replace the motherboard. Then the power supply. Then some other board I can't recall. Nothing helped. Finally, on day two, I brought another identical and definitely working printer and, of course, it failed in the same way. I connected to a new computer, same problem.
Somehow I got my hands on a schematic of the printer, and there, in the power supply, there's a little label pointing to an empty box, saying something like "ground fault detection". Whoa... first I ever heard of that.
Pulled out a multimeter and measured between ground pin and the nearest radiator, and yes, there was a significant voltage difference.
Now, when I told the customer that there was something definitely wrong with his ground wiring, he got all upset. He had paid a lot of money to have all of his electricity rewired months before, and everything worked fine up to a couple of days earlier. He had even had an inspector from the government go through the system just the other day, the day before the problems with the printer occurred. And he approved the system.
My natural reaction was: "where exactly did this inspector go to inspect?". The customer takes me down into the basement and pointed to the ground disconnection bridge and says: "that's where he disconnected the ground to measure it". Yes, you guessed it, it was still disconnected... So much for inspection and approval.
Admittedly, this wasn't a real ground loop problem, it's more a ground missing problem, but the next one was definitely a loop:
Years later, I had a network problem at a customer's site. It took me all of five minutes to suspect ground loop problems, in particular since I saw actual sparks coming from the token ring network connectors. Experience does save time.
Biting the hand that feeds IT
