I’m writing this up, as I couldn’t find any relevant posts when I first had this problem, and also because the solution was very unexpected, and save me from something
potentially quite dangerous? Feel free to skip to the recipe here.
Since setting up an Octopi server on a Raspberry Pi 3B+ last year, I noticed that the red LED would sporadically blink on, even when the power was physically switched off. I never wired the Pi directly into the 3D printer, so it’s connected to an official Pi USB PSU, and that’s connected to an extension strip with individual switches for each socket.
That setup has enough strangeness that I thought maybe the Pi was super sensitive, and picking up some sort of ghost current through the extension lead or something. Maybe the switches were not great on there. The problem was that everything worked fine, so I never really investigated it much further. But it always bothered me.
I had a simple socket tester that I’d picked up in screwfix. It tests for common faults like missing connections, switched wires etc. I plugged it in to the extension lead. Interestingly it started beeping before I’d even switched the power on to the switch. I unplugged the extension, and tried the tester in the wall socket. The sequence of pulses suggested that the Live & Neutral wires were switched over. Later I isolated the power and confirmed that the Live & Neutral were switched. A few minutes later I had them wired back the correct way, and put everything back together.
I know that AC devices will work quite happily with the Live & Neutral either way round (they alternate after all). It must have been like that for at least 8 years and hadn’t caused any other problems. All the wall sockets in the house are double pole switched, which disconnects both Live & Neutral when the switch is off. The problem I had is that the switch was being left on, and then I guess the extension cord was just switching off the Neutral, which should have been the Live. Quite how dangerous that is, I don’t know, but I’m glad I tracked it down and fixed it.
Solved. Now the socket is wired correctly, the tester is happy and the Pi no longer blinks the phantom red LED.
I just couldn’t help finding out how dangerous the switched wires could be. It seems like there is a fair bit of debate, but in short, it can be very dangerous. Devices that would be otherwise disconnected from the mains, will instead be live, and just waiting for a fault to complete the circuit via your own delicate body.