Hot and Neutral Mixed Up - Page 1
Forum section: electronics / projects

author: staticshock
author_rank: Contributor
timestamp: February 26, 2025, 07:23:13 pm
content: I have a unit that I'm validating connections for the power system. I have come across an issue and would like expertise on it. I uploaded a pdf showing the diagram of the system. An AC power cable supplies power to the unit, which goes through a filter and then distributed to a Switch/Circuit Breaker, LED, and Power Supply (115vac to 24vdc). In the process, I determined that whoever built the unit mixed up the Line and Neutral terminals of the filter. The connections that are supposed to be connected to the filter Line terminal was connected to the Neutral terminal and the connections that are supposed to be connected to the filter Neutral terminal was connected to the Line terminal. It baffles me that the unit has been operating for a long time in such a state.What is the significance of the line and neutral being flip-flopped?Checking continuity with a DMM, a continuous beep would mean that there is continuity. I noticed that when I checked two points that should not have continuity, the DMM beeps once then nothing. I don't understand why the DMM does that. Is it because of this mixed up issue?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 26, 2025, 07:52:43 pm
content: For many applications there is no impact from switching hot and neutral.Primary impact is safety. Depending on construction parts of the chassis may be hot and thus dangerous to touch if other parts of the body are grounded.Second impact can come if ground connections on the equipment are not isolated from the "neutral" connection and then connected to another piece of equipment has the ground connected to a correct neutral. This will be exciting with blown fuses or opened breakers as the least exciting possibility.

author: microwhat
author_rank: Contributor
timestamp: February 26, 2025, 08:08:55 pm
content: Quote from: staticshock on February 26, 2025, 07:23:13 pmThere is probably a capacitor between those two points. When you approach the terminals with your meter probes, the capacitor is discharged. The DMM considers the initial charging current as continuity, but as the capacitor charges, the current drops and the apparent continuity disappears.If you measure the voltage between those points after checking continuity, I would expect to see a small voltage: whatever the capacitor is charged to.

author: bdunham7
author_rank: Super Contributor
timestamp: February 26, 2025, 08:32:26 pm
content: With most devices like the ones you have there would be absolutely no difference in how they function. The only safety concern I would have is that when the SW/CB is in the OFF position the LED and PSU are both still energized such that someone taking them apart or immersing them in water might get shocked, or in some cases if there was an internal fault that caused a short from LINE to GND the circuit breaker either would not trip or would not succeed in disconnecting power from the units. So just wire it correctly and be done with it.

author: ajb
author_rank: Super Contributor
timestamp: February 26, 2025, 09:02:29 pm
content: This shouldn't cause any issue at all, because the line and neutral are current-carrying conductors that should ALWAYS be assumed to be at a hazardous potential and protected accordingly. That means neither one is ever left accessible to the user, and neither is ever connected to the equipment ground in an appliance. This is absolutely critical in universal voltage equipment, sincebothconductorswillbe at mains voltage when connected to a 208/240VAC supply.Screw-in lightbulbs are a bit of a historical exception, where the outer contact is somewhat accessible on some sockets -- connecting the outer shell to neutral makes accidental contact safer, but does not make itsafe!