How Can Changing AC Sockets Cure Hum?

[Dionysus]

On a Saturday morning trip to Menards I ran into a local handyman. In my basket were about a dozen white 15 amp receptacles. The handyman asked, "Why are you buying them without ground?"
I replied, "They're going in to my mother-in-law's house. Cosmetic change... after 50 years she's tired of ivory."
Handyman sniffed and said, "Well you just hook the ground up to the neutral... they both go to the same place in the breaker box anyway"
I wish I had thought fast enough to ask, "What about the homeowner or the next handyman who flops back and white in an upstream box?" (I did fish new Romex to specific locations where she had been using a ground lifter. I remember how popular "cheater plugs" were when grounding became code in what was it, 68?)

That is indeed AGAINST CODE. Actually these days to pass an inspection (here anyways) "un-grounded" receptacles (especially those with a ground 'pin') must be protected by a GFCI as there is no bond connection for safety. Done that in many a house where it is not practical to run new wire at the time (only for those; if it is practical new wire with a ground is pulled). Of course there were the ones in the intermediate times where the receptacle had no ground, but the box was bonded; in which case its EASY.

I actually have a fun one coming up where I have to fish new wires as all the circuits on the upper floor are sharing the same ground wire. 14awg. yeah, no.

And I agree with Bill, I'd also check between the two. I would also attempt to trace the circuits to their breaker, and panel.

 

[Catwalker]

I apologize in advance for derailing this thread further, but...

What happens when the live and neutral wire are switched? I have never seen the effects of this, and am curious as to what would happen. Also, in another thread (about video wall power supply, I believe) it was mentioned that what really mattered was not which wire the current was on, but the potential difference between the two. (Leading to the "is two 60V lines possible" question). How does that bit of information relate to this situation?

 

[Wheezy]

I apologize in advance for derailing this thread further, but...

What happens when the live and neutral wire are switched? I have never seen the effects of this, and am curious as to what would happen. Also, in another thread (about video wall power supply, I believe) it was mentioned that what really mattered was not which wire the current was on, but the potential difference between the two. (Leading to the "is two 60V lines possible" question). How does that bit of information relate to this situation?

To the load itself, there is little difference. The issue arises when you have an ungrounded cord, such as a lamp. With a hot neutral, the case of the light bulb is energized, rather than the center electrode. It will find a ground, such as you when you go to change the bulb. Additionally, if the neutral is switched, rather than the hot, you could still have voltage in a receptacle even with the switch off.

 

[FMEng]

I apologize in advance for derailing this thread further, but...

What happens when the live and neutral wire are switched? I have never seen the effects of this, and am curious as to what would happen. Also, in another thread (about video wall power supply, I believe) it was mentioned that what really mattered was not which wire the current was on, but the potential difference between the two. (Leading to the "is two 60V lines possible" question). How does that bit of information relate to this situation?

Without current flowing, there is no voltage drop from the hot through the load. When a switch in the neutral line is open, everything from the load to the switch stays at 120 V potential. That increases the chance for exposure to lethal voltages, which explains why the NEC chose to never switch the neutral.

 

[FMEng]

We've drifted off topic. How the AC outlet is grounded contributes to the audio hum problem, but it is just one factor. The root cause is how the system of audio components are wired together, essentially making a giant, single turn transformer out of the cable shields.

I can walk into any building, plug two audio components into any two AC outlets, and feed audio between them without hum. The problem is most people, including many industry professionals, don't know the tricks and tools to do it. The manufacturers also set us up for failures by designing equipment with inputs and outputs that are prone to the hum problem without special care. I will say that manufacturers are doing better than they were 20 years ago, but compromises are made to hold prices down.

Dealing with the problem at the AC outlet is sometimes easier than other methods, and it has the added benefit of reducing the chance of having hum problems show up unexpectedly when wiring or equipment changes are made in the future. This is why the finger gets pointed at the AC outlet, and why certain myths about AC grounding spread like gospel.