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[Mike Sokol]

I found this really excellent video about why double-path neutral currents from mis-wiring can cause huge magnetic fields that surround power wiring and even entire rooms. This probably explains why some performance stages seem to hum like crazy, while others are inherently hum free. For example, on some stages I can make bass guitars hum when the strings align with overhead and under-stage conduits, etc...

https://www.youtube.com/watch?v=XZ_zzHpGBGw&list=PLRNS1x1jcKbGZ9uIVMBHmm7-Sh2kePbVg&index=3

Now I need one of those EMF gauss meters and come up with some EMF demonstrations in order to understand how it all works.

[Frank Koenig]

Now I need one of those EMF gauss meters and come up with some EMF demonstrations in order to understand how it all works.

A coil of wire plugged into a mic input might be a good start. We can give some thought as to how to calibrate it. -F

[Mike Sokol]

A coil of wire plugged into a mic input might be a good start. We can give some thought as to how to calibrate it. -F

One quick test to determine if a branch circuit has a secondary neutral path is simply to clamp an ammeter around the entire H-N-G bundle and see if it nulls to zero under load. If it doesn't, then the neutral current has taken a secondary path and EMF is being generated. Could be another test procedure for when I'm chasing hums.

[John Roberts {JR}]

One quick test to see if a branch circuit has a secondary neutral path is simply to clamp an ammeter around the entire H-N-G bundle and see if it nulls to zero under load. If it doesn't, then the neutral current has taken a secondary path and EMF is being generated. Could be another test procedure for when I'm chasing hums.

Isn't that almost exactly what a GFCI/RCD does?

While an ammeter clamped around a line-neutral-ground bundle would not detect neutral current flowing in the ground lead.

JR

[Mike Sokol]

Isn't that almost exactly what a GFCI/RCD does?

While an ammeter clamped around a line-neutral-ground bundle would not detect neutral current flowing in the ground lead.

JR

True and True. What I'm hoping to explain from studying double-path neutral currents is why some stages seem to hum a lot more than others. I should be able to duplicate this on a tabletop experiment using some romex with a second path neutral that can be switched into the circuit. Then I'll put a substantial load from a space heater on it (maybe 10 amps) then move a bass guitar around the area and see how much it hums from both setups. One of the other things discussed in the videos was how the flux from unbalanced neutral currents can cause metallic conduit and secondary conductors to carry current and heat up. In fact, this unbalanced neutral current condition can probably induce a current in the EGC ground wire, which would contribute to ground loop differential voltage and hum currents. This is really interesting as it could explain a few hum situations I've been around in recent years where I had no clear explanation for what was occurring.

More to learn... 

[John Roberts {JR}]

True and True. What I'm hoping to explain from studying double-path neutral currents is why some stages seem to hum a lot more than others. I should be able to duplicate this on a tabletop experiment using some romex with a second path neutral that can be switched into the circuit. Then I'll put a substantial load from a space heater on it (maybe 10 amps) then move a bass guitar around the area and see how much it hums from both setups. One of the other things discussed in the videos was how the flux from unbalanced neutral currents can cause metallic conduit and secondary conductors to carry current and heat up. In fact, this unbalanced neutral current condition can probably induce a current in the EGC ground wire, which would contribute to ground loop differential voltage and hum currents. This is really interesting as it could explain a few hum situations I've been around in recent years where I had no clear explanation for what was occurring.

More to learn... 

While I am not a expert on this (either) it seems the co-located line and neutral wiring will result in a small loop area between the opposing current flows that will tend to cancel each other out.  OTOH if some of the neutral current finds a different path back to the panel, the loop area between the original line and that spaced apart neutral return current can be significant so anything inside that loop could see that resulting magnetic field.

JR

PS: I have given more thought to detecting mis-wired or bootleg neutral ground wiring, and my current thinking still favors and audio sniffer to probe for noise on ground "and" a load (preferably switched, between line and neutral) to generate AC noise on the neutral. Further this load and audio sniffer need to be separate so they can load and sniff different outlets in a branch simultaneously.  A bootleg would read as all grounds having same noise as neutral. Reversed would read noisier ground than neutral, and correct wiring a relatively quiet ground.

Instead of a dedicated load something a simple as a couple hundred watt lamp might generate a readable hum, lamp with a dimmer even more audible noise.  For the sniffer I just imagine a small battery powered unit maybe even with a small speaker so no exposed metal parts except the outlet probe and perhaps a contact to grab an environmental ground reference from the tester. This is still a hypothetical WIP so don't try this at home yet.

 

[Kevin Graf]

But the GFCI/RCD needs to be upstream of the current split demarcation point. In areas that still have all metal water pipes, a building's neutral current will happily use the water pipes as one of it's paths back to the power company Neutral.

[Jerome Malsack]

What about the hearing aid  loops ?   This would put a hum on them right ?   Tcoil should pickup and put in ones ear.

[Kevin Graf]

Jim Brown writes that a listener receiver for an inductive loop hearing impaired system is also a convenient and inexpensive magnetic field probe.

page 30 of:
http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf

[Lyle Williams]

I'm not completely sold on the massive magnetic field concept.  This is going to be a single loop coil, not something I think I could intentionally use to make a massive magnetic field.

[ProSoundWeb Community]