Electrical noise field radiating from the house

[Rhodesplyr]

I have a very odd noise problem in the house I'm renting at present, and I'm posting on the chance that someone here has seen something like it.

The noise itself plays havoc with single-coil guitar pickups. It sounds like ~120Hz with a regular phasing effect added, four of these phasing cycles about once per second, then a pause, then four again. It emanates as a very coherent point-source field from a square protrusion in a downstairs exterior wall that, as far as I've been able to determine, contains a heating return duct from upstairs that goes into the slab foundation at the floor. Using a filter choke as a field detector, I measure up to 11mV on an AC voltmeter (battery operated), and it's consistent in strength and direction all the way from floor to ceiling downstairs. I can measure the field outside, but it's weaker, so it seems to be originating inside.

A little backstory: this house was built in 1947 on a slab foundation. It's actually a duplex (though not used as one at present) with two electrical boxes, one upstairs and one downstairs. The original system was two-wire only, but during a partial renovation in the 1980s, some new three-wire circuits were added to handle appliances like a dryer. Also, though she's a good friend, I have to say that the owner of the house had a tendency to hire people to do things like wiring and plumbing who are both cheap and seem down on their luck. (It seems to make her feel good that she's giving them work.) I've found and fixed a number of problems.

Turning the downstairs electrical box off via the Main switch has no effect on the noise. Turning the upstairs box off cuts the noise voltage almost exactly in half. Cutting both boxes off reduces it slightly more, but not much. I measure no field at the ducts in the furnace room, only in the wall. And, based on what I know about the routing of the wiring, I wouldn't expect for there to be any electric wiring where I'm finding the noise.

I can access the metal heating duct upstairs, and I measure 3VAC difference between it and the closest AC socket neutral wire.

Any advice on the next step to track this down? My instinct is to investigate the grounding of both circuit breaker boxes.

Or should I call Ghost Hunters? ;-)

[Chuck H]

FWIW I don't think it's a good idea to alter in any way the electrical on that house. Because it's been worked on by several questionable service people, it's partly rennovated, outdated to begin with and doing strange things, if anything were to go wrong and burn the place down, whether it be your fault or not, you could end up in a lot of trouble. Best, IMHO, not to get involved.

You could try shielding the inside of the pickup covers for the single coils with copper foil. This has worked for me. You could also try using a power conditioner on the off chance that the power itself is causing some of the noise. I've seen this with FET controlled power sources.

JM2C

Chuck

[Rhodesplyr]

Chuck,

I certainly appreciate your advice and recognize the wisdom of it. On the other hand, if something *is* wrong, I'd prefer to know what it is and whether or not it might get worse. I make a clear distinction between diagnosis and treatment. First, I'd like to figure out what's going on. Then, I can determine if we need to do something about it. My family has been involved with the home construction business for ~50 years, so we know a good deal about wiring and codes, etc... There's a reason my friend wanted me to live here :-)

My brother noticed that the main ground rod is installed in a brick planter that was part of the renovation. (The owner is a landscape architect.) And he says that he's read that, to be effective, it's best for a ground rod to have as large an unobstructed area around it as practical. In this case, the rod's only "window" to solid ground is through the bottom of the planter. It may be the case that, due to the slab construction, we have multiple grounds at multiple potentials and that, somehow, this metal duct is acting as a bridge between them.

[Steve Conner]

To me, the phasing effect is a giveaway that the noise is ultimately coming from some SCR controlled appliance or other. The same kind of idea as a lamp dimmer, but it's getting ramped up and down for some other purpose.

Over here a lot of washing machines have a brush motor with the speed controlled by a triac, and I can hear similar noises from mine when I play a guitar with single-coil pickups in the next room.

Now if I'm reading you right, the puzzling thing is that the noise is still there even if you cut off power to the whole building? So, all I can say is that the offending appliance must belong to a neighbour, and somehow the return current from it is finding its way down your heating duct instead of the neutral wire where it belongs.

How's the weather over there? If it's been dry for a long time, you could try watering the ground rod to see if it makes a difference.

Single coil pickups are obscenely sensitive to magnetic fields, and shielding only helps so much, because it only stops electric fields. It has to be that way, because if you fitted a shield that stopped magnetic fields, the pickup would stop working altogether!

My old band used to have a rehearsal room near a railway line. Our other guitarist had a Telecaster, and every 5 or 10 minutes, he would start to pick up a loud buzz. Sometimes it was phasey, other times it had a high-pitched whine mixed in. About a minute later, a train would come thundering past, and very soon after that, the noise would go away.

You'll never guess what was going on there. :-)

[Chuck H]

You'll never guess what was going on there. :-)

The steel wheels friction on the tracks would temporarily magnetize the tracks?
The train, being a large steel structure, was acting as a reflector for EMF from another source?
The train would trip a track switching relay a mile before arriving and the motor for the track switch caused the noise?

What was it???

Chuck

[Tom Phillips]

Crossing gate activation?
Something to do with the huge electric generators and electric motors in the engine?

[Enzo]

Not to take the tangent too far, but aside from the train engines having huge electric motors with their own fields turning the wheeles, there are also large spinning generators making fields to power those motors. And ther are currents through the tracks, and the low impedance of the train car wheels shorting between the rails is part of the train control and detection system. Signals in the rails. ANy of that and more could be involved.


Back to the real issue. Because you have an irritating symptom in your single coils does not mean your house has a wiring problem. it might or might not. House wiring is not installed to avoid guitar pickups.

And while you may detect some form of field in certain areas, it could as easily be that you are detecting a standing wave caused by two fields located elsewhere.

With wireless mic units, we sometimes have troubl;e with pipes and ducts in the wall reflecting the signals and causing reception problems. Your ducts could be acting as reflectors or a sort of wave guide. Not to mention a duct can act as an antenna.

And i wouldn;t read too much into that 3v on the upstairs duct. Place an aluminum cookie sheet on your kitchen table and measure the voltage from it to the nearest ground. I bet you get some. In fact, what is your best high voltage reading from just grabbing the red probe tip with your fingers? A 20 foot hunk of duct would surprise me if it DIDN'T have some voltage on it.

[Steve Conner]

I suppose I should have mentioned that the trains were electric. Like elsewhere in Europe, they use 25kV AC overhead wires, and the current is returned through the track. No weird magnetized wheels, just hundreds of amps of AC current flowing in a 20 foot loop out in the back yard.

Noisy current, too. The older trains used a huge bank of SCRs for power control, the newer ones have a high-frequency chopper.

Is this irrelevant? I'm just trying to make the point that to make really bad magnetic interference, you need a heavy AC current flowing in an open loop. Normally the neutral is bundled up with the live in the same cable, and they carry equal and opposite currents, so their fields cancel and that's why normal house wiring doesn't interfere with your guitar too badly.

But in the case of the train, the "live" is 20 feet in the air and the "neutral" is the rails, so the field doesn't cancel. At least, it didn't in our practice room. I think you're looking for a similar situation, where current is going out in some cable, but instead of going back along the neutral wire in that cable, it's taking a detour somewhere it shouldn't.

[Enzo]

Oh, 25kV then, all returning to ground through thousands of microscopic commutator points along the rims of the wheels against the rail. Like a giant power drill motor.

One wonders, in the dark, well I can do the wondering in the dark too, but when it is dark around the train, aside from pantographs sparking, does one ever see any wheel-sparking from current (as opposed to friction), or is it too well distributed?

[Steve Conner]

Can't say I've ever seen any, and I've been travelling on those trains for years, and it's dark most of the time in winter.

The train cars weigh about 50 tons, and I think all that weight on the wheels probably forces a pretty good contact. I do remember seeing the overhead wires short out as I was riding to work one day. It was a dull morning, and the arc was so bright you'd have thought the sun had come out. It made a hell of a bang, too.

I do know that they can't allow the return current to flow through the wheel bearings, it would wreck them in short order. The neutral wire from the transformer goes to a spring-loaded contact gizmo on the end of one of the axles.

[Chuck H]

The neutral wire from the transformer goes to a spring-loaded contact gizmo on the end of one of the axles.

Kinda like a giant RC car.

You probably know this, Steve. You mentioned seeing the 20kV line short once (must have been very impressive), is there a formula for insulators and voltage potential. As in what's a safe distance for 20kV (or any voltage for that matter) from a conductor to avoid arching?

Chuck

[Steve Conner]

Good question... It's one of these "Believe me, you don't want to know" ones. Many people have written many books about it.

The glib answer is that air has a dielectric strength of 30kV per centimeter. But that's for perfectly clean, perfectly smooth, rounded electrodes in a lab. In the real world, dirt, surface roughness, moisture, kids with carbon fiber fishing poles, and so on, reduce it considerably.

In practice, the insulators used for 25kV overhead wires are about a foot long, and the wires have at least a foot or two of clearance from everything else. In tunnels, they use special low-profile insulators that are basically stout fiberglass poles. They don't work very well, you can hear them sizzling ominously in the low-level stations.

Electric utilities, railroads and so on take safety very seriously, and there's usually at least a mandatory 10ft safety clearance between overhead lines and personnel. When electrification first came in, the railroads spent considerable time agonizing over the extra risks involved.

[Rhodesplyr]

Enzo,

I agree that the duct could be acting as an antenna or a reflector. The only things that caused me to think that it was a conductor in the wall was the fact that the signal I measured had a narrow peak in a vertical line up and down the wall. It was also consistent from floor to ceiling, and the direction of the field would be consistent with a single conductor carrying a current.

I also agree with you that just finding a voltage on the duct doesn't tell much. What I should measure, using a resistor of some sort, is whether or not there is significant current involved or not, the same type of test used to diagnose power transformer leakage.

Back to the real issue. Because you have an irritating symptom in your single coils does not mean your house has a wiring problem. it might or might not. House wiring is not installed to avoid guitar pickups.

And while you may detect some form of field in certain areas, it could as easily be that you are detecting a standing wave caused by two fields located elsewhere.

With wireless mic units, we sometimes have troubl;e with pipes and ducts in the wall reflecting the signals and causing reception problems. Your ducts could be acting as reflectors or a sort of wave guide. Not to mention a duct can act as an antenna.

And i wouldn;t read too much into that 3v on the upstairs duct. Place an aluminum cookie sheet on your kitchen table and measure the voltage from it to the nearest ground. I bet you get some. In fact, what is your best high voltage reading from just grabbing the red probe tip with your fingers? A 20 foot hunk of duct would surprise me if it DIDN'T have some voltage on it.

[Enzo]

Of course. The antenna type voltage won't be able to produce any current. A clip wire to ground and a resistor, added to your voltmeter, should be able to quickly determine if your duct is hot in any way beyond forced air.

[spsimmons]

I'm new to this electronics stuff, but have done many, many remodeling jobs. I have seen, on more than one occasion, a vertical return used as a convenient, though surely illegal, way to get a wire from one floor to another. I'm not sure how this might contribute to the problem. Often, you can remove the grate and see down into the return duct.