A most unusual problem.

[gar]

190922-2222 EDT

Eddie702:

A suggestion that may help you.

You now know the two breakers that are wired together in a loop. Disconnect one of the hot wires at the main panel from its breaker. Obviously both of these breakers need to be off when you do this. Connect a 1500 W space heater between the removed wire and neutral. Turn all breakers in the panel off, and turn on the one connected breaker to the problem circuit. This will cause about 10 A to flow in this loop.

Where the wires of this loop are not close together there will be a reasonably detectable magnetic field around the wire. With a magnetic field sensor you trace where the wires run in the walls. With a test lead from the output of the closed breaker you can measure the voltage drop along the loop.

I don't think the voltage drop measurement will directly take you to the problem point, but it may help you guess where. Logic may lead you to a likely location.

How can you look for the stray magnetic field? I use an air core coil on about a 1/2" square tube with 1000 to 2000 turns. Properly orientated and about a foot from the 10 A current I will see more than 1 mV.

Then I tried using the 120 V primary of a Nutone transformer without removing the core as the sensor coil. Saw something like 10 mV at 1 foot. A 240 V Nutone would be better, don't know if one is made.

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[gar]

190922-2423 EDT

In my last post numbered #21 you want no loads anywhere except the one heater load at the end of the loop.

Suppose the connection of the two circuits together is not at the end of each circuit, then that stub circuit, or circuits, will produce no stray magnetic field, and not traceable by a stray magnetic field. However, this stub circuit will have exactly the same voltage on it as the point at which it became a stub. This stubbing point is probably the common connection of the two circuits. Or at least closer.

See if you can make my idea work.

.

 

[Eddie702]

190922-2423 EDT

In my last post numbered #21 you want no loads anywhere except the one heater load at the end of the loop.

Suppose the connection of the two circuits together is not at the end of each circuit, then that stub circuit, or circuits, will produce no stray magnetic field, and not traceable by a stray magnetic field. However, this stub circuit will have exactly the same voltage on it as the point at which it became a stub. This stubbing point is probably the common connection of the two circuits. Or at least closer.

See if you can make my idea work.

.

gar, thank you. The two hot wires leave the panel and go up separate cables (obviously) and go up in the wall near the kitchen sink . I suspect 1 circuit (probably the #12) is a dishwasher or garbage disposal or something that has been intermixed with a #14 circuit. If I can't find it your suggestions will help.

 

[readydave8]

I would want to ask the landlady whether she recently had a tenant move out. If so, was it under amicable circumstances? To me, this very much sounds like "malicious mischief," something an unhappy tenant might do on their way out of the rental.

I don't think a vandal would have worked that hard to just screw up a few lights and receps

 

[romex jockey]

190922-2222 EDT

Eddie702:

A suggestion that may help you.

You now know the two breakers that are wired together in a loop. Disconnect one of the hot wires at the main panel from its breaker. Obviously both of these breakers need to be off when you do this. Connect a 1500 W space heater between the removed wire and neutral. Turn all breakers in the panel off, and turn on the one connected breaker to the problem circuit. This will cause about 10 A to flow in this loop.

Where the wires of this loop are not close together there will be a reasonably detectable magnetic field around the wire. With a magnetic field sensor you trace where the wires run in the walls. With a test lead from the output of the closed breaker you can measure the voltage drop along the loop.

I don't think the voltage drop measurement will directly take you to the problem point, but it may help you guess where. Logic may lead you to a likely location.

How can you look for the stray magnetic field? I use an air core coil on about a 1/2" square tube with 1000 to 2000 turns. Properly orientated and about a foot from the 10 A current I will see more than 1 mV.

Then I tried using the 120 V primary of a Nutone transformer without removing the core as the sensor coil. Saw something like 10 mV at 1 foot. A 240 V Nutone would be better, don't know if one is made.

.

An amprobe and a bell xformer ? :? Am i reading you right gar?:dunce:

~RJ~

 

[gar]

190925-1105 EDT

romex jockey:

Yes, except I don't know about the Amprobe.

Some quick experiments this morning.

The Newtone primary shunted with a 1 ufd capacitor, this does not get me to resonance which takes about 5 ufd and would be better, and a Beckman 4.5 digit meter that resolves 10 microvolts as my measuring instrument.

One can not be moving the coil during the measurement because you induce voltage from the earth's field.

I got some residual readings as low as about 20 microvolts. Within several feet of my main panel I was seeing 1 to 2 millivolts. About 4 feet from a an 8' Slimline fixture 200 microvolts with the axis of the center leg of the transformer core pointing at the fluorescent. The pointing direction does not make sense to me at this time. I have to do other experiments.

Outside in my yard I can get down to the 10 to 20 microvolt range. Under my 3 phase primary lines possibly 40 to 50 microvolts.

You turn on and off the power source that is generating the field to see the change from that field being tested relative to ambient.

Have to leave now.

.

 

[gar]

190925-1506 EDT

Doing only a slightly more controlled experiment.

Fluorescent fixture is about 7' above the floor. I am hold the transformer at about table height, 30".

In an arbitrary location, but directly below the fixture I get the following results:

Maximum 140 microvolts with plane of core laminations perpendicular to ceiling, and the axis of the center leg of the EI laminations being vertical.

Minimum 30 microvolts by rotating the transformer 90 degrees with the lamination plane still vertical.

With power to the lights off the residual is around 10 to 20 microvolts. Later I will bring the tuning capacitors up and see if I can get a greater change with orientation.

.

 

[gar]

190926-0936 EDT

Some of the information in my posts #26 and #27 is invalid relative to a magnetic field sensed with the Newtone transformer. However, the Newtone or some other transformer is a useful magnetic field sensor to sense a 10 A 60 Hz signal on a single wire as I will explain below.

In my previous experiments I simply connected the meter leads to the Newtone primary leads. This transformer has steel covers on both sides of the laminations. So there really is capacitive coupling from these covers and the magnetic core to the coil of some composite value. In free space if I have a voltage gradient, then I really have a capacitor. Thus, I had a capacitive voltage divider from the hot 120 V AC line in the ceiling to the primary coil in the transformer.

I was hand holding the transformer so as to change its orientation. So I was also part of this capacitive divider.

I was not getting a maximum voltage reading from the transformer in the orientation I expected. Then I sat the transformer on the table, released my contact, and there was a large change in the reading. That kind of result does not come from a magnetic field. This looks more capacitive.

I can simply capacitively shield this transformer primary by connecting one primary lead to the transformer metal. Did this and the voltage dropped to near zero.

Went back down to the basement, and with one primary lead connected to the transformer cover, and the 10 A single wire, and got valid magnetically induced signals.

Thus, the experimental setup must include an electrostatic shield. The Newtone with the use of electrostatic shielding is something many electricians will have laying around that can provide a useful stray magnetic field sensor. More later.

.

 

[gar]

190926-1545 EDT

On how and why a transformer with a closed magnetic core can have an induced voltage in the coil from an external magnetic field.

Consider an EI transformer core with a coil wound around the center leg. Put this magnetic circuit in a uniform widely dispersed magnetic field, a field like that from a current flowing in a single straight wire. Put this magnetic core in a free space uniform magnetic field. This will distort the free space field because the permeability of the core is much greater than free space. However, that does invalidate the following description.

Magnetic flux flows thru the core. If the direction of the entering flux is parallel to the center leg it is also parallel to the outer two legs. And the flux in each of the three legs will be about the same. Rotate the core 90 degrees and little flux flows thru the three legs because the three legs are now perpendicular to the flux direction. And the magnetizing potential difference across a leg is near zero.

The orientation with the maximum flux flow will have the maximum induced voltage in the coil.

The flux field in a leg in the 90 degree orientation is relatively near zero because the magnetizing force across that leg is near zero.

For the single wire magnetic field creator the magnetic flux line have a circular path around the wire.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html

https://www.khanacademy.org/science...rent/v/right-hand-thumb-rule-solved-numerical

Do some of your own experiments.

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