Magnetic field intensity in home

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gar

Senior Member
Location
Ann Arbor, Michigan
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EE
220413-1654 EDT

For persons that make magnetic field measurements in homes ---

1. What field levels do you see in milligauss?
2. What do you consider abnormal?
3. What are the sources or causes of various levels?
4. What is your criteria for excessive?
5. Why do you make these measurements?
6. Do you use these measurements to troubleshoot wiring problems?

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I haven't done a serious investigation for years.

I would have to go back and read up again.

I had a couple customers that worried about EMF back when.

6. Yes, if I feel that we have an issue with bootlegged neutrals. I've also used it to trace wires in the wall.

Readings will vary.
 
Last edited:
ptonsparky:

I do think that item 6 is where one would have the greatest use. I can easily trace a wire 6' away, and in a conduit.

Badly wired houses, and incorrectly wired circuits are where this technique can be quite useful.

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If your house is wired with metallic flex you have less of a problem. See attachment from Southern California Edison.
 

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  • Electric and Magnetic Fields.pdf
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I would not attempt to trace wires in a wall unless I knew they were the only ones energized. Six feet is too far when you want to put a screw within 4" of the wire.

Its a bummer when the electrician puts a wire in the wall forgetting that he also wants to mount shelves and a work bench to that wall. There are better methods for that purpose, but it will work.

Walking thru my 12 hear old house I get an average of 1.1 with a max of 2.4 a couple feet from my glass fronted VFD enclosure. 500mg about 3" from the load reactor. MAGCHECK-95 attached to a Fluke 87, Mv, 100ms
 
220414-1754 EDT

LarryFine:

Not a big cost. More information later.


ptonsparky:

It is not necessary to be 6' away to make a measurement, but if a wire is buried 6' underground you could trace it.



i do like looking at the waveform on a scope to correlate with a meter reading. But hauling a scope around is not easy.

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220414-2120 EST

When I shutdown various experiments I am able to get the maximum residual field intensity down into the 0.1 milligauss range. If I don't turn off the 250 W bulb with a single wire feed it creates a lot of residual flux.

In certain areas I can rotate my single axis sensor so that the output voltage is under 10 microvolts, and 10 microvolts is under 0.1 milligauss.

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220416-1747 EDT

I have refined my magnetic sensor somewhat. In my backyard now I get a maximum reading of about 0.1 mV or about 0.02 mG with the magnetic field being vertical. I suspect the source is my power lines, but I have not taken my scope out to see what the waveform looks like. For the moment I will assume it is a 60 Hz sine wave, or thereabouts.

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220417-1148 EDT

LarryFine:

Several thousand turns, some transformer laminations, some capacitance around 1 mfd, and a suitable AC sub millivolt ( 10 microvolt resolution ) meter.

My current calibration for my combination is about 0.3 milligauss reads 1 millivolt.

For calibration I use a straight wire about 8 ft long with 1.5 A 60 Hz, and either a distance of 20" or 40" from the wire for the coil location.

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gar said:
LarryFine:

Several thousand turns, some transformer laminations, some capacitance around 1 mfd, and a suitable AC sub millivolt ( 10 microvolt resolution ) meter.
Click to expand...
Would you have a schematic we could see?
 
220417-1502 EDT

LarryFine:

The circuit is very simple. It is an inductor with a capacitor in parallel, and the high impedance meter also in parallel. The capacitor is adjusted to make this a parallel resonant circuit at 60 Hz.

I have the capacitor in parallel at the inductor, and then several feet of leads to the meter. But the capacitor could be at the meter.

My inductor has a rather low Q and therefore resonance is very broad. At present I have no electrostatic shielding around the coil.

If I keep the capacitance at the inductor, then I can add some additional filtering after the tuned circuit if I want. Haven't seen the need yet.

When in my basement and testing near the single wire test source, and watching the output on a scope I see a pretty good sine wave, and I can rotate the coil to a position with a null sine wave ( a straight line on the scope ).

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My recollections is >5mg is unwanted. Measurements depend on where you are measuring from. For example, take your gaussmeter and put it up against a dimmer switch that is on and you will get high mg readings but move a few inches away and they should read zero mg.

The most common causes in a home are:

1) Neutrals from 2 circuits tied together-- not much of an issue now with afci circuits

2) Wiring such as knob and tubing where the neutral and hot are not run together. In some homes electricians would wire a 3 way by having a fed at both ends and a 2 wire between the switches along with switch leg from one of the 3 ways. This would cause high electromagnetic field.

3) A neutral and a ground touching allowing 2 pathways for the current to flow.
 
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