Should an Electrician Have a Gaussmeter?

By Karl Riley

It is becoming clear that the electrician who carries a gaussmeter will be able to get a head start when trouble shooting system problems and power quality issues. Now that professional quality instruments are available in the $200 range, there is no good reason not to have one. Only a few years back you had to pay $2,000 for an accurate gaussmeter.

Neutral-to-ground shorts and incorrect bonding of neutral in subpanel busses, generators, receptacles, etc. all produce easily detectable magnetic fields which follow the faulty circuit through the building. These fields interfere with electronic devices by inducing currents or interacting with the fields produced by the devices.

About Gaussmeters:
Gaussmeters measure AC magnetic field strength in millgauss, or mG. They are very simple instruments to use. If you hold a three-axis gaussmeter in your hand and walk through an energized building, you will instantly see whether there are any miswired circuits, which shunt some neutral current either to ground or to a different neutral in another branch circuit.

Most sensitive gaussmeters use coils as sensors. When three coils are arranged in the three directions, it is called a tri-axial meter, and it will read the same no matter how you hold it. The read-out is digital and the resolution is 0.1 mG (milligauss). A single axis gaussmeter has a single coil for gaining directional information. The probe must be turned to orient it to the resultant field; this is done by turning it until the maximum reading is displayed. Both read the same number no matter how many sources are present.

Gaussmeters measure AC magnetic fields generated by circuits carrying "net current". Net current means the amount of current in the circuit, which is not compensated for by the other conductors. In other words, when some neutral return current is diverted to ground through an error or accident, or to another neutral in a separate branch circuit (thus creating parallel conductors in violation of 310-4), the amps on the neutral returning on the circuit will not equal the amps on the hot or hots. If the hot is carrying 10 amps and the neutral only 6 amps, you have a circuit with a 4 amp net current. This is simple to measure. Clamp an ammeter around the whole circuit. It will read 4 amps. A correct circuit will read 0 amps, as the magnetic fields from the conductors cancel each other. Net currents usually occur in pairs, since the diverted neutral current forms another path. Neutral to ground can form multiple net current paths.

Thus, a gaussmeter will instantly show if there is a circuit with net current. A 4 amp net current produces 8 mG at 1 meter, 4 mG at 2 meters, 2 mG at 4 meters, etc. (mG = 2i/m). Computer monitors are sensitive to between 5 mG and 12 mG. Electron microscopes misbehave at 1 mG or more. A world health body has tagged 4 mG long-term exposure as related to childhood leukemia.

The Bell 4080 triaxial meter is the one I recommend. It is very accurate, costs just under $200, and can fit in a shirt pocket. It is most useful in a walk through. To be thorough, one can take a reading every few paces in a grid pattern in each room. The results will show the shape of the field and help identify the faulty circuit.

In my book, "Tracing EMFs in Building Wiring and Grounding" I give a full description of how to use a gaussmeter before and after the trouble-shooting process. The next stage makes use of clamp-on ammeters to trace the net current circuits, starting at the panels. The book explains the logical process, which identifies the miss-wired circuits and leads to finding the incorrect neutral/ground or neutral/neutral connection. (See below for sources for the book). The gaussmeter measurements now drop to the ambient, which is usually 0 mG to 0.2 mG. If the building is within a strong field from a nearby power line or transformer, the gaussmeter will also tell you this and let the client know if this is the source of his problems with his electronics.

A single axis gaussmeter with a flexible sensor attached to the meter by a cord is useful in getting directional information. A very accurate one I use is the MSI-95 which is compensated for harmonic frequencies and has the advantage of being able to read extremely high mG levels, such as on the surface of transformers and motors. The sensor can also be plugged into an oscilloscope for a true picture of the waveform for harmonic identification.

As an EMF consultant, I am often called in to find the source of power frequency magnetic fields, which are causing computer monitors to jitter. In most cases, the first persons to be called in are electricians; next come the electrical engineers. Then a EMF consultant should follow the protocols described in my book to the sources. The building's electricians then correct the errors and everyone is happy.

What a EMF Consultant should do.

  1. Have a gaussmeter.
  2. Have different sizes of clamp-on ammeters, including a flexible one, which can measure around large bus ducts. (Sources for instruments are given in my book).
  3. Be familiar with the various ways neutrals are misconnected to ground and to the wrong neutrals.
  4. Know the Code articles, which back up the EMF recommendations.
  5. Follow the logic of the trouble shooting sequence explained in my book.

To summarize the conditions detected by the gaussmeter:

  1. Net current on circuits, always a result of a Code violation.
  2. Overheated conductors and conduits due to induced currents from the magnetic fields.
  3. Conditions setting up the possibility of shock and fire.
  4. The explanation for computer monitor jitter and malfunctions in other sensitive electronics.
  5. Incorrect grounding.
  6. Objectionable current on water pipes, building steel, HVAC ducts, gas pipes, etc.
  7. Power line or transformer fields, which can easily be distinguished from internal sources.

Caution about gaussmeters: There are many meters on the market, which are inaccurate, particularly if the circuit being measured has harmonic content. One-piece single axis meters are also awkward and slow to use. My recommendations are based on years of experience and I have no financial stake in the products.

Final statistic: A recent California study of magnetic field levels in 90 schools statewide found that 70% of the elevated fields were due to internal sources rather than power lines. The pilot study that I lead verified that the internal sources were wiring connection errors of the kind described above.

Where to buy gaussmeters as well the book, Tracing EMFs in Building Wiring and Grounding:
Bell 4080 triaxial gaussmeter ($199.50), and the single axis MSI-95 gaussmeter with its separate probe ($235), call MSI at (800) 749-9873. Credit cards accepted.

Mike Holt sells the book. Call 1-888-NEC-CODE and ask for Tracing EMFs in Building Wiring and Grounding ($28). Credit cards accepted.

Karl Riley
ELF Magnetic Surveys
(941) 283-8232

Mike Holt's Comment: If you have any comments or feedback, please let me know, Mike@MikeHolt.com


Copyright © 2002 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)