BENER Digest Update.
Home Magnetic Field Surveys and Water Pipe Ground Currents

At high frequencies, electric and magnetic fields are closely coupled and their effects are largely interdependent. At power frequencies, however, electric and magnetic field components behave virtually independently. For example, a transmission line with a 1000-Ampere (Amp) current operating at 345 kilovolts (kV) has the same magnetic field as a line with a 1000-Amp current operating at only 100 volts, but the electric field of the latter is more than three orders of magnitude lower.

Initial research on transmission line fields focused on bioeffects of the electric field component, which is substantially higher than that of most other environmental electric field sources. When research results on animals exposed to high electric field levels were largely negative and epidemiological studies, such as the Wertheimer-Leeper study, suggested the importance of the magnetic rather than the electric field component of power line EMF, attention began to shift to measuring and controlling magnetic fields.

Many people have become concerned about the potential health effects of the magnetic fields generated by common sources of EMF exposure in the home, and their concern is heightened by publicity from the popular media. At times, the information obtained from the media is overly sensational and/or incorrect. For instance, in quoting a study, one journalist stated that those living near pole-mounted transformers were at risk of exposure to high levels of magnetic fields. As a result of this statement, I received a flood of calls from individuals whose worries about living near transformers were unfounded.

The original epidemiologic study, by Wertheimer and Leeper in Denver, actually stated that the magnetic fields of transformers decrease to ambient levels within 6 or 7 feet of the transformer -- much closer than the location of most homes. Wertheimer and Leeper were really discussing an unusual situation in the older parts of Denver, where some twenty or thirty homes are fed by one transformer. Those closest to these transformers were exposed to high magnetic fields emanating from the low voltage load currents of all the other homes fed by the same transformer. In most communities, however, transformers only feed 3 to 6 residences, and the concerns addressed by Leeper and Wertheimer do not apply.

Although concerns about magnetic fields in the home are often misplaced, as with the misinterpretation of Wertheimer and Leeper's study, there are sources of magnetic fields in the typical home that may present a source of unusually high magnetic field exposure for those living there. It is important to be aware of the major sources of magnetic fields in the home and it is necessary, in some cases, to seek and mitigate potential problem areas.

The major sources of magnetic fields in the home are:

1. Transmission lines supported by steel or wooden towers. These lines are used to transmit power from generating plants to substations or between substations. Substations are often large complexes of transformers and switch gear equipment surrounded by chain link fences, or located in special buildings or bricked-in yards. The voltages for transmission lines are from 32 kV to as much as 750 kV. Trees and other conductive objects can block the electric fields, but not the magnetic fields, of these transmission lines.
2. Distribution lines are operated at lower voltage (such as 5, 13.8, or 27.6 kV) and are energized from step-down transformers. The first-step transformers are usually located in substations, but subsequent-step transformers may be located on wooden poles or underground. These transformers bring the voltages down to the typical three-wire drop that provides electrical service for residences. The transformers look like metal milk or trash cans when on poles, and are pad-mounted within rectangular steel boxes for surface and underground systems. There is no inexpensive way of blocking or shielding the magnetic fields from existing distribution lines or transmission lines.
3. Appliances have fields which, while quite large at the source (anywhere from 20 to 1000 milligauss (mG)), decay to ambient levels a few feet away. In conducting a magnetic field home survey, it is important to be aware of the fields from appliances, and to avoid including these fields in the estimates of ambient fields from other sources. Because walls do not stop magnetic fields, appliances can cause fields in adjacent rooms. For example, a dial electric clock in a stove will produce higher fields on the other side of the wall than in front of the stove. Humidifiers and dehumidifiers often have high fields that can extend several feet around them.
4. Water pipe ground currents are found in many suburban communities which have all-metallic water service and mains. However, homes that use wells do not generally have this problem. To prevent electric shocks within a home, all metallic surfaces of appliances and conducting pipes should be grounded. This produces an alternative metallic path through the house which can carry, in some cases, neutral or return currents. Currents on metallic plumbing are not canceled; unlike household wiring in which hot and ground conductors are in close proximity and, as a result, magnetic fields produced by each wire tend to cancel out. Even when all the power in the house is off, there may be ground currents, and thus magnetic fields, in the house from electric use in neighboring homes. It should be noted that the term "ground currents" is also used to describe currents which flow through the earth itself. While ground currents through the earth may constitute a shock hazard, they are generally on the order of a few milliamps which is too small to produce a significant magnetic field. The term "ground current," as used in this paper, refers to current flowing along buried metallic water pipes (with or without water in them) rather than current flowing in the ground itself.
5. Faulty wiring is another source of magnetic fields. Often, a neutral lead inside the house is accidentally, or in some cases improperly, connected to a grounding wire. This allows for multiple return current paths to occur, resulting in an imbalance of wires in close juxtaposition. The resulting magnetic fields are often high.
6. Unusual wiring, such as knob and tube wiring and current loops created by two-way switches, can cause localized higher magnetic fields. Knob and tube wiring is sometimes found in older homes, where two wires inside the walls are separated by two or three inches and supported by porcelain knobs and tubes. An electrician must rewire these circuits with modern Romex or BX cables. Two- (and three- or more) way switches, such as are often found at the tops and bottoms of staircases, can, in some cases, be wired in such a way as to produce current loops that will elevate the magnetic fields when they are turned on. Not using the lights or having them rewired will eliminate this problem. The magnetic fields of unbalanced wires and faulty wiring may be confused with those of water pipe ground currents because they have similar decay characteristics (as described below). Therefore, before attempting to eliminate faulty wiring, water pipe ground currents must be checked for and eliminated.

In most cases, the magnetic field on unbalanced distribution lines (with a non-zero net current) decay at the slower rate of one over the distance from the line. Both service drop lines that have net currents on them and water pipe ground currents also produce magnetic fields which decrease at a rate of one over the distance from the source.

When magnetic field measurements are taken in homes, fields from distribution and transmission lines will appear as a uniformly falling field from points closest to the line to those furthest away from the lines. Aluminum windows and doors and some copper pipe loops can produce magnetic fields by induction, which affect this uniformity. Net current sources will drop off more sharply with distance, and can be recognized by this spatial pattern.

Balance in phase currents in the hot and neutral wires is an important factor in the rate at which the magnetic field decreases with distance from the wire. In general, whenever paired wires carrying equal and opposite currents are close together, the fields drop off at a rate of one over the distance squared. Thus, the wires feeding a 100-watt lamp wire will contribute almost no magnetic field at distances of two or three inches away. However, if the wires are separated, the electricity reaching the lamp will not be affected, but the cancellation of the wires is not as complete, resulting in strong magnetic fields extending several feet away.

Since I specialize in the in-home surveys to detect and eliminate water pipe ground currents, I will now focus on the magnetic fields of such currents.

A home's water service almost always enters the home from the street through underground pipes. When the water supply system consists of metallic water pipes and mains, there are often water pipe ground currents. When a load on the house, such as a toaster, is turned on, a current will normally flow from one of the 120-volt lines on the transformer and return over the neutral wire. With metallic water pipe systems, the National Electrical Code (NEC) requires a connection between the neutral at the electric service entrance (the fuse or circuit breaker box) and the water service where it enters the home. This connection allows for an alternative return path to the street which often results in currents that flow through conducting pipes to neighbors' homes and to their neutrals before returning to the transformer.

When the water service currents exceed approximately one half to one Amp, the resulting magnetic fields can be substantial, and can be found over large sections of the home. Since they do not flow on the home's neutral, these currents will cause a net current on the three wires of the service drop, which results in high magnetic fields in the house in the vicinity of the electric service drop. The magnetic field from these net currents decays slowly (at a rate of one over the distance).

Generally, water pipe ground currents are detected when there are unusually high magnetic fields near the front door and other rooms on the side of the house where the neutral to water pipe connections occur. High fields extending in the room opposite the service drop and sections of the basement are also indicative of fields from water pipe ground currents.

To determine the existence of water pipe ground currents, three currents should be measured:

• I(WS) -- the electric current on the water service pipe as it enters the house;
• I(NG) -- the electric current on the neutral to water pipe cable; and
• I(WP) -- the electric current on the water pipe just after the neutral to water pipe cable.

Electric currents (in Amps) from the water pipe system in a home with water pipe ground currents

Toaster               Lamp
(1500 W, 12.5 Amp)    (60 W, 0.5 Amp)   I(WS)      I(NG)      I(WP)
Off                   Off               1.6 Amp    0.9 Amp    0.7 Amp
Off                   On                1.75 (Change in current: + 0.15)
On                    Off               5.6  (Change in current: + 4.0)

In the above example, turning on the lamp causes the I(WS) current to increase slightly (from 1.6 to 1.75) so we know that the current in that outlet is in the same direction as the existing I(WS) current. Had it decreased (to 1.45), we would need to try other outlets until we found one that made the I(WS) increase. If we test an outlet with too large a load (like the toaster), it is not possible to know what the real change in I(WS) is. For example, if we got an I(WS) reading of 5.6 A with the toaster on, we really wouldn't know if that represented an addition of 4.0 A from the toaster, or an addition of 7.2 A in the reverse direction, which cancels the preexisting 1.6 A I(WS) to give a new net current of 5.6 A.

All the field and current measurements depend on the loads and appliances operating at the time of measurement. As the data show, the values for I(WS) increase with the power demands of the appliances in use, and these levels can change rapidly and frequently, e.g., a refrigerator goes on or off, or a neighbor turns on an appliance in their home. The first set of readings indicate that the water service current returns to the electric neutral in the following way: 56% is on the neutral-to-ground wire, and 44% goes through various pipes in the house on the way to a ground wire before reaching the electric service neutral. Some of the magnetic field readings in the house were due to the latter. The second two sets of readings were conducted to identify an outlet where a load would increase rather than decrease the value of I(WS) as noted above. The resulting readings indicate that about 4 out of 12.5 Amps (32%) of the return current for the toaster finds its way to the water service pipe. The current in the service drop neutral and the water service, water main, and neighbors' service drop neutral, divide according to their relative impedances. The splitting of the currents into I(NG) and I(WP) indicates that in this case they are approximately equal.

To eliminate the ground currents coming into the home and their resultant magnetic fields, a non-conducting section of pipe must be placed between the water main and the house. Placement of a non-conducting section of pipe at the shut-off valve at the property's edge is the recommended practice because it will satisfy the National Electrical Code requirements (a minimum of 10 feet of water pipe under the earth for grounding to the electrical service neutral), and eliminate most magnetic fields from water pipe ground currents entering the home.

The non-conductive pipe coupling should never be placed inside the house, because this seriously compromises the grounding of the home's electric wiring, and can produce dangerous electric shocks. Moreover, placing the coupling in the house would violate the Code. The work should be approved by the local plumbing inspector and performed by a licensed plumber. To meet Code requirements and reduce the risk of electrical shock, a licensed electrician should ensure that the neutral at the service entrance is also connected to a metal rod driven into the earth, and check that the neutral wire along the service drop functions properly. There have been cases in which the service neutral was broken and all the return currents appeared on the water pipe system. The home owners were not aware of this potentially dangerous situation because all appliances worked normally.

In conclusion, there are many common sources of magnetic fields in the typical home. In particular, ground currents from water pipes can frequently produce high magnetic fields because they are net current sources. To be effective, home EMF surveys must not only determine the spot measurements of the magnetic fields, but must also identify the sources and, if possible, suggest ways to reduce these fields.

Maurer, S. J., Ground Current Magnetic Field Study, Research Report EP 90-57 Final Report Prepared for ESEERCO, 1155 Avenue of the Americas, NY, NY, 1992.

Maurer, S. J. and Rosenthal, D. L., Source Characterization of Power Frequency Magnetic Fields in Urban Environments, Proceedings of the American Power Conference, Vol 56, pp 1182-1184, Chicago Illinois, 1994.

Dr. Maurer is with the Department of Electrical Engineering, New York Institute of Technology, 1855 Broadway, New York, NY 10023.