12 January 2000
218-233-8816---------- Forwarded message ----------
Date: Tue, 11 Jan 2000 15:11:16 -0600
From: Edward Maxey <email@example.com>
To: "Roy L. Beavers" <firstname.lastname@example.org>
Subject: Ground Currents
Duane Dahlberg's "UNDERSTANDING GROUND CURRENTS" is a commendable article detailing both the physics of stray currents and the history of how our nation's electric utilities brought us to our present situation. Those seriously concerned about electromagnetic fields in their homes and work environments will do well to study this document.
The following remarks are intended to supplement Duane's fine paper. Several topics bear comment:
A: Transmission lines are DELTA configured, carry energy from power plants to substations and have almost zero net currents. This is because the return current from phase A is totally carried on phases B & C as the phase angle rotates. Transmission lines have very high voltages but are not the major source of 60 Hertz magnetic fields.
B: Primary distribution lines are WYE configured and carry power from
substations to secondary transformers. These are the transformers
commonly seen on utility poles. They are also increasingly placed
in metal boxes in both front and back yards. Primary distribution
lines are of two varieties:
1. Radial feed in which a supply and return line carry power from a source to one or a series of secondary transformers and
2. Looped feed in which the supply and return lines carry power as above but also continue on to a second bonding to the source at a second point. Looping contributes significantly to 60 Hz magnetic field generation since return current paths are often widely dispersed from supply paths.
C: Duane correctly points out that the neutrals of the primary distribution system are metallically bonded to the neutrals of secondary distribution lines in secondary distribution transformers. Thus the return path of primary distribution systems easily finds its way into homes and working environments. The metallic bonding of neutrals in secondary distribution systems is NOT mandatory. D. Lanera in a 1993 paper writes:
"This is the only grounding tie that it is allowable to remove under the strict rules of the NESC and NEC."
His paper describes a 43 square mile area in Michigan where this tie was removed in connection with a US Navy ELF Radio Transmitting Facility. Removing this tie and implementing other measures described by Lanera would do much to reduce stray currents if implemented throughout the nation.
D: Eugene Preston is credited for his work on how underground rural distribution systems generate magnetic fields. This same Eugene Preston in association with Heath Hofmann developed a net current control and installed it in Preston's back yard. A presentation of this work was made before the AMERICAN PUBLIC POWER ASSOCIATION. Such controls either block excess return current or suck additional return current to effect a balance between supply and return flows; thus virtually eliminating magnetic field generation.
E: A significant source of high magnetic fields in apartment complexes and work places is the placement of a bonding screw in circuit breaker panels. This screw bonds the neutral bus to ground and thus provides an alternate return path. This bonding screw was being required by municipal inspectors here in Arkansas until it was demonstrated to them that such a practice was in violation of existing electric codes. It is not unlikely that inspectors in other States are mistakenly requiring such bonding screws.
Kudos to Duane Dahlberg
Back to Top