Pete is pretty close to being right on the money. Slack span insulators, aka
dead end bells, ect. are most commonly made of a combination of porcelain
and steel. Sometimes the porcelain is replaced by Rubber, glass, or
fiberglass. The steel is at times replaced by aluminum.
The problem is at the connection points. Depending on the voltage on the
line, their maybe two or more insulators and their is two connections for
each additional insulator. The metallic connections, although touching each
other, don't have a good electrical connection. The poor connection is due
to the corrosion an the material.
The voltage on the wire creates an electrical field which induces a voltage
into the metallic and conductive hardware in the immediate area. Due to the
difference in size and distance from the live conductor these things are
energized to a different voltage level. The corrosion separates the hardware
and a gap is formed. The voltage difference causes stress in the gap and the
air within the gap breaks down and no longer acts as an insulator. When this
occurs a spark activates and discharges the air gap. then the process begins
again. This occurs at a rate of 60 hertz per sec. and continues as long as
the voltage and gap are present.
Under windy conditions the gap is created and shorted temporarily. This
creates a very intermittent condition that they are famous for.
This situation, the corroded hardware is responsible for over 80% of all
power line interference sources. However, the most common is not the famous
slack span insulators.
To find out more about this attend the Vienna Wireless Society's Winterfest
2002. I'll be doing two power line interference presentations.
Mike Martin k3rfi
RFI Investigator
RFI@pepco.com
Potomac Electric Power Company
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