Power line noise
Distribution lines are the high voltage lines that distribute power along
residential and light commercial feeds. Many years ago these lines were just
over 2200 volts, then 4160 volts (from primary to neutral) became a standard. Many amateurs used surplus
4160 transformers in power supplies! I once had a 572B amplifier using a 4160
pole pig. The line voltage switch (2.5% taps) was at the middle of the winding,
making a great center tapped winding. Back around 1970, I ran about 2900 volts
on the plates of four Cetron 572B’s.
In the 1970’s, I started building noise locating equipment and
repairing noise locating equipment for utilities. 1980’s and later, I
subcontracted work for a few small (and one major) companies locating more
difficult cases of power line noise. As I quit working with utilities, most
residential system were 7200 volts from primary to neutral. My local feed today,
on a rural dirt road with single phase, is 7200 volts.
Power Line Noise
Identify Your Noise!
Distribution power line noise is
generally a raspy
buzz. Power line generated noise on normal distribution lines is modulated at
some low harmonic of
the power line
Almost always the single phase “buzz” is 120 Hz. This is because noise
is from small arcs (usually in insulators or pins), and the arc almost always occurs twice in each cycle near the
negative and positive voltage peaks
of the 60 Hz sinewave. Power line noise is broadband. Power line noise does not
appear in clusters or periodically across any given band, although level can
change greatly from band to band.
Switching noise and digital noise from
consumer devices is almost always periodic. The periodic appearance across a
given band occurs because most consumer noise is generated by a high frequency
clock or oscillator’s harmonics. If the time base or oscillator of the consumer
device is 30 kHz, noise generally appears every 30 kHz across a band. There will
generally (but not always) be clear spots between noise peaks.
Some forms of modern digital noise can sound like power line noise.
One example is my neighbor’s Direct TV recorder system. The Direct TV device
they have far exceeds FCC emission limits. From mid-AM broadcast band to mid HF,
it sounds just like a residential distribution power line noise and the raspy
buzz it generates follows the power lines. It disappears above 10 MHz.
Transmission line systems, the really big lines running from
town-to-town, run at much higher voltage. Because of that, the noise is
generally different. The noise ranges from a sharper buzz to a hissing with a
faint 120 Hz or higher note. High voltage transmission line noise can sneak up
on us, making us think we have normal background noise because at times it
blends into normal background hiss. This is because the arc or discharge, being
from a much higher voltage, often lasts over a much larger portion of the cycle.
It also can involve multiple phases. This smoothes or softens the noise,
but if you listen carefully there is generally a little 120 Hz, 240 Hz, or 360 Hz
slightly musical buzzing present.
Power line noise is relatively frequency
independent, usually having
only a very gradual
change in level with
line noise can be
band specific, but
it is never
Light dimmers and other consumer devices can be the same.
If a noise is frequency periodic, especially a signal repeating with 10 kHz or
more spacing between peaks, it is probably a switching power supply or digital
device of some type.
refers to line noise as land-noise or ground-noise, very unusual
slang since neither
the ground nor the
land is a source of
Distribution system power line noises originate in the following
items, each of
which has subtle but
characteristics in pitch or sound. Some sources also start and stop in response to weather:
or hardware arcs
insulators. Often quiet in wet weather, and almost always wire-motion
Arcs from loose
clamps or bolts
that join wires. Often sensitive to hard hits on the pole, but can drop
dangerous molten pieces of metal
Arcs in hardware,
arrestors. Not usually affected by weather
Arcs in hardware
near, but not
to, power lines. These are generally much worse when dry, but usually
unaffected by pole movement
Poorly wrapped or
wires that secure
power lines to
knob insulators. This is another dry weather arc, usually on VHF and higher
internal arcs in
transformers. These arcs are generally unaffected by weather or wire movement
Pin or Hardware Arcs
These arcs are primarily, but not always, dry-weather noises.
This type of noise
is generally a higher-pitch
raspy or rough noise. Pin noise or hardware arcs between loose pieces of metal
on the pole
almost always go
away in wet weather.
This is because moisture wets the dielectric (oxide or corrosion) in the pin
area making it conductive, and that stops the arc. This particular noise source also “breaks up”
when poles and wires
wiggle or move. When
I did noise
investigation for a
companies, I would
strike the suspected
pole with a large
hammer and listen
for the noise to
“break up”. Another method I used
(after looking to
see the guy wires
were well clear of
any hot lines) was to shake
or push on guy
wires. You should
not do this without
permission of the
pole owner. I had
Insulator pin arcs
are one of the most
common sources of
broadband noise on
power lines. This
noise is caused by
low tension on bell
them to hang with
visible sag or
slack. The noise is
generally a medium
to low level noise
with a higher
pitch because the
arc is weak with
very low current,
but like all noises
it can propagate a
long distance along
The pins on each end
of insulators can be
a common source of
noise. The long
insulator above is a
newer Polymer type.
not have the leakage
capacitance of older
insulators, and is
not as noisy when
span tension is low. The
pins however are the
same in almost all
insulators. With low
tension the pins
corrode and make
poor contact. This can cause a
very tiny arc. The arc
excites the power
line through the
insulator’s stray capacitance
and the power line
acts like a giant
antenna. A few
milliwatts of energy
can radiate a long
distance when using
a long wire antenna
like a power line!
Sources of Noise on typical pole
Loose connections on power factor correction capacitors can arc for many
years without damaging anything. This is generally an all weather noise.
The bracket of the capacitors should be grounded
to the pole ground wire, and the capacitor, solidly. Not bonding generally makes
a dry weather noise, and is a safety hazard to workers
Polymer insulators can have slack spans or low tension with little likelihood
of noise. Since they are very long and have very low end-to-end capacitance,
they are unlikely to have enough voltage at the pole end to arc, even when
loose. They are good trouble-free insulators.
Loose ceramic bell insulators are bad news, especially in dry weather! The large metal caps towards the
pole side capacitive couple to the hot wire side. This bell-style insulator has
considerable capacitance from the hot end of the insulator to the ground end. It
should never be used in slack spans. Slack spans should use post mounted
insulators or long polymers to minimize capacitance and increase leakage path
between the ends.
Most often a noise problem with slack spans is
rooted more in the
capacitance of the
across the insulator
surface, although both
can be involved.
The longer polymer
insulators on the
pole above have a
long fiberglass rod
core and a very long
path around the
bell insulators have a
very large metal
casting capping the
low voltage or
grounded end, and
have an interlocked
center pin and body
cap separated by
ceramic. Spacing is
small and parallel
surface areas are
large in the more
insulators, causing very
between the metal
cap and the center
pin of the ceramic
have very low
capacitance and a
long leakage path,
so they do not
end-to-end nearly as
well as the ceramic
bell insulators. A
span might have to
be left slack if the
pole can not be
preferred when a
span has to be
Pins that secure the
insulator to the
corrode and build up
a thin layer of
insulation. When a
span is slack (under
the insulator metal
end cap, the
floating pin that
locks the end cap to
the eye bolt or
and the mounting
hardware will arc
across the thing
layer of corrosion
in the joints. This
is because the pin
is not pulled
tightly against the
and a small arc
develops across the
corrosion in the
joint. In wet
weather the arcing
will often stop and
the line become
quiet. Slack spans
with bell insulators
are mostly a dry
Loose Clamps and
Hardware on Poles
Loose hardware on
poles and wires is a
common problem. It
is also a safety
issue! This type of
makes a severe raspy
strong noise over
all bands. This type
of noise is
although it can get
get either louder or
quieter in rain. If
it is arcing from
will generally go
away in the rain. If
it is a loose
connection on a
like a loose nut on
it will come and go,
hardware should be
solidly bonded to
the ground wire on
the pole or it
anything else. This is
noise as well as
workers. It also
reduces the chances
Note the eyebolt at pole top is grounded through a wire to the guy wire and
the vertical ground wire running up the pole.
All hardware should ground to the pole ground.
The bracket to the left, for example, should be securely grounded to the ground
wire running down the pole.
Notice this utility let the bracket float. (the line that might be a ground
wire to the bracket is actually a shadow)
While a well-insulated ungrounded bracket
won’t make noise, it does create a safety hazard to linemen.
If the disconnect switch insulator should ever arc through, develop leakage,
or crack the bracket would become hot. If the lightning arrestor would fail
shorted and blow its ground wire off, then the bracket could have full primary
voltage. If the ground wire was close but not touching the ground wire, it could
arc from normal leakage and cause radio noise.
The bracket either needs to be a long distance away from the ground wire,
or it needs to be bonded to the ground wire. The best installation would bond
the bracket into the pole ground wire.
The eyebolt holding
also either be
solidly grounded, or
it should be kept
away from the ground
wire. Again the safest installation for linemen would be with a grounded bracket.
Hot clamps and other
line hardware should
be tight. Some of
the most severe
noise sources are
loose hot clamps and
actually start fires
in dry weather.
Ballasts in street lamps can also be a problem.
Other Non-utility Sources
Off-on buzzing noises, timed with a regular rhythm, are often
small permanently wired electric transformers, heating pads, fish tank heaters,
and older electric fences.
Electric fences are generally about 1 second on and one second
off. Noises are caused by bad splices and connections, or wires contacting
something that causes a small arc.
Small transformers often have a thermal cutoff switch. They
usually pulse slower than an electric fence. The general cause is a shorted
secondary circuit, which in turn overheats the transformer and causes the
thermal bimetal overload switch to cycle off and on as the transformer winding
heats and cools internally. Small heating pads are similar in time.
Heating elements for fish tanks and large electric blankets
are often very slow in cycling with long off times.
Frequency-periodic drifting signals are usually switching
power supplies of some type.
Really broadband almost indescribable trash that is almost
white noise often comes from plasma TV sets.
For you own noise locating equipment, it is best to use AM
detection. It is important that any impulse or arc detector use a wide IF bandwidth and
AM detection. AM broadcast receivers work after a fashion, but make localizing
noise to one particular pole difficult or impossible. This is because the
wavelength of the AM band is very long. Wires and conductors along the electric
system conduct longer wavelength noise with very little attenuation. Another
effect is standing waves, which can make the same noise peak and null as the
receiver moves along the wires. The combination of low attenuation with distance
and standing waves along the wires can make it very difficult to pin down the
exact source. As such, AM receivers have very limited utility for narrowing down
The very best receivers are VHF or UHF AM receivers, like old
television field strength meters. Another commonly available and relatively
inexpensive receiver is a regular portable aircraft receiver. A few FM portables
or mobiles for amateur use include aircraft band, or selectable AM detection.
This is a very wide tuning range battery-powered receiver with a wide IF
bandwidth. It has an AM detector and attenuator. This Sprague noise
interference locator was typical of handheld noise locating devices used by
utility companies. It is one of many RFI locators I used when doing consulting. It tunes from the low AM broadcast range up to UHF in one
The basic receiver is very much like homemade units I built for utility
My units used two detectors. The first detector used a variable capacitor
tuned local oscillator and mixer. It up converted to a 40 MHz IF system driving
a video detector. The IF system was actually an IF and video detector
module from a standard TV set. I switched between the low frequency mixer (40
MHz down) to television set varactor VHF and UHF tuners.
This commercial unit is very similar. It up converts to a UHF TV tuner and IF
system. A single dial tunes from below the AM broadcast band to UHF.
This unit has an internal battery, and uses several different hand-held
antennas shown below in order of descending frequency.
upper VHF and UHF directional antenna
Low VHF to upper HF loop, inside a bigger mid to upper HF loop.
Broadcast to mid-HF rod antenna.
This rod antenna is plugged into the hand unit that contains the tuning
The rod antenna is bidirectional.
This is a home brew loop I made. It includes a built in amplifier and band
switches from AM broadcast up through 10 MHz.. I also use it in conjunction with
a MFJ-259B meter to locate buried cables. I can find the location of underground
cables within one inch using this loop!
The yellow area is yellow heat shrink over 5/16th inch copper tubing. The gap
or split in the tubing is at the very top, opposite the copper tuning box.
The bottom ends of the copper tubing run through and solder to the copper
box. Multiple turns inside are tapped by a switch, and tuned by a 200 PF
This loop, because it employs a JFET amplifier, is very sensitive.
The null is though the loop center.
This loop makes an excellent cable locator.
I easily find the location of underground cables with this loop and a small
signal source, like the MFJ-259B.