Power line noise

Also see Radiated and Conducted Noise

Distribution power line noise is generally a raspy buzz modulated at some low harmonic of the power line frequency.  Some forms of 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.

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. This smoothes the sound of the noise out.

Power line noise is relatively frequency insensitive, having only a very gradual change in level with frequency. Power line noise can be band specific, but it is never frequency specific. 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.

CB jargon sometimes mistakenly refers to line noise as land-noise or ground-noise, very unusual slang since neither the ground nor the land is a source of noise.

Power line noise is from the following causes, each of which has subtle but unique characteristics in pitch or sound, and also in response to weather:

• Pin or hardware arcs on insulators, generally bell insulators
• Arcs from loose clamps or bolts that join wires
• Arcs in hardware, like lightning arrestors
• Arcs in hardware near, but not connected directly to, power lines
• Poorly wrapped or insulated tie wires that secure power lines to knob insulators
• Arcs inside equipment, like internal arcs in transformers or lightning arrestors

Insulator Pin or Hardware Arcs

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 particular noise source also "breaks up" when poles and wires wiggle or move. When I did noise investigation for a few utility 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 permission.

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 insulators, allowing them to hang with visible sag or slack. The noise is generally a medium to low level noise with a higher sounding smoother pitch because the arc is weak with very low current, but like all noises it can propagate a long distance along the lines.

The pins on each end of insulators can be a common source of noise. The long insulator above is a newer Polymer type. It does not have the leakage capacitance of older ceramic bell 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

Loose connections on power factor correction capacitors can arc for many years without damaging anything.

 The bracket of the capacitors should be grounded to the pole ground wire, and the capacitor, solidly.

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.

Loose ceramic bell insulators are bad news! 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 insulator than actual leakage 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 external leakage path around the ribs. Ceramic 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 compact ceramic insulators, causing very high capacitance between the metal cap and the center pin of the ceramic insulator. The longer multi-ribbed polymer insulators have very low capacitance and a long leakage path, so they do not couple from 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 back-guyed. Polymer insulators are preferred when a span has to be left slack.

Pins that secure the insulator to the hardware will  corrode and build up a thin layer of insulation. When a span is slack (under low tension)  the insulator metal end cap, the floating pin that locks the end cap to the eye bolt or mounting hardware, 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 mounting hardware 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 weather problem.

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 problem generally makes a severe raspy strong noise over all bands. This type of noise is generally unaffected by moisture, although it can get get either louder or quieter in rain. If it is arcing from something being ungrounded, noise will generally go away in the rain. If it is a loose connection on a through connection, like a loose nut on the transformer primary connection, it will come and go, being largely independent of moisture.

All metallic hardware should be solidly bonded to the ground wire on the pole or it should be well-insulated from anything else. This is important for minimizing radio noise as well as protecting utility workers. It also reduces the chances of lightning damage.

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. Te 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 the polymer insulator should also either be solidly grounded, or it should be kept away from the ground wire. Again safest to linemen is to ground the bracket.

Hot clamps and other line hardware should be tight. Some of the most severe noise sources are loose hot clamps and corroded disconnect switches. Loose connections can actually start fires in dry weather. 

Locating Noise

It is important that any impulse or arc detector use a wide IF bandwidth and 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 tunes from the low AM broadcast range up to UHF in one tuning range!

 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 capacitor.

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 loaction 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 variable capacitor

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.