Power line noise

Power line noise is generally a raspy buzz modulated at some low harmonic of the power line frequency.  Power line noise is  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. CB operators sometimes mistakenly call line noise 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:

• 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 a higher pitch raspy noise. It almost always goes away in wet weather. It also "breaks up" when the 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", or (after looking to see the guy wires were well clear of any hot lines) shake or push on the 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!

The problem 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.

The bracket above, for example, should be securely grounded to the ground wire running down the pole. Notice this utility let the bracket float. While that won't make noise, it does create a hazard. If the disconnect switch insulator should ever arc through, develop leakage, or crack the bracket would become hot. If the ground wire was close but not touching the ground wire, it could arc from normal leakage and cause radio noise. It either needs to be a long ways away from the ground wire, or it needs to be bonded to the ground wire. Best would be to bond it into the ground wire.

The eyebolt holding the polymer insulator should also either be solidly grounded, or it should be kept away from the ground wire.

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.