Telephone Pole: Part 2 - more than you ever wanted to know

As I wander my neighborhood with Mr. T (my dog) I notice that most of the “telephone poles” in the area are configured as shown in Figure 2. The wiring shown here is the electrical grid wiring, not the telephone and internet wiring that is also on these poles. I’ll talk about those later.

The single line at the top of the pole carries one phase of a 3-phase nominal 15000 VAC distribution circuit. This line is at about 7200 VAC with respect to the Neutral wire, or ground. Although it doesn’t seem like it, the 7200 volt line is called Medium Voltage – and it is, compared to lines further upstream from the neighborhood.

At intervals along the line of poles, you will find a transformer – a big steel canister full of carcinogens (PCBs). One wire lead from the Primary conductor is connected to the primary side of the transformer, the other side of the transformer input is connected to Neutral.

The secondary of the transformer (3 wires) are connected to the Service Drop, providing 220VAC to the residence. This drop is 3-phase and can then be configured in your house for either 110VAC or 220VAC. 

Sometimes the transformer feeds more than one house, so the transformer secondary is connected to three lines called the Secondary Main. Individual houses are supplied by service drops off the Secondary Main lines. This is shown in the configuration on the left in the Figure 2.

As you may notice, the higher the wire on the pole, the higher the voltage. The top-most line is at 7200 volts, the Secondary Main lines are at 110V with respect to ground, or 220V with respect to each other. The top line is obviously a Kill-Me-Quick. The Secondary Main lines are Kill-Me-Almost-As-Quick. Those are genuine electrical engineering terms - trust me.

The pole I climbed as a boy was of the configuration on the right in Figure 2. There were no Secondary Main lines for me to kill myself on, and I stayed well below the transformer and hence avoided the 7200 volt Kill-Me-Quick line at the very top.

Figure 2

As I walk my dog and study the poles. I notice that there is quite a lot more going on. Consider Figure 3.

There is an assortment of details here.  First, there is a thing called a Lightning Arrestor. This gizmo is supposed to provide a short circuit to ground in the event lightning strikes the Primary line.  If you look close you’ll see a wire running from the bottom of the arrestor to the pole and down the pole all the way to ground at the base of the pole.

Also in the wiring between the Primary and the transformer input is a giant fuse. This blows out in the event of a voltage or current surge – and is a common cause for local power outages.

I once witnessed a Primary fuse blow. Well, I witnessed a neighborhood grey squirrel cause the fuse to blow. He was inspecting, too closely as it turned out, the transformer inputs and somehow got himself across 7200 volts. This was coupled with a bright white flash and a sound like a .38-special going off. Those two stimuli got my attention and my eyes swiveled in time to see a rather charred-looking, formerly grey, squirrel executing a parabolic ballistic trajectory from the top of the transformer to the ground. In my mind, he left a faint smoke trail behind. Sure enough, his sacrifice took out all power in the neighborhood for an hour until the lineman came and reset the fuse.

Figure 3

As you move up the electrical distribution inverted pyramid, the electrical stuff on the poles gets more complicated. Consider Figure 4.

The cross piece at the top of the pole carries all three phases of the 7200 VAC. Voltage difference between these wire can be as great as 15,000 volts. This pole supports only a single transformer, but more can be used. Each transformer is connected to one of the high voltage lines through an arrestor and fuse. And each transformer can have a three-line secondary drop to a residence, or to a Secondary main.

Figure 4

A, B and C: Three phases of 7200VAC (only B is connected to the transformer)

D: Neutral

E: Fuse

F: Lightning arrestor

H: Transformer

G, J: Secondary drop

K: Communications lines (cable and telephone)

The lowest line involved in electrical transmission is the Neutral line, located below, or on the same level as, the transformer.

Incidentally, I notice that the 7200 volt lines and the Neutral line are not insulated. They appear to be bare metal. Their height above ground and their distance from each other is apparently sufficient safety margin. The secondary lines, except Neutral, are insulated. 

Any lines appearing below the Neutral electrical line are used for communications, meaning telephone and cable. The conglomeration of communications lines can easily become extremely complicated.

In cases where perhaps there are three Primary lines and maybe three, or more, transformers, and bundles of communications lines as well, the whole situation can look mighty chaotic. Consider this couple of photos I found/took.

Figure 5

Figure 6

Figure 6 shows a single phase 7200VAC feeding a single secondary transformer which in turn feeds a Secondary Main from which there are two Secondary drops to houses. A streetlamp is also powered from the secondary. Cable/phone lines are also present.

Figure 7

Figure 7 shows the end of a power run. A single 7200 VAC line ends. A three-wire Secondary Main also ends with a drop to a house and power to a streetlamp. Two guy wires hold the pole up, and phone/cable lines are present.

Figure 8

Figure 8 shows a drop from one three-phase 7200 volt set to another set at right angles. There is no transformer present, but there is a Secondary Main travelling through (and powering a streetlamp). 

So, there you have it - my assessment of the "telephone poles" in my neighborhood.  Probably not too interesting to most of you, but something that has always nagged me. As a card-carrying Electronics Engineer for 40 years (despite never taking a single course in Electrical Engineering), I always felt this gap in my education. Now I don't. I've finally completed the self-education I began 62 years ago when I climbed a rural telephone pole in Florida.