Saturday, October 3, 2009
Tutorial 6: Identifying the Wires
Last time we learned, at a very high level, about the different voltages of power lines used to get electricity from the generation station to your power outlet. Along with that we touched on the concept of voltage as pressure, how it is raised at the generation station so that the electricity can be efficiently moved closer to where it will be consumed, and as it gets closer to the end user, it is gradually dropped in steps, depending on where it is going, until it is stepped down to household current right before it goes into your home.
Please understand that there is so much more, and I mean a lot more, stuff to be learned about this if you're technically minded and interested. I am intentionally only glossing over a lot of this stuff, as my intent is to make you, my patient and suffering readers, merely aware of what is going on, and not trying to make any of you experts. If I am successful in imparting this basic awareness of how stuff works, then it theoretically will make you understand my rants and grumblings. Isn't that pathetic of me? Deliberately teaching you guys so that I can rant, in order that you can nod in empathy and agree, yeah Grumpy, that sure is lame. Manufactured sympathy!
OK, enough reflection. What a buzz kill.
Today, we take a first look at what is on the transmission towers and power poles.
The overriding rule of always is: EVERYTHING IS DANGEROUS AND SHOULD BE CONSIDERED ENERGIZED. Even if you know what it is, don't touch it, EVER. I'll get into that more later.
First up: High voltage transmission. It's easier to sort out.
Pictures 1 and 2 you'll probably want to click on to view full size at some point. Both illustrate the same thing, though. High voltage transmission is always three-phase, meaning each circuit consists literally of three sets of conductor. In Picture 1, there are two circuits on the tower. In Picture 2, each tower has two circuits, for a total of four circuits in this power line corridor.
You'll note that in Picture 1, each phase consists of a bundle of four individual wires, while in Picture 2, each phase consists of two wires. Added wires is like a larger hose, less resistance, better efficiency. Typically, you'll see single wire phases up to 230kV. Two wires per phase starts to appear at 345kV. Three wires at 500kV and up. But there are exceptions. The number of insulator bells also is a good indication of how high the voltage is, but there are different manufacturers, ratings, and sizes of the bells, so the actual number doesn't mean as much as the overall distance. None of this really matters much from our point of view though, as all of these will pretty much flash-roast you if you touch them.
In both Picture 1 and Picture 2, if you look close, you'll see a couple of faint, thin wires at the very top of the structures. These are called static wires or shield wires, and they are grounded at every structure. They are not energized, per se, but you can frequently detect voltage on them due to induction from being so close to the actual power lines. Literally, the invisible magnetic field around the power lines passes close enough to induce voltage in them. Their real purpose is to provide a preferred target for lightning strikes, by being grounded, they draw the lightning bolt away from the power lines themselves. They are not used to carry a current, but they are still entirely unsafe to touch. Sometimes power companies also run fiber optic cables along side or attached to the static wires, because the existing power grid infrastructure provides a ready-made grid useful for laying down an information/data grid. The power company uses a tiny, negligible portion of this bandwidth for data transfer between stations, and can lease the remaining bandwidth to cable, telephone and data providers.
That's basic transmission. Not too complicated. Subtransmission lines, say at voltages down around 34kV-69kV, will be similar in concept if not design. Smaller towers or more often wooden poles, three phases of single-wire, a single static line sometimes but not always. The main thing is that subtransmission usually remains uncomplicated, unlike the distribution stuff we're about to get into.
Picture 3 is a distribution pole. As a rule, the most dangerous stuff is at the top of the pole, and decreases in its danger score as you go down.
From the top down, then....
Three-phase primary, I am guessing somewhere between 8kV-14kV. The wires coming from either direction are installed as dead-ends, and are attached to the ends of the insulator strings. The dead ends from either direction are then connected with "jumpers" that are attached to the wire on each side and are held in place by an insulator on top. Although not preferred compared to an actual line switch, these jumpers can be detached from either side to "open" the circuit.
The sharply angled wires leading down to the right are guy wires, used to brace or hold the pole in place, entirely expected on poles where the circuit takes a turn, and as you can see the three phase is turning slightly to the left at this pole. Without the guy wires, this pole would want to be pulled to the left by the weight and tension of the primary.
The next single wire down is the neutral. It is grounded at frequent intervals along the circuit (ideally at every pole), and leads back to the big transformer neutral at whatever substation(s) this circuit travels to. The neutral is also extended to each customer service drop and ultimately grounded through the grounding rod at every customer site.
When you see the "braided" service drop at your house, there are usually two "hot" wires, with black insulation, wrapped around an un-insulated cable that carries the tension from the pole to your home. This bare cable is also the neutral. Since there are typically three wires total, we call this "triplex". You can see some of the triplex here, the hot insulated wires coming from a nearby poletop transformer and apparently provided to power the streetlight.
The last wires on the bottom are nondescript telecommunication stuff: Phone, cable TV, etc.
Picture 4 is a single-phase distribution pole with a "fused cutout" and poletop transformer.
Again, from the top down....
Single phase primary.
Hard to make out is where the jumper is attached to the primary, but you can see just to the right of the top insulator, the clamp where the jumper is attached before it runs down to the fused cutout. In plain terms, a 'fused cutout' is a device that can be used to open the circuit leading down to the poletop transformer, and the part that can be opened also serves as a fuse which will blow and cut off power to the transformer if it were to fail.
Neutral and service drop wires.
Clear as a bell or clear as mud?
What I want to leave you with, particularly regarding distribution poles, is this: There are no guarantees of safety about anything. You never know when a high voltage primary is damaged and has dropped down into some of the other stuff. Usually when this happens, if things are grounded properly, the resulting fault will cause a fuse or substation circuit breaker to open. Usually. On the other hand, what if you're touching something "safe" at the moment something "unsafe" falls into it? Depending on the sensitivity of the protection on that circuit, it could take up to several seconds to clear the fault, and in that delay your body would bear the brunt of it: You're dead.
A traffic accident involving a pole can tweak/whiplash all of the lines for blocks in all directions. So let's say you stumble upon a "benign" telecom cable hanging low, and you "know" it is safe, so you grab it to move it out of the way. Little did you know about the car that smacked a pole two blocks away, causing the primary at the wreck to fall onto the telecom, and the recoil/whiplash simultaneously causing the telecom in front of you to break loose from a mount and hang low. You reach out and... hey, where'd all these angels come from?
Until a power company guy is on the scene and says it is safe to touch, keep your hands off of EVERYTHING. Even when he says it is safe, he won't be offended if you ask him to touch stuff first.
Wow... long post. And I'm really only getting started!
What we learned: Forget what you just learned about what is what, all of that stuff can potentially kill you, even when all you see is telecom and no actual electrical power lines are in sight. Leave it all alone, keep people away, and wait for the power company guy.
So... it's time to ask again. Are these generic tutorials helping? Anything you want me to elaborate on? Too much? Not enough? Any requests for a particular topic?
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