Neighbourhood Wiring and Safety Issues
|© Volker Kuhlmann, 12 Aug 2003 (list postings are © their respective authors)|
|1. Introduction||5. Tips / FAQ|
|2. Normal operation||6. References|
|3. Electricity supply faults||7. Comments|
The topic of how to arrange ethernet (CAT5) wiring between houses or sheds has come up a number of times on the New Zealand Linux User Group list, together with warnings about related dangers from various forms of electricity and how to protect against them. This text is an attempt at writing up the information. It also includes related information about PC power supply protection, and some "stories" for easy reading. All quoted text was edited for spelling and grammar.
The threads on the NZLUG list started Sat 26 July 2003. The posts I saved of it are here, in compressed mbox format.
Ethernet uses unshielded twisted-pair cables with 4 pairs in a common sheath, called CAT5 (up to 100Mbit/s) or CAT5e (up to 1Gbit/s). The cable conductors are electrically isolated from the electronics on both sides, by a device immediately behind each RJ45 socket. This is the little square black box on the ethernet card. The isolation is rated at 2kV. The cable conductors are on floating (i.e. unspecified) potential. The maximum cable length is 100m for UTP/twisted pair cabling. The standard is IEEE 802, in its various parts (which also covers what's commonly known as "wireless"). The texts can be downloaded from the IEEE, currently free of charge (get it now and back it up, the "free" isn't guaranteed forever). It is also an international standard (ISO/IEC 8802, ISO/IEC 15802, and others).
It is, generally speaking, not possible to simply connect two different circuits together and carry signals from one to the other. It won't work. Some form of electrical isolation is always required. This also holds for stereos - there is in-built isolation in the signal lines in the form of a capacitor.
When stringing long pieces of metal around, be aware of the following issues.
Different houses/sheds/points in the landscape are not necessarily at the same potential (voltage to earth). Normally this doesn't matter, but a metal wire provides a very good shortcut between two places. Potential difference = voltage, and voltage causes current to flow, given a chance. The wire is that chance.
Walking over some floors in certain environmental conditions (low humidity, shoes, etc) can accumulate charge on the person. When you touch the door handle you get "zapped". You can only feel it, if it exceeds about 2000V (if I remember correctly). It's not harmful because there's little energy behind it. Electric fences operate on the same idea - high voltage so it can be felt, but not enough energy to kill anything. Long before you can feel them, charges can destroy electronic equipment. This is why you use anti-static wrist straps and similar equipment. Long overhead wires can similarly accumulate charge, where it can be much higher.
Atmospheric conditions can concentrate charges which a CAT5 will be happy to dissipate. Stringing one across the valley should be followed with suitable isolation and grounding at both ends.
Houses are not connected to the same supply phase. In fact this will alternate to balance the load. There are three different phases, with 380V between each two phases, and 240V between each phase and ground. New Zealand uses the MEN system for house wiring, this is essentially a star-shaped topology. In your house, the phase is labelled "Active", the return wire is labelled "Neutral", and the protection earth wire is labelled "Earth".
|Never run power cables between dwellings which have their own supply already. This is not only illegal, it's also dangerous (it breaks the star topology).|
Keep in mind that when it comes to electric power, things are illegal for a reason.
The protection already built into ethernet hardware should be sufficient for short cable runs in typical conditions, and is reliable, Roger says :
So strange kamaguza ethernet hubs/switches/RJ45 things do indeed have excellent isolation, on every input/output.
If you suspect it's not sufficient, or want to be extra sure, use a "little double ended fuse unit for Ethernet cabling" [any more info/URLS?] on each end of the cable.
The little plug packs powering ethernet switches etc. also provide electrical isolation to the mains power system to some degree, but it wouldn't be enough to cope with serious lightning.
Substantial faults in or with the electrical supply system can produce similar effects to lightning. Their destructive potential depends on the particular circumstances.
Some faults don't only exist for a brief period. Nic Bellamy has a story about that: 
Providing the difference is not major, the isolating transformers in the network cards should handle it - they're tested to around 1kV if I remember correctly, and there's often a 500-1kV spark gap too. Lightning is considered "major" though ;-)
Wayne Rooney posted an excellent overview :
I worked in the heavy electricial industry for years. Lightning means overtime. If lightning hits the ground, around one time out of three it goes to ground through a high tension power line. If it hits a power line, it knocks out a bunch of high tension fuses. So you spend hours driving around replacing high tension fuses. Easy work and lots of it.
Most 11kV/400V transformers have these spark gap arc horns to divert lightning. The lightning jumps across the air gap and goes to ground rather than going through the transformer. It's reasonably rare that lightning takes a transformer out, although the HT [high tension] fuses come before the spark gap, which is why they blow. So while you definitely get spikes on the low tension side, they tend to be of very short duration, though the voltage can be up to six or eight hundred volts.
Now, if you get a lightning strike on a low tension line [the segments which go to your house], then kiss your appliances goodbye. I've seen it. Everything in the house that is plugged in gets shagged. And your insurance company says "Act of God, tough luck mate." (Always a good idea to check to see if you're covered for lightning strike.)
High tension lines also have the occasional lightning arrester, which when blown apart seem to be made up of lots of disks stacked up into a cylinder.
Lightning does raise the ground potential at the instant of strike, and is something to be aware of. There are cases of cows being electrocuted because lightning struck nearby and the difference in potential between the ground at their front legs and the ground at their back legs was enough to electrocute them.
So we have this situation where you string a cable from one house to another, is it safe or not?
All right. If the two houses are on different supply transformers then don't do it. The reason is this: your cable can carry current caused by differences in ground potential between the two sites. Each transformer has its own earth mat, but the only electrical connection between the two transformers is the ground itself. Your CAT5 can be a better conductor than the ground. Under normal conditions you may get away with it, under fault conditions your cable may heat up until something melts.
OK, two houses on the same transformer are already connected together - by the neutral conductor. So stringing a cable between them is a bit safer. You are paralleling a big fat wire with your little thin wire. However, you can run into trouble like this:
In one of the houses the hot water cylinder gets replaced. The plumber disconnects the main earth conducter from the copper pipes and doesn't reconnect it, leaving the house with no bond to earth. A tree then falls through the overhead line, taking out the neutral, but not the phase. So electricity can get into the house, but has no return path. If you had strung a bit of coax between this house and the next for your 10base2 network, then you're in trouble, because the shield on the coax will try to carry the whole return current for the house. (Actually, you're in trouble anyway, because every metal appliance in the house would bite you if you touched it.)
Now your 10baseT network should actually put up with this problem because of the 2kV isolation. But if it breaks down at one end then you get the full voltage at the other end. The NIC at the other end may put up with this, but it is not good for you if you unplug the cable and touch the end.
Under normal conditions, stringing CAT5 a short distance between 2 houses on the same distribution transformer would be fine. Under abnormal conditions expect trouble. Plan for the abnormal conditions.
... and he also has some ball-park estimates of likelihood :
> How much of your work was "normal conditions"?
That's a bit of a moot question - if you're working 'faults' then abnormal conditions is what you see all the time. A better question may be "out of a hundred houses, how many will have a serious problem regarding electricial supply in a one year period?" Not many.
Working under normal conditions at the moment, seen two abnormals in the last month - one was a burnt out mains cable where it terminated at the isolator on the meterboard, the other when working on a switchboard with old cabling with the bare earth wires and seeing enough difference in potential between the earth wires to cause little arcs where they touched.
> How many of the abnormal conditions could not have been foreseen?
I would say 99% of abnormal conditions can be foreseen, and the way they can be foreseen would be to figure out all the things that can go wrong: cars running into poles, trees falling through power lines, snow snapping lines, high wind banging lines together*) etc, etc. The problem is that such conditions are statistically unlikely and so are not really taken into consideration. And then they happen.
*) I remember one time when it was a bit windy and the power to
the whole town on Amberley kept going out - got sent up to check it out - had
a look at the Amberley end, looked ok, drove up to Waipara, looked ok. So I
say to the guy I'm with, "Shall we follow the lines across country?" He says,
"Nup, we'll sit here for a bit," and pulls out the paper and starts reading
Couple of minutes later there's this enormous ZAAAP! and the sky to the right lights up. "There it is," he says. The wind was blowing the 33kV line up into the earthing conductor that was strung along the top to attract lightning away from the lines.
Generally the CAT5 cable will not present a risk under abnormal fault conditions IF it is plugged into a NIC at each end AND by fault conditions we mean 400/230 volt supply.
For high tension faults you can figure out the volts per meter back to where the HT is earthed (where it is supplied from, which is normally lots of kms away) and get a rough idea if it is safe or not. E.g., if this 11kV line comes down, it has 6.6kV to ground, it is supplied from the substation 4km away, so that's 1.65kV per km, so 300 meters of CAT5 will have 495 volts difference between each end IF each end were connected to ground (which it isn't). You also have to take into consideration the breakdown voltage of the insulation on the CAT5.
When thinking about lightning, well if lighting wants to go somewhere, it goes somewhere.
Personally, I've seen 11kV get through onto the 400/230V once. 11kV to ground is more common. Lightning strike onto HT is common. Lightning strike onto 400/230 I've seen once. Lightning definitely hits things like wires and trees and the ground.
Lightning strikes contain a huge amount of energy and can be very destructive. People's anecdotes suggest they're best watched from a distance! The energy, in form of an electric charge, follows the path to the ground which has the lowest resistance. This path is not necessarily obvious - the point at which flashover occurs first plays a role, and nearly everything changes its electrical characteristics significantly under high voltage, high current, and flashover.
The path to ground is not restricted to a narrow channel. The bulk might go down an obvious path (follow the scorch marks), but it will also spread out among all available paths. If there are 2 paths available with one of them having twice the resistance of the other, 2/3 of the energy flows down the path with the lower resistance, and 1/3 down the path of the higher (basic circuit theory). If lightning hits a power line in a city, appliances are fried in houses within a certain radius, with the closest houses being affected worst.
When lightning (or the short current from a large power supply line) hits the surface of the earth, it doesn't just disappear there. It dissipates into the ground in a radial fashion generating large currents. Basic circuit theory says: large currents cause large voltages. In this particular case it's called step voltage. Take a step, and you can have 1000V between your feet! That's why people walk in small steps over switchyards. The ground potential, always assumed to be zero, rises to levels well above safe limits. It's not necessarily radial either - water ways for example have much lower resistance than soil and will therefore carry more of the energy.
The problem with underground cables of any kind is that they are right inside something that suddenly has a very high voltage. And they're good conductors. The bit of PVC sheathing may be effective at 240V, but whether it's there or not makes no difference at 240kV. Being a good conductor, it goes a looong way.
Be especially aware of this:
lightning rod. A length of metal provided for lightning charges
to flow to earth, as a preferred path to, say, property, to avoid or
Here are some examples:
Any direct hit to your house, or any phone or ethernet cable, is bad news for all the connected equipment. Count yourself lucky if you're unhurt. Using protection gear will probably not save all the hardware, but will reduce injury to the person sitting in front of the PC. Note the "probably", there are few absolutes when it comes to lightning.
Holding a handset against your ear during a thunderstorm can lead to Darwinian selection.
These are not just myths. People's experiences (posted, and private email):
Ooh! This happened to someone I know. He was talking on the phone, with the phone against his ear during a big thunderstorm. He was leaning against the chest freezer in the shed. Leaning with his hip. A shed with a wet floor. So the chest freezer was ground.
The voltage from the lightning strike went into his ear and out his hip, leaving him deaf in one ear, and with severe burns on the side of his head and hip.
If he'd been leaning against the chest freezer with the hip opposite to the ear he had the phone held to -- the zap would have gone through his heart, and most likely would have killed him. It nearly did anyway.
I knew the boy, and the boy's mother. The nurse that took care of him in the burn unit was my aunt.
During a thunderstorm, I answer the wireless phone! Safer!
I've had more than one UPS fuse zapped by lightning -- better the UPS fuse than every computer in the house!
Pete McGhee: 
At the mining site where I work, we run a COAX ethernet LAN which passes from the factory to the office via a 4" steel pipe, buried at a depth of approx 1.5m. Cable run underground is in the order of 10-15m.
Being on top of a hill and surrounded by iron-rich rock, we get more than our fair share of CG [cloud to ground] strikes up here and have had two close strikes in the last 3 years that have affected the cable run.
Strike #1 was approximately 1km away. Induction(?) popped both NICs and smoked the mobo's at either end of the buried line, all other PC's (with unfiltered power then) on site were fine.
We fitted Black Box (no endorsement to be implied) coax arrestors at either end after this event.
Strike #2 was a direct hit (<500m) Both surge arrestors copped the lot, but we only lost one NIC, we did however lose multitudes of other gear.
$25,000 later we now have site-wide UPS.
My personal machines run CorCOM surge suppressors. Two per machine, in series. We have had several CG's within 1km of the house over the years and have escaped damage. The latest was yesterday, with a strike just across the road within 150m of the power and phone lines. I lost 3 out of the 4 suppressors, but the machines are still intact. The strike was hefty enough to cause arcing in the domestic wiring, detonating a lightbulb and leaving that heady aroma of scorched electronics and ozone that says "missed you, this time". Only loss was a modem which died in my arms as I pulled the Telecom plug - Having felt the current go through as I yanked the wire, I now have relays fitted - that was one scary sensation!!
The power surges and spikes caused by thunderstorms are not necessarily limited to direct lightning strikes. Water can get into the neighborhood transformer, wet tree branches or bamboo can blow onto the lines shorting out the whole valley, and lightning+water in the transformer can cause it to explode. That's the green flash that lights up the whole valley prior to it going totally black. The power surges are spectacular during these events.
Even a surge protector with a resettable fuse (many don't have that anymore!) is better than nothing.
If you're short on cash, get one of those first, because you should use them anywhere you're using electronic gear -- your stereo and TV for instance.
The deal is, when the voltage goes way too high, the fuse blows, cutting off the circuit entirely before the zap gets to your gear. Most surge protectors just clip out the highs, but don't really do signal conditioning -- they don't protect you from erratic signals or *under* voltages (brownouts) either.
A good UPS does a whole lot more. Basically, a UPS keeps a battery charged up all the time, and leeches a nice conditioned power signal off the battery. So it should be providing superior signal conditioning well as a little bit of time to flush your buffers and take down your computer systems gracefully during a power outage. If it gets too big a zap, it blows the fuse where it's plugged into the wall similar to your surge protector -- but the signal conditioner should continue to pull nice clean power off the battery during this event.
When you've finally got your UPS you can move the old surge protector to your monitor, and leave the monitor off the UPS, for example. This just means you're drawing less power off the UPS during a power outage, resulting in less downtime on the CPU. And providing *some* protection for your monitor.
A good UPS can also aid you in power managment. You can connect the UPS to the CPU via the RS232 port, and program it to tell the computer to shut down nicely when it sees that there's an outage. This is known as APM, Automatic Power Management.
The old apmd on linux actually destabilised the kernel on SMP machines, but there are newer methods and utilities out there (another topic!).
You can also get additional battery backup to add to your basic UPS if you've got several machines, and have a master CPU running the apmd that can then tell all the other machines to shut down, rather than having a separate UPS for every machine.
In a small town in the New Mexico high desert at ~2300m I was walking down the street (like lucky La Rue) and one of those sudden thunderstorms kicked up. I was in town, not out in the middle of a golf course, so I figured it would be safe to put up my umbrella. Saw a few lightning strikes nearby, so I hold the plastic handle and note that I'm wearing rubber-soled shoes.
Should be safe, right? Wrong!
Hair stood on end and was suddenly surrounded by a ball of St Elmo's Fire that seemed to go on for several seconds.
The umbrella ribs were a great conductor, especially as they were streaming water down to the ground.
What could I do? Take the umbrella down, and have it go through my *head* instead? I took short but very quick steps to duck into the nearest doorway--*then* took down the umbrella.
A workmate who saw me from a passing car said I looked like I was in an electric blue birdcage.
I had gotten a sensation of mild electric shock through my whole body, but I was basically unhurt.
Unfortunately, I did not emerge with super-powers.
Everyone knows that some places are more prone to lightning than others (and reading this, one might get the impression that some people are more prone... but surely not). The biggest factors are weather and terrain. Exposed areas (hilltops, ridges, high plateaus with nothing much else around) will catch more lightning. Fewer houses on the power line in the country side equates to a higher chance of being the shortest path to ground. You'll have found out if you are in an area prone to lightning (fried appliances, light bulbs, etc).
It depends on what you're doing as well. If your job involves asking for trouble, you have good reason to feel a bit "on edge", as Grant Black finds out :
A: Yes they provide isolation which is good enough for typical conditions which exist almost all of the time (it won't extend into the megavolts). They provide no protection against overvoltage between the inputs (they convert voltage at a fixed ratio), or huge energy blasts.
A: Let me put it this way. You have a huge(!) ball of energy coming in at sufficient megavolts, into an isolation device 2cm3 in size. What do you think you'll have a fraction of a second later? 3 guesses. The first 2 don't count.
A: If the surge is big enough to jump the card's isolation, it will destroy the card as well as at least the main board, probably most other things in the PC as well. If the surge is really big, the smouldering remains of the PC will answer that question for you.
A: It provides the computer with power during power cuts and voltage dips (until the battery runs out), and it swallows all small spikes. Bigger surges blow the UPS's fuse, which is more sensitive than the fuse in the switchboard, and it then supplies the computer until the battery runs out. If something has to blow, you might prefer it to be the UPS instead of the computer. It doesn't protect against close-by lightning which strikes the power cables (nothing will).
A: Plug the PC's power cable into the UPS' socket. You can also connect a serial cable between the two, and set it up so that the UPS signals the PC to shut down when the battery is about to run out. You'll need to run a power monitoring daemon, and uncomment some lines in /etc/inittab.
A: It's a cut-off switch in the switchboard of the house, triggered by one or more buttons located whereever you like them. You push one of those buttons, and the whole house goes black. Useful if you often don't get much warning of lightning and want to "get in first".
A: No. Strikes reach a long way underground - several metres. They can also strike buried phone and electrical cables. Compared with an overhead cable, burying should reduce the risk. It won't reduce the voltage in the cable during a strike, but the close proximity of the cable to ground over the distance allows a good part of the energy to drain.
A: Insulation is a matter of degree. Up to the breakdown point, it's fine. Beyond that, the isolation drops to practically zero and it's like a dam break - the only safe place to be is somewhere else. I don't believe the CAT5 cable sheathing by itself provides even a 2kV isolation, and if so, it would have to be undamaged (no small cuts!). Putting it into a plastic pipe would be mainly for physical protection. At a guess, it'll be good enough for close-by faults of the 11/33kV system (if there's not water around providing shortcuts). It won't cut it for lightning.
A: Legally - it depends on who owns the duct and/or its contents.
Contractually - it depends on T&C with respective Telco.
You'd have to take into account easements/R.O.W. etc. Ask the parties involved (Council, Telco, your lawyer) first. In other words, forget it.
A: Alkathene pipe stops it perishing, garden hose will perish after a few years underground. Alcathene is just low-density polyethylene. The higher-density stuff is better. Polyethylene pipes are commonly used for water/gas and sometimes have electric wires through them. They can last at least 10 years and are quite flexible (bend, don't break during earthquakes, etc).
A: This is up to you to decide. In a city you probably get away with doing nothing, after all most PC users don't have surge/spike/whatever protection. If your power supply has many surges (rural areas) it might pay to think about it. If your area is prone to lightning or electrical storms, yes you do (and you didn't need to read this to find that answer).
A: Steve Wright has some ideas  (abridged):
This is all talk-on-a-cereal-box when the energy levels in question are in the "outrageous" category, and the isolation device is the size of your little fingernail. The rules of motion of electrons give way to the "get the fsck out of the way" rule, which in general will take precedence.
You cannot isolate spikes in the "outrageous" category. The only thing you can do is "bond" everything local - keep everything at the same potential. This is not feasible over long cable runs unless you are going to lay a substantial ground mat, or *seriously* isolate one end.
Running any long wire breaks all the protection systems. We really need someone from the heavy electrical industry to comment.
Regarding the questions of underground protection - this will help greatly with lightning strikes, but not with substantial electrical faults. Lightning tries to get to "ground" by the best possible path. Once it has done so, it will proceed to drill a hole at said point on the ground. However, the point on the ground that has suffered the strike will rise *substantially* in potential, causing both ends of the cabling to do so as well.
If I had to run a length of CAT5 underground I would be looking for the following:
§ Both ends of the cabling were roughly equidistant from any substantial mains electrical apparatus, particularly large pole/roadside transformers, and also not passing within 5 meters of the same.
§ A lightning shunt is created by burying the cable to 100mm depth minimum, at least at the points where it entered any building (one meter buried), or near any termination.
§ Cabling to be kept WELL clear of any substantial metal structure - distance to be at least 0.5m (buried) or 4m<wince> for exposed cabling.
§ Tin sheds, buildings with lots of metal (cowsheds), buildings with substantial mains electrical installations, and the like, to be avoided completely.
§ Structures with substantial vertical height must be lightning shunted. 100mm depth, 1 meter length, minimum.
§ Cables that enter a residence must be kept short, and preferably terminated at a hub/switch well away from any possible contact. Be aware that the switch may destroy itself in flames during a substantial fault. Will it burn your house down? Will you know about it? Are you insured for this?
§ Heavier wiring must not be used. The very fine wiring of CAT5 will vaporise and protect you and your property. 2.5mm wiring will not.
A: Not a thing. These RCDs (Residual Current Device) are designed to cut the power in case of insulation faults so you don't get electrocuted. (Note using one doesn't mean being safe from electrocution!) They may also act as a fuse, but that's not their primary purpose and it's not legal to have them as the only fuse in the circuit (their max break current is too low).
A: You should be fine for about 30m, or between adjacent urban houses. Beyond that, you definitely need to start doing something about better insulation and protection.
Nic Bellamy : I must admin I am placing a bit of trust in a couple of cards at the moment; there's a network cable in a shallow trench between my place and my neighbors - so far, no problems, and it's only about a 30m run. LAN-speed WolfET. This way, I can't blame connection speed for the fact that I seriously suck at it :-)
Other options I looked at were free-space optical (FSO) and wireless, but decided to risk it. If I was going much further, I'd definitely look at something with better isolation.
A: Yes. Fibre optic cables, but they're still very expensive, and they're not as convenient. If you put a spade through a CAT5, you can cut the insulation back a bit and wire it into a connector, or carefully solder it back together. It'll lose a little signal-to-noise ratio but it'll work. If you put a spade through an optical fibre, you can cut the insulation/protection back a bit and tie the cores together, and it won't work.
You could also go wireless.
If your electronics skills are good, consider this line-of-sight optical link: ronja.twibright.com
A: Pull all the cables out of your PC, modem and phone before the weather turns bad, or the contractor drives the digger into the 33kV cable up from your house.
A: Wayne Rooney says : I read an interesting article from the Thirties once about ground currents. You drive a couple of metal rods in the ground about 30 meters apart and connect them to a telephone earpiece. You get all sorts of clicks and whistling sounds (like dolphins singing), although these days, with much more electricity used, the interesting sounds tend to be swamped out with 50Hz hum.
A: Theoretically, but it's not exactly off-the-shelf, and you'll have to make at least some the equipment yourself. Wayne Rooney found an article from the Thirties  about building underground radio antennas, which he put online here.
A: Tesla-coils are an all-time favourite. Wayne Rooney reports 
Comments are welcome, if you don't have my email address already, my contact details. Hopefully there aren't any massive blunders. I don't give any guarantees - if some surge blows up something, it's not my fault.
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|Copyright © 2003 by Volker Kuhlmann
Created: 29 July 2003, last updated: 12 Aug 2003 URL: http://volker.top.geek.nz/linux/tech/