Electrical Transmission Lines on Rainy Days

Is the rain a continuous stream of water ?

BTW: Dry air Break down voltage is infact a bit less than the humid air.

If you look at the value of the BDV of air, the answer would be clear.

"Is the rain a continuous stream of water ?"

(BDV = Breakdown Voltage?)

Well, a connection of water molecules (not pure water) could definitely exist between the transmission line and the ground, even up to a person's feet. I guess.

If you were speaking about the breakdown voltage of "water" its a joke. Water could be used as a medium of electrocution for home appliances like hair-dryers which work on 110 to 240 V AC mains - so transmission lines which go to kVs are definitely way past any "BDV".

And yet:

http://www.youtube.com/watch?v=hIkNY5xjy5k

That is a different phenomenon--that was due to a switch opening, possibly too slowly (or with problems with the arc quenching equipment--they mentioned SF6 gas).

Think about it (I always hate it when somebody says that to me)--the switch starts in the closed position and then breaks--so, the initial arc is only a fraction of an inch. But then, the arc path becomes filled with ionized gas, a pretty good conductor, so the arc can be stretched. Then, magnetic (and electric?) forces push parts of the arc apart stretching it even further. The plasma arc moves up due to the buoyancy of the hot gas compared to the cooler air around it. Finally, the arc gets long enough that it can't sustain itself.

Now think about it in the other direction--closing the switch--in the open position, the metal parts are to far apart to allow an arc to be created (if that were not the case, it wouldn't be much of a switch, would it?), as the metal parts approach as the switch is closed, they eventually get close enough to start an arc, and very shortly after that the switch is closed and the arc is extinguished (or, at least, disapears, because the metal to metal path through the switch has less resistance than the path through the ionized gas of the arc).

If you've ever done any arc welding, you've experienced a similar phenomenon--you have to get the metal parts very close together (usually they touch) to get the arc started, once the arc is started you pull the electrode back a little bit but can maintain the arc, because the path through the now ionized gas is a better conductor than the non-ionized air that was there before the arc started.

Wikipedia: Electric arc has a pretty good discussion. An ionized gas is essentially a plasma and you can read more about that at Wikipedia: Plasma (physics)--makes the molecules in the air split into positive ions (the nuclei of atoms) and electrons, which makes the gas much more conductive.

BTW, one of the dangers in manually switching high voltage disconnects is the danger of opening or, more often, closing the switch too slowly. If you strike an arc as you close and let the arc last too long (before getting the switch all the way closed) too much of the metal can melt, and then when the switch is completely closed, the melted metal solidifies and the switch is effectively welded shut.

I think most systems that uses the actual tracks for current are underground subways, so that doesn't represent a problem since it doesn't rain underground.

Surface railways that use electricity source their current from overhead wires, so there is no pathway to ground other than the train's electrodes.

Any rails on the surface are ground rails, so they are at virtual zero potential difference to the earth.

Apparently you've never been to Chicago where the elevated trains have a third rail which is electrified to conduct power to the trains. They also run for great stretches at street level. This third rail is hot.

They also have them in the UK, and not just on the underground.

"Surface railways that use electricity source their current from overhead wires, so there is no pathway to ground other than the train's electrodes.

Any rails on the surface are ground rails, so they are at virtual zero potential difference to the earth."

I am not sure I got what you are saying. I was under the assumption that a the lines above a train are at a high voltage. Now when it rains, there is a conduction path, from the lines, through the wet pole, to a person. Likewise, from the electrode to the wet body of the train, to the people touching the outside body of the train.

My real question is why isn't there a current, when reasonably there should be a current (taking that impure/rain water is a good conductor).

If there were any conduction, it would short to earth first, not to any bystanders.

Correct. And the operative word is if because if there was significant current leakage engineers would change the system so as to not waste energy and create a hazard.

Holzfeller

"If there were any conduction, it would short to earth first, not to any bystanders."

Thats probably the answer. More current would go by the paths of lesser conductance. And so if the ground potential is much lower than the wet conduction path having a person, it wouldn't affect him. But somehow I can't understand how a human's conductance would be much lower (that is resistance much higher) than ground. We are talking of atleast tens of kV, thats a huge amount of voltage. Even, if (by the current divider law), most of the current goes to the low resistance earth, a human would feel some current (I would imagine of the order of tens of volts). But that doesn't happen.

Voltage wants to go to ground. Even if it was a constant "connection" from the water to the person. The voltage wound have to reason to "want" to travel through the human because it would have nowhere to go. Instead it takes the lower resistance path which is through the earth which the water is already in contact with.

I imagined it something more like this:

The blue spray being the effect of rain water having settled on the ground, the lines and the pole. In this, won't it just follow a current divider rule? And from 15 kV (or even more), even if 150 V passes through him, he would feel a good shock.

This is fundamentaly wrong. Voltage does not want to go to ground, beacuse of the potential difference, current wants to flow across the impedance of the circuit.

Also... your statement that....

"The voltage wound have to reason to "want" to travel through the human because it would have nowhere to go. Instead it takes the lower resistance path which is through the earth which the water is already in contact with."

This is very wrong, current will not take the path of least resistance, current takes EVERY PATH at the same time.

We essentially agree with each other. I have heard it said to many times that current will only flow in the path with the least resistance. It flows in all paths at the same time and, yes, the majority will flow in the path that offers the less resistance/impedance to that flow.

"I have heard it said to many times that current will only flow in the path with the least resistance. It flows in all paths at the same time and, yes, the majority will flow in the path that offers the less resistance/impedance to that flow."

Just to add to that North of 60, the currents in different conducting paths will follow the current divider rule - just as how (in the water-pipeline analogy) many pipes of different cross-sectional areas, connected to the same water source at a potential energy ('voltage'), will all have some pressure in them, but the ones of least cross-sectional area will have the greatest pressure.

Sry I have been away, thanks for covering for me dkwarner.

In my statement about current taking the path of least resistance, I was treating the water as a "short" to ground, in which the impedance would be zero and the current would = infinity (or all of the current) through that connection, and thus none would run through the human. Sorry, I didn't state it more clearly.

Also dkwarner, if you are going to quote someone you might change your signature, even though you still offended no one ;P

I understood there is a serious risk from EHV overhead transmission lines, eg 400,000V as the potential is enough to jump about 30metres. Damp air would surely be the most risky? I could be wrong on the distance guys. It was a First Aid course, where advice is to maintain a distance from casualty until power is disconnected.

Not likely. Generally, depending on humidity, air will break down at about 30kV / cm.

That is less than 14 cm or about 33 inches at 400 kV.

Oops... You multiplied by 2.54 instead of dividing. 14 cm is about 5.5 inches.

The 30kV/cm is probably a bit high for rainy weather, but it is certainly of the correct order of magnitude.

I suspect that the 30 ft safety was if a high voltage line was broken and near or touching the ground, tree, etc.

Like this....

Rain water is not a continuous conductor as there are breaks between rain drops. If current and power were to conduct under raining conditions from overhead transmission lines to ground, power systems will fail due to short circuits through rain water to ground, with tripping of relays, breakers etc.

Since power systems are functioning under raining conditions without any such short circuit conditions and tripping, it means rain water is not causing short circuit condition and power systems are remaining healthy under raining conditions.

The situation is different in , say , a swimming pool , where water is continuous and electricity can conduct in this water.

Raghunath7

"Rain water is not a continuous conductor as there are breaks between rain drops. If current and power were to conduct under raining conditions from overhead transmission lines to ground, power systems will fail due to short circuits through rain water to ground, with tripping of relays, breakers etc."

Well, its not only about when it rains. But the throughly wet pole (and other things) itself, during and after it has rained. Common sense and daily experience does indeed tell me too, that its not been shorted. But I would (by theory of the matter) believe it should be. That's why the question.

Re: Well, its not only about when it rains. But the throughly wet pole (and other things) itself, during and after it has rained. Common sense and daily experience does indeed tell me too, that its not been shorted. But I would (by theory of the matter) believe it should be.

There are insulators between the actual high voltage wires and things like the (wet) poles that support them. The insulators are rather carefully designed to work in wet (and slightly dirty) conditions--you should look at an insulator--I should find a picture to refer you to.

Ok, here's one style of insulator. They all have a lot of similarities. Notice the bell like shape, so that water will tend to shed off the top of the insulator and there is still a dry insulation path at the center of the insulation assembly. Hmm, that's not explained very well--the path that a current would have to travel to get from the high voltage area to the low voltage / ground area is over the outside surface of the insulator. So it would have to go down over the bell shape, then travel horizontally from that outside area to the center of the assembly, then down over the next bell shape and so on.

BTW, a rule of thumb is that you see one bell like shape for every 1 kv of insulation--well, at least that used to be the case. We could guess the voltage on a line from the number of bells in the ceramic insulators. I'm guessing that at really high voltages, they've got bigger bells which can handle more than 1 kv per bell.

" There are insulators between the actual high voltage wires and things like the (wet) poles that support them. The insulators are rather carefully designed to work in wet (and slightly dirty) conditions "

Thanks for that Rhkramer. I have of course, seen the type of insulators, but the important point you made is their design, and how the inside of it can't be wetted (in most part). Thanks. But what about a direct path (not containing one of these insulators). A train for example, travels with contact to high voltage lines. When its raining heavily, I assume there is a conduction path due to the rain water from the lines to the outside body of the train. People touching the outside body of the train with their bare hands don't get shocked, through (not that I want them to lol.)

You're welcome!

Something I had never thought about is what JRaef explains in his post #23--I'll quote a little bit, but his entire post is worth reading:

Once the cones get completely covered with moisture, the water can conduct to ground. but at 230kV, the INSTANT it does, the heat created immediately evaporates the water, breaking the circuit. It immediately condenses again, and immediately vaporizes again, ad infinitem as long as the moisture is present. The crackling sound is the energy created in that cycle every time. This is called "leakage" and is something all power lines experience, but the designers are keen on minimizing it, not so much for the safety issues because the grounding takes care of that, but more for the loss reduction.

Correction : 10 kV per rain shed is a good indicator of voltage on lines, 3 Rain shed 33kv, 1 rain shed 11kv. In Australasia 5 rain shed is 66kV so always be careful. The cracking/hissing sound from lines on wet, humid, moist, foggy days, is corona being created by the wet air passing over the conductors and breaking down the air. The air then transmits the electrical energy in the air flow, hence the noise. It is sparks flying off the conductor and ionising the air. A corona camera shows this very well. Corona causes major issues and it creates big losses on conductors so lines get run at elevated voltages and hotter temperatures. normally around 70-90C and 132kV can be run at 190Kv, but that depends on design, loading, weather, span lengths, conductor type and make, etc. On the ends of the insulator strings you will see what looks like horns, these are called arcing horns and they allow any flash over the insulator to go to earth, (the cross arm where all the suspension fittings are connected). These buzz the most on damp foggy days. Lines can flash (arc) to ground where you may be standing, simply due to the conductivity of the air in that area. Grounding is normally done by the very top wire(s) which is the OPGW, Optical Power Ground Wire, commonly called the optical fibre ground wire. Grounding is done above the line for lightning protection. There is no earthing below the lines. Only clearances and the minimum clearance for 132Kv lines, phase to phase is over 3.2m, part safety and part that 132kV will jump this gap under the right conditions and 400kV is 5.3m. Mid span on 11kV conductors is 7m from ground. Trust me it does flash to ground and it does hurt. Makes you eyes water a bit.

Current carrying conductors are separated from wet poles using insulators. Consequently current cannot flow into wet poles.

"Current carrying conductors are separated from wet poles using insulators."

Yes, I understand that and in the ordinary sunny day, its right. But on a rainy day, even the ceramic insulators are wet by the rain water - wouldn't that increase their conductivity?

Wet and dusty conductors will increase the conductivity.

The insulator design does not prevent flashover 100% of the time. Such a design will be expensive. Rather the design will be, to prevent flashover most of the time. This is the understanding I am having.

You already know that water does conduct electricity, however if the air to water ratio is high, that is, more air than water then electricity will not flow, as is the case in rain water droplets.

As we all know electricity is a movement of electrons, and as air does not have free electrons to move then is no conduction, whereas water has free electrons, but they have to move in continuous stream and be able to "jump" from one atom to another.

If they are interrupted as in the case of water droplets or rain as there is air spaces around the droplets, there is no continuous path of conduction, so no electricity is conducted.

Swimming pools as posted by "raghunath7" would be considered continuous as for the "transmission poles and wires"....Next time you use an umbrella in a heavy or even light rain storm, look to see what happens to the rain drops that land on your umbrella.. do they stay on the surface of the umbrella?

No, they roll off because of gravity and water surface tension. Gravity pulls the water to earth and will continue to do so until the force exerted on the water drops does not move them further, and the water surface tension is overcome by gravity causing the water droplets disperse into smaller droplets which, if you watch your umbrella, makes those droplets run/move faster in the direction gravity is pulling it.

Now apply that theory to the transmission poles and wires and you have your answer, and as my friend "Holzfeller" said and a few others the chances of you being electrocuted in a rain storm under a power transmission pole are very unlikely as electricity looking for the path of least resistance to earth.

But why would anyone stand outside in a rain storm?

Here's a question for you.. what is the estimated voltage of a lightening strike? Answer that and it will tell you the voltage required to break down the insulating properties of air, whereas the voltage in a transmission line is no where near than.

Insulators are designed as per IEC standards for various types of over voltages, switching, lightning, dynamic over voltages, unbalanced short circuit resulting in over voltages etc.

When the voltage exceeds the break down insulation levels of equipments being protected, surge arresters provide a temporary path of low impedance to ground to protect the equipment insulation.

Normal EHV line voltages are not sufficient to conduct across conductors [phase to phase] or from conductors to ground [phase to ground], even during rainy conditions. Otherwise our power systems will fail, every time there is a rain.

Rain water is a "semi"-conductor. Pure water (distilled) Is actually a very good insulator. That said, a rain water path from the third rail, across the insulator, to ground will dissipate at such a high gradient that you would get hit by the train before you are close enough to get zapped. P.S. Those catenaries and third rails typically carry 600 V.D.C @ 1000 Amps or more! Don't go near them. They'll kill you before you Know it.

Gazu

"Rain water is a "semi"-conductor. Pure water (distilled) Is actually a very good insulator. "

I understand that fact (as is clear in the start of my post). Its probably the high gradient of dissipation that I am ignoring. Despite the high gradient of dissipation (how high?), I just feel that from the many kVs of transmission lines used in railway systems, people getting shocked due to a wet path from the lines to the poles, to the ground (grass, concrete) to their shoes, slippers to their feet should be quiet common.

No. Everybody here seems to be ignoring the ceramic insulators that are used, part of their design is a bell like shape designed to shed water without allowing conduction. The insulators prevent a conductive path from the lines to the poles (or other mechanical support).

As far as electricity being carried through the air and rain drops, others have explained that quite well.

Jay, if the water is continuous enough to create a current path to the person standing near the power pole, it's usually continuous enough to continue that path all the way to ground. This is also a lower impedance path than through a human body. If you look, typically a power distribution system has a ground or neutral wire under the hot or phase wire(s) which, again, forms a shorter, lower impedance path to ground. On my street there are three hot wires, one for each phase, on a cross-arm with a ground wire about 3 ft below the cross-arm.

The insulators, poles and cross-arms are not perfect. They are the lowest cost system available and are actually very safe, as people are rarely electrocuted.

A former co-worker of mine told of his trip to the Florida Keys. He was standing in a parking lot at a rest stop and heard a crackling sound from above. After a bit of investigating he found that it was high voltage arcing across the salt spray that had settled on the insulators and cross-arms at the top of the power poles.

like this?

I saw something like that once. A big rotating irrigation sprinkler shot up into some 230 KV transmission lines - shorted out 2 phases and made quite a flash between the two lines.

Thanks for that JRaef. But consider this picture, and the point I have circled in red.

Tell me if I am wrong, because I assume that the line that the train is touching (red circle) is a high voltage line. Now when it rains, wouldn't there be a wet conduction path from that line to the body of the train. When the train stops at stations, people are in physical contact with the outer body of that train before they get inside, but don't feel a current. That's what I don't understand.

There is not sufficient resolution in the photo to see the details of the contact strip and its support system. Obviously there must be some form of insulation between the roof of the train and the contact strip, and there must be some form of good conductive path from the contact strip to the electrical system of the engine (either hinged/sliding busbars, or flexible cable).

Clearly under wet conditions there will be some leakage from the contact strip across that insulation to the support system which is hinged on the roof of the train, but that is the key: the leakage is to the roof of the train. There are multiple low impedance paths from the sheet metal of the roof directly to the wheels, which in turn are in very firm contact with the grounded rails. A high quality voltmeter could probably detect a measurable voltage between ground and shoulder level of the chassis under such conditions, but since most people with intact skin can't feel much under 60 Volts, they won't be able to feel the leakage.

Dkwarner

" There is not sufficient resolution in the photo to see the details of the contact strip and its support system. Obviously there must be some form of insulation between the roof of the train and the contact strip, and there must be some form of good conductive path from the contact strip to the electrical system of the engine (either hinged/sliding busbars, or flexible cable). "

The structure I circled is the same for any train that uses the same principle. My underlying reason for posting that image was to get an explaination for a case where the high voltage insulators were not used.

You're right that all trains (and other similar devices, like streetcars) that use overhead electric wires must have essentially the same structure, and that structure must include some good insulators.

As someone else pointed out, if there were no high voltage insulators, then the high voltage source would be shorted to ground (through the train body, undercarriage, and rails), and a circuit breaker would open at the source, removing the voltage.

The same is true of high voltage transmission lines, except that the short would be to ground through the metal tower.

In either case, a wet and/or dirty insulator still has much higher resistance than a wire; it is not a short. Again as someone else pointed out, if any significant current flows through this resistance, the heat will quickly vaporize the water, blowing away some of the dirt in the process, resulting in a cleaner, virtually dry insulator.

Both the train body and the transmission line tower have resistances several orders of magnitude lower that any human body, so any current that might flow through the body of a person touching the train or tower will be miniscule.

the connectig wires/strips will be insulated from the body of the train.

Hopefully just to summarize the points that have been well mentioned and explained by several contributors in this thread:

1) Any conduction will be to ground rather than to bystanders due to least resistance.

2) Live wires and rails are well insulated from anything that could bring contact to peoploids.

3) Voltages and currents are so comparatively low (from this type of live rail or overhead line) that even conduction to ground would be insignificant (otherwise designs would have been changed).

4) Any significant voltage/current would evaporate the liquid path and cut the circuit anyway.

I was amazed to learn, on BBC TV show, that recent Japanese Bullet Trains use 25kV on overhead wires - to give improved current capability. Maintaining close contact is vital and cleverly/ safely engineered using a cam/ spring mechanism.

FYI A motor in each carriage gives better traction and tilting suspension gives better cornering. Earthquake early warning allows safe highspeed shutdown as was shown in the recent disaster.They are even engineering guides to keep wheels on the track when trains stop on a camber.

Quite correct. The pantograph or current collector is mounted on insulators attached to the roof. As explained by others in the thread, clean insulators do not have a problem with rain water, so there is no danger to the public touching the coaches. Should the collector fail or the overhead wires break, causing live wires to touch the metalwork of the coach, the supply system is designed to trip, whether the power supply is DC or AC.

Gideon

" The pantograph or current collector is mounted on insulators attached to the roof. As explained by others in the thread, clean insulators do not have a problem with rain water, so there is no danger to the public touching the coaches. "

Thanks for joining in Gideon. Could you explain "clean insulators fo not have a problem with rain water"? That's the part which I am having trouble with. I am not sure if its the case PWSlack says - that its not connected to a high voltage so no porblem (hoping I am not misquoting his idea). But what I want to ask is this: In an ordinary day, the pantograph (thanks for posting the name btw), insulator and body of the train work well - no question on that for me. But on a rainy day, the insulator on which the pantograph is mounted can be thoroughly wet - and becomes line a sheet of conductive material. Wouldn't this give a conductive path from the line, through the body, to the person? My common experience of course tells me that a person is insulated, what I can't understand is how.

Take a look at posts #23, #27, and #28. (Hope I didn't mix up those URLs.)

I understood the matter of the ceramic insulators explained earlier. But my follow-up was when such insulators are no present (like in the case of the picture I provided).

I was going to respond here and say those insulators are always there, somewhere, and I was going to point you to those .pdfs that someone else posted, but you've found them, so all is good! ;-)

The superstructure of the train is connected electrically to the return running rail. Thus, the potential of the train with respect to earth is so low that there is no risk of electrocution when a person bridges the gap between train and platform.

No problem.

What voltage is that point generally? In anycase, this "rainy day question" could be applied to transmission lines which carry high voltages too. So how does it explain?

If you look at the details there (I could get at least two links)

Page 4 and page 2

The Insulation will be clear. The Pantograph is supported by insulators and these are with the required creep length and the profile to ensure the required insulation. All these are depending upon, as pointed out cleanliness of the insulator, especially in the lower webs (which more critically defines the creep length) which actually are more important than the top portion. If these areas gets contaminated eg with salt or other deposition then arc through may strike and most likely the system will trip before anyone gets shocked (afterall the metal frame is highly conductive with the huge crossection available and soldily earthed through the rail) And finally if any one do provide the conducting path it will be afterall the potential difference across his hand where he touches the wall and his legs (on floor) and that too is likely to be low.

The whole thing is quite highly protected afterall they do run in worse condition than rain (Snow which accumulates unlike rain)

Thanks for those pdfs. They seem to illustrate that the entire pantograph structure is supported on ceramic insulators. I guess it explain it then, because those insulators (as Rhkramer and others said) are designed carefully so as to not provide a conduction path even during snowy or rainy days.

I guess that explains it then.

@ Thccontrols

What's the explaination for why this man is not shocked by the transmission lines? I can assume the jet of water is not continuous, but tend to think it is. But what's the valid explaination? Sorry if I missed it above.