A tree trimmer puts a branch on a line that causes a sustained arc that leads to fallen lines and more arcing. What is amazing, this goes on for hours stopping and starting perhaps from automatic protective equipment. The utility can't turn this off? Heavy fault currents probably find their way down the feeders to homes causing smoke and small fires and the gutters to spark. Incredible video:
| A tree trimmer puts a branch on a line that causes a sustained arc that | leads to fallen lines and more arcing. | What is amazing, this goes on for hours stopping and starting perhaps from | automatic protective equipment. The utility can't turn this off? Heavy fault | currents probably find their way down the feeders to homes causing smoke and | small fires and the gutters to spark. | Incredible video: |
You got an MPEG or WMV format file of this?
Sorry, I have no file, just the Youtube link.
I can't believe the massive fault currents available to burn up the feeders to all these houses at once!
John
My father has a summer cottage. One winter, the top of a transformer pole broke off and the distribution wiring contacted the feeders. Two cottages supplied by that transformer burned to the ground. My father's place was also fed by that same transformer but suffered no damage, most likely because he always throws the main breaker when closing the place up for the winter.
That distribution system there is quite old - either 4800V or 7200V line-line, with delta connected primaries. The top of that pole was loaded with carpenter ants.
| My father has a summer cottage. One winter, the top of a transformer pole | broke off and the distribution wiring contacted the feeders. Two cottages | supplied by that transformer burned to the ground. My father's place | was also fed by that same transformer but suffered no damage, most likely | because he always throws the main breaker when closing the place up for | the winter.
It could still have damaged the breaker and the wiring leading in to it.
I did see a video on TV once of a house fire with a downed service drop to it. When the power company guy came to cut off the service so the firemen could douse the flames safely, he ended up getting quite a shock from the triplex when he picked it up from the ground. Fortunately he survived due to probably the combination of the triplex insulation and his LV gloves. But he was pretty much staggering back after it hit him. The video also showed some kind of arcing along the wire, but I don't know that that was not news media touch up. Apparently there was a primary to secondary fault inside the transformer that caused this and was not apparent to the lineman that tried to cut it off.
I don't disagree, however this happened 10-15 years ago, and the wiring and main breaker work fine (at least as a switch).
I had one experience with a step charge: I was walking home after a windstorm along a street, which had a cop blocking the far end. I noticed a downed powerline on the ground buzzing quietly across the street from where I was walking. I noticed a tingling in my legs. When I realized what was happening, I turned around to go back the way I came, making sure to take very small steps. The cop at the far end was really blocking traffic from coming toward me, and there should have been another one at the end of the street I started down.
On Fri, 18 May 2007 21:48:17 +0000 (UTC) Michael Moroney wrote: | snipped-for-privacy@ipal.net writes: | |>On Fri, 18 May 2007 15:23:13 +0000 (UTC) Michael Moroney wrote: | | I had one experience with a step charge: I was walking home after a | windstorm along a street, which had a cop blocking the far end. I noticed | a downed powerline on the ground buzzing quietly across the street from | where I was walking. I noticed a tingling in my legs. When I realized | what was happening, I turned around to go back the way I came, making sure | to take very small steps. The cop at the far end was really blocking | traffic from coming toward me, and there should have been another one at | the end of the street I started down.
I came across something somewhat like that (a cop was there, too). I didn't notice any step potential, but I did notice the cop was a bit too close to the downed line which at that point wasn't really doing anything. I warned him about it and convinced him to back away from it some more. After a few minutes while giving him a hand taping off the area, suddenly the downed line starts to arc and smolder, then a loud buzz, then a boom with sparks everywhere. The look on his face I will remember forever.
I have no idea how that line came down. There hadn't been any bad weather nor was there an accident there.
Hello, and assuming the usual multi-grounded neutral distribution system, that's another good reason to have the neutral solidly tied to earth ground at the service entrance. Not perfect but it helps. Sincerely,
John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory
4555 Overlook Avenue, SW Washington, DC 20375-5337On Mon, 21 May 2007 11:32:04 -0400 J. B. Wood wrote: | In article , snipped-for-privacy@world.std.spaamtrap.com | (Michael Moroney) wrote: | |> My father has a summer cottage. One winter, the top of a transformer pole |> broke off and the distribution wiring contacted the feeders. Two cottages |> supplied by that transformer burned to the ground. My father's place |> was also fed by that same transformer but suffered no damage, most likely |> because he always throws the main breaker when closing the place up for |> the winter. | | Hello, and assuming the usual multi-grounded neutral distribution system, | that's another good reason to have the neutral solidly tied to earth | ground at the service entrance. Not perfect but it helps. Sincerely,
And to not have the neutral tied metallically to the service drop and especially not tied to the MV distribution neutral.
Where do you think the power goes when the MV hot touches the MV neutral? Does it take the shortest path? No, it takes all paths. One of them is through the service drop. At least a very well grounded neutral at the service drop will help. But isolation would help even more, as long as the isolation can hold up to distribution medium voltage. And a MV->LV transformer should have that ability to withstand MV. But for reasons of lightning protection, utilities cross MV neutral to LV neutral, increasing the exposure of service drops to distribution voltage. Of course if a MV hot touches the LV network, isolation at the pole transformer won't help.
I understand that there are countries where the primaries of the distribution transformer are not connected to the ground/earth in any way (from the perspective of the customers ground). This is not necessarily a 'better' or safer system. There are complex trade-offs involved and there are a lot of good reasons (for increased and other reasons) for making sure that at least one primary lead of that transformer is earthed.
In the USA, there is a special case exception in rural areas where there are problems with stray voltage traveling through the earth in farm situations. One solution is to use a special neutral isolator to keep the secondary neutral/ground/earth disconnected from the primary ground unless the voltage difference reaches a certain threshold, in which case the bond is re-established.
Not everybody does it that way. France is one such country with the TT Earthing Scheme. In other words, the power company does not supply any grounds to the customer on their distribution lines. The customer is responsible for providing their own local earth connection that is not connected to the larger distribution network. For safety, an RCD (Residual Current Device) is required for such installations. An RCD is similar to what we know in America as a GFI or ground-fault interrupter. However it is not exactly the same. Typically, an RCD is set to trip on a ground fault with a greater current than the typical GFI 5ma threshold and also it contains complex timing circuits and often covers more than just one circuit.
In my opinion, the North American scheme gives several advantages when safety is concerned as you have the following unique features when compared to the typical Euro Systems that use the RCDs.
I've seen some of the wiring in France and I'm not sure what they did before RCD's became widely used. Did they just not care if anything was grounded? I'll bet it wasn't all that safe.
I'm old enough to remember the days in the USA (late 1950s and early
1960s) when we switched to plugs and outlets with 2 prongs to 3 prongs). The electrical system became a lot safer, as a result, particularly for users of portable power tools. Double insulated appliances and the later GFI requirements provided further safety improvements.Beachcomber
I can still remember using my dad's old metal case, two prong, non-polarized drill motor in the garage in bare feet. Only did that once! Should one happen to plug it in the wrong way, one gets more than drilling action. I guess I am lucky I am still here. :-]
Hello, and I think it's more important in this scenario to have the neutral properly earthed at the service entrance. Any floating conductor hot with respect to earth ground (including metallic water pipes, etc) presents a shock hazard that exists both internal and external to the building. If a 7200 MV wire should contact a LV service entrance wire one hopes that fault current is either conducted to ground (accompanied by tripping of the MV fuse) via the grounded neutral (at the pole and/or service entrance) or a building internal LV distribution circuit breaker/fuse is blown (MV wire touches LV "hot" wire rather than neutral.)
The purpose for MV multi-grounded neutral distribution systems is not safety (other than lightning protection); it is to provide for rapid operation of over-current protection devices under "hot" conductor-to-earth fault conditions and facilitate timely fault location and repair. This helps stabilize system voltages by not having leaky current faults (conductors touching tree branches, etc) that can accumulate over a period of time. In a few locales (e.g. desert areas with few trees in western U.S.) where this is not an issue MV distribution is done sans neutral and the cost of stringing an additional conductor is avoided. In those systems MV-to-LV transformers are connected line-to-line rather than the usual line-to-neutral.
Of course intentionally earthing one side of a voltage source makes the other side "hot" and introduces an electrical hazard that must be dealt with. Sincerely,
John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory
4555 Overlook Avenue, SW Washington, DC 20375-5337| I understand that there are countries where the primaries of the | distribution transformer are not connected to the ground/earth in any | way (from the perspective of the customers ground). This is not | necessarily a 'better' or safer system. There are complex trade-offs | involved and there are a lot of good reasons (for increased and other | reasons) for making sure that at least one primary lead of that | transformer is earthed.
Being earthed is one thing. Connecting primary and secondary neutrals is another.
| In the USA, there is a special case exception in rural areas where | there are problems with stray voltage traveling through the earth in | farm situations. One solution is to use a special neutral isolator to | keep the secondary neutral/ground/earth disconnected from the primary | ground unless the voltage difference reaches a certain threshold, in | which case the bond is re-established.
So are you saying stray voltage never happens anywhere else by in rural areas? How clever of those electrons to recognize that.
| Not everybody does it that way. France is one such country with the | TT Earthing Scheme. In other words, the power company does not supply | any grounds to the customer on their distribution lines. The customer | is responsible for providing their own local earth connection that is | not connected to the larger distribution network. For safety, an RCD | (Residual Current Device) is required for such installations. An RCD | is similar to what we know in America as a GFI or ground-fault | interrupter. However it is not exactly the same. Typically, an RCD | is set to trip on a ground fault with a greater current than the | typical GFI 5ma threshold and also it contains complex timing circuits | and often covers more than just one circuit.
In many countries, an RCD main is required. I've seen the requirements of such for Japan.
| | In my opinion, the North American scheme gives several advantages when | safety is concerned as you have the following unique features when | compared to the typical Euro Systems that use the RCDs. | | 1. Individual circuits can be protected by inexpensive outlet-type | GFI's or slightly more expensive breaker GFI's. As mentioned | previously, RCD's are expensive and complex. | | 2. Unlike a typical RCD installation, the whole house is not plunged | into darkness should a single GFI trip due to a real fault or from | testing. | | 3. In the North American System, 240 V. is available (via the split | phase wiring scheme) for the larger appliances that need it (range, | dryer, large air conditioner). However, at no point is any voltage | inside the house higher than the nominal 125 volts to ground.
... except when the effects of MV distribution systems are factored in.
| 4. Bathrooms are arguably safer and more convenient to wire | electrically. You don't need to worry about 240V wiring near the sink. | Light switches can be conveniently mounted on the wall (instead of | using pull chains), you don't need to install isolation transformers | as they do in the UK for shavers. Outlets don't need to have switches | on them and because of the lower voltage, you have less paranoia about | bonding everything together in the bathroom. Hair dryers can be | safely plugged into convenient bathroom outlets protected by GFI's, | etc. | | I've seen some of the wiring in France and I'm not sure what they did | before RCD's became widely used. Did they just not care if anything | was grounded? I'll bet it wasn't all that safe. | | I'm old enough to remember the days in the USA (late 1950s and early | 1960s) when we switched to plugs and outlets with 2 prongs to 3 | prongs). The electrical system became a lot safer, as a result, | particularly for users of portable power tools. Double insulated | appliances and the later GFI requirements provided further safety | improvements.
Safer, yes. But I still think the safety can be improved even more by better isolation from the MV distribution.
This discussion of MV neutrals is interesting, but I'll mention that in this specific case, there is no MV neutral. This is an old system, all the transformer primaries are connected phase to phase (delta), the poles have crossarms with two conductors on top (3 conductors further upstream where 3 phase is available) with no neutral. The secondary neutrals of different distribution transformers are all isolated from each other. I have no idea how (or even if) the primary circuitry is referenced to earth ground.
I have a "souvineer" from this incident, a rectangular fuse holder/switch marked 7200V from the primary wiring.
Do you really mean "Another power line indecent video"?
Bill
...
In other words, no direct connection between the MV and LV neutrals anywhere, correct?
Out of curiosity, how do you think the MV neutral should be dealt with?
Not grounded at all? Grounded only at the substation/transformer bank that produces it? Grounded but only at poles where the 120V/240V secondaries aren't grounded (or some other "minimum distance" between groundings of MV and LV systems)? Grounded at all poles with distribution transformers (like currently done), but using a separate ground stake from the secondary (i.e. grounded at the same pole, a few inches away)? Something else?
Since the MV neutral would no longer connect to the LV neutral, should the LV neutrals be connected together or isolated from each other?
As I mentioned in my last post, the old system supplying my father's place has no MV neutral (delta), and the LV neutrals of different distribution transformers aren't connected to each other.
TT earthing is unrelated to this. The supply of a TT system is earthed at the distribution transformer, but the supply company doesn't provide you with an earthing terminal (or it does, and you choose not to use it).
I've heard some french supplies are IT rather than TT.
An RCD is exactly the same as a GFI. RCD's come in a variety of trip current ratings and delayed trip timings. In the UK, an RCD which is being used to protect against high earth fault loop impedance as you might have with an earth rod is required to be rated at least 100mA trip and can be time delayed (e.g. 200mS). In France, a 500mA trip is commonly used for this, but I don't know exactly what their regs require.
In the UK, TT systems tend to be limited to rural locations with old supply infrastructure, although anyone is permitted to make their installation TT by ignoring the earth connection provided by the supplier. Constructing your own TT system like this is often done for outbuildings and outdoor supplies where it can be considered safer to have a more local earth connection, and is explicitly mentioned in the regs as a permitted practice. My house is a TN-C-S system, but a circuit for outdoor sockets is an RCD protected TT system with its own earth rod which is not connected to the rest of the house internal earthing.
I don't believe TT systems are very common across Europe, although they certainly do exist.
Earth leakage breakers for TT systems have been around in the UK since 1950's that I know of.
It's interesting that you cite so many things which are safer in the US. What do you think are the things which are not safer in the US that cause so many more electrical deaths per capita in the US than we have in the UK (and I suspect across much of Europe)?
On Tue, 22 May 2007 19:30:58 +0000 (UTC) Michael Moroney wrote: | snipped-for-privacy@ipal.net writes: | |>Being earthed is one thing. Connecting primary and secondary neutrals |>is another. | | ... | |>Safer, yes. But I still think the safety can be improved even more by |>better isolation from the MV distribution. | | In other words, no direct connection between the MV and LV neutrals | anywhere, correct? | | Out of curiosity, how do you think the MV neutral should be dealt with? | | Not grounded at all?
No, it should be grounded. Just not at the same place the LV neutral is.
| Grounded only at the substation/transformer bank that produces it?
No. Multiple grounding is generally needed.
| Grounded but only at poles where the 120V/240V secondaries aren't grounded | (or some other "minimum distance" between groundings of MV and LV systems)?
Some reasonable distance apart from LV grounding. I don't know if other side of pole is enough, but it might be.
| Grounded at all poles with distribution transformers (like currently done), | but using a separate ground stake from the secondary (i.e. grounded at | the same pole, a few inches away)?
180 degrees on other side of pole might be enough. I don't know if that would work or not. It might.| Something else? | | Since the MV neutral would no longer connect to the LV neutral, should the | LV neutrals be connected together or isolated from each other?
I assume you mean in a network LV system. They normally are connected to each other. But I'm not so sure I trust such a system. I have not put much thought into it.
| As I mentioned in my last post, the old system supplying my father's place | has no MV neutral (delta), and the LV neutrals of different distribution | transformers aren't connected to each other.
Then the risk is reduced to the risk of the MV lines contacting the LV lines or the transformer having a fault between primary and secondary. Certainly that is a non-zero risk. But it is better than having neutrals solidly connected between MV and LV.
-----snip----------------
----------- Cheaper and less robust outlets, lamp sockets, switches, etc. along with less respect for the potential hazards might be a factor.
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