flashover

[altec]

I know that this is a diy forum, but as BR works with the esb in Ireland i was wondering if i could ask 2 slightly off diy questions anyway. Here goes,

What is flashover, what causes it and how can it be prevented? I am interested to know as a friend of my brothers was killed recently in a power station in Ireland when flashover occurred and would like to know more about it.

Why are high voltage circuit breakers filled with oil under pressure?

 

[paulh53]

This one needs to be answered very carefully? Think about it?

 

[breezer]

in reverse

when you switch off an ordainairy switch you do just that switch it off (two contacts separate or break) .end of story.

as voltage increases so does its ability to jump

now lets use an ordainary switch and switch off high voltage (not that you would)

you turn off the switch and the contacts move apart as they move the elecltricity is still flowing, but as the contacts get further apart the elctricity starts to jump the gap (it arcs) and continues to flow, now the switch is in the off position but the high voltage electricty is still flowing (oh dear)
that is what happens.

Now imagine that you have a "proper high voltage switch" with a much bigger gap, as the switch moves the same thing will happen, the electricty will start to jump from one contact to another and even at maximum distance it will still arc.

Don't know how or who, but it was realised that if the switch is immersed in oil the switch will not arc (or at least not as much so when maximum distance is achieved you really have now turned it off.)

(there is also such a thing as an airblast circuit breaker but you can guesswhat it does by its name

Flash over

this is again to do with arcing. if you are too close to high voltage and it "sees you" it will jump to you and........ instant death.

You may be unaware at the time but you may have something you shouldn't i.e metal ladder / pipe etc this will make you more "attractive" and so the elctricity will jump to you.

Electricty will always follow the shortest path.

The above is in "easy speak" my understanding of the questions asked, although i did not mention arcing causes heat rise and other such info.

High voltage electricians and electricans are different, so i may be wrong.

before any one asks you may have seen "linemen" running down the cables on overhead pylons ( i have seen them) which are live why are they not killed if what i have said is true? the answer is their baskets are hooked on by helicopter which is not actually touching anything.

I am also sorry to hear of the loss of your friend

 

[paulh53]

Electricty will always follow the shortest path.

Electricity will always follow the least path of resistance to earth,

 

[breezer]

thats what i said. I also said The above is in "easy speak"

you forget this is a diy forum

 

[paulh53]

Soz Breezer, But I was only trying to make things a little clearer for the "DIYer (As I do in most of my posts)
It seems to me that most "Tradesmen" on here forget that they are replying to "DIYers", And tend to get wrapped up in their own conversations, Which totally sidetrack the "DIYers" original subject/post
Can we please stick to the original post topic in all our replies

 

[breezer]

funny you should say

most "Tradesmen" on here forget that they are replying to "DIYers",

i couldn't agree more

 

[paulh53]

We all make little mistakes Eh?

 

[BR]

I presume the man you speak of is **************** power station. As I am an employee of ESB, I cannot comment on the details of that particular case.

Flashover is an extremely complex issue and this is certainly not the place to go into it all. I think breezers explanation is pretty good, but it is of course very much in "easy speak".
One of the big dangers with flashover it that it can "flash over" insulators and as some of the work in power stations has to be conducted on live apparatus (possibly up to and over 38KV) major precautions have to be taken. The chances of been "hit" by a bolt of flashover in a power station are extremely unlikely due to precautions that are invariably taken. The danger arises when a flashover triggers an explosion (which is what happened in XXXXXXX).

The dangers of high voltages can never be underestimated; the pictures below (graphic, however i think people need to be aware of the dangers) were taken of casualties who were electrocuted from 3KV lines in America. Should somebody come in contact (I'm talking within 15in. of the conductor) with 38KV their body would most likely explode into minute pieces. I will, to this day never forget a scene which I witnessed in Britain when a southern electric worker came in contact with 42KV apparatus on a pylon. ESB crews were assisting in the repair of severe storm damage at the time.

Yes supply board workers can in emergencies slide along live cables however there are technical issues associated with this.

Oil filled breakers will reduce the arc of electricity to an acceptable level; it will not extinguish it completely.