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VK2WET > ALL 02.02.09 04:25l 94 Lines 4488 Bytes #999 (0) @ WW
BID : 3D0193VK2WET
Read: GUEST
Subj: Re: Pole 1 & Pole 2
Path: IZ3LSV<IK2XDE<F5GOV<CX2SA<N9PMO<K9BBS<VE3UIL<VK2DOT<VK2TV
Sent: 090202/0305Z @:VK2TV.#MNC.NSW.AUS.OC #:53390 [Kempsey, QF68JX] $:3D0193VK
From: VK2WET@VK2TV.#MNC.NSW.AUS.OC
To : ALL@WW
> From: VK2AAB@VK2AAB.#SYD.NSW.AUS.OC
> To : ALL@WW
>
> Tony Ray may be able to answer this;
> What length of water stream could be considered an insulator ?
> I know that they clean insulators on big power lines from a helicopter but
> I have often thought they could do it with a pulsed stream of water.
> That way there would be no continous water path.
> Perhaps the risk of the pulsing failing would be too great.
>
> At one emergency management committee meeting I attended there was a discussion
> about the problem when a big storm brings down lines that might be live.
> The problem is after a really big storm it might be a day or two before the
> electricity supplier could get to the site.
> The question was could some one from SES be trained to make safe.
> With wires laying in the street for a considerable time it presents an
> increasing risk that the area cannot be guarded and access has to be provided.
> The whole idea of some one else sorting out the problem was booded down
> very smartly.
> However no solution was offered.
> 73 Barry VK2AAB
Hello Barry,
What a good question. When we had pole-top fires the fire brigades would
extinguish them from the ground with a water stream that was more like a
concentrated spray. It was reasoned that a spray contains many pockets of
air which interrupt a possibly conductive stream. The brigades didn't
hesitate to act on fires on live 11kv mains (about 6.4kv phase to ground).
As for wires on the ground, one first needs to be able to identify whether
it's low voltage (240/415) or HV (11, 22, 33 or 66kv). It would be rare
for higher voltage lines to come down and, generally, they are not run
through suburbia.
For low voltage, there is every probability that fallen mains can still be
alive because the only protection that's usually provided is for
over-current, and not earth leakage. Unless there has been a phase to
phase or phase to neutral short, the fuse(s) are unlikely to have
operated.
For HV, Sensitive Earth Fault (SEF) protection "should" operate to isolate
the line, but should is the operative word. It's quite possible in rural
areas that such protection is ineffective or non-existent. We didn't
utilise SEF on some 11kv lines, but that was 20+ years ago. A case in
point regarding SEF is when one of my trees decided it was time for its
top to come tumbling down - right across my neighbour's 11kv supply. The
conductor was on the ground spitting and farting for some time before the
protection finally operated. The ground here is very dry and is a poor
conductor.
I'm not sure of what's done in the case of a SWER (Single Wire Earth
Return) system (used widely in rural Australia), because earth is the
normal return path. SWER lines are generally 22kv. We didn't have any SWER
lines.
A good rule of thumb for any fallen HV conductor is stand well clear and
assume it is still alive. Even with my experience of 21 years at a county
council, and mostly in System Operations, the section that deals with
transmission and sub-transmission, I would not go anywhere near a fallen
HV conductor.
Even for a fallen conductor, and where that line has been isolated, normal
safe working practices would still apply. The line would be tested as
dead, earths applied, and an Access Permit issued before anyone could
touch that wire.
Applying earths means shorting the active conductors together and, those
conductors being connected to an earth stake that is driven into the
ground at the point of earthing. Everyone who is going to be working on
that section of line has their name and signature on the Access Permit,
and prior to that section being re-energised after repairs, they must
individually sign off the permit.
For testing, a proximity device is used that lights up when placed near a
live conductor. That tester is tested at the time of use by checking on
both the live and dead sides of, say, an air-break switch. If a complete
section of line is out such that no live section is available, two testers
are used to verify the line is dead.
I think that in this day and age there could be some serious workplace
safety issues if unsuitably trained personnel were to "work" on LV or HV
mains.
Given your position and involvement with the emergency services arena you
are bound to know that first rule of rescue - don't endanger any other
lives. I think it applies perfectly with regard to fallen conductors.
Cheers ... Ray vk2tv
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