Electricity problems (was Re: [plug] A Quote)

[David Campbell]

Date sent:      	Wed, 23 Jun 1999 10:01:58 +0800
From:           	Kenworthy Family <billk at opera.iinet.net.au>
To:             	plug at linux.org.au
Subject:        	Re: Electricity problems (was Re: [plug] A Quote)
Send reply to:  	plug at linux.org.au

> Hi,
>  Ive believe that underground power can be more exposed to lightning than above
> ground in some circumstances - this is because power lines carry some lightning
> protection so while problems occur, they are localized.

ROFL - The lightning protection you talk about ain't lightning 
protection at all. Taking the example of 11kV lines (a little higher 
voltage than your normally household wiring) you have the three 
insulated cables wrapped in something which resembles carbon fibre 
(moderately conductive but not metalic). This is for electric field 
smoothing (to reduce the chance of electricity jumping out of the 
centre bundle). This is overlaid with another insulator jackect then 
the steel wire guard "wrap" which is more to stop the odd spade than 
a lightning strike. Then there is the final PVC coat.

Your 440v/3 Phase [1 phase = 240v] cables are basically three cores, 
PVC jacket, guard wires, PVC jacket.

> Underground however looks like a low resistance circuit buried in a higher resistance (the "ground") - thus
> providing a longer path for the current to dissipate. I would not be surprised if
> underground power was not protected against strikes in as effective a way as above
> ground.

Generally the guard sheath is almost as good as one of the centre 
conductors. You would need a decent amount of amps down the cable 
before the local voltage would exceed the 20,000 volts (or whatever) 
to punch through to the inner core. Electrical authorities like 
connecting the neutral (mid-voltage to all three phases) to a ground 
stake and also connect the wire guard to this (sometimes).

> In tropical areas, strikes within 1km of airport communication
> lines (i was in the RAAF at the time) were believed to cause the highly visible/audible
> flashovers on telephone distribution blocks inside the buildings during storms.  -
> in short, disconnect and not be sorry (ALL connections, modem,  network cabling &
> power, including pulling plugs, not just turn off.)

Those strikes would of been cloud-to-cloud strikes which induce some 
hideous voltage in cables running the same direction. Above ground or 
below ground will make absolutely no difference (unless they were 
totally enclosed in a magnetic shield).

David Campbell
=======================================================
campbell at torque.net
"This is not an office, rather Hell with fluorescent lighting"