[LINK] Aerial vs underground Cabling....Re: NBN and Batteries

[David Boxall]

On 1/03/2011 3:32 PM, Marghanita da Cruz wrote:
> Last year, Energy Australia (electricity) upgraded the
> aerial cabling, in my street, to Aluminium - which is
> more robust, according to the sparky incharge.
> ...
> I seem to recall, that the underground cables melted in the
> Canberra Bushfires. ...
To take the last point first: Highly unlikely. I remember reports of 
overhead wires falling, but none about underground cabling.

Heat rises. Anything underground is, by definition, below the source of 
bushfire heat.

Soil is a surprisingly good insulator. Much forest regeneration 
following bush fires is attributable to seeds. Those seeds survive, 
commonly less than a centimetre below the soil surface. Insects and 
lizards survive in burrows, often less than three centimetres deep. I 
know of no underground cabling installed less than 30 centimetres below 
the soil surface. Would heat that will not sterilise seeds melt wire?

I've seen a cable buried straight up a hillside. The next rainfall 
runoff followed the line of least resistance: the soil disturbed during 
the cable installation. The loose soil eroded, leaving the cable 
exposed. Had there been a bushfire at that stage, the insulation might 
have melted. The resulting short circuit could have fused the wire.

Apparently, aluminium runs hotter than copper. In the insulated 
environment of underground installations, heat can be a problem. I 
gather that underground electricity cabling is usually, if not always, 
copper.

In overhead installations, the lower weight of aluminium gives it an 
advantage. To overcome tensile strength issues, the aluminium cable is 
typically reinforced with a core of steel.

-- 
David Boxall                    |  The more that wise people learn
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http://david.boxall.id.au       |  How much they don't know.
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