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

[Richard Chirgwin]

On 5/03/11 10:35 AM, David Boxall wrote:
> 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.
>
It's only fair to mention that NBN Co does not intend to use overhead 
cable more than it absolutely must.

At the moment, it lacks access to Telstra ducts, which is why there are 
more overhead cables in pilot sites than are intended for the rest of 
the country.

RC