Heating Pad v.s. Substrate Heating Cables

Andrew Roberts wrote:

 <<I have read with much interest the articles on the Krib by Dan Resler
and Uwe Behle regaring DIY substrate heating cables.  I work as a CAD
operator in an electrical engineering firm, and my colleagues all say that
the inslulation on normal wire will quickly break down  (mind you, these
folks are all pretty paranoid engineers in the high voltage/utility field
:P). Has anyone used the plans for the DIY cables, and can you make
comments on the longevity of the "normal" wires underwater??>>

What are the reaons for using heating wires in the substrate versus a
heating pad under the tank? A pad under the tank can be run off line
voltage without the need for a heavy and energy wasting transformer, or any
electrical power wires in the tank. A pad will lose some heat from the
lower side, however with thermal insulation I am sure this could be reduced
to well below the losses in a transformer system powering a low voltage
heating system. Other than a catostrophic failure of the tank glass which
would cut into the pad (admittedly a low probability) the risk of
electrical shock from this system is remote.

Some have seen mentioned waterbed heating pads, does anyone know what
wattages these are availble in? A waterbed could need a lot of heat. It is
desirable to use a heater that has sufficient output to keep the tank at
temperature, but not so much that a failure would lead to rapidly rising
and excessive temperatures.

I have seen heating pads made of red silicon rubber with embedded
resistance wires used in industrial applications. A review of the Thomas
Register would turn up several manufactures of heating pads, something I
plan to do when I set up the BIG tank.

Can anyone justify why internal substrate heating cables should be used
over an external heating pad?