RE Electricity - again

>I think the reason that lower voltage is more attractive for undergravel
>heating cables is that it allows for more reasonable cable dimensions to
>be used.  To generate 100W of power, cables subjected to 120 volts
>require a resistance 25 times greater than cables subjected to 24
>volts.  When we are talking about using continous cable to develop the
>resistance (like in UG heating cables), the cables have to be pretty
>thin or really long to get enough resistance.  The upshot is that
>reasonable size wire and shorter lengths can be used at 24V DC than at
>110V AC.  

No, the reason that 24V was chosen for Dupla's undergravel heater, and 
that they discontinued their 42V system is due to european standards.

Some years ago, the low voltage safety standard was changed from
50V to 30V. Dupla, to continue to get european approval for their
cable heater, had to drop the voltage to below 30V. 24V was settled upon
as this allowed a sufficient margin allowing for the tolerance of mains 

Swimming pool lighting and garden lighting come under the same
approval ratings, so you may notice that a great deal of pool lighting
is either 12 or 24 volts.

Also, providing that the transformer meets the approval standards,
this includes short circuit protection, the low voltage side is considered
safe, and no approval is needed for the equipment hooked up to it.

The CE approval mark that is placed on many items means that the 
device has also met the emmitted radiation and receptive radiation
standards of EMI.

Marque APD - ANGFA(NSW) - Sydney