Re: Heating Cables & electricity

> From: krsfert at citilink_com (karl schoeler)
> Date: Thu, 7 Mar 1996 09:28:57 -0600
> Subject: Substrtate heating
> Has anyone tried FlexWatt mylar sheeting for substrate heating?
> The better reptile shops have used it for years and it is really
> easy to work with. 
This suggestion makes very good sense to me. The concentration of
temperature necessary to create thermal stress would be non-existant
with this method. I suspect that reptile heating pads also have
relatively low temperature concentrations, after all, the reptiles
sit on them without getting cooked! ;-)

> From: nguyenh at nosc_mil (Hoa G. Nguyen)
> "electricity takes all available paths."  Given a voltage source with
> parallel paths to ground, electricity will flow through all paths, with
> current varying inversely to the resistance in each path.
> The reason the ground wire works is that no voltage source is an ideal
> voltage source.  If we have a ground path with near zero resistance, the
> current drawn by that path will be so great that either:

I think the important lesson here is to ensure that the tank water
itself is grounded. If you have an electricity source submerged in
the tank, the water itself CAN become electrified without creating
an internal short circuit in the faulty device. Your body could be
providing the pathway to ground. The reason why low voltage sources
are safer is the skin's resistance is sufficient that with dry
skin you can touch a 12volt current source with little danger of
electrocution. With wet skin this is not true. The use of ground fault
protection is also an extremely good idea. Again, this relies upon
there being a significant low amount of current making a path to
the earth ground.

I must say, the notion of using wire-wrap type wires for UG heating 
aroused safety concerns in me from the beginning. I've suggested 
the use of a flexible PVC tube as a mechanical protection to encase
the wire wrapped core for applications where you want to use thinly 
insultated wires for heating under water (but I can't think why you 
need to). The insulation on wire wrap is extremely fragile; it's 
designed that way because it is used by electrical engineers and 
designers to prototype printed circuit devices (or at least it used 
to be in the days of low frequency electronics)

Steve in Vancouver

PS. A group of us Vancouverites are drumming up support for an aquarium
plant club here. Interested people can contact me, Olga or Jos.