heating cables and voltages

---------- Forwarded message ----------
Date: 13 Dec 96 08:39:37 +1100
Subject: heating Cables and voltages
Autoforwarded: FALSE
To: Aquatic-Plants at actwin_com (Receipt Notification Requested)

From: Erik Olson <eriko at wrq_com>
Date: Tue, 10 Dec 1996 15:14:46 -0800 (PST)
Subject: Re: Aquatic Plants Digest V2 #375

>One small caveat is that such cables are in fact 120 volt units, and
>having high voltage drops ACROSS your tank is a potential disaster if the
>insulation should wear thin at opposite ends of the tank. 

Actually, when I was making some heating cables I thought long and hard about
the hazards.  If you try to measure the resistance in a typical(?) fresh water
tank you will find that there is a VERY large resistance across even the
smallest of gaps.  Therefore, if the insulation wore thin at two ends of the
tank I don't think that your fish would receive an electric shock - the heating
cable would still have a much lower resistance and so this is where the
electrons would flow.

If you stuck your hand in there and picked up the cable at the bear conductor
part then I could not guarantee anything! (Although, once again you still have
a higher resistance than the cable).  I'll have to think more about this,
because of course you would be supplying a path to earth.

As an aside, 110V (and I suppose 120V - what voltage do you US guys run at
anyway?) and below is considered low voltage - the definition of which is that
it won't kill you. (That's why you use 110V for a supply I guess).  In my job,
using 110V for field instrumentation allows you to get away with less safety
protection devices.

Also, a cable cannot be a "120V Unit" - it simply has a maximum voltage rating
(which I am sure would be well above 120V).

What the manufacturer is probably saying is that the resistance of the length
of cable has been made such that you need to apply 120 Volts to give a total
power dissipation of x Watts (Remember that Power = Voltage squared over
Resistance. (ie the more voltage the more power).

What I was actually thinking of at the time is that insulation is not required
at all.  Does anyone agree with this? (Particularly at low voltages of say 24V

Steve Amor
Steven.S.Amor at Shell_otc.au