Voltage, Current, Shut UP!
OK, I see I can't try macro-ing any information without having people
regurgitate information that may or may NOT be relevant. In my original
post on 120 Volt vs. 24 Volt cable heating, I noted that the 120 volt
cable is more dangerous should insulation wear thin on opposite sides of
We have now gotten two "lectures" on how voltage is irrelevant and it's
the current that's important. Though the statement "It's the Current That
Kills" is true (yes, it was written on my freshman lab instruction books
12 years ago), saying voltage is irrelevant is misleading. V=IR. The
resistance of the tank (and your body) is constant in this picture! Thus,
voltage is proportional to current.
If you have a 120 volt heater vs a 24 volt heater, five times as much
current can also be pulled through the tank if the insulation should
weaken. I just popped a probe into a couple of my tanks, which measured
their resistance at around 100K ohms across the tank. This means 1.2
milliamps could flow through the tank if the insulation is exposed at both
ends of a 120v cable, but only 0.24 mA for the 24 volt cable.
Yes, this is neglecting the issue of whether the cable heating is isolated
from the power mains or not (with 120 volts, no. With 24, yes), and
whether you are standing on a copper plate in bare wet feet (perhaps
working on that magic grounding probe that also kills all your snails?).
But again, I think you'll find the 24 volt potential produces 1/5 as much
current (at most! probably less because it's isolated from ground) as the
120 volt case.
Erik D. Olson ex-physicist
eriko at wrq_com