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Re: pruning power cords
- To: aquatic-plants at actwin_com
- Subject: Re: pruning power cords
- From: Bill Wichers <billw at waveform_net>
- Date: Tue, 12 Aug 2003 11:56:54 -0400
Here is a question for our electrical gurus...
This weekend, I accidentally cut the side of the power cord for my Dupla
undergravel cables instead of the vals. This was not the actual coated
cables but the power cord that is connected to them and plugs into the
Be glad there was only 24 volts there. This kind of thing is sooo much more
exciting with 277 volt commercial lighting circuits but we'll leave that
for another time ;-)
transformer. It was underwater and there was a spark and that was it. I
immediately unplugged them and dropped the water level down below the cut.
If I bent the cord sideways, it was obvious where I had sliced through the
insulation to the wire but it wasn't all the way around the cord, just on
one side. I let it dry out for a couple hours and then painted the area
with 2 coats of brush-on electrical tape. After it dried, I refilled the
tank and turned them back on. They didn't come back on because the
temperature controller indicated it was too warm. The next day, the
temperature had dropped to 79 (it stays around 84) so I finally
checked/replaced the fuse in the Dupla transformer. The light on the
transformer came on, the cables powered up, in theory, and all seemed well.
Hours later, the temperature hadn't changed so I reached in and grabbed the
cable, it was not on. The output of the transformer registers 24V so it is
working. I measured the resistance of the cables (unplugged from
everything) and got 4.3 ohms.
I would recommend putting some heat shrink tubing over the liquid
electrical tape. I don't know how well the liquid electrical tape will hold
up underwater exposed to high light levels over time. I usually use some
heat shrink tubing and silicone sealant when I need a water tight splice
(and there are ready-made assemblies similar to this available). If you
want to email me off list I can send you a few inches of heat shrink tubing
freebies since I have lots (several 100+ foot reels :-)
Here is my question: Is it possible that I fried the cables? The
temperature controller is working. The transformer is working or the light
wouldn't be on. That only leaves the cables doesn't it?
If you had fried the cables you would register infinite resistance (or at
least megohms) and not 4.3 ohms across the cables. If you measured around
24 volts on the output of the transformer then it is probably OK -- which
makes sense since the fuse blowing *should* have protected the transformer
from any serious damage.
I think you have one of two things going on here:
1 - you nicked the wire in the cable in your tank, and it is now an
intermittent connection that is OK in certain positions but open in others
when the wire is flexed enough that the conductor ends separate inside your
2 - your transformer has suffered an internal fault and is now putting out
24 volts but at too little current to run anything (I've seen this happen
before in other kinds of transformers). This is easy to check, just connect
the cables AND measure the voltage WHILE the cables are connected (plug the
bannana plugs in part way so that there is some metal to connect to for
testing). If the voltage falls off to some very low value then this is the
problem. If the voltage is maintained in the general vicinity of 24 volts
(it will drop a little) then you are OK.
If it is your splice you're best bet is to either solder it and then
insulate it with sealent and heat shrink tubing, or use a waterproof splice
assembly. You can get a ready-made splice that 3M makes that is a crimp
"butt-splice" (takes two ends of two wires and connects them together) that
has some heat-activated adhesive inside and a nylon outer jacket. You can
get them from http://www.digikey.com but they aren't exactly cheap.
BTW, running the wires in the tank inside some rigid aquarium tubing will
help protect them from wandering pruning tools...
UNIX Systems Administrator