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Re: pruning power cords

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 splice.

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...


Waveform Technology
UNIX Systems Administrator