Heat Tape + Foil Tape = Dangerous Inductance

So I have built several racks and have built several for others and this is the first time I have noticed this. I, like most use foil tape to hold the heat tape in place. I was finishing one of my racks today and had it all plugged in. I had to reach to the back of it and in the mean time shocked the crap out of myself.

At first I thought, great I have a short somewhere, but then I realized that all of the heat tape with foil on them do this.

With the herpstat on full blast, and putting a multimeter across the two pieces of tape, this is what I get.

http://i871.photobucket.com/albums/a...psrlfnbj5s.jpg

I thought I would put that out there to serve as warning, and to ask what other tapes that people out there may use as alternatives. As clearly, conductive tape is probably not a good choice and is most likely a disaster waiting to happen.

Don't you cover the exposed part of the wire with electrical tape? The rest is covered in plastic so I don't know how that is happenening. I right now am using heat cable, though I'll be going to flexwatt shortly.

Yeah, the exposed ends are all sealed off. Well, in short, this happens because AC has a magnetic field. When you interrupt this magnetic field with a conductive material, such as aluminum tape, you create inductance.

So the easiest way to remove the problem is to remove the tape and use a non-conductive one, which sounds easy, but I think if you use something like duct tape, the tubs will not slide very well. Not to mention, the duct tape will never come off the heat tape, if you ever have a reason to. So just trying to find out what else is out there that people are using.

how about that black electrical/electricians tape?!

I've used foil tape and flexwatt on a few different racks and an incubator and have never been shocked...:confusd:

Yeah, i thought about using that, but I was unsure how well it would stick, and if they made it in anything wider than the 3/4"

I use clear packing tape and have never had this issue! I've never actually used the aluminum foil tape.


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Originally Posted by sho220

I've used foil tape and flexwatt on a few different racks and an incubator and have never been shocked...:confusd:

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I originally thought about this myself, being this is the first time I have any problems. The only thing that I can think of is that the thermostat was on full power at the time, since the heat tape was at ambient temps.

I measured the voltages of my other racks and most of them are showing about 20V across the foil tape, and according to the herpstat are only putting out 30%ish power, so I am guessing that because the heat tape rarely has full power to it, this is why I have never noticed it.

100 volts seems a little high, im around plenty of 480v circuits twirling next to each other almost daily, sometimes for hundreds of feet. Which yes they create enough inductance to set a tick tracer off, but 3 volts is the most I have measured. getting 100 volts induced in a couple feet, running in a straight line? I can buy some foil tape tomorrow and do my own test, but I find it extremely hard to believe it is inducing that much. A bad connection to the heat tape would easily do that though.

With that said I did always think foil tape was a ridiculous thing to use, but it seems to be very well established in the rack making world. I use duct tape myself, never had an issue.

*edit*

I'll try to remember to buy some tape tomorrow, after reading your last comment, interesting you are even getting 20v. I'll report what I get.

I noticed a few times while soldering up some heat tape the rosin core boiled out of the solder and spit around. where it landed it left tiny little burns in the heat tape. and when I put that piece in like usual thinking the tiny little pin hole would be fine it shocked me. if you soldered your tape I would bet money that is where your short is coming from and the reason your getting shocked by it.

I had this on the last rack.

Cover the ends of the heat tape with non-conductive tape first and then, on top of that, use more foil tape to connect the front and back strips of the foil in the pic and the problem disappears.

I've been shocked only once when I was building my first rack. Needless to say I take every precaution I can now!

What I do is use black electrical tape to tape down heat tape. Then I go over the electrical tape with foil tape. I haven't had any problems since!

Sounds like you may have an area on your heat tape that isn't completely sealed.

I haven't ever been shocked by foil tape before and I touch it all the time.
My ends are sealed with electrical tape, then the heat tape is secured down in the rack by the foil tape.

Ive tried duct tape on heat cable, it seems to slowly erode away because of the elevated temps.

I built my own rack and used THG heat tape. I soldered all connections and taped both ends with electrical tape. then used foil tape to hold it down. As long as the cut portion and soldered part are sealed there should not be any shocks.

You did something wrong or have a bad section.
Never had any problem here and I use either a 6 inch or 12 in piece of 12 inch wide per shelf so I have a lot of soldering and tape going on.

Everyone recommending electrical tape to hold down teat tape, electrical tape really only sticks to itself and not great at that either.

Use blue painters tape.. Cheap and effective!

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Originally Posted by OhhWatALoser

100 volts seems a little high, im around plenty of 480v circuits twirling next to each other almost daily, sometimes for hundreds of feet. Which yes they create enough inductance to set a tick tracer off, but 3 volts is the most I have measured. getting 100 volts induced in a couple feet, running in a straight line? I can buy some foil tape tomorrow and do my own test, but I find it extremely hard to believe it is inducing that much. A bad connection to the heat tape would easily do that though.

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Yeah, I thought that was crazy myself. As an electronic technician by trade, when I first noticed this, I thought for sure there had to be a short somewhere causing this. I am still finding it hard to believe that this heat tape is that efficient at creating inductance. But then again, if you think about what this stuff is electrically, its one large linear length of coil wire.

I will note that this rack does have THG heat tape on it. The others where I am seeing 20V are flexwatt, but like I said are only running at 30% power. I am also going to do some testing at full power with a few strips of both tapes to see what I can come up with.

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Originally Posted by dillan2020

I noticed a few times while soldering up some heat tape the rosin core boiled out of the solder and spit around. where it landed it left tiny little burns in the heat tape. and when I put that piece in like usual thinking the tiny little pin hole would be fine it shocked me. if you soldered your tape I would bet money that is where your short is coming from and the reason your getting shocked by it.

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I have soldered some of my racks in the past, but the one in question, I used the actual connectors.

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Originally Posted by dr del

I had this on the last rack.

Cover the ends of the heat tape with non-conductive tape first and then, on top of that, use more foil tape to connect the front and back strips of the foil in the pic and the problem disappears.

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My ends are all covered with electrical tape, and on top of that, I do not take the foil tape all the way to the edges, just to prevent issues like this.

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Originally Posted by PitOnTheProwl

You did something wrong or have a bad section.
Never had any problem here and I use either a 6 inch or 12 in piece of 12 inch wide per shelf so I have a lot of soldering and tape going on.

Everyone recommending electrical tape to hold down teat tape, electrical tape really only sticks to itself and not great at that either.

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That is what I thought myself, but every piece of heat tape I have does this where foil tape is used.

Yep,

You made the perfect inductive plate with no outlet to ground.

Trust me linking the back and front pieces of foil tape will fix it by giving it the needed outlet. ;)

Well I was surprised in more way than one, but for starters, I need to eat my own words:

http://i1269.photobucket.com/albums/...psb9vwdte7.jpg

measuring 60 volts across a 1 foot piece of heat tape. but here is the odd part:

http://i1269.photobucket.com/albums/...psn8fhbbvy.jpg

I wasn't able to actually shock myself even with wet fingers, I didn't take a picture but I even did something stupid and stuck my tongue in between them, still nothing. I also could not get a wiggy to read the voltage and I know that will read 24 volts, yet the digital said there was 60. So honestly, I don't know whats going on. Maybe I should try a longer piece of tape and see if I can get something to hit me or at the very least set the wiggy off.

Some other things to note: The above pictures are with foil tape over the bus bar. If i moved the tape to the side, but still holding the heat tape down, i only got 7 volts
http://i1269.photobucket.com/albums/...psqr7oehf0.jpg

smaller piece over the bus, at 22

http://i1269.photobucket.com/albums/...pskzawofs8.jpg


even smaller, I got 10,
http://i1269.photobucket.com/albums/...psqm25qhhu.jpg

smaller piece and outside of the bus, only 1 volt
http://i1269.photobucket.com/albums/...psmhzn0dn3.jpg

adding a bridge still left potential between the ends
http://i1269.photobucket.com/albums/...psw4wigjnv.jpg

just showing it is highly conductive:
http://i1269.photobucket.com/albums/...psbd94oglw.jpg

120v feed
http://i1269.photobucket.com/albums/...psludi6v9f.jpg

I got a busy rest of the day, I'll lay out a bigger piece and see what I can do tomorrow. Still find it odd I could not shock myself with 60 volts even with my tongue.

http://i60.photobucket.com/albums/h1...psdbdiztja.jpg

http://i60.photobucket.com/albums/h1...psfgkmzyzy.jpg

I would love to try this BUT I have a feeling that the sacrificial strip on the front has something to do with me not having any problems. :rofl::rofl:

I dont mean to offend you but ive been doing electrical work on sh-60 helicopters for 8 years now and will say confidently that you are doing something wrong

6 foot piece of heat tape, I couldn't get a reading over 40 volts, still nothing to set a wiggy off, and I couldn't create a scenario to actually shock myself. The readings on the digital meter are interesting, but I still can't seem to find any actual danger.

So I talked to my dad today, got an answer that explains all of it. So basically adding the foil makes a crappy capacitor (2 conductors separated by a insulator) and it holds a tiny charge. When you go to read it with the digital meter, it has a resistance in the megaohms, thus not discharging much at all and reading the voltage. With a mechanical meter or body part the resistance is very small and discharges it instantly, thus no reading on the meter. When you got hit, you must of had a condition where it stored enough to give you a shock. I wasn't able to recreate that condition but I have no doubt it is possible. My smaller pieces had less of a charge because there is less to hold it. I had way less of a charge when the piece weren't sitting over the bus, thus having air also act as an insulator.

So knowing all of that, the op did nothing wrong, just accidentally made a bigger capacitor than the rest of us.

I would suggest if you must use conductive tape, use small pieces or try not to place it directly over the bus bar, it won't make as good of a capacitor.

When you start reading into mega ohms your pretty much looking at an open. I see what your trying to say but he probably has a nick or small tear in the heat tapes insulation

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Originally Posted by OhhWatALoser

So I talked to my dad today, got an answer that explains all of it. So basically adding the foil makes a crappy capacitor (2 conductors separated by a insulator) and it holds a tiny charge. When you go to read it with the digital meter, it has a resistance in the megaohms, thus not discharging much at all and reading the voltage. With a mechanical meter or body part the resistance is very small and discharges it instantly, thus no reading on the meter. When you got hit, you must of had a condition where it stored enough to give you a shock. I wasn't able to recreate that condition but I have no doubt it is possible. My smaller pieces had less of a charge because there is less to hold it. I had way less of a charge when the piece weren't sitting over the bus, thus having air also act as an insulator.

So knowing all of that, the op did nothing wrong, just accidentally made a bigger capacitor than the rest of us.

I would suggest if you must use conductive tape, use small pieces or try not to place it directly over the bus bar, it won't make as good of a capacitor.

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What is the bus bar?

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Originally Posted by OsirisRa32

What is the bus bar?

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The copper bars that run down the sides.