Jump to content
Light-O-Rama Forums
Sign in to follow this  
Robert G.

GFI Outlets.

Recommended Posts

Jeff, let me offer a thought, but let me say beforehand that I'm not suggesting that we don't use GFI's.

However, the sheer nature of what we do virtually assures that we're going to have a horrible time with GFI circuits. Extension cords laying on the ground, even completely dry, create leakage situations. Heck, any piece of insulated wire, whether or not it's on the ground, introduces leakage into a circuit.

Last year, I borrowed a Fluke leakage meter (cool device, but expensive). Then I compared a few different extension cords that I had. I basically laid out 100 feet of cord with a 200 watt light bulb at the end. The cheap green cords from Lowes were absolutely the worst, and had excessive leakage that would blow any GFI outlet, and this was laid out on my driveway in dry weather!

The better cords, which cost significantly more, did a lot better.

Then, I took one of the better cords and ran it through my swimming pool, with the connectors on both ends completely dry. The leakage current nearly tripled, and no connectors were involved! This was a totally new extension cord that cost nearly $60.

The higher the amperage, the more leakage our cords/lights/etc are going to create. Also, the sheer nature of dimming circuitry creates leakage as well.

I know many of the local 'city' displays around us do not use GFI's at all, due to this problem. All it takes is high humidity, and the display crashes.

Of course, it is technically possible to build lights and extension cords so that leakage current isn't really a problem, but nobody is ever going to spend that kind of money.

It simply boils down to the fact that GFI's weren't designed for high current, long run, fast switching applications. It was to keep people from killing themselves when they knocked the radio into the bathtub.

Share this post


Link to post
Share on other sites

chuckd wrote:

The higher the amperage, the more leakage our cords/lights/etc are going to create. Also, the sheer nature of dimming circuitry creates leakage as well.

I would not have thought that the amount of current through the cord had an effect on leakage current. Am I over simplifying it when I view the leakage paths as fixed resistance (for the conditions present) and the leakage current being a function of Voltage/Resistance?

BTW, I have a 100' green two conductor cord that I just bought at Lowe's last month. Over 80' of it is staked down across my fairly damp and currently snow covered yard and I'm not see any GFCI trips. I know the GFCI is working because the cord I had there for Halloween had a cut in the insulation and did trip the GFCI.

Share this post


Link to post
Share on other sites

Yes, you're oversimplifying it. Leakage current for our purposes has two major sources, resistance and capacitance. There also is an inductive component, but it's usually much less. Also, depending on the load at the end of the line, these can vary quite a bit. LED's for example, are very capacitive.

So, if you decide to dim a strand of lights, thus drastically changing the power waveform resulting in tons of harmonics, non-linear effects come into play quite a bit (resulting in more leakage current). This is why so many people blow their GFI's when fading lights.

The app note for the Fluke 360 leakage current clamp meter has a nice high level description of some of this, and how to measure:

http://support.fluke.com/find-sales/Download/Asset/2788368_6115_ENG_A_W.PDF

Share this post


Link to post
Share on other sites

Hi Chuck,

I think what you are seeing with the megger is what is called "Capassitive Coupling". AC current will flow through a capassitor. Where as D.C. current does not.

Share this post


Link to post
Share on other sites

Yikes Chuck, with all that considered it's a wonder anybody can get away without tripping. It still seems like the real world results are fewer trips than your numbers predict. But most people (including me) probably aren't running a 200 watt load at the end of a 100' cord lying on the ground.

Did you take any measurements on a Lowe's three conductor cord versus a Lowe's two conductor cord (or SPT-2)? Taking the capacitive effect into consideration adds more support to the thought that the removing the ground conductor should eliminate a significant leakage path.

BTW, thanks for the waveforms and assorted data on the snubber thread. My TRIAC knowledge is lacking, so I'm just following along and absorbing.

Share this post


Link to post
Share on other sites

I only tried their cheap, green 100 footer versus their expensive name brand cord. Both were three wire (with ground). I was only looking to see if the quality of cord made much difference, and how much leakage there was in a dry versus wet condition (but only what difference that made to the insulation, not a path to ground).

I liked in the Fluke app note how they said that if you simply used DC current, you'd see quite a different resistance versus an AC waveform. And with dimmers, it gets really hairy.

What I wish we had was intelligent GFI's, that would know what 'normal' leakage was, and if it grew too quickly. But I pretty much know why we'll never see those.

Share this post


Link to post
Share on other sites

You can also find the item below at HarborFreight.com or at your local store if you have one.

Frank A. wrote:

Happy Thanksgiving

here is what I use

http://www.littlegreenhouse.com/accessory/electrical.shtml

most people do not like covering their connections

but

not my choice I use these on 110v and dmx connections

Frank A.

Share this post


Link to post
Share on other sites

I test my cords each year by taking a bucket of tap water.. putting a copper ground into it.. plugging one end of the cord into the GFCI and the other end into 2 of the 100 watt spot light bulbs.. I drop the cord except for the ends into the bucket.. if it trips the GFI i throw the cord out...

these are the lowes 2 wire or similar cords.. I have only had a couple fail since 06 when I began animated displays....

did have a couple brand new fail....

and no I never touch the water with my hands.. yes I wear electricians protective gloves when im working on this...

-Christopher

Share this post


Link to post
Share on other sites

All my mini tree's were blowing the GFI's. So I read somewhere where the mini tree's themselves being a wire frame causes to much drain when it is raining. I put 3/4 boards below them and they work again. I cant find any one thing that causes it. All the lights work, all the plugs are elevated and they still pop. It has been raining for the past few days non stop. Hopefully by putting them on 3/4 board they wont pop anymore.

Share this post


Link to post
Share on other sites

My mini trees use plastic tent stakes as legs. The frames have three 1/4-20 stainless nuts welded to them, and then I install anchor bolts into plastic tent stakes. I drill a pilot hole in the stake, then grab the anchor bolt in the middle with a pair of vise grips, heat the wood screw end, and screw it into the tent stake, as it melts the plastic out of the way. Then I try to remember to spray them green, and screw them into the bottoms of the mini trees when I install them in the yard. No GFCI trips from the mini trees doing it this way. (as long as all the channels on the tree are on the same circuit)

Share this post


Link to post
Share on other sites
Guest
This topic is now closed to further replies.
Sign in to follow this  

×
×
  • Create New...