HI, new to the forum. Please forgive me if this has been covered before. I wanted to make some holding tank heaters for my rv. I had a spool of nichrome laying around from a previous project so that was my heater medium. I made two heaters using fiberglass as the backing fabric and silicon for insulation along with aluminum to reflect heat back towards the tanks. I chose AC as the power source instead of the 12v coach batteries to conserve power and because my rv has a generator if shore power isn't available.

I got it all wired up, two heaters in parallel showing just over 40ohms total resistance so they're pulling roughly 3 amps but my rv seems to have a real problem accepting the heaters. The rv has GFI circuits and they pop every time I plug it in to the rv while it's on shore power from my house. The house GFI also pops. I can plug them into an extension cord directly to a GFI from my house with no problems and getting right around 80 degrees heat but every approach I have tried using the rv on shore power will pop the rv and house GFI.

I can run the heater using the rv generator but using shore power refuses to hold the load. I tried grounding the plug to the rv chassis and that made a small difference by poppiing the GFI after a second or two instead of immediately but this still isn't satisfactory of course.

The circuit is simple with a coil of nichrome wire laid flat on the backing connected through copper wire to the terminals of a standard a/c outlet plug and a separate ground to the rv chassis. Each heater offers roughly 85 ohms of resistance. They are connected in parallel but will also pop the GFI when connected in series but that configuration provides too little heat to be any good to the holding tanks.

Any thoughts?

 

Is the ground connection to the chassis required for some other reason? The small current that flows from the nichrome wire to ground is probably what’s tripping the GFI. That’s its job.

 

Is the ground connection to the chassis required for some other reason? The small current that flows from the nichrome wire to ground is probably what’s tripping the GFI. That’s its job.

I originally wired the heaters without a ground. The GFI popped immediately. Adding the ground slowed things down a second or two but that's all. I am considering attaching the ground directly to the heaters at the aluminum backing but I thought it might cause more trouble if there is a bit of current leakage there.

 

It is interesting that the heaters exhibit no GFI related problem when an extension cord from shore power is used. The unique characteristic of extension cords is that they typically produce an AC voltage drop at the far end, due to an inductive reactance per unit length that is inherent in extension cords. As an experiment, try inserting a 100 watt incandescent light bulb in series with the heater plug, thereby reducing the voltage at the heater.
It may be that the layout of the heater construction is somehow producing a stray inductive current path and a reduced voltage will bring it into compliance with the GFI units, as observed with the extension cord.

 

Use a two-wire circuit - take the ground terminal out. The GFI should only trip if it finds current on the GND side of the circuit. I've got an electric smoker that trips the GFI with the three pronged connector (the element does leak to GND). I put a ground adapter plug on the end of the cord and my tripping problem went away. Just make SURE that you aren't putting AC on the GND. Hit your contraption with a DC meter and check the resistance between the plug terminals and your RV ground. Anything less than ∞ Ω is not good and needs fixing.

 

Use a two-wire circuit - take the ground terminal out. The GFI should only trip if it finds current on the GND side of the circuit. I've got an electric smoker that trips the GFI with the three pronged connector (the element does leak to GND). I put a ground adapter plug on the end of the cord and my tripping problem went away. Just make SURE that you aren't putting AC on the GND. Hit your contraption with a DC meter and check the resistance between the plug terminals and your RV ground. Anything less than ∞ Ω is not good and needs fixing.

I'll check it with a meter tomorrow but I'm skeptical. The heaters work fine plugged into my house which is also on a GFI. That tells me the heater isn't shorting or inducting to AC ground. Is that thinking correct?

 

I made two heaters using fiberglass as the backing fabric and silicon for insulation

When you say you used fiberglass fabric, did you use the resin also? If not there could be leakage through your silicone, not silicon, insulation into the water in the tank. Even the best application of silicone rubber(RTV) unless done under pressure in a mold can have voids in it.

 

When you say you used fiberglass fabric, did you use the resin also? If not there could be leakage through your silicone, not silicon, insulation into the water in the tank. Even the best application of silicone rubber(RTV) unless done under pressure in a mold can have voids in it.

That would be pretty amazing. I t makes good sense since nothing else so far is. Is there another decent insulation I could add that still conducts enough heat to make them useful?

 

That would be pretty amazing. I t makes good sense since nothing else so far is. Is there another decent insulation I could add that still conducts enough heat to make them useful?

Why put it in the tank? If your talking about the waste tank won't stuff get caught in the "wire'? Don't they usually put the heaters on the outside of a tank? I think if I was doing this I would buy something like this - https://www.frostking.com/products/heat-cables/electric-roof-cable-kits and install it around the outside of the tank.

Or they make something called a heat blanket, a flat silicone rubber strip with nichrome embedded in it. I just bought one from Ebay (China made) for bending wood. One of those could be glued to the bottom of the tank, or they can be bought with adhesive that is made for them already on it. If interested I can find the companies store information for you.

 

Why put it in the tank? If your talking about the waste tank won't stuff get caught in the "wire'? Don't they usually put the heaters on the outside of a tank? I think if I was doing this I would buy something like this - https://www.frostking.com/products/heat-cables/electric-roof-cable-kits and install it around the outside of the tank.

Or they make something called a heat blanket, a flat silicone rubber strip with nichrome embedded in it. I just bought one from Ebay (China made) for bending wood. One of those could be glued to the bottom of the tank, or they can be bought with adhesive that is made for them already on it. If interested I can find the companies store information for you.

I made pads that go under the tank. They attach to the bottom I assumed you meant current is leaking through the polypropolene into the water. I know current can be induce in weird ways but you were thinking of something completely different.

 

I made pads that go under the tank. They attach to the bottom I assumed you meant current is leaking through the polypropolene into the water. I know current can be induce in weird ways but you were thinking of something completely different.

And I was assuming you meant they were in the water.