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Old 07-23-2013, 07:16 AM   #41
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Originally Posted by sdagro View Post
Hi Mike,
Would like to see this tester you made or a plan for it. Also, all my problems were from shore power. Bighorn was prepped for genny but didn't have one in it. Thus the transfer switch. Steve
Here you go. This is the reverse of what I built as a kid to find short circuits in the wiring of my cars back in the 60s. Since I couldn't afford to keep blowing fuses, I wired up a brake light bulb on a pair of alligator clips, and used it in place of the fuse. When the light was bright, that meant you had a short to the chassis in that circuit, and you could wiggle all the wires to see when the light went out. This tester could do the same thing for an RV with a short in the 12-volt DC wiring.

However, in the case of ground testing, we WANT a short circuit between the ground pin of the shore power plug and the chassis of the RV. Please note this is used on an UN-PLUGGED, UN-POWERED RV which is disconnected from shore power. The RV's battery will supply around 2 amps of current through the brake-light bulb and should glow brightly (and measure full battery voltage with the optional DMM voltmeter, if there's a solid safety ground connection to the chassis. After the initial test of the shore power plug, you can then add on all your extension cords and adapters one at a time, and confirm you still have a bright light and full voltage (a low resistance safety ground), while flexing and twisting all connections.

If you add a jumper wire inside the RV's circuit breaker panel to TEMPORARILY bond the Ground and Neutral Bus together, you can also use this tester to verify the neutral conductor. As mentioned earlier in this thread, and open or loose neutral wire will allow an over-voltage condition on a 50-amp/240-volt shore power connection which WILL damage your RV electronics and appliances.

The beauty of this test procedure is that it's perfectly safe since there's only 12-volts DC in your test, and it won't be fooled by a high resistance or intermittent connection in your safety ground path. If you measure the voltage across the battery first and compare it to the voltage across the bulb, this is basically one-half of a Kelvin Bridge, something I used to measure missile guidance system components back in the 80's. It's very difficult to get an accurate low-resistance reading with any voltmeter since there's not enough current to test for corrosion. But the 2-amps of the bulb is about the perfect amount of current to discover any corrosion. And you can really see it flicker if you're on an intermittent connection.

The only caveat to be aware of is that a brake light bulb gets very hot in just a minute, so I'm going to buy a cheap plastic light trailer brake light housing for the bulb, instead of the old brake light socket I had in my tool box. I think the hot bulb could put a burn mark in your carpeting if you let it run for a few minutes while laying on the floor of your RV. And you'll need a pretty long cable running to the battery on some larger trailers. But in all, this is a $10 build at most.



Mike Sokol


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Old 07-23-2013, 10:56 AM   #42
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Quote:
Originally Posted by sdagro View Post
Hi Mike,
Would like to see this tester you made or a plan for it. Also, all my problems were from shore power. Bighorn was prepped for genny but didn't have one in it. Thus the transfer switch. Steve
Here you go. This is the reverse of what I built as a kid to find short circuits in the wiring of my cars back in the 60s. Since I couldn't afford to keep blowing fuses, I wired up a brake light bulb on a pair of alligator clips, and used it in place of the fuse. When the light was bright, that meant you had a short to the chassis in that circuit, and you could wiggle all the wires to see when the light went out. This tester could do the same thing for an RV with a short in the 12-volt DC wiring.

However, in the case of ground testing, we WANT a short circuit between the ground pin of the shore power plug and the chassis of the RV. Please note this is used on an UN-PLUGGED, UN-POWERED RV which is disconnected from shore power. The RV's battery will supply around 2 amps of current through the brake-light bulb and should glow brightly (and measure full battery voltage with the optional DMM voltmeter. After the initial test of the shore power plug, you can then add on all your extension cords and adapters one at a time, and confirm you still have a bright light and full voltage, while flexing and twisting all connections.

If you add a jumper wire inside the RV's circuit breaker panel to TEMPORARILY bond the Ground and Neutral Bus together, you can also use this tester to verify the neutral conductor.

The beauty of this test is that it's perfectly safe since there's only 12-volts DC in your test path, and it won't be fooled by a high impedance connection in your ground path. If you measure the voltage across the battery first and compare it to the voltage across the bulb, this is basically one-half of a Kelvin Bridge, something I used to measure missile guidance system components back in the 80's. It's very difficult to get an accurate low-resistance reading with any voltmeter since there's not enough current to test for corrosion. But the 2-amps of the bulb is about the perfect amount of current to discover any corrosion. And you can really see it flicker if you're on an intermittent connection.

The only caveat to be aware of is that a brake light bulb gets very hot in just a minute, so I'm going to buy a cheap plastic light trailer brake light housing for the bulb. I think it could put a burn mark in your carpeting if you let it run for a few minutes. And you'll need a pretty long cable running to the battery on some larger trailers. But in all, this is a $10 build at most.



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Old 07-23-2013, 11:22 AM   #43
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I know that using it as half of a bridge would be invalid, but how about just drawing the power from a DC outlet instead of a long lead to the battery post? That would make it a little easier to use. Maybe I'm overlooking something else, though?

And do you, as a rule, check every AC outlet for this condition, or just one?
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Old 07-23-2013, 11:47 AM   #44
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I know that using it as half of a bridge would be invalid, but how about just drawing the power from a DC outlet instead of a long lead to the battery post? That would make it a little easier to use. Maybe I'm overlooking something else, though?

And do you, as a rule, check every AC outlet for this condition, or just one?
This is not designed used on an outlet, but on the shore power cord itself. However, you could wire up a kludge cable and use it for interior outlet ground testing, but you would have to be VERY careful not to plug in the RV to shore power, or things could get VERY dangerous.

And yes, plugging into an 12-volt DC outlet would work. Here's my cigarette lighter version.

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Old 07-23-2013, 12:17 PM   #45
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Yeah, I understand that ALL shore and generator power must be OFF.

My question was whether you check for this bootleg ground on ALL outlets , or just one or a few.

I'm guessing that checking one outlet would be enough because, assuming that the camper and power cord and its adapter plugs are properly wired, this problem could ONLY be at the power source (park outlet or generator).

The issue might be verifying that all the camper wiring is OK. But if the tester on the outlet shows proper H, N and C connections on ALL outlets, and your tester shows no faults on any ONE outlet, you should be good to go.

At least, that's my line of thinking.

Just trying to make sure I understand all of this. Sorry to be a pest but the aviation maintenance side of me wants to know all of this stuff.
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Old 07-23-2013, 12:19 PM   #46
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Sorry. Touched SEND too soon by mistake.

I also see that the simplest way is to do it at the cord.
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Old 07-23-2013, 12:51 PM   #47
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Sorry. Touched SEND too soon by mistake.

I also see that the simplest way is to do it at the cord.
Yes, that's always the first test. Shore power cord, then add on extension cables and adapters one at a time to confirm they're all good.

And remember, this is not a "bootleg ground" tester, but rather a ground fault resistance test. Not the same thing at all.

And yes, if your shore power connection has continuity to the frame of the RV using this tester, and you're still showing an open ground on all inside outlets of the RV when you're plugged into power, then it's the campground pedestal or home outlet at fault. However, if you have only one internal RV outlet showing an open ground, then the fault is in that outlet itself. Just basic wire tracing at that point to find the open circuit.

Mike Sokol
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Old 07-23-2013, 04:57 PM   #48
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Got it. Makes perfect sense, too.


I'll leave you alone now. Thanks for the assist.
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Old 07-23-2013, 05:40 PM   #49
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Got it. Makes perfect sense, too.

I'll leave you alone now. Thanks for the assist.
No problem at all. I believe that the failure of the student to learn is actually the failure of the instructor to teach. So if something is not clear to you all, then it's my fault for not describing and illustrating the concept correctly.

One of my next challenges is to teach farm kids at agricultural education centers about electricity and electrical safety. And it appears to be a K-12 situation since farm kids start doing chores around electrical pumps and machinery at a very early age. So now I have to figure out how to break them up into perhaps four different age groups, then customize a presentation for each one.

Yes, there will be Tesla Coils and Van de Graff generators....

Mike Sokol
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Old 07-23-2013, 05:42 PM   #50
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You're lucky; you get to teach kids.

My students are all adults, most well over 40 and VERY set in their ways.
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