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Author Topic: Ground Continuity Current Testing  (Read 8622 times)

Mike Sokol

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Re: Ground Continuity Current Testing
« Reply #10 on: January 27, 2014, 09:50:40 pm »

I think it would be valuable to be able to switch between a lamp and a fixed resistor with a dvm in parallel to allow you to quantify a reading.  Maybe that is over thinking its use, but I am thinking a .5 ohm resistance will only give 1 V drop and, especially in a bright setting, it may be difficult to notice the difference between a 12 V lamp at 12 V and at 11 V?  Especially without a side by side comparison.  That .5 ohms at a full 50 amps could translate into a 25 V drop.

But then I like working with numbers!

Guys, don't over-think this. I'm not trying to get accurate resistance numbers, just testing an RV's shore power system to see if its ground fault path can carry enough current to successfully trip a 20-amp circuit breaker. To that end, I'm sure a bulb will be a good enough intermittent indicator, especially if you combine it with a voltmeter.

As I noted in my original post, this circuit is essentially one half of a Kelvin 4-wire Bridge with separate current and voltage paths used to measure a resistance by nulling it against a known calibrated resistance. Even with very long test wires, a Kelvin Bridge can easily measure down to fractions of an ohm resistance with ppm accuracy. That could be done with this half-bridge if we used a calibrated resistor instead of a bulb, but again we're only looking for under 1 ohm resistance to be code compliant. The general grounding failures in an RV are due to loose frame bolts, contact corrosion, and broken wires in dog-bone/pig-tail adapters used to connect to various campground pedestal outlets.

Once it gets warm outside I'm going to do some testing in various real-world scenarios and see how well it works.
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Mike Sokol
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Frank DeWitt

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Re: Ground Continuity Current Testing
« Reply #11 on: January 27, 2014, 10:07:37 pm »

I like it.  If you are concerned about the leave it connected start a fire issue then put a push button switch is series and make it a switch that requires a lot of pressure. 
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Lyle Williams

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Re: Ground Continuity Current Testing
« Reply #12 on: January 27, 2014, 10:20:54 pm »

I like it.  If you are concerned about the leave it connected start a fire issue then put a push button switch is series and make it a switch that requires a lot of pressure.

If you don't want it to stay connected don't put a spring clip on the end!
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Stephen Swaffer

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Re: Ground Continuity Current Testing
« Reply #13 on: January 27, 2014, 10:58:49 pm »

Or maybe a timer of some sort-that would let you keep your hands free to work connections/harnesses to find intermitents.

As to overthinking-I guess I was thinking more along the lines of general use.  I don't do much RV work, but I can think of a lot of places where testing with more than mA would be helpful-from trailer lights to grounding on AV equipment to general equipment troubleshooting.  I repaired a motor starter on a grain bin today (in -3 actual temp, - double digits windchill) where the fault would never had showed with an ohmmeter.  Common wire was touching frame-actually held by a screw that had arced in two-I noticed it arcing under load-a high current ohm test should have showed something most likely.  (And yes, it was using 240 volt 3 wire, with the same wire providing ground as well as neutral for control wiring-consider the location.)
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Mike Sokol

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Re: Ground Continuity Current Testing
« Reply #14 on: January 28, 2014, 08:06:03 am »

If you don't want it to stay connected don't put a spring clip on the end!

The idea of the spring clip is to allow you to wiggle all the connections looking for an intermittent open. That's one of the biggest problems with RV wiring since there's so much vibration and salts from road travel. Often the hot-ground issues seem to come and go randomly, but I think a current load test will help find those intermittent open circuits.
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Mike Sokol
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Lyle Williams

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Re: Ground Continuity Current Testing
« Reply #15 on: January 28, 2014, 02:50:42 pm »

The idea of the spring clip is to allow you to wiggle all the connections looking for an intermittent open. That's one of the biggest problems with RV wiring since there's so much vibration and salts from road travel. Often the hot-ground issues seem to come and go randomly, but I think a current load test will help find those intermittent open circuits.

I don't mind the spring clip.  I just thought the simplest solution for those who were worried about a long term connection would be to just chop the thing off!  :-)

The Australian Standard for routine testing of portable appliances gives a few options for earth continuity testing.
  1. Just a VOM.
  2. 12v/200ma.
  3. 12v/10a.
  4. 12v and the greater of 1.5x appliance rating and 25a.
My read on this is higher currents are preferred, but they still want tests done by people who don't have access to high current testers.

I agree that there are some combinations of faults and poor wiring practices that could result in heat, smoke or even fire if exposed to 25a.  Isn't that the point of testing though?  Find the fault while you are looking for one (and can put out the fire etc) ...
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Mike Sokol

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Re: Ground Continuity Current Testing
« Reply #16 on: January 28, 2014, 04:13:30 pm »

I agree that there are some combinations of faults and poor wiring practices that could result in heat, smoke or even fire if exposed to 25a.  Isn't that the point of testing though?  Find the fault while you are looking for one (and can put out the fire etc) ...

In the states we call that a "smoke test".  ;D
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John Roberts {JR}

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Re: Ground Continuity Current Testing
« Reply #17 on: January 28, 2014, 05:22:42 pm »

You could sense for voltage at the low side of the light bulb. If it is low volts, that is OK, or if it high volts, that's OK (for not starting a fire)... Mid volts is the dangerous scenario.

A resistor feeding the base of a transistor from the bottom of the lamp lead, will turn in the transistor (driving a LED?), if the ground voltage is more than .5V or so. At 2A, .5V is generated across 250 mOhms. You could set this threshold higher if you think 1/4 ohm is a low enough impedance ground path. 

JR
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frank kayser

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Re: Ground Continuity Current Testing
« Reply #18 on: January 28, 2014, 06:08:27 pm »

In the states we call that a "smoke test".  ;D

Heck, Mike.  That's what I call the point where I power up the sound system for the night!   ;D
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Mike Sokol

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Re: Ground Continuity Current Testing
« Reply #19 on: January 28, 2014, 08:10:42 pm »

Heck, Mike.  That's what I call the point where I power up the sound system for the night!   ;D

Indeed...  8)

Be careful not to release the magic smoke as that's what makes the electronics work. ;)  Of course, you can get a Lucas Smoke recharging kit if you really need it. http://www3.telus.net/bc_triumph_registry/smoke.htm

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Mike Sokol
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