Leaking Current Tester...

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1793

1793

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Location
Louisville, Kentucky
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Inspector
I'm looking for a tester that will show the amount of leakage of current, like a bad refrigerator, that is tripping say a GFI receptacle. I would like to keep the $$ low, so does anyone have any ideas?
 
100310-1504 EST

If we can assume that that the leakage is from hot or neutral to the enclosure of the refrigerator (or whatever), and there is a 3 conductor cord to the appliance with hot, neutral, and EGC as the wires, and that EGC is connected to the enclosure, then make an adapter as follows:

Three prong socket and plug. Connect hot and neutral straight thru with individual wires. Between the two EGC terminals put a 1 ohm 1 W resistor. If there is much over 1 A on the EGC wire the resistor will burn up. Put a digital voltmeter across the 1 ohm resistor. MV translate to MA. This technique saves burning out expensive fuses in the current mode of DVMs in the case of a large unexpected current.

For this test to work the enclosure must not have any conductive path from the enclosure that would bypass the current measuring resistor.

.
 
Not to push and brand name but Sears craftsman has this one that I caught on sale for $10.99 AC Line Splitter 81066
Cat II 600v 50/60 Hz input 120V 15A max.
images


If you have an Amp clamp that will clearly add a test for your hot and neutral clearer.
At the bottom there's also two inlets for voltage reading via needles.
This equipment is three prong.
 
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Cow said:
Is an amp clamp around both the hot-neutral at the same time not precise enough?

Megger works well also.
Click to expand...
They have amp clamp meters but they are bit more sensative for this. I have a AEMC 565 for around $270 but a Fluke will cost more.
 
gar said:
100310-1504 EST

If we can assume that that the leakage is from hot or neutral to the enclosure of the refrigerator (or whatever), and there is a 3 conductor cord to the appliance with hot, neutral, and EGC as the wires, and that EGC is connected to the enclosure, then make an adapter as follows:

Three prong socket and plug. Connect hot and neutral straight thru with individual wires. Between the two EGC terminals put a 1 ohm 1 W resistor. If there is much over 1 A on the EGC wire the resistor will burn up. Put a digital voltmeter across the 1 ohm resistor. MV translate to MA. This technique saves burning out expensive fuses in the current mode of DVMs in the case of a large unexpected current.

For this test to work the enclosure must not have any conductive path from the enclosure that would bypass the current measuring resistor.

.
Click to expand...


Thanks Gar - I learned something new today that could help me out in the future.

~Matt
 
Cow said:
Is an amp clamp around both the hot-neutral at the same time not precise enough?

Megger works well also.
Click to expand...


I had one of these (leaking fridge) yesterday. Couldn't use the Megger because of the electronics in a modern fridge. Just got my drop-cord GFCI. Plugged fridge into that (it tripped immediately). Fridge must be bad.

Mark
 
I always spread out some old newspapers under the appliance and look for the leaky spot.
This seems to work real well for my cars and trucks so why not for the fridge?

You said in the original post that the appliance was already tested, ?the GFCI is tripping?
 
jwelectric said:
... You said in the original post that the appliance was already tested, “the GFCI is tripping”
Click to expand...

While I will not argue with your statement I can't count the number of times I've been told: "well it worked when plugged into another outlet, so it cannot be the appliance."

I am looking for something that I could use to "show" why the appliance is or could be the culprit.
 
1793 said:
While I will not argue with your statement I can't count the number of times I've been told: "well it worked when plugged into another outlet, so it cannot be the appliance."

I am looking for something that I could use to "show" why the appliance is or could be the culprit.
Click to expand...

That's why I grab the extension cord and portable GFCI and figure out whether it's the appliance or the circuit.

Mark
 
wptski said:
Hmm, I never noticed the lack of that feature before. I like AEMC stuff but their French>English converted manuals are a challange at times!:roll:
Click to expand...

I have owned a couple of AEMC current probes and liked them with one exception. One model I owned was sensitive to higher frequency electrical noise. It was pretty much worthless when used to measure current in a servo drive line.

I looked at both the AEMC and the Fluke specifications and did not see where either talked about noise immunity or filtering. I think it would be desirable to have a good filter on a current leakage meter to help make it immune to noise currents. Especially when attempting to measure such small levels.

Gars suggestion of using a resistor may work in theory but you are attempting to read voltages in the 0.005V range and that makes this method very susceptible to noise currents. I have attempted this method many times and found I was unable to tell the difference between actual "expected current flow" and noise currents. Of course the sucess rate is variable depending upon the environment you are tesing in and the the "expected current level" being tested.
 
Correction, I was a little slow. They do talk a bit about filtering and this is a good value to compare between meters..
 
100311-0959 EST

ELA:

Using the "shunt" method of current measurement is not a big problem with respect to noise currents except when measuring fast rising large currents.

1 ohm carbon composition resistors are very low inductance. Neither my Fluke 27 or 87 are much good above 20 to 40 kHz. The 27 is inherently quite noise free with shorted leads, reads 0 on 0.1 millivolt increment range. Labeled millivolts on the meter. The 87 has some internal noise and reads several tenths. I do not know if this was true when it was new.

The problem with high currents is that one has to connect the meter leads to the resistor. This inherently creates a one turn loop. Large fast rising currents induce a voltage in this loop that can be larger than the intended signal. This problem I ran into many years ago when I was testing the opening of a circuit breaker. The experiment consisted of a number of 6 V car batteries in series, heavy wire, a mercury relay, a shunt, and the circuit breaker. The scope picture was not anything like what was expected because of this induced voltage. The shunt I used was a bar about 8" long and by running the center wire of the coax cable as close as possible to the bar the cross sectional area of coupling was greatly reduced and the resulting picture was as expected. A better arrangement would be a tubular rod for the shunt with the lead run coaxial in the tube.

I suggested the "shunt" method because the original post wanted an inexpensive method.

.
 
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What tools do you happen to already have?

For example, if you already have a current clamp that can read in the mA range, then the only tool you need is an extension cord with H and N separated from G. Run the clamp around the H and L wires, and any net current that you measure is current leaking someplace else, just as Cow suggested in the second post. But this approach depends upon having an amp clamp rated for the mA range, otherwise you won't see currents at the level that will trip a GFCI.

If you already have a digital multimeter, then the resistor approach that gar presented will work if the leakage is returning on the EGC. This is a pretty good bet, but not guaranteed, especially if there is plumbing involved.

You can build a CT based leakage detector, using a small current transformer made for ground faults, and a suitable burden resistor. Just thread the H and N leads through the CT. For example this: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=582-1018-ND with a 1K burden resistor would have a sensitivity of about 1V/A; just place a multimeter across the resistance (similar to the system that gar described). It would cost a bit more ($10 for the CT, plus shipping) but would have better isolation between the circuit and your meter, and would catch any residual current.

-Jon
 
gar said:
100311-0959 EST

ELA:

Using the "shunt" method of current measurement is not a big problem with respect to noise currents except when measuring fast rising large currents.

1 ohm carbon composition resistors are very low inductance. Neither my Fluke 27 or 87 are much good above 20 to 40 kHz. The 27 is inherently quite noise free with shorted leads, reads 0 on 0.1 millivolt increment range. Labeled millivolts on the meter. The 87 has some internal noise and reads several tenths. I do not know if this was true when it was new.

.
Click to expand...

Gar,
As I mentioned results will vary based on the equipment being measured and the ambient environment. Glad that you have had good success, mine has been less than stellar when measuring low currents near the noise floor.
If you are going to open the circuit anyway (to insert a resistor) then I just use the current function on the meter.

My Fluke 87 measures 1.4mv when shorted with a 1 ohm carbon resistor.
Appears that my meter is quite noisy internally.
 
100311-1452 EST

ELA:

I am am quite cautious these days about using the current function in the meter unless I am real sure what may happen. Too many expensive fuses. $15 or whatever vs a couple cent resistor.

This application requires no great accuracy. Ideally the clamp-on probe is easier to work with, but not inexpensive if you do not have one.

.
 
gar said:
100311-1452 EST

ELA:

I am am quite cautious these days about using the current function in the meter unless I am real sure what may happen. Too many expensive fuses. $15 or whatever vs a couple cent resistor.

This application requires no great accuracy. Ideally the clamp-on probe is easier to work with, but not inexpensive if you do not have one.

.
Click to expand...

You mean like ... when you take a current measurement and then fail to switch the leads back to volts before you take a voltage measurement :-?

I hate to admit it but I have done that once or twice and I agree those fuses can be hard to locate in a hurry if you do not have extras on hand.

When measuring current I always start out on the highest scale (10 amps on my meter) and work my way down.
Considering that if more than 10 amps were flowing in the ground lead you would most likely know that in advance.


I agree that you do not need high accuracy for this leakage test but it is nice to be able to distinquish between actual 60 hz leakage currents and noise induced readings.
 
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