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[TJ (Tom) Cornish]

Any thoughts?

a] Many DMM have AC Current ranges.  My 30+ year old bench meter has a 200 uA full scale range, my Radio Shack DMM may go even lower.  So you loop the wire through the DMM and the clamp ohmmeter at the same time.

b] Trying to make accurate reading using the AC power line has it's frustrations.  First the line voltage bounces all around, second the high frequency noise may be read differently with different meters.  So instead of the AC line, use a big power amplifier.  Use a music player with a 60 Hz sine wave signal.  It will be safer and more accurate.

All of these techniques have their place.  I have a .1 ohm 100w shunt resistor that is great for high-current applications, and being a 1% resistor, is plenty accurate enough:
http://www.digikey.com/product-search/en?pv1=1323&pv2=14&FV=fff40001%2Cfff80488&k=shunt+resistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

Whether this is easier than looping the ground wire around a clamp meter probably depends on the circumstances.  The advantage of the shunt resistor is you can make an enclosure with the resistor and a receptacle with measurement test points via banana plugs, and you don't have to cut up a cord.  The disadvantage is that you potentially need more than one shunt resistor in your jig, depending on what you're trying to measure - one on the hot lead, one on ground, etc.  The hot lead shunt resistor will also be at line voltage potential, so care in measurement is necessary.

A clamp meter requires a cord with the wires separated, and can become a little bit messy looking if you're wrapping multiple turns (you can't over-under your turns or you cancel the current), and of course requires a clamp meter.

Using the current range of your multi-meter (which is really a voltage measurement over a current shunt) is potentially convenient, but there are limits to the range of current that can be measured - most meters are 10A on the high range, and a couple hundred mA on the low range.  Since most meters are banana plugs, some kind of test adapter must be made, and it can be easy to damage the meter with overcurrent and/or plugging into the wrong banana jacks.

[Mike Sokol]

a] Many DMM have AC Current ranges.  My 30+ year old bench meter has a 200 uA full scale range, my Radio Shack DMM may go even lower.  So you loop the wire through the DMM and the clamp ohmmeter at the same time.

Not a bad idea. I've thought about using one or the other, but certainly could use both for this test. I worry about the legal aspects of opening up the EGC to insert a DMM during a field test, but perhaps I'm being over cautious. I would need to make a simple break-out box with banana jacks to connect the meter inline with the EGC. There could be a shorting strap for when you don't have a DMM in the circuit and a 10-loop coil for the clamp meter to boost sensitivity of milliamp range. 

b] Trying to make accurate reading using the AC power line has it's frustrations.  First the line voltage bounces all around, second the high frequency noise may be read differently with different meters.  So instead of the AC line, use a big power amplifier.  Use a music player with a 60 Hz sine wave signal.  It will be safer and more accurate.

True for in the lab, but not practical for field testing. I want something you can throw on a backline quickly to troubleshoot GFCI tripping issues. This needs to be a quick and dirty test that doesn't take too long, but gives you a pretty definitive go/no-go answer. 

[Mike Sokol]

All of these techniques have their place.  I have a .1 ohm 100w shunt resistor that is great for high-current applications, and being a 1% resistor, is plenty accurate enough:
http://www.digikey.com/product-search/en?pv1=1323&pv2=14&FV=fff40001%2Cfff80488&k=shunt+resistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

That could eliminate my worry about opening up the ground accidentally. I'll have to a few calculations to figure out the best resistor value to use that will produce sufficient voltage drop at 1 mA to show up on most meter AC voltage scales. Then it just needs a couple of binding posts for the meter leads, which will only carry voltage but no real current. This is only one-half of a Kelvin Bridge, so it won't give an accurate leakage reading at any incoming voltage other than 120-volts, but it's accuracy will only vary by the percentage that the line voltage deviates from 120-volts. Again, if I want PPM accuracy I'll do it on a lab bench. But for real world applications this could be a pretty good fit.   

[Mike Sokol]

I have a .1 ohm 100w shunt resistor that is great for high-current applications, and being a 1% resistor, is plenty accurate enough:
http://www.digikey.com/product-search/en?pv1=1323&pv2=14&FV=fff40001%2Cfff80488&k=shunt+resistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

Inside my head calculations suggest this should be a 1 ohm resistor since a standard DMM goes down to 1 mV AC and 1 mV across a 1 ohm load equals 1 mA. So a standard accuracy DMM could read 1 mA of hot-to-chassis leakage, which is plenty close enough for this test. A better meter could measure sub-milliamp currents as well. Wattage for the resistor is a little more tricky since it could be possible to pass 20 amps (circuit breaker limit) and 120 volts through this resistor, which would result in 2,400 watts of heat. Of course, the meter would be off-scale at that point which is a pretty good indicator that something is terribly wrong. I'll look for a 100-watt/1-ohm resistor for this test. Most of the time there would be fractions of a watt resistor heating with normal leakage currents. It would only get interesting (read hot) if there was a line-to-chassis short happening during the test procedure. 

[TJ (Tom) Cornish]

Inside my head calculations suggest this should be a 1 ohm resistor since a standard DMM goes down to 1 mV AC and 1 mV across a 1 ohm load equals 1 mA. So a standard accuracy DMM could read 1 mA of hot-to-chassis leakage, which is plenty close enough for this test. A better meter could measure sub-milliamp currents as well. Wattage for the resistor is a little more tricky since it could be possible to pass 20 amps (circuit breaker limit) and 120 volts through this resistor, which would result in 2,400 watts of heat. Of course, the meter would be off-scale at that point which is a pretty good indicator that something is terribly wrong. I'll look for a 100-watt/1-ohm resistor for this test. Most of the time there would be fractions of a watt resistor heating with normal leakage currents. It would only get interesting (read hot) if there was a line-to-chassis short happening during the test procedure.

The 100 watt resistors are not available in anything higher than .1ohm - there is probably a smaller one.  Keep in mind that the resistor won't stand alone - at the very least, there is resistance in the wiring loop; and other than in a catastrophic short, this resistor is part of a normal circuit.  If we calculate from a 20A loop current, that gives a 2v drop over the .1ohm resistor, so the resistor should never dissipate more than 40 watts - if the loop current is truly 20A. 

I did this exercise when I built my test jig, and it seemed to me that I wouldn't have any trouble getting an accurate voltage measurement across a .1ohm resistor.

Disclaimer - I wrote this post in about 90 seconds - apologies if the math and/or logic is wrong.

[Mike Sokol]

I did this exercise when I built my test jig, and it seemed to me that I wouldn't have any trouble getting an accurate voltage measurement across a .1ohm resistor.

Disclaimer - I wrote this post in about 90 seconds - apologies if the math and/or logic is wrong.

While you can get an accurate reading of BIG current with a 0.1 ohm resistor, remember this needs to have readable voltage drop with 1 mA of current, yet survive a complete hot-to-chassis short and just enough series wire to allow 20 amps of fault current. Yes, I have to sit down, draw this out, and do the math, but I'm pretty sure a 1 ohm resistor is what's needed here. However, I've been wrong by an order of magnitude before and will be wrong again, so I'll do the math first.

I did a quick eBay search on Chinese resistors and found a 1 ohm, 100 watt resistor for less than $5 with free shipping. Not sure how accurate it is, but I have a serious Kelvin Bridge from my old missile guidance system building days that will measure down to 5 ppm accuracy at low ohms. I'll bet it's close enough for this field tester and the price is right. http://www.ebay.com/itm/1-Ohm-1R-100W-Watt-Power-Metal-Shell-Case-Wirewound-Resistor-/370550984569?pt=LH_DefaultDomain_0&hash=item56468f4f79

[Kevin Graf]

This is exactly what you need!



Shunt 150 Amp 100Mv. | EMPRO HA150100
http://www.electronicsurplus.com/Item/149886/EMPRO%20-%20Shunt%20150%20Amp%20100Mv_%20-%20HA150100/

Electronic Surplus was in downtown Cleveland for decades.  EE's and other would stop by on lunch break.  But they moved to a far east suburb. Now you have to pack a lunch to go shopping.

[Mike Sokol]

This is exactly what you need!

Electronic Surplus was in downtown Cleveland for decades.  EE's and other would stop by on lunch break.  But they moved to a far east suburb. Now you have to pack a lunch to go shopping.

Wow, that is cool...! Might be a bit of overkill, but this looks like something you could use to bring Frankenstein's monster to life. I do love surplus stores. 

[Mike Sokol]

Here's an idea on how to keep the resistor from blowing up if something goes wrong during the test. What about a pair of husky diodes in reverse parallel across the sensing resistor? Once the voltage drop across the sensing resistor hits 1/2 of a volt, it will short out (essentially) and prevent any more current through the shunt resistor (which I still contend needs to be 1 ohm). So this would measure up to 500 mV across the shunt resistor which works out to 500 mA of measured ground leakage current. In that case, since the resistor could only ever see 500 mA of current times 500 mV of voltage, the max wattage it needs to dissipate would be 1/4 of a watt. Or I could use 5 volt Zener diodes instead which would allow a max linear reading of 5 amps leakage current producing 5 volts of sensing voltage, which works out to a 25 watt resistor required. I'm sort of liking that since there's no way it could overheat my 100-watt/1-ohm resistor, and it would allow more than sufficient leakage current test levels.   

[Lyle Williams]

My cheap earth current rig is just an AC milliammeter inserted directly in the earth line.  It is bypassed by two pairs to diodes that limit the voltage across the meter movement.

TWO diodes each way to get a high enough voltage limit across the meter.  EACH WAY to ensure that it can conduct on both sides of an AC cycle.  200A diodes in the hope that they won't blow before the fuse/breaker should a fault develop in the gear under test. The diodes were scrapped/surplus items.

Of course I can't use it to test anything that is bolted in a rack or has an alternate ground.  You need a differential current meter for that and they are a couple of grand from memory.

[ProSoundWeb Community]