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[Mike Sokol]

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.

I think that a big old bridge rectifier with the DC outputs bonded together would give me a bi-directional 1-volt clamp. It's an unorthodox usage since the PIV rating would never come into play. Once there's around 1 V peak (or 3/4 of a volt RMS) voltage across the resistor, then it would shunt. So my top leakage current limit would be around 750 mA before the diode shunt kicks in.  I would trust something like this 100-amp 1600 PIV unit. http://www.ebay.com/itm/MDQ100-16-Single-Phase-Diode-Bridge-Rectifier-100A-Amp-1600V-LJN-/151294539123?pt=LH_DefaultDomain_2&hash=item2339db7573

or even this 50-amp/100-volt bridge rectifier could work. Remember, the reversed junction will never see more than the forward bias of the stacked P-N junctions, maybe 1 volt or so. http://www.ebay.com/itm/50-Amp-100-Volt-Bridge-Rectifier-MP15010-50-Amp-Full-Wave-Diode-Rectifier-/130905566376?pt=LH_DefaultDomain_0&hash=item1e7a946ca8

Also, something rated for 50 or 100 amps steady state should be able to absorb the limited energy of a shorted 20-amp circuit breaker on a typical branch circuit. I'm not hooking directly into cam-locks with this thing (yikes).

[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.

Diodes less than 200A would probably work.  I just couldn't find any good data on the momentary current handling of rectifier diodes.  You need to take a momentary current possibly 20x greater than the breaker value.  230vac/10a is the common household circuit here, so finding surplus diodes with a their continuous = my required peak allowed a very conservative choice in ratings.

If you test in a way that assumes the device under test has a live chassis, then just directly inserting a meter into the ground line is ok.  That is actually one of the options in the portable appliance test standard I work to.


I also have a separate test jig that simply has a 100 ohm resistor in the earth line.  An AC voltmeter put across the resistor will show 0.1V per mA of earth leakage.  That jig has a RCD/GFCI plug on it where it connects to mains power.

[Lyle Williams]

I also have a separate test jig that simply has a 100 ohm resistor in the earth line.  An AC voltmeter put across the resistor will show 0.1V per mA of earth leakage.  That jig has a RCD/GFCI plug on it where it connects to mains power.

[Stephen Swaffer]

Unless you plan to routinely hammer your resistor with over voltage a simple 20 A fast blow fuse would give you reliable protection-if you are really concerned go with Buss's Lo peak fuses-they are designed to limit arc flash (I know OT) so they blow really quick-I would hazard a guess as to how fast but probably have to admit I was wrong.

[Lyle Williams]

Unless you plan to routinely hammer your resistor with over voltage a simple 20 A fast blow fuse would give you reliable protection-if you are really concerned go with Buss's Lo peak fuses-they are designed to limit arc flash (I know OT) so they blow really quick-I would hazard a guess as to how fast but probably have to admit I was wrong.

That would be fine for protecting the low ohm shunts suggested earlier.  My 100 ohm shunt would not trip a breaker/fuse if a chassis became live.  The 10w shunt resistor would then burn out under 500+w of load, leaving the chassis live.  Hence my RCD/GFCI.

[John Roberts {JR}]

Clamp diodes will work fine as you intended and you don't have to size them that much oversize. Diodes tend to fail as short circuits when they overheat and melt so the shorted diode will make even less heat than before, A few amp diode bridge will probably be more than adequate. You just want enough thermal mass that it doesn't vaporize from the initial surge.

I tested this back in the '70s when I was looking into alternate grounding schemes, and UL was receptive to accepting diodes for safety grounds if i paid them the $$ to set up a file and certify them (I didn't). 

JR

[Lyle Williams]

Thanks for the info JR.

Engineering has been defined as "doing for $1 what any fool can do for $2".    :-)

As I only wanted one test jig I took the $2 (well, probably $200, had the parts been new) path.

[Stephen Swaffer]

That would be fine for protecting the low ohm shunts suggested earlier.  My 100 ohm shunt would not trip a breaker/fuse if a chassis became live.  The 10w shunt resistor would then burn out under 500+w of load, leaving the chassis live.  Hence my RCD/GFCI.

Just keep in mind that if you place diodes across a 100 ohm resistor limiting the voltage to 1 volt your test range will max out at 10 mA.  Really not a problem as long as you keep in mind that any 10 mA reading might be much higher.

Also, a 100 ohm load will only draw 1.2 amps at 120 V, so it will not trip a breaker-but it will also create "only" 144 watts of heat-still too much for a 10 watt resistor though.  Not trying to be picky-just don't want someone to be confused if they are trying to do the math.

[Mike Sokol]

Just keep in mind that if you place diodes across a 100 ohm resistor limiting the voltage to 1 volt your test range will max out at 10 mA.  Really not a problem as long as you keep in mind that any 10 mA reading might be much higher.

Also, a 100 ohm load will only draw 1.2 amps at 120 V, so it will not trip a breaker-but it will also create "only" 144 watts of heat-still too much for a 10 watt resistor though.  Not trying to be picky-just don't want someone to be confused if they are trying to do the math.

Steve, I'm proposing a 1 ohm resistor which will output 1 mV per 1 mA of current. So the max current could be 120-amps under a dead short to the chassis (of course, the breaker would trip before that's reached steady-state, but a peak current around that is possible if the input line is stiff enough). Remember, I can get a 100-watt/1-ohm resistor for less than $6 with shipping, so I only have to clamp it around 5 volts or so for a max heating of 25 watts. With that setup I could measure up to 5 amps of leakage current (that's a lot more than required) which would output 5 Volt AC to my meter. With a 1-volt clamp, this would measure up to 1 amp leakage current with 1 volt output. Again, I'm looking for perhaps 10 or 20 mA, so even 1 amp (1,000 mA) is WAY over the expected values. I'm just trying to make this idiot proof in case something goes wrong (and yes, I admit to being an idiot sometimes).

[Lyle Williams]

Just keep in mind that if you place diodes across a 100 ohm resistor limiting the voltage to 1 volt your test range will max out at 10 mA.  Really not a problem as long as you keep in mind that any 10 mA reading might be much higher.

Also, a 100 ohm load will only draw 1.2 amps at 120 V, so it will not trip a breaker-but it will also create "only" 144 watts of heat-still too much for a 10 watt resistor though.  Not trying to be picky-just don't want someone to be confused if they are trying to do the math.

Sorry, I should have typed using my twangy 230v Australian accent.  :-) 

Under the standard I am required to test to, 5ma is a fail under all conditions and 1ma a fail under most.  Actually most stuff just gets meggered at >1Mohm to pass.  Gear without a mechanical on/off switch (gear that needs power connected for the power switch to work) needs a direct (cheap jig and multimeter) or differential (expensive gear) earth current measurement.

De-racking gear for testing with the cheap jig would be a pain, but testing earth leakage a whole roadcase at a time avoids the hassle.

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