Auto jumper cables to verify conduit GND continuity? ;')

fixizinOctober 2, 2009

Seriously, if you have a 50+ yr. old residential structure, nicely outfitted with EMT indoors (and RIGID outdoors) but good safe GND depends on solid connections all the way back to the main panel, how do you VERIFY that your EMT can actually carry 20AMPS in a worst-case ground fault?

EMT connectors have always seemed a bit lame to me, in terms of CONTACT AREA, and there's all those long runs, hidden behind OLD plaster walls, subjected to who-knows-what kinds of abuse, errant nails, improper anchors, ig-nernt/expeditious "modifications", etc...

So... do you (breaker OFF) disconnect a receptacle neutral at the farthest extent of a conduit run, wire the neutral screw directly to the metal box w/ AWG12 copper, and plug in a 20A angle grinder?... then, while it's running, check voltage drop back at the panel? =:O

Or do you just ASSume it's lame, and pull a new green/bare GND wire?

... what say ye RKIs???

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hexus

**sigh**.......

/exits thread

    Bookmark   October 2, 2009 at 7:40PM
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brickeyee

There are testers available to verify the quality (current capacity) of grounds.

They are expensive, but if you are really worried you can purchase one.

Makeshift testing runs the risk of causing fires when trying to test grounds that turn out to be poor.

The electronic equipment uses very brief and slowly increasing low voltage waveforms with current monitoring to avoid creating heating if the current flow starts to limit as the voltage rises.

Mine cost over $1000.

    Bookmark   October 2, 2009 at 9:30PM
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fixizin

LOL... that test equipment sounds a lot safer and more sensible. Will look into rentals.

hexus apparently missed the smilies... and sarcasm... or he went to check if his jumper cables are where he left them. :D

    Bookmark   October 3, 2009 at 4:53AM
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fixizin

... and would a waveform generator and oscilloscope be able to accomplish the same feat? (Albeit with more difficulty, but, if that's what's available...)

    Bookmark   November 27, 2009 at 10:39AM
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wired_lain

There are several types of equipment you can use:

1. Ground Bond Tester.
2. Digital Low-Resistance Ohmmeter or DLRO.
3. Loop Impedance Tester (used in Europe, not readily available in US).
4. Ideal SureTest Circuit Analyzer - some models measure ground impedance.

#4 is probably the most affordable of these.

    Bookmark   November 27, 2009 at 12:30PM
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billhart

You could start with a relatively inexpensive digital multimeter that measures 0.1 ohm resolution. If the resistance isn't in the tenths, you KNOW you have a problem.

Turn off the power to the circuit, and verify near 0 volts AC between hot and neutral, and neutral and ground.

Switch the meter to ohms. Note the reading when the meter probes are shorted together (might be a little more than zero). Measure resistance from ground to neutral and subtract the value from shorted leads.

Note that if you get a hot to ground short, you have 120V divided by perhaps 0.1 ohm, so you get 1200 amps. That's good, because it trips the breaker fast, before much heat accumulates. If you have 0.5 ohm, that's 240 amps and the breaker will be slower (but still less than a second?), while 29 kw is heating up the bad connection. That could be dangerous. If you have 6 ohms and only get 20 amps, the breaker won't trip for hours and meanwhile you are dumping 2400 watts somewhere you don't want it and are guaranteed to start a fire.

    Bookmark   November 27, 2009 at 2:36PM
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brickeyee

"2. Digital Low-Resistance Ohmmeter or DLRO. "

Or any small ohm meter are not adequate.

The ability of the ground to carry current is what is important, and measuring resistance does not tell you if the ground path can current enough current to trip a breaker.

A correct bond tester should be used.
It applies a slowly rising voltage and measures the current that is flowing.
As soon as it hits the desired current it switches off to prevent excessive heating.
It also takes the voltage off if the current decreases at any time.

The most casual metalic path can produce a low resistance connection.
When a significant amount of current starts to flow the connection can simply act as a fuse and open, removing the ground path completely.

Before the bond testers became easily available, all sorts of equipment was used to verify ground quality.

A variac and an ammeter can be used.
You slowly turn up the variac until the desired current flows.
Since you will never be as fast as electronics at shutting the current off, it can create risk.

    Bookmark   November 27, 2009 at 4:14PM
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fotostat

If you really care about it, why not just run a grounding conductor? It should be easy, get a fiberglass snake and pull a ground in each pipe. Or use one of the conductors that is already in the pipe, such as the white wire, to pull a new white wire and a ground wire. Simple, straight forward, and it will give you the peace of mind that you are looking for.

    Bookmark   November 27, 2009 at 5:28PM
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fixizin

... excellent points to cogitate upon. True that an Ohmmeter sources only mA to test a circuit, but it's a great STARTING point, as a high reading there would obviate ANY higher current testing, automated or not.

My waveform generator idea would have to drive a J-FET or similar power amplifier device. At that point I'm jury-rigging something when I could rent/borrow a proper version. Just needed to know it's name(s) in the trade.

Meanwhile, I have no tangible reason to doubt the integrity of any of the conduit, aside from some of it being 50+ years old, and the rest of it 25+ yrs. old. Even though ~200 yds. from Atlantic coast, NO visible rust anywhere, even j-boxes (yet), exterior or interior. (Apparently, a perpetual accumulation of latex paint IS salt spray-proof! ;')

If you really care about it, why not just run a grounding conductor?

Short answer: human nature + $$. On the one hand, running a separate GND appeals to my belt+suspenders side, but OTOH, it offends my thrifty+lazy side, lol.

    Bookmark   November 29, 2009 at 4:32PM
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