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Bonding question-continuity and resistance

Bluedog225

Bluedog225

Texas
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Nov 18, 2019
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I’m wondering how one can ensure that bonded electrical components are functioning as intended. Specifically, I can imagine measuring continuity between two points, and low resistance, when, in fact, the two points are not sufficiently electrically conductive to provide the path of least resistance to ground.

In other words, everything can be bonded together but one of the connection points could be loose, or corroded.

How would you know? Is the resistance as measure by a multimeter linear at higher current? I can imagine a big, corroded bonding lug passing the small current of a multimeter easily but failing to protect at higher voltages and currents in a short.

I hope this makes sense. I’d like to know generally and specifically as I’m using a large, multipart file cabinet to house a battery and inverter. Doors, drawers, various connected pieces, etc. Simple continuity seems inadequate.

Thanks
 
My gut feeling is that bigger currents will face less resistance on like like a loosely assembled conduit than the multimeter would, cause the bigger current will get some arcs going through the loose connections.

I did EMT this weekend and was disappointed to find the end of the conduit not beeping to ground and neutral, it was reading about 1 ohm.

I did later find an untightened set screw earlier in the run.

But I was running a ground conductor too. I would expect that conductor to beep to neutral for sure and it did.
 
I was thinking a large current might burn away the bit of conductive material that was allowing the circuit to read as continuous. Like a filament on a bulb burning out.
 
I think both things can be true in different circumstances. What bluedog described is very common on loose battery connections on cars, where once you try to flow a few hundred amps to the starter motor the tiny bits of metal that are actually conducting fry, and then the whole thing goes open circuit and the car has NO power until someone messes with the battery terminals again.

I think HWY17 is referring to 'dielectric breakdown' but im not sure that actually occurs at 120v or not.. i just dont know enough to say. But it is certainly a thing at higher voltages that can arc across air gaps.

So i think the premise of the initial question is sound, ie you can totally measure low resistance across a path that can't handle high current. I think the caveat is that IF the circuit was to actually 'burn open' then there would no longer be any risk to touching it.. i.e. to get shocked in split phase you have to be in series with a 'path back to source' which means you have to touch one line and either a. the ground/neutral, or b. the other line.

If you are only touching line and ground and the ground 'conductor' goes open, you will no longer be shocked! Downside is, instead of tripping a resettable circuit breaker you've probably done some actual damage in an unknown location and now have to find and fix THAT.
 
I was particularly interested in the grounding conductor opening in the event of a large fault. And the current going through me to ground instead of through the grounding system. Continuity seems like a false assurance.

Maybe in reality, it’s fine.
 
robbob2112 said:
What sort of meter do you use to test that a ground rod is installed correctly?
Click to expand...
You can't verify how it's installed, without pulling it back out.
But you can test it's connection to the earth. (Which is all that matters)
A good quality digital multimeter, is trustworthy.
You will have to drive a second rod, and measure the resistance between the two. Usually at 50', and 100'. 25 ohms or less is the goal.

Edit to add.
Soil composition is always the enemy.
 
timselectric said:
You can't verify how it's installed, without pulling it back out.
But you can test it's connection to the earth. (Which is all that matters)
A good quality digital multimeter, is trustworthy.
You will have to drive a second rod, and measure the resistance between the two. Usually at 50', and 100'. 25 ohms or less is the goal.

Edit to add.
Soil composition is always the enemy.
Click to expand...


OK, so no particular meter.... but the wire between the two rods would have resistance as would the meter leads. Do you just string an insulated wire and subtract the resistance of the wire to get your figure or is that small enough that it doesn't matter in the overall measurement?
 
timselectric said:
Hint, it's very rare that you will get an acceptable measurement.
Click to expand...


Ok, thx. I just got a 1035+ 4 wire meter in the mail yesterday and like a kid with a new toy I am measuring all sorts of things. Internal resistance of my 12v200ah battery, across cables, through fuses, etc. A new toy that will provide endless fun until I am bored with it.

And of course it made me wonder how they measure the ground rod resistance.
 
robbob2112 said:
Ok, thx. I just got a 1035+ 4 wire meter in the mail yesterday and like a kid with a new toy I am measuring all sorts of things. Internal resistance of my 12v200ah battery, across cables, through fuses, etc. A new toy that will provide endless fun until I am bored with it.

And of course it made me wonder how they measure the ground rod resistance.
Click to expand...
I love getting new toys (I mean tools).
 
We use a ground bond tester, about 10A for seconds or minute, to check chassis grounding. That's for electronic chassis and the like.

Sometimes, difficulty is contact of the probe because anodized aluminum. But screws provide contact.


Some things break down at higher voltage. Not what you want to rely on for safety ground, but I've had old switches that didn't show continuity on DMM, worked fine at 120VAC.

Skin is an interesting one. About 200kohms according to DMM, but apply 240V and the skin as dielectric breaks down leaving 500 or 1000 ohms for wet internal tissue, carries 1/4 to 1/2 amp. (ask me how I know. Well, not really, I didn't have an ammeter in series. But it better explains physiological effects than 200k ohms would.)
 
Hedges said:
We use a ground bond tester, about 10A for seconds or minute, to check chassis grounding. That's for electronic chassis and the like.

Sometimes, difficulty is contact of the probe because anodized aluminum. But screws provide contact.


Some things break down at higher voltage. Not what you want to rely on for safety ground, but I've had old switches that didn't show continuity on DMM, worked fine at 120VAC.

Skin is an interesting one. About 200kohms according to DMM, but apply 240V and the skin as dielectric breaks down leaving 500 or 1000 ohms for wet internal tissue, carries 1/4 to 1/2 amp. (ask me how I know. Well, not really, I didn't have an ammeter in series. But it better explains physiological effects than 200k ohms would.)
Click to expand...


I've done the test of 30kv across a flyback transformer in an old radiation king TV. Burned an entry hole at my finger tip and an exit hole at my first knuckle on that finger. My arm vibrated for a few hours.

And who hasn't touched an o'scope probe to their skin while under flourescent lights? 60~80v ac... no current past it.
 
There is a meter specific to test ground rod resistance. I was thinking there was wording in the code that the ground should be 25 ohms or less and if not you need to drive another rod. Cheaper to install two rods all the time rather than the cost of a tester.
 
Yup
I couldn't care less about how conductive the soil is.
I install the required electrodes and connect them to the grounding system.
And move on to the next item on the list.
If the soil isn't conductive enough for an OCPD to clear the fault. Then it's not conductive enough to be a hazzard for people.
No point in worrying about a problem that doesn't exist.
 
Dry soil: 2-3 rods down (or sideways) to ensure low ohm resistance.
Under 25ish is good.
Always wet soil: one normally suffices.
 
f the soil isn't conductive enough for an OCPD to clear the fault. Then it's not conductive enough to be a hazzard for people.
No point in worrying about a problem that doesn't exist.
Click to expand...
This is how i generally feel about it, although my knowledge of the subject is sparse.

Ground rod discussions also seem sort of silly in the context of my mobile home which has 10 or 12 huge steel auger screws tying the steel frame of the structure into the dirt. Not only are there a bunch of ground points but a crapload of surface area on each point. So discussion of ground loops and such has always seemed sort of odd coming from that context.
 
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