What is the highest acceptable ohms reading for "bo

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What Is An Acceptable Bonding Resistance Reading on a meter? Or Better yet, what is the highest resistance reading one can get and still consider a bonding point properly bonded?

More Insight to my question:

Here at our place of work, we have an area that has water pipes and exposed structural steel; one of my many tasks is to verify that all pipes and exposed structural steel are still properly bonded together and to ensure that any bonding clamps and cables have not lost their bonding integrity thus compromising the safety of employees working in the area.

And ultimately to verify that all is bonded and connected back to the ?I-beams? that are encased in concrete since these ?I-beams? are grounded to the building service. (I hope I?ve painted a good enough picture for you to understand where I?m going with this).

Anyway, I?ve verified all pipes and exposed metal is bonded back to the concrete encased eye-beams using a DLRO meter thus the reason for my question of how high of a resistance reading can I expect to get on my meter and still accept any bonding point as being ?properly bonded?? and well within the NEC code.

Is there a "set" industry standard that clearly states what ?that? acceptable value should be?

If there is, please refer me to it as I would like to acquire it firsthand and share it with my colleagues and co-workers.

I would appreciate everyone's input as well as that of other electrical engineers who might have a sound insight to the subject hand at matter.

Thank you all, and God Bless you all.

Roberto Clemente

[ December 08, 2005, 05:43 PM: Message edited by: charlie b ]
 
Re: What is the highest acceptable ohms reading for "bo

The answer is thus:

1. Determine the largest overcurrent device that is likely to energize a specific EGC at a specific point.

2. Determine the maximum impedence that could exist and still cause the OCPD reliably open on a short circuit (this is typically about 6X the setting of a breaker) at that point. Ohms law will tell you this with a slight correction skin effect (especially on larger conductors).

3. Measure the EGC impedance from the OCPD to the specific point in question. If it is less the the value determined in step 2, than you are OK.

The problem is that you have to check EVERY point to be sure you are OK, something that is physically impossible.

Best bet is to just accept that the people that wrote the code knew what they were doing and be sure the code requirements for bonding are followed AND that the installation is done properly and is not corroded.

Not all that long ago (only 15 years or so ago), I was at an auto plant where a short circuit occured in a control panel. The OCPD never tripped, even though the resulting fire destroyed a control panel.

Eventually someone decided the reason for the fire was an inadequate bond between the control panel and the ground bar. The copper bar had been installed without removing the paint under the bar and somehow the unplated screws used to fasten it to the sheet iron had corroded so badly there was virtually no electrical continuity between the copper bar and the sheet iron.
 
Re: What is the highest acceptable ohms reading for "bo

In my opinion the most important part of the bonding system is the equipment grounding conductor. The bonding of the steel and piping systems, have very little to do with electrical safety and should never be a significant part of the fault clearing path.
Don
 
Re: What is the highest acceptable ohms reading for "bo

The bonding of the steel and piping systems, have very little to do with electrical safety and should never be a significant part of the fault clearing path.
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This has become true in the last 25 years or so, if you are using the piping and steel as electrodes to return electricity to its source. In the 1960's and earlier, city water systems were fabulous grounding grids and clearly that is much less the case today.

I would suggest that bonding of building steel and water pipes, if not effectively grounded on their own, would be critical if those parts became accidentally energized.

This may be a misuse of the term bonding by Don's standards, I'm not sure.

Jim T
 
Re: What is the highest acceptable ohms reading for "bo

Jim,
In the 1960's and earlier, city water systems were fabulous grounding grids and clearly that is much less the case today.
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Very true, but even then the fault return path should be via the EGCs, main bonding jumper and the grounded conductor.
Don
 
Re: What is the highest acceptable ohms reading for "bo

In regards to the original question (and a good question, at that), Ive never heard of a max imp. reading for a bonding connection. One would think there should be. If there is, i would like to know. If there is not, i would megger the connections and use my best judgement.
 
Re: What is the highest acceptable ohms reading for "bo

My reason for this question is because according to UL, the bonding of any piece of metal which is attached to, or part of a luminaire must not exceed a bonding "resistance / impedance reading of more than .020 ohms and someone within our department here decided that "NO BONDING POINT" should exceed 30 milli-Ohms (.030) and I personally think it is to stringent and not completely a realistic value. Certainly not when you take into account that there are different types of metals (within our environment) that are being bonded together.

My desire (if possible) is to find an acceptable and more realistic value that applies otherwise, by the current standards, we would have to fail a number of items that are reading values greater than 30 milli-ohms but less than 1 ohm. When you consider that companies are spending thousands upon thousands of dollars trying to achieve values of 3ohms or less, I think WE are doing quite well.
 
Re: What is the highest acceptable ohms reading for "bo

Tom:

A megger would not give a good indication of connection resistance the scale is set for Megohms.

Typically an earth ground resistance tester, sometimes called an earth megger is utilized to verify low resistance between different parts of an electrical distribution system. A straight connection resistance can be measured utilizing a Ducter/micro-ohm meter/DLRO (digital low resistance ohm meter).

In answer to the original question AS LOW AS POSSIBLE. Properly cleaned mating surfaces, properly sized hardware, installed per manufacture recommendations, torqued to proper value.

[ December 12, 2005, 02:58 PM: Message edited by: brian john ]
 
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