Effective? Ground Fault Current Path

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iwire said:
Ryan I don't think I am struggling at all. :)
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You know I didn't mean it that way :)

I am going to change all the speed limit signs around my house to 'Don't go too fast' and each inspector..(opps).. I mean cop can decide what is to fast.

That should work well. :D
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It seems to work well in some European countries.
 
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Bob,
That would do it for sure. However tests have shown 4 0r 5 times the OC rating will do it.
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Where do those numbers come from? I would expect that a 1 cycle trip would be in the magnetic trip range and not the inverse time trip range. The lowest mag trip point on any standard breaker currently on the market is 8 times the handle rating. Most are in the 10 to 13 times rating with one at 40X.
Don
 
iwire said:
Pierre I am not going to convinced an open ended section can be enforced.

I say I have provided an effective ground fault path, the inspectors says I have not.

Who's right?

Who can tell?

The NEC has not told me the perimeters of the requirement.


JMO, Bob
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Bob
This is very true, and I guess it could be a potential issue for some inspectors. Most inspectors are not engineers and the math in this industry is not their best friend ;) .

I think that Dereck's last post is good for those who are willing, but for the average installing, not much is going to change due to our thread.
 
I started thinking about this problem when reading Soares. The contention there is that an effective ground fault current path is one in which the short circuit current is 8 times the trip rating of the breaker, to place the breaker in the 'instantaneous magnetic trip' regime.

I see no support in the code for the 8x trip rating goal, rather than 6x or 10x or 0.1x with ground fault detection. This discussion has opened my eyes to even more issues, including the fact that if the goal is 'instantaneous magnetic trip' some breakers would require far more than 8x trip rating.

IMHO a mandatory voltage drop requirement would _not_ be sufficient in all cases; it would certainly work for small circuits where the EGC is as large as the circuit conductors, but once the EGC is smaller than the circuit conductors, ground fault current starts to become less than 'voltage drop limited short circuit current'.

I pretty well have to side with iwire on the enforceability issue. Without any specification for just what constitutes an effective ground fault path, about the only thing that these code sections actually require is that the EC _think_ about the issue and decide that the ground fault path is effective.

The discussion has also answered my original question. An EGC that meets all of the explicit code requirements by which is not 'effective' would probably pass; an EGC that does not meet explicit code requirements but which is effective would _not_ pass.

Back to the issue of AC cable without the bonding strip, this would _not_ be a suitable equipment grounding conductor for extending a circuit, even if the AC cable is short enough for the armor to constitute an effective ground (by any reasonable standard, eg fault current of 8X trip rating).

-Jon
 
Jon, all modern AC cable has a bonding strap along it's length, I guess my question would be, what are the properties of the sheath and strap combined, ohm/kft? This would give us something to use in a calculation.

Roger
 
winnie said:
I pretty well have to side with iwire on the enforceability issue. Without any specification for just what constitutes an effective ground fault path, about the only thing that these code sections actually require is that the EC _think_ about the issue and decide that the ground fault path is effective.
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The code is pretty clear that you have to provide an effective ground fault path, and goes on to state what an effective ground fault path is (what it takes for the bc protection to open). What is unclear is the time frame of the fault clearing. There are other requirements you have to meet such as installing wire in accordance with the manufacturer's instructions. Every piece of wire comes with data on just how much current it can take and for how long before it is damaged. You can't claim you are code compliant by meeting some of the code. You have to meet it all. Thus you cannot have an install where the EGC is inadequate to protect the conductors.

The discussion has also answered my original question. An EGC that meets all of the explicit code requirements by which is not 'effective' would probably pass; an EGC that does not meet explicit code requirements but which is effective would _not_ pass.
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In practice you are correct, but I am not convinced the code suppports this conclusion taken in its entirety.

Back to the issue of AC cable without the bonding strip, this would _not_ be a suitable equipment grounding conductor for extending a circuit, even if the AC cable is short enough for the armor to constitute an effective ground (by any reasonable standard, eg fault current of 8X trip rating).

-Jon
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The reason regular AC is not considered an effective ground fault path is not because the armor cannot carry the current, because it can certainly do so. It is because there is no guarantee that the path is continuous
 
petersonra said:
The reason regular AC is not considered an effective ground fault path is not because the armor cannot carry the current, because it can certainly do so. It is because there is no guarantee that the path is continuous
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Bob type AC cable is considered an effective ground fault path.

Check out 250.118

The only question that remains is for how long a run does it remain 'effective'

I agree there are other sections of code that need to be followed.

That is kind of my point, 250.4(A)(5) is not one of them. ;)
 
ryan_618 said:
Do you have an exception somewhere that I have never read?
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No I don't.

I don't have one for "Neat and Workmanlike" either but the NFPA admits that is vague and possibly unenforceable.

Let me elaborate on Roger's question.

At what length of run would a 12/2 AC cable supplying a 20 amp 120 volt circuit become an ineffective ground fault path?

At what footage does the installation become a 250.4(A)(5) violation?

How do you determine that?

Even 110.10 is left a bit vague 'to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit.'

What is extensive damage?

It sounds like more than a bit of overheated insulation.
 
don_resqcapt19 said:
Bob,
Where do those numbers come from? I would expect that a 1 cycle trip would be in the magnetic trip range and not the inverse time trip range. The lowest mag trip point on any standard breaker currently on the market is 8 times the handle rating. Most are in the 10 to 13 times rating with one at 40X.
Don
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Don
I was using a study done by Ga. Tech University. I re-read the study and
I could not find a reference to the operating time of the breaker. After looking at some breaker curves I agree with your reference to 10 to 13 times the breaker rating.
 
Bob wrote

" agree there are other sections of code that need to be followed.

That is kind of my point, 250.4(A)(5) is not one of them."



This may be a tough section to enforce, but I believe that it is one of the most important sections in the NEC to try and comply with, even if it means doing more than what the NEC requires to try and reach the goal.
 
Pierre,
This may be a tough section to enforce, but I believe that it is one of the most important sections in the NEC to try and comply with, even if it means doing more than what the NEC requires to try and reach the goal.
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how do we do this and how would a inspector call foul with AC cable if there are no values to go by?

In a conduit or tube a conductor can be installed, but if AC is used the installer is stuck, and it could be quite an expense to completly rip out and replace an installation that has no writen rules (numbers for calculating compliance) to follow.


Roger
 
I need to amend my stance a bit. While this bit of code may be difficult or even impossible to enforce, I want to distinguish 'how hard it is to enforce' from 'how valid or important is it'. I believe that these code sections address an important issue. I believe that these code sections would be improved by having specific performance guidelines, however at some point one needs to trust human judgement.

I believe that this comes down to a judgement call by the EC. I don't see how an inspector could enforce any particular values of trip time, conductor damage, or elevated voltage in materials bonded to the EGC, but I believe that it would be entirely reasonable for an inspector to ask 'What happens if there is a short circuit on this circuit? How quickly do you expect the breaker to open? Do you expect any damage?' I believe that lack of even considering this issue would be grounds for a red tag, just like not doing a load calculation; but that with the necessary calculations if the EC simply says 'I conclude that its good enough.' then I don't see how the inspector could fail the installation.

iwire brought up the example of speed limits. Massachusetts, like many other states, has a 'Basic Speed Rule'
http://www.mass.gov/legis/laws/mgl/90-17.htm
http://www.mass.gov/rmv/dmanual/chapter4.pdf

In a nutshell, no matter what the posted speed limit is, you are speeding if you are going too fast. And I'm sure that we can come up with situations in which we would all agree that a reasonable and prudent speed is far less than the posted speed limit, but there are likely many grey areas. Rather difficult for a cop to enforce :) yet important because the law _cannot_ describe every single possible situation, and at some level has to trust people simply to be reasonable. At a practical level, this sort of law would never be enforced by a speeding ticket, but instead would come up in a lawsuit and only if there were an actual accident.

For what its worth, marketing literature that I've seen makes the claim that the armor and bonding strip of AC cable has lower resistance than the required equipment grounding conductor if the EGC were wire sized by 250.122.

-Jon
 
For what its worth, marketing literature that I've seen makes the claim that the armor and bonding strip of AC cable has lower resistance than the required equipment grounding conductor if the EGC were wire sized by 250.122

I've measured some pretty low gnd resistance with a SureTest on branches done in aluminum armor AC. That was using expensive AMC-50/SpeedLock type connectors though.
 
Here is some data for you to think about. I ran some numbers for a very common circuit, your Plain Jane 20-amp circuit using 12-2 with G. Following is the results of the maximum distances, X fault current and expected voltage drop with 16-amps load current.

93 feet, 16 X fault current, 5% VD
150 feet, 10 X fault current, 8% VD
250 feet, 6 X fault current, 13% VD

From the numbers, on a 20-amp circuit, you do not have much to worry about until you push beyond 250 feet with respect to fault current impedance. Like I said if you design with a max 5% voltage drop, who cares (assuming P & G are the same size).
 
Dereck

I am assuming those numbers are for the length of cable run. Does it account for junction boxes, fittings and splices????


I am not taking the stance that this is enforceable. I could not even begin to think how to enforce something like this. What I am saying is that the Effective Ground Fault Current Path is something that is overlooked sometimes and is very important. It would not hurt to have this dealt with in a better fashion... how is a different story.
 
Pierre C Belarge said:
Dereck

I am assuming those numbers are for the length of cable run. Does it account for junction boxes, fittings and splices????


I am not taking the stance that this is enforceable. I could not even begin to think how to enforce something like this. What I am saying is that the Effective Ground Fault Current Path is something that is overlooked sometimes and is very important. It would not hurt to have this dealt with in a better fashion... how is a different story.
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how about every ground wire installation has to have a PE seal on a set of plans before a permit is issued?

:)
 
Interesting discussion.....I have located what could be a problem in my inspection jurisdiction as a result of a local electricians complaint.

There is a new rehab facility being built that I was unaware of. I'm not sure if it is in my jurisdiction, as it is smack on the line bewtween county and city. I went by there today to take a look and the elecrical contractor has apparently decided that none of it qualifies as a healthcare facility and has used tons of MC cable....12/2wg. He has also used about 100 NM connectors to attach it to metal boxes. About half are the correct rated connector.

My question is, assuming this is indeed my jurisdiction and they did not pull a permit, would not some portions of this facility where machines, tubs and therapy devices are located be classed as a health care area and then be subject to a redundant ground path, which MC does not fill? In fact it fails the healthcare requirement because of the weakness in the ground path.

This exact topic regarding a Rehab facility has never come up before, and if anyone has some guidance, it would be appreciated before I look into this on Monday.
 
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dereckbc said:
Here is some data for you to think about. I ran some numbers for a very common circuit, your Plain Jane 20-amp circuit using 12-2 with G. Following is the results of the maximum distances, X fault current and expected voltage drop with 16-amps load current.

93 feet, 16 X fault current, 5% VD
150 feet, 10 X fault current, 8% VD
250 feet, 6 X fault current, 13% VD

From the numbers, on a 20-amp circuit, you do not have much to worry about until you push beyond 250 feet with respect to fault current impedance. Like I said if you design with a max 5% voltage drop, who cares (assuming P & G are the same size).
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dereckbc
Are the fault current figures( 16 x fault curent) 16 x the OC device rating?
And is this caculated as a bolted fault or and arcing fault?
 
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