The Sufficiency of NEC rules

[GoldDigger]

The other sequitur is: the sign of insanity is repeatedly doing the same thing expecting different results....

Extremely erudite! This is the first time I have seen sequitur used without the "non".

BTW, since "non sequitur" translates as "It does not follow", I am fond of throwing in "nolite sequamur", which I translate as "let's not follow", before going off on a tangent.

 

[Besoeker]

Inability to follow the thread. What a pity.

Indeed. It happens. And you are not the first.

 

[weressl]

.......I am fond of throwing in "nolite sequamur", which I translate as "let's not follow", before going off on a tangent.

....an another appropriate response to this thread.

 

[kwired]

kwired:
The reason the topic from another thread is brought here is the same as why this was started in the first place: to know about the sufficiency NEC rules.Also the OP of that thread specifically states there is no receptacle in the balcony. I found the first reply from Don not mentioning about the simultaneous touching of exposed metal part of a receptacle and grounded balcony metal railing and the risk of shock associated with. So I thought the code rule could have a word about it and so brought it here.

So your quoting of NEC rule below

indicates there is no mention in the Code about the risk associated with 'simultaneous touching' of exposed non-current carrying metal part of an electrical system and any other adjacent extraneous grounded metal other than what is mentioned in 250.104 above. Isn't?

Quite frankly I'm not even sure I understood what you are trying to say here. My point is that any metal object is going to be conductive and if introduced into the right situation can create a shock hazard. 250.104 mentions some common things that apparently have been a problem before, and maybe are somewhat "likely to become energized" or "likely to be grounded" even if not intentionally bonded. You seem to bring with you the idea that every imaginable metal object needs to be bonded "just in case" Do you realize how difficult that concept is to practice.

 

[Sahib]

Quite frankly I'm not even sure I understood what you are trying to say here.

I understand. I want to say faith will come to you as you keep on trying.

My point is that any metal object is going to be conductive and if introduced into the right situation can create a shock hazard.

Yes. Correct.

250.104 mentions some common things that apparently have been a problem before, and maybe are somewhat "likely to become energized" or "likely to be grounded" even if not intentionally bonded.

The problem here is when things such as metallic hand railing is liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.as it is not effectively connected to the EGC, it may create a hazard.

You seem to bring with you the idea that every imaginable metal object needs to be bonded "just in case" Do you realize how difficult that concept is to practice.

No. Obviously, you can not bond movable objects.
For fixed metal objects of considerable extent, other than those mentioned in 250.104, you need to check up the operation time of the upstream OCPD and depending it, you could easily decide whether the bonding is required or not in a residence.

 

[kwired]

The problem here is when things such as metallic hand railing is liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.as it is not effectively connected to the EGC, it may create a hazard.

So is a concrete slab on grade, and we generally don't bond to those - except special cases like around swimming pools.

The only thing bonding the railing will protect you from is if there is a low impedance fault to the railing, then the overcurrent device of the faulted circuit is what protects you. Otherwise if you bring an exposed ungrounded conductor within reach of any grounded metal object you still have a risk of shock. Drag an extension cord out to a swimming pool area where all metal objects are usually well bonded and you still can be electrocuted because the ungrounded conductor is still ungrounded. We do however try to compensate having the need to drag the extension cord out there by requiring certain receptacle placements and GFCI protection of those receptacles or other specific circuits at times. NEC still will not stop someone from dragging a cord in from elsewhere if that is what they want to do.

 

[Sahib]

The only thing bonding the railing will protect you from is if there is a low impedance fault to the railing, then the overcurrent device of the faulted circuit is what protects you. Otherwise if you bring an exposed ungrounded conductor within reach of any grounded metal object you still have a risk of shock. Drag an extension cord out to a swimming pool area where all metal objects are usually well bonded and you still can be electrocuted because the ungrounded conductor is still ungrounded. We do however try to compensate having the need to drag the extension cord out there by requiring certain receptacle placements and GFCI protection of those receptacles or other specific circuits at times. NEC still will not stop someone from dragging a cord in from elsewhere if that is what they want to do.

Unfortunately, you do not seem to grasp the last para of my last post, the gist of which is below:

Suppose a ground fault takes place to the exposed metal conduit of the receptacle in the balcony with nearby metallic hand railing not bonded to the EGC, but, nevertheless, at ground potential. Also suppose at the same instant a person touches simultaneously both the metal conduit and the metal railing. Will he be electrocuted? No if the upstream OCPD of the receptacle operates for example within 0.4 sec to remove the voltage on the conduit. Suppose the OCPD takes longer to clear the fault. What will then happen? The person is liable to be electrocuted unless the conduit is bonded effectively to the railing in which case there is negligible difference of potential and so no hazard.

 

[kingpb]

Plane and simple the NEC does not cover step and touch potentials, period. So, any discussion in that regard, if carried out in the context of meeting the NEC, is superfluous.

Therefore, this post has been answered and further discussion regarding earthing, bonding, and step and touch potentials should be moved to the grounding forum.

Cheers~

 

[kwired]

Unfortunately, you do not seem to grasp the last para of my last post, the gist of which is below:

Suppose a ground fault takes place to the exposed metal conduit of the receptacle in the balcony with nearby metallic hand railing not bonded to the EGC, but, nevertheless, at ground potential. Also suppose at the same instant a person touches simultaneously both the metal conduit and the metal railing. Will he be electrocuted? No if the upstream OCPD of the receptacle operates for example within 0.4 sec to remove the voltage on the conduit. Suppose the OCPD takes longer to clear the fault. What will then happen? The person is liable to be electrocuted unless the conduit is bonded effectively to the railing in which case there is negligible difference of potential and so no hazard.

I fully understand that, but you have modified the OP's (from the thread in which this balcony situation started) installation. He had no conduit or receptacle in the balcony or railing, the balcony was supposedly insulated from ground, If the balcony or railing should become energized somehow there is nothing grounded in the vicinity to create much of a risk of shock.

To have a shock risk doesn't matter what is or is not grounded, what really matters is what is possible to have a difference in potential.

We bond things to try to minimize differences in potential, but introducing ungrounded conductors to the vicinity still increases the risk of shock.

 

[Sahib]

you have modified the OP's (from the thread in which this balcony situation started) installation.

To suit the purpose of this thread.

We bond things to try to minimize differences in potential, but introducing ungrounded conductors to the vicinity still increases the risk of shock.

You already stated it is not practicable to bond all metallic objects.
I gave you a criterion for bonding of any fixed metallic object.
What else is required?

 

[jim dungar]

What else is required?

The likely hood of coming in contact with an energized conductor, and any likely resultant injury.
Remember the NEC does not use the word possible.

Article 517 has very specific details for bonding some conductive surfaces, including window frames, however the code making panels saw no reason to extend these requirements to other general locations.

 

[Sahib]

Plane and simple the NEC does not cover step and touch potentials,

Cheers~

No, 250.104 and other relevant articles are for that purpose, IMO.

 

[Sahib]

The likely hood of coming in contact with an energized conductor, and any likely resultant injury.

Refer to the accident statistics if you have any.

Remember the NEC does not use the word possible.

They try to make you think statistically, guys.

Article 517 has very specific details for bonding some conductive surfaces, including window frames, however the code making panels saw no reason to extend these requirements to other general locations.

So?

 

[kwired]

To suit the purpose of this thread.

You already stated it is not practicable to bond all metallic objects.
I gave you a criterion for bonding of any fixed metallic object.
What else is required?

If you have a home with metallic door or window cladding would you run bonding conductors to every door and window cladding? How about metal roof gutters and downspouts that are otherwise electrically isolated? Not every metal object has a very strong need to be bonded to the electrical grounding system.

 

[Sahib]

If you have a home with metallic door or window cladding would you run bonding conductors to every door and window cladding? How about metal roof gutters and downspouts that are otherwise electrically isolated? Not every metal object has a very strong need to be bonded to the electrical grounding system.

If not stated in the NEC explicitly, I would rather use the criterion stated earlier to make a decision.

 

[kwired]

If not stated in the NEC explicitly, I would rather use the criterion stated earlier to make a decision.

If not stated in the NEC explicitly, then there is no requirement. Design issues are just that - design issues. I do not work on cell phone towers, all the grounding and bonding that is done at those sites and is frequently mentioned on this site is just design practice common to the application and not any NEC requirement. Same can probably be said for many other specific applications.

 

[Sahib]

If not stated in the NEC explicitly, then there is no requirement.

Correct. It is no requirement. It is discussed to decide whether there is any need to make it a requirement.

 

[kwired]

Correct. It is no requirement. It is discussed to decide whether there is any need to make it a requirement.

Well my answer (probably back in the first 10 posts in at least one of the threads that was incorporated into this one) was no there is no need to make all metal objects bonded to electrical grounding system.

You can throw literally millions of "what if's" at any installation and never design something you can positively say will never have potential for injury or property damage in some kind of failure.

 

[Sahib]

Well my answer (probably back in the first 10 posts in at least one of the threads that was incorporated into this one) was no there is no need to make all metal objects bonded to electrical grounding system.

Neither did I say that there was any need to make all metal objects bonded to electrical grounding system.

You can throw literally millions of "what if's" at any installation and never design something you can positively say will never have potential for injury or property damage in some kind of failure.

The scope of discussion is much more limited: whether the criterion is worthy of becoming a requirement.

 

[kingpb]

No, 250.104 and other relevant articles are for that purpose, IMO.

A few things:

#1; Is an opinion I have, but will keep it to myself.
#2; Plane and simple, the NEC does not ever mention step or touch potentials.
#3; "other relevant articles" means nothing to me. Either state which articles you are using to support your claim, otherwise your opinion does not hold water
#4; Engineered grounding systems are required for electric generating stations, substations, switchyards, and petrochemical facilities especially, and any other location where on-site generation is concerned. Grounding systems for these facilities are governed by IEEE 80, IEEE 665, and the NESC. The NEC primarily governs grounding systems for industrial, commercial and residential; without generation.

Minimum design inputs that are required for an engineered system (no were covered in NEC) :

• Soil resistance or resistivity data.
• Single line-to-ground and double line-to-ground fault current magnitude and phase angle per contributing source, including Ia and 3Io values and X/R ratio. The fault study shall include all on-site buses 13.8 kV and above, including any auto-transformer contributions (as applicable). Each fault study should include all modeled levels within the facility and at least one level outside the facility per transmission line. For projects that include local generation, the fault study shall include two fault scenarios, one with all local generation "on" and one with all local generation "off."
• Backup breaker clearing time, based on documented confirmation from either the client or the Responsible Protection Engineer.
• One-line diagram, including all electric generating units and transmission and distribution lines.
• Site and plant arrangements.
• Client's grounding standards, if any.

The following needs to be considered:

• Fence location and grounding strategy, e.g., temporary versus permanent.
• Location of motorized gate operators or other equipment that could unintentionally ground the site fence.
• Location and extent of insulating surface layers, e.g., crushed rock, asphalt.
• Required electrical properties of surfacing material, e.g., minimum resistivity.
• Location of roadway light poles and grounding method.
• Above grade pipe routes, particularly in areas falling beyond the main power block.
• Requirements and considerations governing connections of services to off-site sources or destinations, such as fuel gas, telecommunications, power supply to off-site consumers, steam and/or condensate connections to off-site consumers, railroad spurs, etc.
• Construction power source and the grounding and safety philosophy.
• Foundation rebar bonding to ground grid and routing of ground conductors within and around foundation rebar cages.

In the case of a ground fault, protection should ensure safety for personnel by either automatic disconnection of supply, or by the limitation of the touch and step voltages to acceptable limits; achieved by the following design objectives:

• Provide a low impedance ground fault current return path (in order to activate the protective relaying and clear the ground fault as soon as possible).
• Maintain safe voltages on station structures and accessible equipment during normal operation and electrical transients.
• Minimize noise interference in control and instrumentation systems.
• Minimize the effect of lightning surges to personnel, equipment, and structures.

It is necessary to provide safety grounding for structures, buildings, skids, and non-current carrying metal objects.

It is recommended to have an equipment ground and safety ground for electrical and mechanical equipment.

Structures that cannot become energized as a result of electrical equipment failure (structural support steel) needs only a ground grid connection.

Metal fencing needs to be grounded. The method will depend on the proximity to the ground grid and the transmission lines.

Finally, A serious hazard may result during a power system fault from the transfer of potentials between the ground grid areas and outside points, by conductors such as pipes, train rails, and communication and signal circuits. The danger is usually from touch type contacts. The touch potentials encountered may exceed the ground potential rise of the facility itself.