Ignorant questions from "quallified persons"

[xformer]

I have read many good replies to this OP. In my humble opinion, I am a student of the electrical industry and as the industry evolves, I will be required to learn more. I don't know everything nor will I ever. I am not the God of the electron. As an Instructor, or even a boss for that matter, one always wants people to know what they are doing. People can not really know what they are doing unless they ask questions. I have always had the mentality that I could learn something from everyone I worked with, or be refreshed on what not to do. If we want to see progress in our industry, we must allow the seemingly dumb questions to be asked.

 

[Strife]

Most people, even the "qualified ones" think in terms of Ohms laws. Very few know Kirshaw (damn I can't spell it right now, the one that says that sum of voltages in a a series circuit are 0, while the sum of ampacities in a parallel circuit are 0) law.
Most people think the EGC will somehow save them from electrocution, when in fact is only designed to trip the overload device. Again, looking at Kirschaw law would make it pretty self explanatory.

I would not include this among the ranks of "stupid questions." It is a good question, for anyone who has never come across the situation before, and who has not had a chance to think it through. I think a reasonable person could say that the N and G points are already connected together (i.e., at the service panel), so the two are electrically identical anyway. The reason that this reasonable person would be wrong in so saying is not an easy thing to come up with independently.

I will admit thinking, in my younger-and-more-foolish days, that it is safe to use an extension cord that is lying on wet grass. My reasoning at the time was that electricity seeks its source, and that every point along the extension cord is closer to the source than I am, so that its movement back to the source could not be in my direction. Then I read a news article about a woman in Chicago who was electrocuted by a damaged extension cord she was using outdoors. I had to draw a picture of the circuit and prove to myself that current could indeed find a path through a person holding a tool at the end of the cord, even though the damaged point was in the middle of the cord.

I will hold to the theory that the only stupid question is the one that is not asked.

 

[Rick Christopherson]

Most people think the EGC will somehow save them from electrocution, when in fact is only designed to trip the overload device. Again, looking at Kirschaw law would make it pretty self explanatory.

This is a common mistake I see on the forum quite often, including this thread. If the sole purpose of the EGC was clearing a fault and tripping the OCPD, then we wouldn't need both a grounded conductor and a grounding conductor, as they both are capable of clearing a fault and tripping the OCPD. The difference between the two is the voltage drop across a current carrying conductor.

The reality is that the EGC has several duties, and clearing a fault is only one of them. Another important factor that seems to get forgotten is that the EGC also maintains a voltage reference to surrounding equipment and structures.

This latter issue is why the original question was not entirely stupid (ignoring code for the moment). The potential exists for a floating chassis voltage that is not the result of a fault condition. An induced voltage on a chassis will not trip a GFCI, nor the OCPD, yet can still bite the end user.

By the way, the name you were searching for is Kirchoff.

 

[George Stolz]

Another important factor that seems to get forgotten is that the EGC also maintains a voltage reference to surrounding equipment and structures.

What benefit does that aspect of the EGC provide? I would call it a caveat of having an EGC, not a benefit. You're putting half of the circuit in the hand of the end user.

 

[Jim W in Tampa]

What benefit does that aspect of the EGC provide? I would call it a caveat of having an EGC, not a benefit. You're putting half of the circuit in the hand of the end user.

It is important that we keep all the frames of appliances such as range,frig,micro wave, dish washer at same potential so as to prevent a shock hazard should we touch one with left hand and other with right hand.
The EGC does this.

 

[K8MHZ]

Most people, even the "qualified ones" think in terms of Ohms laws. Very few know Kirshaw (damn I can't spell it right now, the one that says that sum of voltages in a a series circuit are 0, while the sum of ampacities in a parallel circuit are 0) law.

Do you mean Kirchoff as in KVL (Kirchoff's Voltage Law)?

"The directed sum of the electrical potential differences around a loop must be zero."

Then there is Kirchoff's Current Law.

"The sum of all currents entering a node is equal to the sum of all currents leaving the node." It is definitely not, "while the sum of ampacities in a parallel circuit are 0" Ampacity has nothing to do with Kirchoff's equations, it's a capacity rating for conductors, cables and devices and the term wasn't even in existence when Kirchoff was around.

I'll bet I could ask 100 electricians what Kirchoff's Law was and less than 10 would know.

Most people think the EGC will somehow save them from electrocution, when in fact is only designed to trip the overload device. Again, looking at Kirschaw law would make it pretty self explanatory.

What on Earth does that have to do with the trip rating of a breaker being in excess of the amount of current that would be able to electrocute someone?

 

[jap]

Some of the best questions come from home owners in the box stores.

" I need to hook up a A/C and need 600v wire which should I buy"? while holding a roll of 14-2 and 12-2


Am I missing something here?
NMB is 600v rated.

 

[pfalcon]

What benefit does that aspect of the EGC provide? I would call it a caveat of having an EGC, not a benefit. You're putting half of the circuit in the hand of the end user.

It is important that we keep all the frames of appliances such as range,frig,micro wave, dish washer at same potential so as to prevent a shock hazard should we touch one with left hand and other with right hand.
The EGC does this.

Depending on your environment the voltage difference between two metallic objects can be quite high. Industrial shops were notorious for inducing voltages in machine tools. Van de Graaff has nothing on a poorly wired shop.

 

[wbalsam1]

"Shocked" by attendees

"Shocked" by attendees

A few years back I conducted a ?Service Entrances? Power Point slide presentation for a mixed group of over 200 electricians, inspectors and building officials. About midpoint through the slides there were so many questions coming from a majority of the electricians in attendance that were centered around 250.32 (re-grounding at additional buildings and structures) that I actually loaded in a presentation on ?Grounding and Bonding? and had to go through 15 minutes of that presentation to ?refresh? the attendees on the purpose of the equipment grounding conductor. I was ?shocked? to discover how few understood the functions of an EGC or a grounded conductor and yet were actually out in the field, day after day, performing electrical installations admittedly in violation.

 

[zbang]

I suppose some of the problem is that many people do, but don't think. They never stop to consider why something is done a particular way, even if the answer is "Don't know, that's how we do it." At least now the questioner knows that the other person doesn't know, either, and might keep asking around.

 

[stevenje]

I'll bet I could ask 100 electricians what Kirchoff's Law was and less than 10 would know.

Wasn't that a TV show from the 70s?

 

[kwired]

I'll bet I could ask 100 electricians what Kirchoff's Law was and less than 10 would know./QUOTE]

I had a relative who married a person with the last name Kirchoff. She did not claim to know much at all about electricity:grin:

 

[glene77is]

I'll bet I could ask 100 electricians what Kirchoff's Law was
and less than 10 would know./QUOTE]

Kwired,
Ever read about Kirchoff's Second Law, the Voltage Law ?
Had not thought about it in a while, so I looked into Wiki for a clear statement
"The principle of conservation of energy implies that
The directed sum of the electrical potential differences (voltage)
around any closed circuit is zero."

I have had master ECs lecture me on the non-existence
of any variation of Kirchoff's Current Law.
In fairness, I should note that:
(1) When a technician moves into an engineering position,
then his scope of skills become more project management oriented.
(2) When an electrician moves into a contracting position,
then his scope of skills become more business oriented.
I had read articles in "EDN" about this, several decades ago,
but never noticed it much
until I had to associate with JMs and ECs on a daily basis.

 

[kwired]

I'll bet I could ask 100 electricians what Kirchoff's Law was
and less than 10 would know./QUOTE]

Kwired,
Ever read about Kirchoff's Second Law, the Voltage Law ?
Had not thought about it in a while, so I looked into Wiki for a clear statement
"The principle of conservation of energy implies that
The directed sum of the electrical potential differences (voltage)
around any closed circuit is zero."

I have had master ECs lecture me on the non-existence
of any variation of Kirchoff's Current Law.
In fairness, I should note that:
(1) When a technician moves into an engineering position,
then his scope of skills become more project management oriented.
(2) When an electrician moves into a contracting position,
then his scope of skills become more business oriented.
I had read articles in "EDN" about this, several decades ago,
but never noticed it much
until I had to associate with JMs and ECs on a daily basis.

I can not recite word for word any of Kirchoff's laws, but the concepts to those laws are in memory someplace and do get used a lot. I don't know how anyone could claim to be a service tech to much of any degree without some basic electricity knowlegde. Understanding basic electrical theory makes much of the code make more sense also.

There are too many installers and not enough electricians. On construction I get that. In service work a little theory makes a big difference in productivity.

 

[iceworm]

KCL: Sumation of currents entering a node equals zero. Or, the currents entering a connection point equal the currents leaving the point.

KVL: Sumation of voltages around a loop equals zero.

So sayeth the worm:roll:

 

[Strife]

Ahh, you call it tomato, I call it tomatoes.
So to you:
E=E1+E2 or
E-E1-E2=0 (for a series circuit do anyone can understand it)
and
I=I1+I2 or
I-I1-I2=0 (for a parallel circuit, again, so everyone can understand it)
doesn't mean the same thing?

So sorry I called it ampacity, you're right, it is current. Actually ampacity refers to a conductor capability of withstanding a certain "CURRENT" going through it.

Personally I think less than one would know, but that's my opinion, which seems to be wrong, considering I say "potatoes" (which BTW both spellings checked the spell checker).
Last, what does it have to do with it? Mike Holt had a newsletter on that a few years ago, but I'll try to explain it. Based on Ohms law, many people FALSELY believe that current will chose the fastest path to ground (which is not true, but it gives that false impression). That's why (falsely) we have ground rods installed at light poles.
Now, if I have a path to ground (of let's say 1 ohm) and a path through my body (of let's say 1,000 ohms) and I look at Kirchoff law, I know my body still gets zapped by the same voltage as the one going through the ground path as the voltage is same in a parallel circuit. So unless an OCP trips I'm screwed.

Do you mean Kirchoff as in KVL (Kirchoff's Voltage Law)?
"The directed sum of the electrical potential differences around a loop must be zero."

Then there is Kirchoff's Current Law.

"The sum of all currents entering a node is equal to the sum of all currents leaving the node." It is definitely not, "while the sum of ampacities in a parallel circuit are 0" Ampacity has nothing to do with Kirchoff's equations, it's a capacity rating for conductors, cables and devices and the term wasn't even in existence when Kirchoff was around.

I'll bet I could ask 100 electricians what Kirchoff's Law was and less than 10 would know.




What on Earth does that have to do with the trip rating of a breaker being in excess of the amount of current that would be able to electrocute someone?

 

[Strife]

Really?
And I always thought the EGC was a redundant system. Because the grounded conductor being "under load" has a tendency of failing. I mean, isn't that the reason we went from 3 wires for dryers and oven to 4 wires? Isn't that the reason we went from no ground to ground wire and receptacles?
And all my life I thought the argument for the EGC was made that the neutral fails with time under load so we need a redundant null point. I didn't mention the secondary function of the ground as keeping POCO's transformers to a null point because most people don't know it anyway.
Oh look, I'm all for a null point, and I'm not against a redundant null point, but prey tell, what other function other than those two the EGC performs? (not talking about electronics protection and such)

This is a common mistake I see on the forum quite often, including this thread. If the sole purpose of the EGC was clearing a fault and tripping the OCPD, then we wouldn't need both a grounded conductor and a grounding conductor, as they both are capable of clearing a fault and tripping the OCPD. The difference between the two is the voltage drop across a current carrying conductor.

The reality is that the EGC has several duties, and clearing a fault is only one of them. Another important factor that seems to get forgotten is that the EGC also maintains a voltage reference to surrounding equipment and structures.

This latter issue is why the original question was not entirely stupid (ignoring code for the moment). The potential exists for a floating chassis voltage that is not the result of a fault condition. An induced voltage on a chassis will not trip a GFCI, nor the OCPD, yet can still bite the end user.

By the way, the name you were searching for is Kirchoff.

 

[Strife]

Isn't it what I said?

KCL: Sumation of currents entering a node equals zero. Or, the currents entering a connection point equal the currents leaving the point.

KVL: Sumation of voltages around a loop equals zero.

So sayeth the worm:roll:

 

[kwired]

Really?
And I always thought the EGC was a redundant system. Because the grounded conductor being "under load" has a tendency of failing. I mean, isn't that the reason we went from 3 wires for dryers and oven to 4 wires? Isn't that the reason we went from no ground to ground wire and receptacles?
And all my life I thought the argument for the EGC was made that the neutral fails with time under load so we need a redundant null point. I didn't mention the secondary function of the ground as keeping POCO's transformers to a null point because most people don't know it anyway.
Oh look, I'm all for a null point, and I'm not against a redundant null point, but prey tell, what other function other than those two the EGC performs? (not talking about electronics protection and such)

Equipment grounding is for improving overcurrent device performance when there is a ground fault, and for keeping everything that is bonded together at the same potential.

Earth grounding - the grounding electrode system - rods, plates, CEE, metal piping, is primarily for lightning.

They are two different concepts and are not that closely related in what they do. In a good installation they are like one conductor in the fact that there should be little impedance between them. But if you were to disconnect the grounding elecrode conductor from the grounding electrode(s) and connect a 120 volt ungrounded conductor to the electrode in many cases you will not even trip a 5 amp fuse. Earth is not a good enough conductor to let this much current flow.

The reason for running separate grounded and equipment ground conductors is not because the grounded conductor will fail. It is partly because if it does fail you have elevated voltage between anything connected to it and ground.

There is also an elevated voltage between ground and a grounded conductor with a load on it, even with nothing wrong in the circuit. The longer the conductor is the more resistance it will have meaning a higher voltage between it and ground.

 

[Rick Christopherson]

Earth grounding - the grounding electrode system - rods, plates, CEE, metal piping, is primarily for lightning.

Why do so many electricians here repeat this without taking the time to really think this over? You've been sold on this concept, and no one bothers to question the validity of it. Of the functions of the GEC, lightning is one of the more trivial aspects.

• If there is a direct strike on a building, the GEC isn't going to protect a single thing.The entire electrical system is still going to be elevated to a huge voltage.
• If you think the lightning is going to follow the grounding system and go out through the ground rod, thankfully it doesn't, because it would damage that much more of the structure as it traveled from the point of entry to the GEC.
• Is lightning selective? If it strikes a building, is it only going to follow the green colored wires? (If you are about to say it wants to find a path to ground (zero volts) 120 volts is pretty trivial in the general scheme of things and constitutes a fraction of an inch worth of spark gap.)
• If the lightning strike was nearby but not a direct strike, the GEC is going to experience an elevated voltage and actually carry it into the premises.
• If the strike is on the utility, the premises wiring is still going to be elevated and the premises GEC won't protect a thing.
• If the primary purpose of the GEC was for lightning, then it wouldn't matter if the resistance to earth was 25 ohms or 2500 ohms. There would be no voltage difference without current flow, and once current flows in a lightning strike, resistance is meaningless because plasma is the current carrying conductor.

The GEC serves many functions, but lightning is far from its primary purpose.