Sub panel question

[brian john]

Re: Sub panel question

eprice:

In the "legal" system the only time there is current flow on the EGC (other that leakage current) is when there is a short/fault downstream from the grounded conductor/neutral. If the person or bodies you mentioned happen to inadvertently become this path the downstream voltage is typically too low to have a shocking impact on the inadvertent human conductor.

 

[bennie]

Re: Sub panel question

If you take an equipment ground wire and stick each end in the dirt, at different locations, you will get current flow, even with the service switch open.

[ July 11, 2003, 06:42 PM: Message edited by: bennie ]

 

[karl riley]

Re: Sub panel question

Brian,
yes, I have been happily using the flexible amp probe for years. First available only from a Scandanavian firm, now AEM sells a good one. I have 2 3' probes which can be connected in series and give a correct reading. Good for going around large bussways and snaking around a whole circuit of SE cables.

The gaussmeter will take care of everything you can't get the probes around, like steel beams in the wall, HVAC ducts, etc. It saves time because you don't have to snake anything around the pipe, etc. Just walk along beside it and watch the numbers. You can make rough estimates of amperage, but sooner or later you have to get an amp measurement to know exactly what is happening.

Karl

 

[brian john]

Re: Sub panel question

Karl:

I/we (the company) own several gauss meters, We utilize these for troubling shooting a variety of problems. Some are wiring errors and some are accidental neutral to ground faults. Both instruments make work easier.

There was a grounding seminar given recently in our area, one of the other techs went to this seminar and told me the following.

During the discussion on bonding at the main service an attendee inquired why the manufacture always seemed to ship a 1" to 1-1/4" green screw with every panel. Some attendees gasped others just looked quizzically. The presenter inquired what type of work this electrician did. The answer- Mostly residential, had been at it 20 plus years and was licensed.

 

[bennie]

Re: Sub panel question

Gentlemen: Analyze a service system with a separate ground wire from the transformer X-0 to the service panel ground bus.

Separate the neutral and ground bus. The neutral will be floating and earthed only at the pole.

Only fault and coupling current should flow on the ground wire. Load current will not.

The neutral will not be earthed after the main.

No need to earth the ground wire, except at the pole.

Check this out, I may be in the twilight zone again.

 

[karl riley]

Re: Sub panel question

Bennie, I am told this is how they do it in Germany and probably some other advanced European countries. Makes sense to me.
Karl

 

[karl riley]

Re: Sub panel question

Hey Bennie, good use of the word "earth" (using a noun as a verb, tsk tsk!) But it's a good usage. If we were to say "to earth" instead of "to ground" we could avoid confusion, because "to ground" can mean different things, but "to earth" has only one obvious meaning.

Then if we mean bond, we say bond. We could get rid of the word "ground" altogether.
Karl

 

[Ed MacLaren]

Re: Sub panel question

concord,
The answer to your question has been given by a couple of the guys. The sketch might help illustrate what they said.

To sum up, the illegal bonding jumper puts the (green) equipment grounding conductor (EGC) in parallel with the (grey) grounded neutral, and this permits some of the unbalanced load current to flow back to the transformer through the EGC which is not sized for such use.

Also, metallic conduits often serve as the EGC and can be exposed. A loose EMT coupling, for example, could have a considerable voltage drop across it.

An open neutral situation could result in the EGC carrying all of the unbalanced load current.

Ed

[ July 13, 2003, 10:22 AM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: Sub panel question

Karl: You caught me I used the "ed" incorrectly. I tacked it on to a noun.

I do agree with you, Don, and others, that using terms that are more technically correct, will take some confusion out of grounding. I am not sure the word "bond" is technically correct.

The word "fault" means failure. I would suggest "fault conductor" or something along that line.

 

[Ed MacLaren]

Re: Sub panel question

During my 35 years of teaching apprentices, I had often wondered why the "grounding/bonding" subject seemed to be so difficult for a majority of the students.

I became convinced that it was because we were trying to use the word "ground" for too many different purposes.

The word "ground" no longer has any meaning in a technical discussion, except to refer to the dirt itself, because any conductor that is associated with the word could be -
1. a system grounding conductor,
2. an equipment grounding conductor,
3. or the grounded (neutral) conductor.
Without specifying which one, it is impossible to communicate effectively.

Immediately after (about 15 years ago) we started using the term "bonding conductor" in place of "equipment grounding conductor", the level of confusion was greatly reduced.

Ed

 

[tom baker]

Re: Sub panel question

Exactly right Ed. The code requires a receptacle to be
406.3
To Be Grounded. Receptacles and cord connectors that have grounding contacts shall have those contacts effectively grounded.

The defintion of of effectively grounded is

Grounded, Effectively. Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.

So the code leads us to belive we connect the "ground" on a receptacle to earth, when what we are doing is making a low impedance path back to the source to clear a line to case fault or bonding.

No wonder we are confused.

 

[gwz2]

Re: Sub panel question

Let change Ed's diagram of July 13, 2003 10:16AM to:

At the left hand enclosure, remove the Switch and Fuse symbols and install a 1? 3W 120/240V 100A meter.
And remove the word "Main" from the Main Bonding Jumper text.

At the right hand enclosure remove the word "Illegal" from the Illegal Bonding Jumper text and name the panelboard as the Main Service panelboard.

In the 'Stick Man circle' add text that his hands are on each sice of a RMC (Rigid Metal Conduit) coupling.

Per 310.15(B)(6) the 100A panelboard could be supplied with 4 AWG conductors if a dwelling unit.

For a "Ball Park calculation using the DC ohms law and the following values:

120V at the meter location.
5 feet of 4 AWG for each of the conductors.
5 feet of 1" RMC
30 Amperes of Un-Balanced current on the circuit with;
R1 as the ungrounded conductor resistance to panelboard.
R2 as the resistance of the load to the grounded conductor.
R3 as the resistance of the Grounded conductor from the panelboard to the meter enclosure.
R4 as the resistance of 5 feet of the RMC.
R5 as the equivalent resistance of R3 and R4 in-parallel.

Chapter 9 Table 8, 1000' of 4 AWG is 0.308 ohms (R). [ R1 and R3 ].
Soares book on Grounding Table 3 shows 100' of 1" RMC to have 0.0154 ohms [ R4 ].

R1 = 0.308 ohms / 1000 feet = 0.000308 ohms times 5 feet = .00154 ohms = R1.

R3 = same as R1 above.

R4 = 0.0154 / 100 feet = 0.000154 ohms times 5 = 0.00077 ohms = R4.

R5 = [ 1/Rt = 1 / R3 + 1 / R4 ].
R5 = [ 1/Rt = 1 / 0.00154 + 1 / 0.00077 ]
R5 = [ 1/Rt = 649.35 + 1298.701 ]
R5 = [ 1/Rt = 1948.051 ]
R5 = 1 / 1948.051
R5 = 0.000513 ohms

E = I * R
R1vd = 30A * R1
R1vd = 30A * 0.00154 ohms = 0.0462 vd to the panelboard load.
R1vd = 0.0462.

R5vd = 30A * 0.000513 = 0.01539 vd from the load at the panelboard back to the meter enclosure.

120V ( at the meter ) - 0.0462 vd and the 0.01539 vd = 119.93841 vd at R2 ( the load).

Amps on R3 is: Evd / R3
Amps on R3 = 0.01539 / 0.00154 = 9.9935 amps.

Amps on R4 is: Evd / R4
Amps of R4 = 0.01539 / 0.00077 = 19.9870 amps.

If the Stick Man is holding a defective RMC coupling , He is subject to 19.9870 amps at 0.01539 volts.
R1 vd (voltage drop) = I * R1.

 

[don_resqcapt19]

Re: Sub panel question

gwz,
If the connection at the coupling is poor the current through the pipe will be limited by the resistance of the defective coupling. If the coupling is "open" the stick man will be sujected to a voltage equal to that of the grounded conductor voltage drop. In this example the voltage will be 0.00000256 volts and if you assume the stick man has a resistance of 500 ohms, then he will be sujected to 0.00000005 amps.
Don

 

[bennie]

Re: Sub panel question

Glenn: I agree with the figures. The stick man will only feel the potential difference between the two panels.

Remove the equipment ground wire, and with no other conductive paths, what difference does it make?

 

[gwz2]

Re: Sub panel question

Please note that the RMC, of this example, carries 2/3rd's of the Neutral current ( the un-balance current) between the meter and the Main Service panelboard.

When metal raceways are used between the meter and the Main disconnect of any typical service in witch the grounded circuit conductor goes thru the meter to the main diconnecting means.

Also, there will be a third parallel path ( thru the earth) with the grounded conductor if the Meter has a ground rod GEC and the metal water electrode GEC is from the panelboard.

Also, on every service with a Grounded conductor, there will be an 'earth' parallel path between the Main panelboard Grounding Electrode (GE)and the grounding of the utility companys' transformer grounding system.

Always, there will be parallel paths when there is a system with a grounded conductor circuit.

I think, more importantly, is the fact that the metal raceway will most likely carry more than 50% of the 'neutral current' between the meter and the Main disconnecting means.

 

[hurk27]

Re: Sub panel question

Glen the third path through Earth would only have the amoutn of current that the resistance drop of earth would allow. which would be the current applied from the voltage drop of the service neutral, if this voltage drop was a full 120 volts and the resistance of the electrod was 25 ohm's the current would be only 4.8 amps but since the actule voltage drop accross the service neutral would only be 2 or 3 volts the current on the gec would only be .12 of a amp of course the water ground would have more current on it but it to would be limited to the circuit resistance.

 

[websparky]

Re: Sub panel question

Gentlemen,

Very interesting and educational thread!

One question;

In the examples above with the stickman, as long as the other path for grounding exists, would he not receive zero voltage?

Thanks,
Dave

 

[gwz2]

Re: Sub panel question

I would think, that for the low voltage involved in the neutral parallel path of that 5 feet or so, that the current, even if very high, would not hurt most individuals because of the ' body ' resistance.

Really, some of the points I was trying to have comments on were:

1) The current flow on the RMC could reach welding status.

2) The current, hopefully, will not hurt a healthy person.

3) That an AC guru engineer would make some AC vs DC correction calculations - de-bunking the 'ball park' DC ohms law calculation. I do not know how!

4) On a 1? 120/240V system where one ungrounded conductor opens, it would be possible for the neutral current to approach the Amps of the service rating, ie. 100A, 200A, 400A, 600A, etc.
That short metal raceway could be carrying 50% or more of those AMPS!

5) On a Grounded Conductor 3? 3W system, It should act as the presented senerio.
On a 3? 4W system and losing either one or two phases will cause neutral current to 'run' high.
Seen many electricians always chose A & B for feeders rather than AB, BC, CA for feeders probably un-balancing the neutral current.

[ July 14, 2003, 07:17 AM: Message edited by: gwz2 ]

 

[don_resqcapt19]

Re: Sub panel question

Dave,
If our stickman is holding on to both sections of the conduit with an "open" connection at the coupling, he will be subjected to a voltage equal to that of the voltage drop on the other path(s). Under normal conditions this should not be enough voltage to cause any problems, but under fault conditions, until the OCPD clears the fault, this could be enough voltage to cause harm.
Don

 

[gwz2]

Re: Sub panel question

Don,

Quite a deference of my and yours Voltage Drop (vd)

across R4 when R5 is open. Mine would be 0.0462 +/- volts and yours is 0.00000256 volts.

Where did the calculations get mixed-up?